Fisher X202B
- Thread starter AlTinkster92
- Start date
Output Stage Modifications Complete
Pic shows the results. The old screen grid and cathode wiring, and the old cathode resistors have all been removed now, and matched Screen Stability resistors have been installed at each output tube socket. To facilitate this, the originally empty pin #1 has now become the new screen grid supply buss at each socket for the SS resistors to attach to. Additionally, matched, <1% tolerance cathode current sampling resistors have been installed at each output tube socket as well.
The second pic shows the as received last stop in this unit. The low voltage DC heater/bias supply will get new discrete caps installed underneath the chassis to replace the duties of the existing (can positive!) can cap serving that function. A previous tech has already replaced the electrolytic cap used in the bias tap position.
Once this is complete, the tubes will be tested, and a performance check completed.
Back soon --
Dave
Pic shows the results. The old screen grid and cathode wiring, and the old cathode resistors have all been removed now, and matched Screen Stability resistors have been installed at each output tube socket. To facilitate this, the originally empty pin #1 has now become the new screen grid supply buss at each socket for the SS resistors to attach to. Additionally, matched, <1% tolerance cathode current sampling resistors have been installed at each output tube socket as well.
The second pic shows the as received last stop in this unit. The low voltage DC heater/bias supply will get new discrete caps installed underneath the chassis to replace the duties of the existing (can positive!) can cap serving that function. A previous tech has already replaced the electrolytic cap used in the bias tap position.
Once this is complete, the tubes will be tested, and a performance check completed.
Back soon --
Dave
Attachments
Hi Dave, any of the 12ax7's test low I left a couple RCA black plates and a couple Telefunks, amp is looking great underneath and glad to know the outputs will last longer now and not run so hard. AL
Modifications and Repairs Finished
Finished up the heater/bias supply this evening. Installed the output tubes back in their as received sockets, and powered the unit up. A rather boring event on the whole (a good thing!) except for finding out the quiescent current that the output tubes were drawing as received, since the DC bias and balance controls had not been touched up to this point.
Two tubes were drawing 41 ma each, one was drawing 43 ma, while the fourth was drawing 52 ma. It appears that the last effort in setting the DC balance controls simply had them adjusted for equal negative grid bias voltage to all four output tube control grids. The pair of tubes drawing 41 ma each were in the same channel, and represent a good match to each other with regards to quiescent current draw. The other pair was not well matched at all (being mismatched by just over 20%), and all four tubes were drawing a high level of quiescent current for the B+ conditions present. As a result, all four tubes were backed down to reach a balanced 33 ma of quiescent current draw for each of the four tubes. After warmup, the bias remained stable as adjusted.
Tomorrow, this unit should make it to the test bench to verify the performance of both channels.
Pics include:
1. The finished heater/bias supply. The area has been tidied up with stress relieved on one of the resistors from the original build. It was much the same scenario as the tension of the resistor pulling on the can cap terminal discussed earlier. Note that significant space has been allowed around the 15 ohm power resistor, since this device runs quite hot in this design.
2. The finished underside shot. The modification/repair work I did can clearly be seen if you look for it, but it blends easily enough into the look of the original build.
3. All buttoned back up ready for a battery of tests tomorrow.
Dave
2.
Finished up the heater/bias supply this evening. Installed the output tubes back in their as received sockets, and powered the unit up. A rather boring event on the whole (a good thing!) except for finding out the quiescent current that the output tubes were drawing as received, since the DC bias and balance controls had not been touched up to this point.
Two tubes were drawing 41 ma each, one was drawing 43 ma, while the fourth was drawing 52 ma. It appears that the last effort in setting the DC balance controls simply had them adjusted for equal negative grid bias voltage to all four output tube control grids. The pair of tubes drawing 41 ma each were in the same channel, and represent a good match to each other with regards to quiescent current draw. The other pair was not well matched at all (being mismatched by just over 20%), and all four tubes were drawing a high level of quiescent current for the B+ conditions present. As a result, all four tubes were backed down to reach a balanced 33 ma of quiescent current draw for each of the four tubes. After warmup, the bias remained stable as adjusted.
Tomorrow, this unit should make it to the test bench to verify the performance of both channels.
Pics include:
1. The finished heater/bias supply. The area has been tidied up with stress relieved on one of the resistors from the original build. It was much the same scenario as the tension of the resistor pulling on the can cap terminal discussed earlier. Note that significant space has been allowed around the 15 ohm power resistor, since this device runs quite hot in this design.
2. The finished underside shot. The modification/repair work I did can clearly be seen if you look for it, but it blends easily enough into the look of the original build.
3. All buttoned back up ready for a battery of tests tomorrow.
Dave
2.
Attachments
tube-a-lou
Addicted Member
Another great job done by Dave, it's gong to sound sweet!
Tube
Tube
Dave takes a quality standard (as set by FISHER in this case) and throw's it out the window, creating a standard for detail that is orders of magnitude more orderly, seemlessly blends in with existing hardware, and damned hard to match, period. His attention to the minutia is unswerving, creating a restoration that looks better (once you figure out where his work is!!!) without calling attention to itself, and from all reports sounds as good as it looks.
Now if i could just bend my resistor and cap leads 1/2 as good as his.............
Congrats on another JEWEL in the FISHER Crown, Dave.
Now if i could just bend my resistor and cap leads 1/2 as good as his.............
Congrats on another JEWEL in the FISHER Crown, Dave.
Tube Testing
Thanks for all the kind words! It is everyone here that makes the AK Fisher Forum so special -- and it is special -- far and away above so many other (good) sites out there. The neat thing here is that we ALL learn from each other......
The tubes in Al's X-202B warrant comment:
OUTPUT TUBES:
The two tubes mentioned previously that were closely matched quiescently in the unit, were in fact proved to be so in the power output tube tester. There quiescent balance is within .54%, while their dynamic (power output) match is within 1.3%. This means that these two tubes are matched within better than 2% of each other, which is a very tight match indeed. These tubes produced 93.53% and 94.74% of ANPO, producing these figures immediately at the beginning of the test, which held steady though out the test period.
Of the other two tubes, one is showing some age, with a rising power output with the application of drive. This is a classic indicator of a tube with some age on it, as the heat produced from generating the power output indication aids the heater in supplying heat to the cathode, so its efficiency rises during the test, as shown by the increasing output level -- all as opposed to an immediate and steady indication that a very healthy cathode will produce, as indicated by the two tubes mentioned above. This tube rose quickly enough to produce 85.24% of ANPO during the test period, and was continuing to rise when the test was terminated. Leaving the test on longer would have produced a higher reading no doubt. But that is generally not the way the tube is used, with the important point being that the true condition of the tube has clearly been spot lighted from this observation.
The other tube -- the one that required much more bias voltage to achieve a normal quiescent current -- is physically the most worn looking tube. But using Gomer Pyle's best accent, Surprise, Surprise! That sucker produces 107.97% of ANPO! And it did so quite immediately, and held it, too.
As I have said before, beside the accuracy that a power output test provides in testing power output tubes, how they act during the test is as much of an indicator of their condition as the power they produce is. Tubes that give an immediate and steady indication indicate plenty of life left within a tube, while one that rises during the test indicates one further along in its useful life.
I would also emphasize, that the tester tests a DUT with a heater voltage of precisely 6.300VDC. For tubes that rise in power during the test, further testing will home in on their condition more accurately. It doesn't take much extra heat to give them the push that they need if they are still very useful as "good" devices. Such is the case with the tube indicating a rising power output. Retesting this tube with an applied heater voltage of 6.60 VDC caused the tube to produce 99.05% of ANPO -- a rather remarkable recovery indeed! With this being the oh-so typical voltage that today's typical 121 vac AC line levels provide to the heaters, the tube then -- in this application -- is good enough indeed. All of the tubes were tested with the tester set up for Class AB1 operating conditions -- just as they operate under in the X-202B.
Overall then, all four tubes are rated as "GOOD". Because the X-202B includes both AC and DC balance controls, the unit can be adjust to account for the less that stellar match between the two right channel output tubes.
SMALL SIGNAL TUBES:
All of these tubes were tested using Gm as a standard of quality. Within these 8 tubes, one phono preamp tube is considered as WEAK in both sections and the other as weak in one section, while the tone control amplifier tube is also weak on one section, as judged again their Gm standard. The tubes were tested in my other proprietary tester, that tests a DUT's Gm using the conditions as outlined in the RCA Receiving Tube Manual for the tube being tested. But as with power output tubes, further testing can help to determine a tube's true condition more accurately, relative to the application it is being used for.
As stated before, Gm is a poor indicator of power tube performance, because the test is conducted at such low signal levels. It is therefore more accurately considered a test of characteristic, rather than quality, being a more suitable test for small signal tubes. But for 12AX7 tubes used in the positions of phono preamp and tone control amplifiers, once again, Gm is still really not the best characteristic to test for, relative to what they do in the amplifier.
And exactly what do they do? They provide gain -- and tons of it. After all, how do you think that the X-202B achieves that voltage gain of some 80,000(!) from the phono input jacks to the plates of the output tubes?? The 12AX7 tubes provide the vast majority of this gain.
Now gain is a measure of VOLTAGE output achieved by a voltage input applied to a tube. But Gm is a measure of CURRENT output achieved by a voltage input applied to a tube. Since 12AX7 tubes generate miniscule amounts of current output, Gm testing is not the best indicator of their performance when asked to produce prodigious amounts of gain in a circuit. Obviously then, a GAIN test is a much more useful indicator --overall -- of 12AX7 performance then, which is the reason I built this type of test capability into my Gm tester when testing such tubes.
Using a gain test then as a measure of quality, the three tubes in question all produced gain levels that were better than 95% of the typical gain that NOS 12AX7 tubes will display in my tester (a value of 76). With this characteristic judged as producing "weak" performance for DUTs with gain levels of <90% of their standard, these tubes are in fact performing quite will in the positions they occupy in the Al's X-202B. In positions where 12AX7 tubes are used to actually produce some small amounts of current output (cathode follower positions for example), these three tubes would be judged differently. But for the current position they occupy, they are just fine indeed.
So when is a Gm test appropriate? It is ideal for Med Mu tubes like 6SN7s, 12AU7s, and the like. After all, these tubes primarily turn high impedance voltage levels at their grids into higher voltage, low impedance values at their plate -- which requires a measure of current gain to produce -- and THAT is exactly what Gm testing is defined by.
With good tubes then, the unit can now be performance tested hopefully later on today.
Back with those results when they are produced.
Dave
Thanks for all the kind words! It is everyone here that makes the AK Fisher Forum so special -- and it is special -- far and away above so many other (good) sites out there. The neat thing here is that we ALL learn from each other......
The tubes in Al's X-202B warrant comment:
OUTPUT TUBES:
The two tubes mentioned previously that were closely matched quiescently in the unit, were in fact proved to be so in the power output tube tester. There quiescent balance is within .54%, while their dynamic (power output) match is within 1.3%. This means that these two tubes are matched within better than 2% of each other, which is a very tight match indeed. These tubes produced 93.53% and 94.74% of ANPO, producing these figures immediately at the beginning of the test, which held steady though out the test period.
Of the other two tubes, one is showing some age, with a rising power output with the application of drive. This is a classic indicator of a tube with some age on it, as the heat produced from generating the power output indication aids the heater in supplying heat to the cathode, so its efficiency rises during the test, as shown by the increasing output level -- all as opposed to an immediate and steady indication that a very healthy cathode will produce, as indicated by the two tubes mentioned above. This tube rose quickly enough to produce 85.24% of ANPO during the test period, and was continuing to rise when the test was terminated. Leaving the test on longer would have produced a higher reading no doubt. But that is generally not the way the tube is used, with the important point being that the true condition of the tube has clearly been spot lighted from this observation.
The other tube -- the one that required much more bias voltage to achieve a normal quiescent current -- is physically the most worn looking tube. But using Gomer Pyle's best accent, Surprise, Surprise! That sucker produces 107.97% of ANPO! And it did so quite immediately, and held it, too.
As I have said before, beside the accuracy that a power output test provides in testing power output tubes, how they act during the test is as much of an indicator of their condition as the power they produce is. Tubes that give an immediate and steady indication indicate plenty of life left within a tube, while one that rises during the test indicates one further along in its useful life.
I would also emphasize, that the tester tests a DUT with a heater voltage of precisely 6.300VDC. For tubes that rise in power during the test, further testing will home in on their condition more accurately. It doesn't take much extra heat to give them the push that they need if they are still very useful as "good" devices. Such is the case with the tube indicating a rising power output. Retesting this tube with an applied heater voltage of 6.60 VDC caused the tube to produce 99.05% of ANPO -- a rather remarkable recovery indeed! With this being the oh-so typical voltage that today's typical 121 vac AC line levels provide to the heaters, the tube then -- in this application -- is good enough indeed. All of the tubes were tested with the tester set up for Class AB1 operating conditions -- just as they operate under in the X-202B.
Overall then, all four tubes are rated as "GOOD". Because the X-202B includes both AC and DC balance controls, the unit can be adjust to account for the less that stellar match between the two right channel output tubes.
SMALL SIGNAL TUBES:
All of these tubes were tested using Gm as a standard of quality. Within these 8 tubes, one phono preamp tube is considered as WEAK in both sections and the other as weak in one section, while the tone control amplifier tube is also weak on one section, as judged again their Gm standard. The tubes were tested in my other proprietary tester, that tests a DUT's Gm using the conditions as outlined in the RCA Receiving Tube Manual for the tube being tested. But as with power output tubes, further testing can help to determine a tube's true condition more accurately, relative to the application it is being used for.
As stated before, Gm is a poor indicator of power tube performance, because the test is conducted at such low signal levels. It is therefore more accurately considered a test of characteristic, rather than quality, being a more suitable test for small signal tubes. But for 12AX7 tubes used in the positions of phono preamp and tone control amplifiers, once again, Gm is still really not the best characteristic to test for, relative to what they do in the amplifier.
And exactly what do they do? They provide gain -- and tons of it. After all, how do you think that the X-202B achieves that voltage gain of some 80,000(!) from the phono input jacks to the plates of the output tubes?? The 12AX7 tubes provide the vast majority of this gain.
Now gain is a measure of VOLTAGE output achieved by a voltage input applied to a tube. But Gm is a measure of CURRENT output achieved by a voltage input applied to a tube. Since 12AX7 tubes generate miniscule amounts of current output, Gm testing is not the best indicator of their performance when asked to produce prodigious amounts of gain in a circuit. Obviously then, a GAIN test is a much more useful indicator --overall -- of 12AX7 performance then, which is the reason I built this type of test capability into my Gm tester when testing such tubes.
Using a gain test then as a measure of quality, the three tubes in question all produced gain levels that were better than 95% of the typical gain that NOS 12AX7 tubes will display in my tester (a value of 76). With this characteristic judged as producing "weak" performance for DUTs with gain levels of <90% of their standard, these tubes are in fact performing quite will in the positions they occupy in the Al's X-202B. In positions where 12AX7 tubes are used to actually produce some small amounts of current output (cathode follower positions for example), these three tubes would be judged differently. But for the current position they occupy, they are just fine indeed.
So when is a Gm test appropriate? It is ideal for Med Mu tubes like 6SN7s, 12AU7s, and the like. After all, these tubes primarily turn high impedance voltage levels at their grids into higher voltage, low impedance values at their plate -- which requires a measure of current gain to produce -- and THAT is exactly what Gm testing is defined by.
With good tubes then, the unit can now be performance tested hopefully later on today.
Back with those results when they are produced.
Dave
Thanks Dave, I have had to re read your post 5 times to understand what you are saying but I think I got it, will re read it many more times I know. Also glad the tubes were ok, my new Westies weren't needed and will save them for future use. I admire your knowledge on all this and have learned a lot! Will look foward to the performance test. Thanks Al
Well, Hold the Phone......
The bench testing was going along smoothly, with the unit individually producing 35.4 watts RMS @ 1 kHz in the left channel, and 34.5 watts RMS at the same frequency in the right channel, at the onset of clipping. At 1 db down from a 35 watt power rating, the channels produced an average of .5 % THD at this frequency -- which was largely a product of biasing the tubes to 33 ma each, versus the factory specification of 40 ma each. This is an acceptable tradeoff in return for the extra output tube life expectancy this move produces.
Power output was strong over a 30 Hz to 20 kHz bandwidth, with the left and right channels measuring 34.5 watts and 33.1 watts respectively at 30 Hz, and 33.1 and 31.6 watts respectively at 20 kHz. With both channels driven, each channel produced 29.57 watts RMS at 1 kHz at the onset of clipping. This is very respectable performance for the stock design, with the output tubes now running notably cooler than before.
Since no particular issues were noted here, I moved onto some square wave testing to check the response characteristics. While the left channel was very good, here the right channel was showing obvious problems -- not so much you may have directly heard it, but problems none the less. That lead to using the scope to signal trace the right channel in a troubleshooting effort, where I ultimately came up on the Recording Output jack. This jack can be wonderful troubleshooting point when tracking down issues with gain and response, so I plugged in the scope to check response through the unit at that point -- where upon the unit promptly went nuts.
Shutting it down, I disconnected the scope, checked everything again and powered it back up. Plugged in the scope, and it went nuts again. This really was nuts. So, off came the bottom plate again, and troubleshooting began in earnest.
When trying different scope cables didn't resolve the issue, I started checking ground continuity, and found the Recording Output jack was open to ground. Hummmmm. The ground lead was good at the jack, so I checked the ground of the jack next to it. No ground. Hummmmmm. In fact, the whole bottom jack array containing the reverb jacks, Recording Output jack, and the Center Channel Output jack had no continuity to ground what so ever.
While this didn't particularly cause a hum problem (because the wire was located within the enclosed chassis), it did cause a response issue, since the ungrounded cable snaked all throughout the power amp area, allowing coupling from the circuits of that area to leak into the cable, causing reduced response in the right channel.
Further checks showed that the end of the shielded cable that should have been grounded to ground the Recording Output jack array, was in fact snipped off. When/how this happened, who knows? But grounding the end of the cable then corrected the vast majority of the response issue, with the remaining difference between the channels due to construction tolerances of the assembly line.
Pics show the tale:
1. The left channel showed a good 2 kHz square wave from the AUX 1 input, on through to the left channel speaker output. There is slight overshoot, but hardly an amount of any concern.
2. The poorly performing right channel by comparison. Response of the right channel is down over 2 db at 20 kHz.
3. Here's the little bugger causing all the commotion, dead center in the middle of the pic. The ground lead has been snipped off, compromising the ground connection for the rear panel Recording Jack array.
4. Reestablishing a ground for the jack and cabling associated with it produced a notably improved 2 kHz square wave, since no outside signal could influence the signal running with the cable anymore. The difference between the two channels now is within the realm of component tolerances between the two channels. Response of this channel at 20 kHz is now down just .5 db. The response of the left channel is down .25 db at 20 kHz.
I will correct this issue tomorrow, where upon that should then finish up this unit.
Finally, I did a quick check of my own X-202B, which did not display this problem, so it appears to be a mystery at this point as to how it occurred in this particular amplifier. In any event, it points out one more time, why thorough testing of a unit is imperative after work is performed on a unit, to make sure its producing all the performance it was meant to produce!
Dave
The bench testing was going along smoothly, with the unit individually producing 35.4 watts RMS @ 1 kHz in the left channel, and 34.5 watts RMS at the same frequency in the right channel, at the onset of clipping. At 1 db down from a 35 watt power rating, the channels produced an average of .5 % THD at this frequency -- which was largely a product of biasing the tubes to 33 ma each, versus the factory specification of 40 ma each. This is an acceptable tradeoff in return for the extra output tube life expectancy this move produces.
Power output was strong over a 30 Hz to 20 kHz bandwidth, with the left and right channels measuring 34.5 watts and 33.1 watts respectively at 30 Hz, and 33.1 and 31.6 watts respectively at 20 kHz. With both channels driven, each channel produced 29.57 watts RMS at 1 kHz at the onset of clipping. This is very respectable performance for the stock design, with the output tubes now running notably cooler than before.
Since no particular issues were noted here, I moved onto some square wave testing to check the response characteristics. While the left channel was very good, here the right channel was showing obvious problems -- not so much you may have directly heard it, but problems none the less. That lead to using the scope to signal trace the right channel in a troubleshooting effort, where I ultimately came up on the Recording Output jack. This jack can be wonderful troubleshooting point when tracking down issues with gain and response, so I plugged in the scope to check response through the unit at that point -- where upon the unit promptly went nuts.
Shutting it down, I disconnected the scope, checked everything again and powered it back up. Plugged in the scope, and it went nuts again. This really was nuts. So, off came the bottom plate again, and troubleshooting began in earnest.
When trying different scope cables didn't resolve the issue, I started checking ground continuity, and found the Recording Output jack was open to ground. Hummmmm. The ground lead was good at the jack, so I checked the ground of the jack next to it. No ground. Hummmmmm. In fact, the whole bottom jack array containing the reverb jacks, Recording Output jack, and the Center Channel Output jack had no continuity to ground what so ever.
While this didn't particularly cause a hum problem (because the wire was located within the enclosed chassis), it did cause a response issue, since the ungrounded cable snaked all throughout the power amp area, allowing coupling from the circuits of that area to leak into the cable, causing reduced response in the right channel.
Further checks showed that the end of the shielded cable that should have been grounded to ground the Recording Output jack array, was in fact snipped off. When/how this happened, who knows? But grounding the end of the cable then corrected the vast majority of the response issue, with the remaining difference between the channels due to construction tolerances of the assembly line.
Pics show the tale:
1. The left channel showed a good 2 kHz square wave from the AUX 1 input, on through to the left channel speaker output. There is slight overshoot, but hardly an amount of any concern.
2. The poorly performing right channel by comparison. Response of the right channel is down over 2 db at 20 kHz.
3. Here's the little bugger causing all the commotion, dead center in the middle of the pic. The ground lead has been snipped off, compromising the ground connection for the rear panel Recording Jack array.
4. Reestablishing a ground for the jack and cabling associated with it produced a notably improved 2 kHz square wave, since no outside signal could influence the signal running with the cable anymore. The difference between the two channels now is within the realm of component tolerances between the two channels. Response of this channel at 20 kHz is now down just .5 db. The response of the left channel is down .25 db at 20 kHz.
I will correct this issue tomorrow, where upon that should then finish up this unit.
Finally, I did a quick check of my own X-202B, which did not display this problem, so it appears to be a mystery at this point as to how it occurred in this particular amplifier. In any event, it points out one more time, why thorough testing of a unit is imperative after work is performed on a unit, to make sure its producing all the performance it was meant to produce!
Dave
Attachments
Interesting, The Hum that is. I once tryed to use the center channel hook up (both) and got a hum as well, no audio, just a hum.. :scratch2:
It's a Wrap
I couldn't find any evidence of a lose end for the shielded cable with the snipped off ground lead, so it must be that it left FRC that way so many years ago. In any event, it's fixed now, with response from the high level input jacks to speaker terminals being within .3 db from 30 Hz to 20 kHz in both channels. Below 30 Hz, the built in LF filter starts to make its effects known, which is a Fisher standard feature of all their designs.
Pics include:
1. The repaired ground lead -- connected to the most logical point that it should have been connected to.
2. As a last detail, some appropriately colored Magic Marker on the speaker terminal board screws helps to cut through the confusion of properly connecting your speakers with respect to polarity. Connected as shown, the speakers are properly phased with the Phase Reverse switch turned OFF. The X-202B maintains absolute polarity from all selector switch input jacks, through to the speaker post connections. The impedance selector wire's terminal was hanging by a thread, and was reattached as well.
3. The output tubes definitely like the new arrangement, running notably cooler than when first received.
4. Singing a happy tune now. And most importantly, there's no hum, and you can even turn the volume down, too!
Dave
I couldn't find any evidence of a lose end for the shielded cable with the snipped off ground lead, so it must be that it left FRC that way so many years ago. In any event, it's fixed now, with response from the high level input jacks to speaker terminals being within .3 db from 30 Hz to 20 kHz in both channels. Below 30 Hz, the built in LF filter starts to make its effects known, which is a Fisher standard feature of all their designs.
Pics include:
1. The repaired ground lead -- connected to the most logical point that it should have been connected to.
2. As a last detail, some appropriately colored Magic Marker on the speaker terminal board screws helps to cut through the confusion of properly connecting your speakers with respect to polarity. Connected as shown, the speakers are properly phased with the Phase Reverse switch turned OFF. The X-202B maintains absolute polarity from all selector switch input jacks, through to the speaker post connections. The impedance selector wire's terminal was hanging by a thread, and was reattached as well.
3. The output tubes definitely like the new arrangement, running notably cooler than when first received.
4. Singing a happy tune now. And most importantly, there's no hum, and you can even turn the volume down, too!
Dave
Attachments
And the defective ground lead is why. It is part of the Recording Output jack array with the missing ground connection. Restoring the ground for the Recording Output jack restored it for all of the jacks in that jack group for channel B. The powered center channel speaker output connections always worked, because the ground connection of this discussion is not part of that circuit.
Dave
Dave
Ain't she Purdy Folks??? Cant wait to get her home and singing again. Great work Dave, I take a bow to you and your talents Sir! 
Hi Folks, wanted to say I will be making a road trip to pick up my X202B and my 800C tomorrow, anxious to get these back home and enjoy them. My thanks to Dave for all his work and expertise on these, also for the phone conversations we have had where I have learned a ton. My new Westie 7591's now have a home in the 800C along with a IBAM circuit to keep things smooth and cool... I consider myself very lucky to meet and talk with Dave, the guy knows his stuff... Thanks Dave and see ya tomorrow. Al
Congrats Al, it should give you many years of service. Enjoy!
Thanks Steve, these 2 are lifetime keepers for sure. Will have to hold off from future audio stuff for a while, have a 9 year old cat that has cancer, gotta take care of him first. Glad to have 2 fine pieces of Fisher gear...Al
A Few Last Ghosts
I have been running Al's X202B pretty good this past week -- as I always do in an effort to shake out any last issues that might crop up. And whadda ya know -- two did.
The first one was easy, with an intermittent loudness function -- in each channel individually when it felt like it, and with different symptoms in each channel when it would decide to go on hiatus. That was traced to the two loudness switch caps that would independently work when they felt like it. With those replaced, the loudness feature now engages and disengages dependably, providing the boost that it should in both channels when activated.
The second one is specifically for Larry and Naptown Rob, since apparently Larry lost significant amounts of hair over Nap's X-202B, and Nap is still without it even yet.
In looking over Al's unit for any last Gremlins to take care of (he's picking it up tomorrow), I noted that the left channel (only) would display a very notable hum when an open (as in, no input connected) line level input was selected (most notably on the Tuner setting), and the volume was advanced. On the other hand, the right channel only emitted a soft hiss in the same scenario. Thinking back to Larry's comments in this thread, as well as Nap's, it occurred to me that Al's unit was likely displaying the very same socially unacceptable behavior that Nap's unit was. It took an afternoon of detective work, but I finally found the cause of the little bugger -- at least in Al's unit. But being as the cause almost surely goes back to the original build, it's very likely that what I found could also be the cause of the hum that has been so elusive in Nap's unit as well. The symptoms he described are exactly the same that Al's unit displayed.
The pics tell the story, but basically, the problem is exactly in the location that Larry suggested -- that being the Mode Selector switch. The hum was 60 Hz in nature, which ruled out power supply hum (it would be 120 Hz), and any hum from the heater wiring, since all the heaters of all the small signal tubes in the X-202B operate from a DC power source -- and uniquely, even the phase inverter tubes! No other Fisher unit I am aware of operates the phase inverter tube heaters from DC. This sucker ought to be as quiet as a church mouse!
But just as with (part of) the same battle I recently fought with my Fisher 400C preamplifier, there is something in the area that does operate off of AC -- the jeweled indicator lights, and it's the Mode Selector switch that controls them!
To Fisher's credit, the Mode Switch itself has a shield plate between the wafer that switches the lamps, and the two wafers that switch the audio circuits. This does a credible job of shielding the sensitive audio circuits from the AC power passing through the lamp switching wafer. Buuuuuuut, it's only effective if the lead dress of the wires connected to this switch keep the audio leads appropriately separated from the AC power leads. In looking at this issue in Al's amp............Whoops, folks on the assembly line didn't get that memo about the importance of lead dress in this area.
Pics include:
1. In this pic, the back two wafers handle the audio signal switching, while the front most wafer controls the light display. At about 8:00 on the rear wafer is a terminal with a short bare lead going to the T-strip behind the switch. That lead represents the input into the left channel 1st AF amplifier stage from this switch -- which is all well fine and good. Except that, see the brown wire lurking close by, stalking this terminal behind it? That's the power lead to the front wafer of this switch to power the lamps according to what position the switch is set at. That's all it took to induce a noticeable hum into the left channel -- and flexing the chassis would of course flex the distance between the brown wire and the terminal, making the symptoms identical with Nap's experience.
2. The offending wiring neatly tucked away from the audio wafers, so that the left channel is now as quiet as the right channel is.
Thought I'd pass this one along for all you techs if you ever come up on this symptom in any other X-202Bs that might pass your way.
This one was a humdinger for sure! (sorry, couldn't resist.....)
Dave
I have been running Al's X202B pretty good this past week -- as I always do in an effort to shake out any last issues that might crop up. And whadda ya know -- two did.
The first one was easy, with an intermittent loudness function -- in each channel individually when it felt like it, and with different symptoms in each channel when it would decide to go on hiatus. That was traced to the two loudness switch caps that would independently work when they felt like it. With those replaced, the loudness feature now engages and disengages dependably, providing the boost that it should in both channels when activated.
The second one is specifically for Larry and Naptown Rob, since apparently Larry lost significant amounts of hair over Nap's X-202B, and Nap is still without it even yet.
In looking over Al's unit for any last Gremlins to take care of (he's picking it up tomorrow), I noted that the left channel (only) would display a very notable hum when an open (as in, no input connected) line level input was selected (most notably on the Tuner setting), and the volume was advanced. On the other hand, the right channel only emitted a soft hiss in the same scenario. Thinking back to Larry's comments in this thread, as well as Nap's, it occurred to me that Al's unit was likely displaying the very same socially unacceptable behavior that Nap's unit was. It took an afternoon of detective work, but I finally found the cause of the little bugger -- at least in Al's unit. But being as the cause almost surely goes back to the original build, it's very likely that what I found could also be the cause of the hum that has been so elusive in Nap's unit as well. The symptoms he described are exactly the same that Al's unit displayed.
The pics tell the story, but basically, the problem is exactly in the location that Larry suggested -- that being the Mode Selector switch. The hum was 60 Hz in nature, which ruled out power supply hum (it would be 120 Hz), and any hum from the heater wiring, since all the heaters of all the small signal tubes in the X-202B operate from a DC power source -- and uniquely, even the phase inverter tubes! No other Fisher unit I am aware of operates the phase inverter tube heaters from DC. This sucker ought to be as quiet as a church mouse!
But just as with (part of) the same battle I recently fought with my Fisher 400C preamplifier, there is something in the area that does operate off of AC -- the jeweled indicator lights, and it's the Mode Selector switch that controls them!
To Fisher's credit, the Mode Switch itself has a shield plate between the wafer that switches the lamps, and the two wafers that switch the audio circuits. This does a credible job of shielding the sensitive audio circuits from the AC power passing through the lamp switching wafer. Buuuuuuut, it's only effective if the lead dress of the wires connected to this switch keep the audio leads appropriately separated from the AC power leads. In looking at this issue in Al's amp............Whoops, folks on the assembly line didn't get that memo about the importance of lead dress in this area.
Pics include:
1. In this pic, the back two wafers handle the audio signal switching, while the front most wafer controls the light display. At about 8:00 on the rear wafer is a terminal with a short bare lead going to the T-strip behind the switch. That lead represents the input into the left channel 1st AF amplifier stage from this switch -- which is all well fine and good. Except that, see the brown wire lurking close by, stalking this terminal behind it? That's the power lead to the front wafer of this switch to power the lamps according to what position the switch is set at. That's all it took to induce a noticeable hum into the left channel -- and flexing the chassis would of course flex the distance between the brown wire and the terminal, making the symptoms identical with Nap's experience.
2. The offending wiring neatly tucked away from the audio wafers, so that the left channel is now as quiet as the right channel is.
Thought I'd pass this one along for all you techs if you ever come up on this symptom in any other X-202Bs that might pass your way.
This one was a humdinger for sure! (sorry, couldn't resist.....)
Dave
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SNEAKY Electronics foreplay at it's best! Keep them leads "G" rated, Dave!!