As I had my 600 open I did the Low Filter "mod". Quite easy to do. Didn't have to change C108-110 as mine were 600V already from the recap. I haven't played it thru phono yet, but should soon.
TA-600 Service Bulletin -- Early Version
- Thread starter dcgillespie
- Start date
Are you saying that you had already changed out C108/C110 to be .1 uF caps from their original .01 uF designations?
Dave
Dave
Dammit. I've got to keep these reading glasses on more. I had put .01 630v in when I recapped it and read them as .1uf's yesterday. I've got some .1uf/600v I'll change them now.
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Changed out the .01uf for .1uf. Bit better.
Checked the plate, screens and bias voltage also tube draws across the 10ohm R
VARIAC set and verified at 120VAC
Plate 369 within 1v of schematic
Screens 325v Screen about 5V high per schematic.
bias voltage -18.91 to -18.95 all 4 tubes
ma's all 4 tubes. varied from 27.6 to 27.9ma.
V19 27.7 10.2213W
v20 27.6 10.1844W
v21 27.9 10.2951W
V22 27.9 10.2951W
Heater voltage for the 12ax7's averages 11.9 with the phono tubes getting about 9.45vdc per tube.
Output tubes are Russian 6P14p-EV's.
Still has the problem of the AM being about 1/2 the FM volume (gain) and the MPX wiring problem. I'll get to that in the next few days.
Larry
Checked the plate, screens and bias voltage also tube draws across the 10ohm R
VARIAC set and verified at 120VAC
Plate 369 within 1v of schematic
Screens 325v Screen about 5V high per schematic.
bias voltage -18.91 to -18.95 all 4 tubes
ma's all 4 tubes. varied from 27.6 to 27.9ma.
V19 27.7 10.2213W
v20 27.6 10.1844W
v21 27.9 10.2951W
V22 27.9 10.2951W
Heater voltage for the 12ax7's averages 11.9 with the phono tubes getting about 9.45vdc per tube.
Output tubes are Russian 6P14p-EV's.
Still has the problem of the AM being about 1/2 the FM volume (gain) and the MPX wiring problem. I'll get to that in the next few days.
Larry
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I have to admit, the AM tuner on my TA-600 works very well indeed. FM is also no slouch, and they can give fairly comparable performance, especially if you can feed one an AM source without the "brick wall" frequency limiting of modern broadcast stations. Handily, my homebrew AM transmitter and my HP 8640B both lack any sort of filtering so I can get the full wide-band experience. At some point I'll put that MPX-65 I have in it's own box, and see if I can couple the TA-600 to the MPX-65 as an external MPX decoder to see how that sounds. I know the MPX-65 will run external, as I've done it on a Bogen tuner but I have not paired it with the 600 yet.
Adjustable bias would be very cool. Mine also runs the Russky tubes Larry is using, and I wouldn't pretend mine are matched. Mine also still rocks original filter caps at the moment, so its not in rotation until I change that out. What are you thinking here, something akin to the old Williamson bias balance adjustment rig, or positive voltage feed into the grid like I believe Scott (or was it Fisher?) did on some models?
Will look into the frequency caps mentioned too. Not sure if some of those parts have different numbers in the late or not, but I'll give it a gander.
Adjustable bias would be very cool. Mine also runs the Russky tubes Larry is using, and I wouldn't pretend mine are matched. Mine also still rocks original filter caps at the moment, so its not in rotation until I change that out. What are you thinking here, something akin to the old Williamson bias balance adjustment rig, or positive voltage feed into the grid like I believe Scott (or was it Fisher?) did on some models?
Will look into the frequency caps mentioned too. Not sure if some of those parts have different numbers in the late or not, but I'll give it a gander.
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Gadget; The frequency caps are the same # from Early to late.
Larry -- Those are some very tightly matched current draw levels between the tubes, but with the phono tubes only getting 9.45 vdc each, you'll likely have another grandkid before phono preamps warm up!
I suspect you are using the full 369 vdc at the plate to determine your Dp level. Remember however, you must subtract out the voltage at the cathodes to determine the real potential between the plate and cathode. When you do that, and subtract out about 3 ma for screen current per tube, your real Dp is down in the 8 watt territory -- which is pretty cool for the tubes -- really too cool: Distortion will be elevated, and more grandkids before listening to vinyl. Shoot for about 11 watts of Dp. This will represent about 83% of the Design Max Dp rating for good tube life, provide a good operating point for the output stage (low distortion), and heat the phono preamp tubes in a reasonable amount of time. This will typically produce somewhere between 21 and 22 vdc across the phono preamp tube heater string.
Dave
I suspect you are using the full 369 vdc at the plate to determine your Dp level. Remember however, you must subtract out the voltage at the cathodes to determine the real potential between the plate and cathode. When you do that, and subtract out about 3 ma for screen current per tube, your real Dp is down in the 8 watt territory -- which is pretty cool for the tubes -- really too cool: Distortion will be elevated, and more grandkids before listening to vinyl. Shoot for about 11 watts of Dp. This will represent about 83% of the Design Max Dp rating for good tube life, provide a good operating point for the output stage (low distortion), and heat the phono preamp tubes in a reasonable amount of time. This will typically produce somewhere between 21 and 22 vdc across the phono preamp tube heater string.
Dave
I know it's an approximation. The math gives me migraines every time too. This also gives me a fudge factor in the right direction. Yeah she distorts a little over 1/2 way on the volume pot although I think the minimus 7's might be at there limit too.
NOT likely. Maybe Great Grandkids. We're changing R158 and 159, Right? with the above voltages, what would be a "get me in the general area" size for the resistors, or if I'm just changing R158, can I sub in a 10K pot to get -21v then swap it out for a combination of resistors that match the value. I suspect with the heaters on the circuit, a standard 3362 series Bourns ain't gonna work. Depending on the resistors I'm gonna have to make a day trip across B-more to hit the nearest electronics store that carries them.
NOT likely. Maybe Great Grandkids. We're changing R158 and 159, Right? with the above voltages, what would be a "get me in the general area" size for the resistors, or if I'm just changing R158, can I sub in a 10K pot to get -21v then swap it out for a combination of resistors that match the value. I suspect with the heaters on the circuit, a standard 3362 series Bourns ain't gonna work. Depending on the resistors I'm gonna have to make a day trip across B-more to hit the nearest electronics store that carries them.
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Stay tuned. I'm putting in an IBAM variation in the one here, because while the tubes in it are really great NOS (Westinghouse) types that test nearly as new, they are only marginally matched for the job they must do in the 600.
Dave
Dave
I found a post from Dave in this thread ( http://www.audiokarma.org/forums/showpost.php?p=8140493&postcount=36 Post 36,37,& 38 ) on how and what to change for a quicky, single pot bias adjustment. No 2K7's but plenty of 3k3's and 2k2's in stash. I'll try it with the 3k3's and a 10K pot. Should be able to get in the neighborhood. And if I don't like the way it works or the range isn't sufficient, I can always change it back and wait for Dave's IBAM variant. So out on the porch for another fun filled day of sunburn's, hot solder slinging, and sweat stung eyes.
Back in a bit with the results.
Back in a bit with the results.
I removed R158 (5.6K) and R159 (4.7K) from the following:
R158....Pin 3 of V22 and the terminal strip on the other end of the resistor.
R159....Removed from the terminal strip end terminal that had the other end of R158, and the next terminal.
Then setup the new 10K pot(NOTE: Dave originally spec'ed a 5K but I didn't have 5K so useda 10K just for testing) with a 3.3K resistor on pins 1 and 3. Solder and heatshrink to reduce/remove shorts potential.
The Wiper lead on the pot attaches to the END TERMINAL of the STRIP where I removed R158 and 159. This will still have 2-330K (R150-151) resistors and the wire to the mirror circuit on the other channel.
One resistor will attach to pin 3 of V22 and the other to the next terminal on the strip. I initially set the pot to 1/2way, and connected meters to the junction of the wiper terminal and Pin 3 on V22. As it was warming up I adjusted the pot to maintain 20.5V to 21V or as close as i could get. (With a 5K I'd have a little more rotation for adjustment). It settled down after about 6 minutes. Voltages shown are after 10 minutes.
VOLTAGES
WALL VOLTAGE 111VAC (Direct to wall socket). Selector Mode in FM-AM with no Signal at antenna leads, Mono-Stereo switch in STEREO, Volume @ ZERO. I can redo it all on the variac @ 117vAC if needed.
Pin 7 365VDC
Pin 9 320VDC
Pin3 21.16VDC
Pin2 varied between 10.26 and 10.32 depending on tube.
Pot Wiper 10.86VDC
V17 Pin-4(HTR) 20.58VDC
V18 Pin 4(HTR) 10.68VDC
Tube draw in mV across 10 ohm r
V15..........337.1mV
V16..........360.8mV
V21..........358.2mV
v22..........301.1mV
These voltages are after 30seconds to settle down.
I'm going to re-arrange the output's so they are better matched. Swap V22 with V16. Also I have 7 other 6p14p-EV's that I can swap in to see if I can come up with a closer matched quad based on tube draw.
ma's
V15.....33.71mA
V16.....36.08mA
V21.....35.82mA
V22.....30.11mA
Subtract 3ma for grids/screens and get
30.71
33.08
32.82
27.11
With 365 on the plates, these figure to be ABOUT.....
11.21w
12.07w
11.98w
09.90w
A bt more spread than I anticipated considering the #'s I got Tuesday. But with all of the voltages closer to their intended nominals, it's probably the tubes showing their age. Anyway swap V16 and 22, re-check the numbers and then check the other 7 (pull the worst one in there now and rotate thru the bunch and see if there is a better tube.).
Larry
R158....Pin 3 of V22 and the terminal strip on the other end of the resistor.
R159....Removed from the terminal strip end terminal that had the other end of R158, and the next terminal.
Then setup the new 10K pot(NOTE: Dave originally spec'ed a 5K but I didn't have 5K so useda 10K just for testing) with a 3.3K resistor on pins 1 and 3. Solder and heatshrink to reduce/remove shorts potential.
The Wiper lead on the pot attaches to the END TERMINAL of the STRIP where I removed R158 and 159. This will still have 2-330K (R150-151) resistors and the wire to the mirror circuit on the other channel.
One resistor will attach to pin 3 of V22 and the other to the next terminal on the strip. I initially set the pot to 1/2way, and connected meters to the junction of the wiper terminal and Pin 3 on V22. As it was warming up I adjusted the pot to maintain 20.5V to 21V or as close as i could get. (With a 5K I'd have a little more rotation for adjustment). It settled down after about 6 minutes. Voltages shown are after 10 minutes.
VOLTAGES
WALL VOLTAGE 111VAC (Direct to wall socket). Selector Mode in FM-AM with no Signal at antenna leads, Mono-Stereo switch in STEREO, Volume @ ZERO. I can redo it all on the variac @ 117vAC if needed.
Pin 7 365VDC
Pin 9 320VDC
Pin3 21.16VDC
Pin2 varied between 10.26 and 10.32 depending on tube.
Pot Wiper 10.86VDC
V17 Pin-4(HTR) 20.58VDC
V18 Pin 4(HTR) 10.68VDC
Tube draw in mV across 10 ohm r
V15..........337.1mV
V16..........360.8mV
V21..........358.2mV
v22..........301.1mV
These voltages are after 30seconds to settle down.
I'm going to re-arrange the output's so they are better matched. Swap V22 with V16. Also I have 7 other 6p14p-EV's that I can swap in to see if I can come up with a closer matched quad based on tube draw.
ma's
V15.....33.71mA
V16.....36.08mA
V21.....35.82mA
V22.....30.11mA
Subtract 3ma for grids/screens and get
30.71
33.08
32.82
27.11
With 365 on the plates, these figure to be ABOUT.....
11.21w
12.07w
11.98w
09.90w
A bt more spread than I anticipated considering the #'s I got Tuesday. But with all of the voltages closer to their intended nominals, it's probably the tubes showing their age. Anyway swap V16 and 22, re-check the numbers and then check the other 7 (pull the worst one in there now and rotate thru the bunch and see if there is a better tube.).
Larry
Attachments
The increased spread you are seeing is due to the tubes now working on a different part of the curve -- and not uncommon. If anything, this is EXACTLY why output tubes need to be (ideally) tested, and at least matched under real world working voltages.
Dave
Dave
Talk about a Goat Rope (Cluster F***). I marked and ID'ed the spares as follows:
#1 RCA 7189
#2 FISHER Branded 7189
#3 GE 6BQ5
#4 GE 6BQ5
#5 Raytheon 6BQ5 Made in JAPAN (4 seams on top) Mullard Tooling? Matsushita???
#6 UNKNOWN 6BQ5. Font on remaining lettering looks same as FISHER used.
#7 Russian 6n14n Nov 64 dated.
HAd to use the Variac to get the voltage up to a level that would get decent results. As the morning went on and it got hotter, the voltage dropped to a low of 108VAC @ 1:30 compared to 116@ 10:30. It's a wonder more stuff doesn't give up the ghost due to undervoltage. And all the wimps have to have their A/C. They could get healthy real fast....sweat some of the FAT off them. Cheap too. No need for a gym, just get out and walk around.
Anyway, RANT OFF. Initial test used as above with tubes previously used. The numbers for those are V15,v16,v21,v22. The spares are numbered 1 thru 7.
No sense repeating the test results from the previous session this morning. See post # 31.
voltages after 5 minute warm up.
After Variac 117.00vAC
Plate 365vDC
Screen 320vDC
HTR's 5.9vAC to 6.2vAC depending on which circuit.
Bias voltage @ pin 3 21.02vDC
Voltage @ junction of wiper for pot and grid return's 11.22vDC
Tube POSITION Tube changes noted at bottom with (*) ALL VALUES IN millivolts.
.........V15..................V16......................V21....................V22
Test
1.........324................(*)346.....................302.................354
2.........288................(*)294.....................485(**)...........315
3.........328................(*)344.....................289(**)...........371
4.........326................(*)331.....................330(**)...........364
5.........320................(*)334.....................357(**)...........362
6.........332................(*)346.....................297(**)...........381
7.........333................(*)339.....................312(**)...........376
(*) Tube #1 was swapped into V16 and left for all subsequent tests.
(**) Tube 2 thru 7 were swapped in V21 socket for each different test.
Using Test #4 as the best baseline I swapped V15 and V21
319.......347.......343..........370
Then I put the tubes back as in #4 then swapped V16 and 21
326........325.........362......368
With the current tubes I have, this is probably as close as i can get them without a fixed bias, IBAM set-up.
HEY DAVE! I don't know what the hell happened but the AM volume is BACK! And all I did was move it from that table outside to inside at night and back this morning. X-ing toes, fingers, and anything else that can be painlessly x-ed.
Larry
#1 RCA 7189
#2 FISHER Branded 7189
#3 GE 6BQ5
#4 GE 6BQ5
#5 Raytheon 6BQ5 Made in JAPAN (4 seams on top) Mullard Tooling? Matsushita???
#6 UNKNOWN 6BQ5. Font on remaining lettering looks same as FISHER used.
#7 Russian 6n14n Nov 64 dated.
HAd to use the Variac to get the voltage up to a level that would get decent results. As the morning went on and it got hotter, the voltage dropped to a low of 108VAC @ 1:30 compared to 116@ 10:30. It's a wonder more stuff doesn't give up the ghost due to undervoltage. And all the wimps have to have their A/C. They could get healthy real fast....sweat some of the FAT off them. Cheap too. No need for a gym, just get out and walk around.
Anyway, RANT OFF. Initial test used as above with tubes previously used. The numbers for those are V15,v16,v21,v22. The spares are numbered 1 thru 7.
No sense repeating the test results from the previous session this morning. See post # 31.
voltages after 5 minute warm up.
After Variac 117.00vAC
Plate 365vDC
Screen 320vDC
HTR's 5.9vAC to 6.2vAC depending on which circuit.
Bias voltage @ pin 3 21.02vDC
Voltage @ junction of wiper for pot and grid return's 11.22vDC
Tube POSITION Tube changes noted at bottom with (*) ALL VALUES IN millivolts.
.........V15..................V16......................V21....................V22
Test
1.........324................(*)346.....................302.................354
2.........288................(*)294.....................485(**)...........315
3.........328................(*)344.....................289(**)...........371
4.........326................(*)331.....................330(**)...........364
5.........320................(*)334.....................357(**)...........362
6.........332................(*)346.....................297(**)...........381
7.........333................(*)339.....................312(**)...........376
(*) Tube #1 was swapped into V16 and left for all subsequent tests.
(**) Tube 2 thru 7 were swapped in V21 socket for each different test.
Using Test #4 as the best baseline I swapped V15 and V21
319.......347.......343..........370
Then I put the tubes back as in #4 then swapped V16 and 21
326........325.........362......368
With the current tubes I have, this is probably as close as i can get them without a fixed bias, IBAM set-up.
HEY DAVE! I don't know what the hell happened but the AM volume is BACK! And all I did was move it from that table outside to inside at night and back this morning. X-ing toes, fingers, and anything else that can be painlessly x-ed.
Larry
Icing on the Cake
As previously discussed, because the output tubes in the 600 must pass a significant amount quiescent current for the plate voltage they operate at, a tight match is required between the tubes to make sure they all share the current load evenly. Otherwise, it becomes quite easy for the majority of the tubes to exceed their plate dissipation rating if just one is a slacker. The real answer therefore, is to provide individual bias controls to ensure that even current distribution exists between all the tubes. If a tight match already exists with your tubes, the adding an IBAM will simply make it tighter.
As usual, space is always tight for the installation of such circuits, but in this particular unit, space was available in a portion of the area originally occupied by the under chassis can cap. This cap had previously been replace with discrete caps, and an inductor mounted in the area this freed up. But in addressing all of the hum problems this had, the inductor was removed, and the original power supply format reinstalled. Finally, the discrete caps were secured in a proper manner, and the wiring to them redressed neatly. Still, this left plenty of space behind these caps to mount a small circuit board that contains all the necessary circuits to install such a modification.
A board was constructed that includes the new adjustments, as well as the individual cathode current sensing resistors, to make for a nice, neat installation. Finally, an appropriate umbilical cable (nine conductors shielded) was used to connect the board to the various connections in both output stages.
During the course of this work, it was determined that the optimum output tube cathode current draw is 34 ma per tube. With 4 ma of this attributable to screen current, that leaves a plate current of 30 ma per tube. With a plate/cathode potential of 365 vdc, this means that each tube is dissipating just under 11 watts (10.95 watts) at the ideal operating point. If anything, this shows why such a tight balance is required between all four tubes: Increasing the plate current draw of any one tube just 3 ma causes it to reach the Design Center rating for the tubes.
With a total output stage current draw then of 136 ma, this will produce a 21 vdc drop across the phono preamp tube heaters (10.5 vdc/tube), providing a good balance of low distortion operation for the output stages, adequate current to operate the phono preamp tube heaters, and good output tube life as well, since the ratings for these tubes were established under the more conservative Design Center standards. Coincidentally, a 21 vdc drop across the preamp tube heaters coincides with the level that Fisher specified for this voltage as well in the later version of the 600 with reduced output stage current draw. This particular unit here you might recall is an early production unit of the early version, so it originally operated the tubes at, and (certainly today), over Design Center ratings.
With the individual bias controls, current draw is equalized between the tubes, and all traces of any plate color are gone -- even when using time exposure pics, which is the best way to check for such concerns.
Pics include:
1. The original cathode and grid bias network wiring in the process of being removed.
2. The new IBAM board has been mounted down as I have done so many times in the past: the circuit board is mounted to an insulator board by wire wraps, and the insulator board is then mounted to the chassis with silicone adhesive. Note that the board also includes the matched 10 ohm cathode sampling resistors as well.
3. The finished output stage area. The new IBAM board interconnects to the connections in this area via the gray umbilical cord running from the board, along the side of the chassis, and over to this area. The installation comes off as un-intrusive as possible, retaining virtually all of the original build qualities by Fisher when the unit was originally manufactured.
4. The complete underside of the finished 600.
5. Finally. The output tubes now show no plate color even when time elapse pics are taken of the tubes to check for coloration.
With this, all of the design elements noted have now been addressed in this unit. Installing an IBAM circuit in the 600 is important to hitting that small target where output stage and phono preamp performance, as well as output tube life all come together for optimum operation. Therefore, I would strongly encourage its installation. The circuit is quite simple, so I will draw it up and post it asap.
Besides the four matched 10 ohm 1/4 watt resistors and the perf board, you will also need four circuit board mount 10K trim pots (1/8 watt is more than adequate), an 8.2K 1/4 watt resistor, and a 4.7K 1/4 watt resistor.
For those of you lucky enough to have one of these units, they are truly an enjoyable piece. The items noted in this bulletin will help you gain the most they have to offer.
Dave
As previously discussed, because the output tubes in the 600 must pass a significant amount quiescent current for the plate voltage they operate at, a tight match is required between the tubes to make sure they all share the current load evenly. Otherwise, it becomes quite easy for the majority of the tubes to exceed their plate dissipation rating if just one is a slacker. The real answer therefore, is to provide individual bias controls to ensure that even current distribution exists between all the tubes. If a tight match already exists with your tubes, the adding an IBAM will simply make it tighter.
As usual, space is always tight for the installation of such circuits, but in this particular unit, space was available in a portion of the area originally occupied by the under chassis can cap. This cap had previously been replace with discrete caps, and an inductor mounted in the area this freed up. But in addressing all of the hum problems this had, the inductor was removed, and the original power supply format reinstalled. Finally, the discrete caps were secured in a proper manner, and the wiring to them redressed neatly. Still, this left plenty of space behind these caps to mount a small circuit board that contains all the necessary circuits to install such a modification.
A board was constructed that includes the new adjustments, as well as the individual cathode current sensing resistors, to make for a nice, neat installation. Finally, an appropriate umbilical cable (nine conductors shielded) was used to connect the board to the various connections in both output stages.
During the course of this work, it was determined that the optimum output tube cathode current draw is 34 ma per tube. With 4 ma of this attributable to screen current, that leaves a plate current of 30 ma per tube. With a plate/cathode potential of 365 vdc, this means that each tube is dissipating just under 11 watts (10.95 watts) at the ideal operating point. If anything, this shows why such a tight balance is required between all four tubes: Increasing the plate current draw of any one tube just 3 ma causes it to reach the Design Center rating for the tubes.
With a total output stage current draw then of 136 ma, this will produce a 21 vdc drop across the phono preamp tube heaters (10.5 vdc/tube), providing a good balance of low distortion operation for the output stages, adequate current to operate the phono preamp tube heaters, and good output tube life as well, since the ratings for these tubes were established under the more conservative Design Center standards. Coincidentally, a 21 vdc drop across the preamp tube heaters coincides with the level that Fisher specified for this voltage as well in the later version of the 600 with reduced output stage current draw. This particular unit here you might recall is an early production unit of the early version, so it originally operated the tubes at, and (certainly today), over Design Center ratings.
With the individual bias controls, current draw is equalized between the tubes, and all traces of any plate color are gone -- even when using time exposure pics, which is the best way to check for such concerns.
Pics include:
1. The original cathode and grid bias network wiring in the process of being removed.
2. The new IBAM board has been mounted down as I have done so many times in the past: the circuit board is mounted to an insulator board by wire wraps, and the insulator board is then mounted to the chassis with silicone adhesive. Note that the board also includes the matched 10 ohm cathode sampling resistors as well.
3. The finished output stage area. The new IBAM board interconnects to the connections in this area via the gray umbilical cord running from the board, along the side of the chassis, and over to this area. The installation comes off as un-intrusive as possible, retaining virtually all of the original build qualities by Fisher when the unit was originally manufactured.
4. The complete underside of the finished 600.
5. Finally. The output tubes now show no plate color even when time elapse pics are taken of the tubes to check for coloration.
With this, all of the design elements noted have now been addressed in this unit. Installing an IBAM circuit in the 600 is important to hitting that small target where output stage and phono preamp performance, as well as output tube life all come together for optimum operation. Therefore, I would strongly encourage its installation. The circuit is quite simple, so I will draw it up and post it asap.
Besides the four matched 10 ohm 1/4 watt resistors and the perf board, you will also need four circuit board mount 10K trim pots (1/8 watt is more than adequate), an 8.2K 1/4 watt resistor, and a 4.7K 1/4 watt resistor.
For those of you lucky enough to have one of these units, they are truly an enjoyable piece. The items noted in this bulletin will help you gain the most they have to offer.
Dave
Attachments
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WOW!!! That IS NICE! Inadvertantly, the readings I took on mine re-enforce the need for a GOOD MATCHING QUAD of tubes, even with them being new tubes (<200 hours on them). Rolling tubes in to try and "level the playing field" as it were, only goes so far. You can see by the results above that changing one tube can cause drastic level changes as the circuit tries to level the total draw out. With one weak or strong tube it upsets the apple cart and causes all kinds of problems, the least of which is the phono tubes not getting enough juice.
Dave has come up with another "ELEGANT" Solution to 1.)Marketings shortcomings, and 2.) the penny pinchers constant "bayonetting" of good engineering practices by eradicating any kind of adjustments to allow longer tube life. Now owners can put in reasonably "mis-matched" tubes (within 10ma or so of each other), and set them to a current level that all of them can live with, allowing the unit to sound better, the tubes to last longer by running them at their "sweet spot", and the phono tubes to heat up before your great grandkids are born.
Thank you Dave. Cut another notch on yer belt.
Dave has come up with another "ELEGANT" Solution to 1.)Marketings shortcomings, and 2.) the penny pinchers constant "bayonetting" of good engineering practices by eradicating any kind of adjustments to allow longer tube life. Now owners can put in reasonably "mis-matched" tubes (within 10ma or so of each other), and set them to a current level that all of them can live with, allowing the unit to sound better, the tubes to last longer by running them at their "sweet spot", and the phono tubes to heat up before your great grandkids are born.
Thank you Dave. Cut another notch on yer belt.
What a wild journey this 600 has had, sure makes me happy knowing it has been in good hands. Now it's worthy to take its place back in my Fisher museum. 
I like the individual bias adjustment upgrade, it helps with the lack of matched quads these days, and should make these tubes last much longer. :thmbsp:
I like the individual bias adjustment upgrade, it helps with the lack of matched quads these days, and should make these tubes last much longer. :thmbsp:
Larry -- You're absolutely right about how sensitive the apple cart is on this receiver. The reason it is can be traced back to the basis of the design itself: As different tubes change the total current flow through the cathode circuit, it changes the very voltage source providing the bias voltage to begin with. It's much like changing a tube not only produces the change from the different tube, but also changes the effective value of the total cathode bias resistance as well. As a result, this causes the four adjustment pots to have greater cause/effect than you would normally be used to when adding an IBAM to a conventional fixed bias output stage, where the bias voltage is supplied by a source separate from the cathode current itself, and therefore remains quite stable independent of actual the actual cathode current drawn. Therefore, this IBAM will require more tweaking to zero it in to the desired current draw, but once it is set, it remains quite stable. The extra adjustment steps required is simply a product of the overall design of the bias system.
As for your AM issue, interestingly, Steve's unit here reacted EXACTLY the same way. I had initially reported to him that the AM section was DOA when first received. Later I reported that cleaning an obviously dirty AM bandwidth switch seemed to clear the issue up. I should also note that the unit was in rather continuous physical motion as well, moving between the work bench, performance verification bench, and listening room. In the course of all that, the AM section quit again, so I reported that ultimately, it was intermittent -- that the dirty AM bandwidth switch was certainly part of the problem, but obviously not all of it.
I initially suspected an intermittent IF transformer, but found them all to be good and functioning well -- even when AM section was in its "off" mode. To be fair, I found that the section never really did go completely dead, but just lost so much gain that you had to turn the volume up to 3:00 to hear anything.
It took quite a bit of trouble shooting, but the little booger ended up being nothing more than a cold solder joint -- that was defective from the day the unit was originally manufactured. When freshly built, the mechanical connection made for the joint in question likely carried the unit through years of dependable operation. But then as heat, oxidation, and operating cycles all took their toll over time, the mechanical connection ceased being electrically sound, causing the AM section to become intermittent.
In particular, it was the solder joint at the center barrel shield of the tube socket at V9, the AM IF amplifier stage. At pin #1 of this tube, there is a .005 uF ceramic cap connected between this pin, and the center barrel shield, which itself is then grounded by a short bare wire to a nearby chassis ground lug. This cap was well soldered to pin #1 and the barrel shield, but at the barrel shield, the connection of the short wire connecting it to the ground lug was compromised. To make matters worse, this absolutely could not be seen, as the opening of the barrel filled in with solder -- making for good integrity of the cap's connection there -- but hiding the fact that the solder never made it down the barrel to catch the bare ground wire. That hidden connection within a connection was absolutely the problem. Once I discovered that and remade the connection, the AM section immediately became rock stable in its operation. In effect what was happening, is that cap is a screen bypass cap for that stage; when it effectively went open, the gain of that stage dropped quite notably due to the degeneration this would create within the stage. With the cap in place, the stage is then allowed to produce full amplification, which then produces a strong AM output.
Of note, this type of problem will never produce an off value voltage reading, as all voltages for the stage remained the same whether the cap was making connection, or not. Since likely the same person at Fisher was making that connection, and because the symptoms are so identical, you might just check that connection in your receiver as well. It absolutely nailed the problem in Steve's unit, and it just might do so in your unit as well!
Dave
As for your AM issue, interestingly, Steve's unit here reacted EXACTLY the same way. I had initially reported to him that the AM section was DOA when first received. Later I reported that cleaning an obviously dirty AM bandwidth switch seemed to clear the issue up. I should also note that the unit was in rather continuous physical motion as well, moving between the work bench, performance verification bench, and listening room. In the course of all that, the AM section quit again, so I reported that ultimately, it was intermittent -- that the dirty AM bandwidth switch was certainly part of the problem, but obviously not all of it.
I initially suspected an intermittent IF transformer, but found them all to be good and functioning well -- even when AM section was in its "off" mode. To be fair, I found that the section never really did go completely dead, but just lost so much gain that you had to turn the volume up to 3:00 to hear anything.
It took quite a bit of trouble shooting, but the little booger ended up being nothing more than a cold solder joint -- that was defective from the day the unit was originally manufactured. When freshly built, the mechanical connection made for the joint in question likely carried the unit through years of dependable operation. But then as heat, oxidation, and operating cycles all took their toll over time, the mechanical connection ceased being electrically sound, causing the AM section to become intermittent.
In particular, it was the solder joint at the center barrel shield of the tube socket at V9, the AM IF amplifier stage. At pin #1 of this tube, there is a .005 uF ceramic cap connected between this pin, and the center barrel shield, which itself is then grounded by a short bare wire to a nearby chassis ground lug. This cap was well soldered to pin #1 and the barrel shield, but at the barrel shield, the connection of the short wire connecting it to the ground lug was compromised. To make matters worse, this absolutely could not be seen, as the opening of the barrel filled in with solder -- making for good integrity of the cap's connection there -- but hiding the fact that the solder never made it down the barrel to catch the bare ground wire. That hidden connection within a connection was absolutely the problem. Once I discovered that and remade the connection, the AM section immediately became rock stable in its operation. In effect what was happening, is that cap is a screen bypass cap for that stage; when it effectively went open, the gain of that stage dropped quite notably due to the degeneration this would create within the stage. With the cap in place, the stage is then allowed to produce full amplification, which then produces a strong AM output.
Of note, this type of problem will never produce an off value voltage reading, as all voltages for the stage remained the same whether the cap was making connection, or not. Since likely the same person at Fisher was making that connection, and because the symptoms are so identical, you might just check that connection in your receiver as well. It absolutely nailed the problem in Steve's unit, and it just might do so in your unit as well!
Dave
That makes a lot of sense, Dave. I've seen just that type of problem before on older set's that used the barrel shield The bottom wire or lead doesn't get any solder due to the way they solder up the shield. I'm definately going to check that one and the rest of the barrel shields that are soldered up.
It developed a startup 120Hz hum y'day for some odd reason. It's really slight, barely audible, and fades out in about 5-6 seconds. It starts about 20 seconds from turning the switch, and is accompanied by what sounds like a click. I'm thinking it's a tube as it started after I began rolling in the output tubes. I think puttig the original output tubes may eliminate it.
It developed a startup 120Hz hum y'day for some odd reason. It's really slight, barely audible, and fades out in about 5-6 seconds. It starts about 20 seconds from turning the switch, and is accompanied by what sounds like a click. I'm thinking it's a tube as it started after I began rolling in the output tubes. I think puttig the original output tubes may eliminate it.
A very subtle imbalance hum during warmup at turn on will almost surely be a product of your output tubes -- particularly if using tubes of different manufacturers, that can have slightly different warmup characteristics. When its grossly obvious however -- as this unit here initially displayed -- that implies power supply problems from inadequate filtering, and is what necessitated returning the power supply in this unit back to its original configuration.
Dave
Dave
Thanks Dave. I thoughtit might be related to the tubes. I'll put the orginals back in it and adjust it with the IBAM circuit.
Question concerning the 9lead shielded cable. Is that something I can get locally at an electrical supply house, mouser/digikey or was it something you had laying around in the "junk box" for something like this???????
Nothing done on it today. 1 sister and my brother came up from FLA, for a End of summer get together along with one niece that I haven't seen since she was 12. Now she's 34 and has 3 kids of her own. There were tears going down quite a few faces today. Had a good time and going back tomorrow to finish up the punishment.
Question concerning the 9lead shielded cable. Is that something I can get locally at an electrical supply house, mouser/digikey or was it something you had laying around in the "junk box" for something like this???????
Nothing done on it today. 1 sister and my brother came up from FLA, for a End of summer get together along with one niece that I haven't seen since she was 12. Now she's 34 and has 3 kids of her own. There were tears going down quite a few faces today. Had a good time and going back tomorrow to finish up the punishment.