Correct. Z4 (your #1) is the Limiter Transformer, while Z5 (your #2) is the Ratio Detector Transformer.
Dave
Dave
Fisher 500B: the story continues
- Thread starter dyche01
- Start date
1 is Z4 listed in the parts list as "FM LIMITER COIL"
2 is Z5 listed in the parts list as "FM Ratio Detector" IIRC it's also the 2nd limiter.
The drawing on the Left is missing Z1 which is next to V1 (6AQ8). Read them left to right and then up or down, or diagonally whichever applies.
I edited your drawing to reflect the correct orientation.
2 is Z5 listed in the parts list as "FM Ratio Detector" IIRC it's also the 2nd limiter.
The drawing on the Left is missing Z1 which is next to V1 (6AQ8). Read them left to right and then up or down, or diagonally whichever applies.
I edited your drawing to reflect the correct orientation.
dyche01
"Buy high. Sell low"
Thanks for the help! I am almost afraid to ask but, are there aftermarket sources for any of these coils? The coil form in Z4 seems very loose and I worry that it might be causing problems with reception. I know that replacing any of these coils is opening up a can of worms with respect to alignment of the tuner and I wonder whether anybody here thinks it's worthwhile.
fred soop
Super Member
If the problem is only a loose coil form, you may be able to open the can and glue it in place. There are replacements (not exact Fisher replacements) sometimes available on eBay that might be made to work. I had a sticky slug in a ratio detector coil and turning it broke the entire coil loose. If it had been any other coil (with only 4 wires) I would have attempted repair but not this coil. Turns out that the Elenco AM-FM radio kit has almost the same ratio detector circuit, so I replaced the original ratio detector coil with these 2 coils (slug that was on the bottom is now the second one on the top). Yes, they look really cheap, but the internal coils are almost the same size as the original Fisher in that big can with a lot of dead space.
1425
I offered to purchase the 2 coils as parts from Elenco to replace those taken from the kit in order to have a complete kit. They sent them at no charge.
1425
I offered to purchase the 2 coils as parts from Elenco to replace those taken from the kit in order to have a complete kit. They sent them at no charge.
Another option is to copy ALL of the info, line by line and put up a WTB ad in Bartertown parts. Make note that it's from a 500B, which coil / transformer it is, and the info you got off the old one. Some older shops may save these coils as they aren't being made anymore. BTW which coil/transformer is it. Depending on which one more than one model used the same transformers. If you can match that transformer to multiple model units, you have a better chance of getting one in good shape.
Mark Oppat @ oldradioparts.net may have an original replacement or a equivalent replacement. Check with him. Give him all the info off the transformer, (pictures help a lot). Who knows you might just get lucky.
Mark Oppat @ oldradioparts.net may have an original replacement or a equivalent replacement. Check with him. Give him all the info off the transformer, (pictures help a lot). Who knows you might just get lucky.
dyche01
"Buy high. Sell low"
Spent part of the weekend replacing can capacitors in the 500B as well as troubleshooting a few small issues. I ordered a full set of replacement capacitors from Hayseed Hamfest, which arrived on Saturday. They were not cheap but they are quite beautiful and, as their website notes: "Life is too short to stuff capacitors."
On Sunday I popped the unit open and got to work. I love the old point-to-point wiring in these guys. It reminds me of a better, vanished time.
Before removing anything I photographed everything and carefully drew all of the connections to each capacitor on the bench paper. The center terminals were not always in the same place but the square circle and triangle glyphs made installation (almost) idiot-proof. I brutalized some of the terminals of the old capacitors getting them unstuck and extracting them from the case but, luckily, nothing else was damaged.
Although it put some wear on the tubes, I powered up the unit after every capacitor swap to verify that everything was okay.
The job isn't completely finished, There is one replacement remaining, but now that I have 'burned in' the four new capacitors for a few hours I notice that the 60Hz hum has virtually disappeared. In fact, the volume control-independent hum has completely disappeared from the right channel and attenuated dramatically in the left. In 'aux' there is absolutely no volume control-dependent hum in either channel, while in 'phono' there is only a faint static when the volume control is turned all the way to maximum. I did this test with nothing connected to the inputs so it is possible that the faint static will disappear when a turntable is connected. The tuner sounds fantastic, very clear and mellow.
On Sunday I popped the unit open and got to work. I love the old point-to-point wiring in these guys. It reminds me of a better, vanished time.
Before removing anything I photographed everything and carefully drew all of the connections to each capacitor on the bench paper. The center terminals were not always in the same place but the square circle and triangle glyphs made installation (almost) idiot-proof. I brutalized some of the terminals of the old capacitors getting them unstuck and extracting them from the case but, luckily, nothing else was damaged.
Although it put some wear on the tubes, I powered up the unit after every capacitor swap to verify that everything was okay.
The job isn't completely finished, There is one replacement remaining, but now that I have 'burned in' the four new capacitors for a few hours I notice that the 60Hz hum has virtually disappeared. In fact, the volume control-independent hum has completely disappeared from the right channel and attenuated dramatically in the left. In 'aux' there is absolutely no volume control-dependent hum in either channel, while in 'phono' there is only a faint static when the volume control is turned all the way to maximum. I did this test with nothing connected to the inputs so it is possible that the faint static will disappear when a turntable is connected. The tuner sounds fantastic, very clear and mellow.
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Sounds Good. The phono "Hiss or static" with no source connected is NORMAL as it's a high gain stage. make a couple of shorting plugs (rca plug with the center and outer connctions shorted together). Plug them in to the phono jacks and crank. the volume to 11. it should be dead quiet now. Same thing when you engage the "tape head". Dead quiet is good!
Looks like you've got it all on the run.
On the Z4 tuning coil the part # is L670-145
The 800b also uses this coil @ Z6.
Both the TA-600 and TA-800 use this coil @ Z3.
So there are a few options out there from salvage chassis'
Looks like you've got it all on the run.
On the Z4 tuning coil the part # is L670-145
The 800b also uses this coil @ Z6.
Both the TA-600 and TA-800 use this coil @ Z3.
So there are a few options out there from salvage chassis'
dyche01
"Buy high. Sell low"
I previously listed four problems:
1. Volume-dependent and volume independent 60 Hz hums.
2. Rolled off/muted left channel.
3. "Channel Reverse" wiring issue
4. Unblinking magic eye and scratchiness in the FM MPX signal.
Except for a faint, residual hum in the left channel, the first problem was solved by replacing the can capacitors. The scratchiness of the FM MPX signal was fixed by thorough Deoxit cleaning of the blue channel-balance pot. I have now discovered the source of problem two and, as it turns out, I am the author of my own misfortune. Soon after buying the unit I replaced the old metal "U" jumpers on the back (the ones that bypass the 'reverb') with some fancy, store-bought jumpers. I have used the same kind of jumpers to connect the pre-amp and main amp stages of many receivers and never had a problem. Well, after several hours of hair-pulling and troubleshooting the source of the bad sound in the left channel turns out to be a fault in one of the jumpers. Although it looks like a short piece of shielded cable, the fancy jumper does not connect the grounds and has only a center conductor. The jumpers do, however, have a metal collar that engages the signal ground on the outside of the RCA jack. As it turns out, one side of the left jumper has a significant leak between the metal collar and the center conductor that plays hell with the signal.
I swapped out the left jumper and, voila, beautiful sound from both channels! The left channel is still weaker than the right but this appears to be due to a known issue with the balance pot in this particular unit - an issue that the previous owner alerted me to.
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dyche01
"Buy high. Sell low"
Okay, so it's like this: The last time I opened up the 500B I checked the bias current on the output tubes, making use of the handy 10 Ohm resistors installed by the previous owner (who also reduced the grid return resistance from 330 kOhm to 220 kOhm). The tubes are a "matched" set of Raytheon 7591's and they sound pretty good but I measured bias currents of (from left to right): 19 mA, 16 mA, 20 mA, and 21 mA. These seem quite low so I re-read the thread here on AK from the previous owner (and did a little circuit tracing) and found that he had reduced the 5.6 kOhm bias resistor to 2.2 kOhm to cool down the original tubes and extend their life. This seemed prudent but I wondered how close the amp was to slipping into class B operation.
Last night, in a fit of obsessive behavior, I built a little modular circuit to enable independent biasing of all four tubes. I used a cute little circuit design by Drew Bolce that I found in another thread (http://www.audiokarma.org/forums/index.php?threads/fisher-500c-burned-resistors.335522/page-4)
Luckily (or unluckily) I had enough parts lying around to put the whole thing together last night, so I just went for it. I substituted 47 uf capacitors for the 33 uf in the original schematic and I left the original 220 kOhm resistors where they were, mounted near the sockets of the output tubes. I used an old bit of perf board from Radio Shack that has been sitting in the bottom of a drawer for 20 years, waiting for this moment to fulfill its destiny.
For input wiring I used some old 20 ga doorbell wire and for outputs I used 18 ga, 4-conductor wire. In my obsessive fervor, I even matched the color of the wires and heat shrink tubing. I heat-shrink insulated everything coming out of the bottom of the board to prevent accidental grounding to the case.
I cut down the board as small as I could, and then hot-glued nylon sleeves to the bottom for mounting to the case. Before mounting, the output wires were bent 90 degrees to the right (compared to the photo above on the right) so that they all pass between the two nylon studs on the right side of the photo. I hot glued two of the nylon studs to the case and used a machine screw to mount the third to the case, via a ventilation hole.
I passed the 'input' wires down through a fenestration in the wall separating the two halves of the case and then ran the 'output' wires up under the wiring that connects the output tubes to the OPTs. Finally, long past midnight, after double-checking all of the wiring, I fired up the unit and measured the bias currents. I have to say that it made me RIDICULOUSLY happy to bias all of the tubes. I bumped them all up to 30 mA and made sure that the current remained stable for an hour or so after everything had warmed up to steady-state temperature.
This morning I listened to the local jazz radio station and played classical music for about an hour and the sound is more rich and luscious than ever.
My next (and probably final) goal for this unit is to replace balance and volume pots. Anybody got a couple of these lying around? I will trade you a pair of vintage Fisher cases made (I think) for a model 400. I can send specs if you are interested.
Last night, in a fit of obsessive behavior, I built a little modular circuit to enable independent biasing of all four tubes. I used a cute little circuit design by Drew Bolce that I found in another thread (http://www.audiokarma.org/forums/index.php?threads/fisher-500c-burned-resistors.335522/page-4)
Luckily (or unluckily) I had enough parts lying around to put the whole thing together last night, so I just went for it. I substituted 47 uf capacitors for the 33 uf in the original schematic and I left the original 220 kOhm resistors where they were, mounted near the sockets of the output tubes. I used an old bit of perf board from Radio Shack that has been sitting in the bottom of a drawer for 20 years, waiting for this moment to fulfill its destiny.
For input wiring I used some old 20 ga doorbell wire and for outputs I used 18 ga, 4-conductor wire. In my obsessive fervor, I even matched the color of the wires and heat shrink tubing. I heat-shrink insulated everything coming out of the bottom of the board to prevent accidental grounding to the case.
I cut down the board as small as I could, and then hot-glued nylon sleeves to the bottom for mounting to the case. Before mounting, the output wires were bent 90 degrees to the right (compared to the photo above on the right) so that they all pass between the two nylon studs on the right side of the photo. I hot glued two of the nylon studs to the case and used a machine screw to mount the third to the case, via a ventilation hole.
I passed the 'input' wires down through a fenestration in the wall separating the two halves of the case and then ran the 'output' wires up under the wiring that connects the output tubes to the OPTs. Finally, long past midnight, after double-checking all of the wiring, I fired up the unit and measured the bias currents. I have to say that it made me RIDICULOUSLY happy to bias all of the tubes. I bumped them all up to 30 mA and made sure that the current remained stable for an hour or so after everything had warmed up to steady-state temperature.
This morning I listened to the local jazz radio station and played classical music for about an hour and the sound is more rich and luscious than ever.
My next (and probably final) goal for this unit is to replace balance and volume pots. Anybody got a couple of these lying around? I will trade you a pair of vintage Fisher cases made (I think) for a model 400. I can send specs if you are interested.
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And THAT my friend is the IBAM (Individual Bias Adjustment Modification) board. If your B+ to the plates is anywhere close to normal spec (393V) You are running about 11.79w per tube. 70 to 80% dissipation is 13.3 to 15.2 watts, so you cam bump them up a little. 34ma will give you 13.36w , 38ma will give you 14.93w. This does not include screen dissipation of 2 to 3ma so actual will be lower by this amount but will get you closer or in the 70-80 percentile dissipation range.
Nicely done. I would caution you about using hot glue to hold the stanchions to the board. they tend not to hold due to the overall heat in the chassis after a while and will drop the assy off the chassis. You can however use the hot glue as a insulation layer on the solder side of the board. It will peel off when needed. Suggestions for mounting the board include a small sheet metal bracket soldered to the partition, or screwing the standoff's thru existing holes in the chassis.
You could send the balance and volume pots to Mark Oppat @ oldradioparts.net and he'll rebuild them for you. Might be a little pricey tho.
Nicely done. I would caution you about using hot glue to hold the stanchions to the board. they tend not to hold due to the overall heat in the chassis after a while and will drop the assy off the chassis. You can however use the hot glue as a insulation layer on the solder side of the board. It will peel off when needed. Suggestions for mounting the board include a small sheet metal bracket soldered to the partition, or screwing the standoff's thru existing holes in the chassis.
You could send the balance and volume pots to Mark Oppat @ oldradioparts.net and he'll rebuild them for you. Might be a little pricey tho.
dyche01
"Buy high. Sell low"
Yep. You are correct. I considered doing the cute balance/bias modification but ended up going with the garden variety IBAM instead.
Thanks for the suggestion about Mark Oppat. I will check him out.
And like I said, the board IS actually attached to the chassis by a machine screw that passes through one of the ventilation holes, so there is no danger of it dropping off. You can see this better in the photo below. I don't trust hot glue either and I used it mainly to help keep things from shifting around while I was making all of the solder attachments.
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It's almost as addicting as drugs, but ABSOLUTELY LEGAL and MORE FUN. And just as expensive in a lot of cases.
dyche01
"Buy high. Sell low"
Hmm.
Just measured the B+ voltage and, instead of 393 V, I got 419 V.
Should I be concerned?
Concur. No problem on the B+. With 419 on the plates 30ma is 12.57w -3ma for the screens (27ma) or 11.31w.
36ma will get you 15.084w -3ma(33ma) for screens and you get 13.827w. Bump them up to 36ma and see how they sound. Max dissipation would be right around 45ma-45.5ma per tube. Ideally you want between 70 and 80% max of 19W for the output tubes now. They sound just as good and they'll last longer. The EH's and Tung-Sol Reissues won't hold up to 100% dissipation for long. Life expectancy is measured in hours, or if you're lucky a couple weeks. I've got a quad of EH's here that have run at 75% of max for 6 years now on a fisher 800c and a Sansui 1000A. And another quad of Tungsolds in the Executive for 2 years now. Haven't had a hiccup outta any of them. They're like the energizer bunny............
36ma will get you 15.084w -3ma(33ma) for screens and you get 13.827w. Bump them up to 36ma and see how they sound. Max dissipation would be right around 45ma-45.5ma per tube. Ideally you want between 70 and 80% max of 19W for the output tubes now. They sound just as good and they'll last longer. The EH's and Tung-Sol Reissues won't hold up to 100% dissipation for long. Life expectancy is measured in hours, or if you're lucky a couple weeks. I've got a quad of EH's here that have run at 75% of max for 6 years now on a fisher 800c and a Sansui 1000A. And another quad of Tungsolds in the Executive for 2 years now. Haven't had a hiccup outta any of them. They're like the energizer bunny............
dyche01
"Buy high. Sell low"
Okay. I rebiased all the output tubes to 34 mA and then took a stab at balancing the inverters. Of course, the gold standard would be to hook up a distortion analyzer to the output and then adjust the inverters for minimum harmonic distortion. In addition to imbalances in the inverter stage, this approach can help compensate differences in the output stage. On the other hand, the simplest approach to adjusting the inverters (pinned at the top of the forum) is a kind of 'dead reckoning' that involves matching resistances with the unit powered off (http://audiokarma.org/forums/index.php?threads/quick-and-easy-400-phase-inverter-adjust.559415/). This method doesn't require complicated test equipment but seems to rely heavily on the assumption that all of the tubes are in spec. Another method, proposed by a user named Hi-Q, is to balance the output of the inverter stage with an oscilloscope (http://audiokarma.org/forums/index....hase-inverter-pots-on-a-push-pull-amp.405394/). As long as the output stage is properly balanced, this method should work pretty well so I gave it a shot.
The original procedure involved measuring the voltage across the output tube grids; adding both signals from one channel together; and then adjusting the inverter to minimize the sum. Unfortunately, my fancy Tektronix scope is on loan and my old Heathkit scope does not have the ability to invert or add signals so I came up with a slight variation on the method. Here it is in gory detail:
1. Attach dummy loads to the A and B output channels. I used 100 W 8 Ohm resistors.
2. Set the input to AUX and apply a 1000 Hz sine wave signal. I use a waveform generator app on my iPad, running through an outboard DAC.
3. Set the volume control moderately high, but not high enough to see clipping in the output.
4. Calibrate the scope by applying both sets of test leads to the same output tube (across pins 5&6 on 7591 tubes) and adjusting gain on the two channels of the scope so that the waveforms match exactly. The easiest and most accurate way to do this is to change from timebase to x-y display, so that voltages across the two sets of leads are plotted on the x- and y-axes. For proper calibration this line should have a slope of exactly 1. Using the grid drawn on the face of the scope, I adjust the x-position of the trace so that it passes through the origin and then adjust the scope gain until the trace passes as exactly as possible through the corners of all the grid squares.
5. Without changing any of the settings, move one set of scope leads to pins 5&6 of the other output tube in the same channel. Because this signal is inverted with respect to the first one, the output trace will be a line with a negative slope.
6. With the scope properly calibrated the slope of this line reveals reveals any imbalance in the inverter stage. Changing the phase balance control causes the line to rotate clockwise or counter-clockwise.
7. Adjust the phase balance control to bring the slope of this line as close as possible to -1 (i.e. make it pass through the corners as exactly as possible). You will probably have to re-center the line several times during the process.
8. Voila.
The original procedure involved measuring the voltage across the output tube grids; adding both signals from one channel together; and then adjusting the inverter to minimize the sum. Unfortunately, my fancy Tektronix scope is on loan and my old Heathkit scope does not have the ability to invert or add signals so I came up with a slight variation on the method. Here it is in gory detail:
1. Attach dummy loads to the A and B output channels. I used 100 W 8 Ohm resistors.
2. Set the input to AUX and apply a 1000 Hz sine wave signal. I use a waveform generator app on my iPad, running through an outboard DAC.
3. Set the volume control moderately high, but not high enough to see clipping in the output.
4. Calibrate the scope by applying both sets of test leads to the same output tube (across pins 5&6 on 7591 tubes) and adjusting gain on the two channels of the scope so that the waveforms match exactly. The easiest and most accurate way to do this is to change from timebase to x-y display, so that voltages across the two sets of leads are plotted on the x- and y-axes. For proper calibration this line should have a slope of exactly 1. Using the grid drawn on the face of the scope, I adjust the x-position of the trace so that it passes through the origin and then adjust the scope gain until the trace passes as exactly as possible through the corners of all the grid squares.
5. Without changing any of the settings, move one set of scope leads to pins 5&6 of the other output tube in the same channel. Because this signal is inverted with respect to the first one, the output trace will be a line with a negative slope.
6. With the scope properly calibrated the slope of this line reveals reveals any imbalance in the inverter stage. Changing the phase balance control causes the line to rotate clockwise or counter-clockwise.
7. Adjust the phase balance control to bring the slope of this line as close as possible to -1 (i.e. make it pass through the corners as exactly as possible). You will probably have to re-center the line several times during the process.
8. Voila.
dyche01
"Buy high. Sell low"
Here are some photos to illustrate the method.
Below is the calibration signal (both leads attached to the same pins on the same tube), with the time-base trace on the left (with a little y-offset added to separate the traces) and the x-y trace on the right. Note that the scope has not yet been calibrated and the slope is not exactly 1.
Here are the two out-of-phase signals measured on two output tubes in the same channel. Note the negative slope in the x-y trace.
Here is the left channel signal before (left) and after (right) phase balance adjustment. Note that to obtain a slope of exactly -1 the line must be rotated counter-clockwise (red arrows).
Below is the calibration signal (both leads attached to the same pins on the same tube), with the time-base trace on the left (with a little y-offset added to separate the traces) and the x-y trace on the right. Note that the scope has not yet been calibrated and the slope is not exactly 1.
Here are the two out-of-phase signals measured on two output tubes in the same channel. Note the negative slope in the x-y trace.
Here is the left channel signal before (left) and after (right) phase balance adjustment. Note that to obtain a slope of exactly -1 the line must be rotated counter-clockwise (red arrows).
