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Fisher 400 both channels distortion

S

streetwise

Active Member
My Fisher 400 seemed to hit some trouble today. I went to listen to dizzy gillespie for xmas eve and what I heard didn't sound like the dizzy I know. The music is there but choppy and distorted almost like poorly received FM stations crossing bands. This is happening on both FM and Aux, I fiddled with tubes so far, checked the phase inverter adjustment and also looked at the tube bias to find that the tubes are all at 3.05ma? so somethings up with the power supply or efb. I also noticed that for the second that I unplug the receiver the sound cleans up before fading away. Any help and direction on where to start looking would be great. It's been 7 years roughly since I rebuilt this receiver and modified it with all of Dave Gillespie's mods so im a little reluctant to get back into diagnosing this receiver after so long out of the game.
 
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Street -- Sorry you're having problems. With such low quiescent current flow, it sounds like the output tubes are getting too much negative voltage applied to their control grids. Measure and record the voltages on pins 2/6 and 1/7, and let us know the results. Then we can have a better idea of what's going on.

Dave
 
d
dcgillespie said:
Street -- Sorry you're having problems. With such low quiescent current flow, it sounds like the output tubes are getting too much negative voltage applied to their control grids. Measure and record the voltages on pins 2/6 and 1/7, and let us know the results. Then we can have a better idea of what's going on.

Dave
Click to expand...
Thanks for the fast response dave, I will record my findings and get back to you very soon!
 
Yup, the the EFB Control Grid Regulator is supplying too much negative voltage to those grids. Either something has happened to that regulator, or something is causing it to produce that level of output. Take and post readings for the three leads of both transistors in that regulator and let us know!

Dave
 
dcgillespie said:
Yup, the the EFB Control Grid Regulator is supplying too much negative voltage to those grids. Either something has happened to that regulator, or something is causing it to produce that level of output. Take and post readings for the three leads of both transistors in that regulator and let us know!

Dave
Click to expand...

the first transistor has -.2mv, -45.9v and .917v
the second transistor has -46v, -52v, -46v

thanks Dave It would have taken me ages to get this far without your help
 
Humm, without designating which reading is associated with which transistor lead it leaves it open to interpretation but in any event, the first transistor normally can't show any voltage at all on it's Emitter lead, since it is grounded in the design. The Base lead would nominally operate at about -0.6 vdc, and the Collector at about -15 to -18 vdc. You are also showing a positive voltage on one of the terminals as well, which again, normally just cannot be. From my vantage point, it would appear that the first transistor has gone open, which is not the normal failure mode for SS devices. Also normally, that would be all but impossible to happen, as the specified transistor is a 160 volt, 1A device, with the circuit demands being but a fraction of that capability. The second transistor may still be good, but would need more delineation between the two 46 volt readings to be sure. If one is reading about 0.6 volt higher than the other (or lower depending on which lead you are measuring), then that transistor is likely still good. Again, being the same transistor as the first one, it too is way, way over-rated for the function it provides.

The transistors normally produce virtually no heat in operation, and with any kind of reasonable installation, chassis underside heat would normally pose no problem as an environment for them to operate in.

That makes it quite a mystery why either one would fail, but it certainly appears that at least the first transistor has. Maybe some sort of AC line spike? I think I'd replace both of them to be safe. Also, I'd update the circuit to add a 1000 volt PIV 1A diode (IN4007 is perfect) from the Base of the first transistor to ground -- banded end to ground. The diode is normally out of the circuit electrically, but provides protection for the circuit in case the bias pot shown in the schematic should ever open up, or the B+ should linger around after the negative supply voltage is gone at shut down. It does this by preventing the Base of the first transistor from ever going positive more than 0.6 vdc -- but I've never heard of that happening before and again, normally can't happen because the regulator circuit is powered from it's own negative DC supply (point being that even when the main DC heater supply drops to zero near immediately at shut down, the supply to the regulator does not).

Sorry you're having problems, but I'm certain that attending to these things will get things back to normal once again.

I hope this helps!

Dave
 
dcgillespie said:
Humm, without designating which reading is associated with which transistor lead it leaves it open to interpretation but in any event, the first transistor normally can't show any voltage at all on it's Emitter lead, since it is grounded in the design. The Base lead would nominally operate at about -0.6 vdc, and the Collector at about -15 to -18 vdc. You are also showing a positive voltage on one of the terminals as well, which again, normally just cannot be. From my vantage point, it would appear that the first transistor has gone open, which is not the normal failure mode for SS devices. Also normally, that would be all but impossible to happen, as the specified transistor is a 160 volt, 1A device, with the circuit demands being but a fraction of that capability. The second transistor may still be good, but would need more delineation between the two 46 volt readings to be sure. If one is reading about 0.6 volt higher than the other (or lower depending on which lead you are measuring), then that transistor is likely still good. Again, being the same transistor as the first one, it too is way, way over-rated for the function it provides.

The transistors normally produce virtually no heat in operation, and with any kind of reasonable installation, chassis underside heat would normally pose no problem as an environment for them to operate in.

That makes it quite a mystery why either one would fail, but it certainly appears that at least the first transistor has. Maybe some sort of AC line spike? I think I'd replace both of them to be safe. Also, I'd update the circuit to add a 1000 volt PIV 1A diode (IN4007 is perfect) from the Base of the first transistor to ground -- banded end to ground. The diode is normally out of the circuit electrically, but provides protection for the circuit in case the bias pot shown in the schematic should ever open up, or the B+ should linger around after the negative supply voltage is gone at shut down. It does this by preventing the Base of the first transistor from ever going positive more than 0.6 vdc -- but I've never heard of that happening before and again, normally can't happen because the regulator circuit is powered from it's own negative DC supply (point being that even when the main DC heater supply drops to zero near immediately at shut down, the supply to the regulator does not).

Sorry you're having problems, but I'm certain that attending to these things will get things back to normal once again.

I hope this helps!

Dave
Click to expand...

Thanks for the write up sorry i didn't identify what transistor was what, I read the pins left to right I have my board mounted very similarly to the way you had shown when you did your improving the fisher 400 thread. I will try replacing both transistors and adding the diode than post back. Im not sure if this failure might have been caused by my sister inlaws cat was visiting, and liked inspecting the back of the receiver where I have the IBAM/IBBA board mounted. I don't know if there's a way she could have shorted something or caused a static charge.

Thank you, thank you as always Dave!
 
I think I found the issue I was replacing the transistor and adding the diode when I noticed 100uf 50v caps on the efb is very bloated I will replace and report back
 
The two voltage doubler caps are perfectly adequate with a 50 volt rating. The output cap from the doubler circuit however needs to have a higher voltage rating due to the doubling action of the configuration. A 100 volt cap is required it that location. Remember that your problem is one of too much negative voltage -- not too little. Any chance one of the voltage doubler caps was installed backwards? The circuit would still double its output (although not as efficiently), but the reverse installed cap would no doubt melt down.

Dave
 
dcgillespie said:
The two voltage doubler caps are perfectly adequate with a 50 volt rating. The output cap from the doubler circuit however needs to have a higher voltage rating due to the doubling action of the configuration. A 100 volt cap is required it that location. Remember that your problem is one of too much negative voltage -- not too little. Any chance one of the voltage doubler caps was installed backwards? The circuit would still double its output (although not as efficiently), but the reverse installed cap would no doubt melt down.

Dave
Click to expand...
When I get a replacement I will look double check the polarity of the caps, I don't know why but the cap looks ready to burst. Maybe just premature failure of the cap. Would the doubler cap have lasted 6+ years of near daily use with the polarity reversed?
 
any time I've had a cap in backwards, it has such a gross amount of leakage that the output voltage ends up very low. If there is enough of a current supply, they tend to get hot and blow up pretty quickly. Doubt it would suffer like that for 6 years without blowing up or otherwise not working right, but the world is a strange place.
 
The doubler configuration I developed for this application is quite unique in that it is a true full wave voltage doubler -- that is, the full doubling action takes place on both cycles of the AC waveform, whereas traditional "fullwave" doubler circuits also conduct on both cycles of the waveform, but only at one half of the doubled voltage on each cycle. It is essentially two half wave circuits connected in series, so if one of the doubler caps goes bad, the output voltage drops significantly as Gadget said. With my design, since the full doubling action takes place on both cycles of the waveform, if one half of the circuit fails due to a bad doubler cap in that half of the circuit, the other half will still double to minimize any loss of output.

Dave
 
OP stated -52v as the highest voltage on one of the transistor legs, assuming the collector of the output transistor. Would that be a result of shorted doubler cap to drop 10v from spec -62v rail? Can then excessive ripple cause the driver transistor to open?
 
No. Even if the buffer transistor were passing all the ripple current of an unfiltered true halfwave rectifier circuit, those currents would never approach the 1A rating of the transistor used, because the over all current flow through the circuit is so small.

Dave
 
dcgillespie said:
The two voltage doubler caps are perfectly adequate with a 50 volt rating. The output cap from the doubler circuit however needs to have a higher voltage rating due to the doubling action of the configuration. A 100 volt cap is required it that location. Remember that your problem is one of too much negative voltage -- not too little. Any chance one of the voltage doubler caps was installed backwards? The circuit would still double its output (although not as efficiently), but the reverse installed cap would no doubt melt down.

Dave
Click to expand...

Dave I believe I have found the issue, the caps installed have the correct polarity but the connection on the negative side of the caps are linked 6ways, meaning both negatives are connected between the diodes. I noticed right away because I looked at your original drawing showing the bump where the negatives cross to alternate diodes. I was most likely confused by the "fisher 400 with EFB,IBAM,PS,PH_INV and control section update_REV7.pdf" not having those bumps I probably thought those were all tied together. It worked for several years with this mistake not sure what that means electrically but I will remedy this and report back
 
streetwise said:
Dave I believe I have found the issue, the caps installed have the correct polarity but the connection on the negative side of the caps are linked 6ways, meaning both negatives are connected between the diodes. I noticed right away because I looked at your original drawing showing the bump where the negatives cross to alternate diodes. I was most likely confused by the "fisher 400 with EFB,IBAM,PS,PH_INV and control section update_REV7.pdf" not having those bumps I probably thought those were all tied together. It worked for several years with this mistake not sure what that means electrically but I will remedy this and report back
Click to expand...

Ok, now i have replaced the transistors mentioned added the In4007 from the base of the first transistor banded end to ground. replaced the 2 doubler caps corrected the positive leads being connected on the caps. still same problem 3.05ma at the output tubes
 
If the Negative terminal of the two voltage doubler caps were shorted together, the circuit would still rectify and double to some degree -- but it would have been very hard on the caps as they would have effectively been placed in series and then across the AC voltage supply, making them pass a significant amount of current -- no doubt more than they were rated to handle. This would absolutely explain the physical deterioration seen in these caps -- or more likely, one gave up the ghost, and then the other allowed the circuit to function as a doubler, but as a half-wave voltage doubler similar to the action I described earlier, where if one side of the doubler is compromised, the other side will still allow the circuit to effectively double -- just not as efficiently. That's why the unit still operated for years, with the one doubler cap likely blown out long, long ago. This also concurs with my original assessment that while this issue certainly needed to be fixed, it wasn't the heart of your problem, because it is based on the fact that too much negative voltage is being supplied to the output tubes. It is not surprising then that the original issue remained.

So now it would seem that we're into a two pipe problem. I was originally surprised that the transistors would have failed, but since they are the most obvious component whose failure would cause the problem at hand, that was the suggestion given. While I'm somewhat surprised that replacing the transistors didn't resolve the problem, I'm not entirely surprised that they are not the problem, either. As I said, all of the SS components used are way, way over-rated for the duties they handle. So now it's time for more troubleshooting: Using all necessary caution of course:

1.Remove all four output tubes (remembering location of course), and turn the unit upside down with the bottom cover removed.

2. Disconnect the 1M 1% resistor of the EFB™ Control Grid Regulator from the point where it connects to the screen grid buss. With the resistor disconnected from the screen buss, measure it's value. Next, with the resistor still disconnected:

3. Connect your voltmeter between the output of the EFB Control Grid Regulator (Emitter terminal of the Buffer Transistor -- the transistor that does not connect to the 1M resistor just mentioned), and ground.

4. Turn the unit on and note the reading on the voltmeter. Operating the unit with the output tubes removed will cause no harm to any circuits of the receiver.

5. Turn the unit off and unplug it. Leave the output tubes removed. The unit will self discharge as normal, but take much longer to do so.

Report back the measured value of the resistor and the voltage indication from the test.

Dave
 
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