Fisher 250TX Advice

potoooooooo

potoooooooo

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I picked up a 250TX a while ago that I've been itching to dig into. I have some hobbyist level electronics experience, but I'm not an EE by any means.

The right channel speaker is really quiet and has this strange distortion (clipping maybe?) at certain volumes, but otherwise everything seems to be working well except for the dead bulbs.

I replaced a couple of fried resistors, hoping it would help, but it didn't. Not surprising, I always assumed it was caps.

The question is, there are a bunch of caps that aren't electrolytic. Do I need to replace those or is it just the electrolytic caps I should be replacing? What about the ones that aren't polarized, do they get replaced as well? Does anyone have a list of caps I should be buying?

Thanks
 
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Usually, only the electrolytics need replacement. Have you tried swapping channels at the record out/tape monitor in jacks? This will tell you if the problem is before or after the tape monitor. If it is after the tape monitor, you can swap the wires feeding the power amp. This can also be done by swapping the wires on the center terminals of the volume control sections if this is easier to access.
 
fred soop said:
Usually, only the electrolytics need replacement. Have you tried swapping channels at the record out/tape monitor in jacks? This will tell you if the problem is before or after the tape monitor. If it is after the tape monitor, you can swap the wires feeding the power amp. This can also be done by swapping the wires on the center terminals of the volume control sections if this is easier to access.
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I tried swapping the L/R channels via the aux input, and the same speaker was affected. I'm a little confused as to what you mean at the jacks. Do you mean to connect the RCDR out and MON in jacks on the rear?

Also I compared my boards to some images of original boards and while many of the caps have been replaced, seemingly a long time ago, there are also quite a few late 60's originals
 
2 RCA CABLES. 1st cable LEFT REC OUT to RIGHT MONITOR, 2nd cable RIGHT REC OUT to LEFT MONITOR.

Also check to see if the 250-T has PREAMP OUT and MAIN IN Jacks (If so, there should be a bridging pin between the out and in NOT THE REVERB IN/OUT JACKS). Left Preamp to Right Main, and Right PREAMP to Left Main. This will tell you if the AMP is at fault (no change in channel when swapped) or the preamp (low channel will swap with cables).
 
No preamp out/power amp in. Have to move the wires as mentioned above. Reverb jacks are at same circuit location as tape monitor and this is the case on most units.
 
larryderouin said:
2 RCA CABLES. 1st cable LEFT REC OUT to RIGHT MONITOR, 2nd cable RIGHT REC OUT to LEFT MONITOR.

Also check to see if the 250-T has PREAMP OUT and MAIN IN Jacks (If so, there should be a bridging pin between the out and in NOT THE REVERB IN/OUT JACKS). Left Preamp to Right Main, and Right PREAMP to Left Main. This will tell you if the AMP is at fault (no change in channel when swapped) or the preamp (low channel will swap with cables).
Click to expand...

fred soop said:
No preamp out/power amp in. Have to move the wires as mentioned above. Reverb jacks are at same circuit location as tape monitor and this is the case on most units.
Click to expand...

I did the L/R swap using 2 RCA cables and the right channel remained distorted and crackly, but the left channel became quiet instead. Seems everything has issues.

I redid the caps on the power amp, and it doesn't seem to have made a difference. I'll have to order in the other caps, my local store has limited selection. Is there anything else that could potentially need replacing? Transistors, etc?
 
So i redid the caps on the preamp, and still no change. I'm officially at the limits of my electronics knowledge.

What should I be looking at, exactly? How do I check if a collector is bad?

Thanks
 
Ok so I did a lot of reading on transistors, did the tests, found a bad one, and replaced it.

Everything works now, sort of. I tested the center voltage and it was around 70mv on one channel, 30mv on the other. At this point i smelled some burning and shut it off. One of the resistors (R829 on the board) was overheating. I checked the bias voltage and it was super high so I put it down to zero and checked the other one (R830, it's twin), which had just started burning up in the same way. Same thing, lowered the voltage via the blue potentiometer and it stopped burning up. As i went to set the bias it would go rapidly from ~0mV to ~65mV with no room in between. This happens on both sides. Are the potentiometers bad? Can i hit them with contact cleaner like other pots, or are they special and more sensitive? Or is this something else?

I noticed that when the resistor was hot so was it's matching transistor, usually. And the voltage on the resistor was around 26V (not mV, yikes).
 
Measure voltage between the negative side of R817 (end away from bias spreader) and collector of Q803. Turn the bias pot and see if the voltage varies smoothly or if it's erratic. If it is erratic then the bias pot(s) either need cleaning or replacement. Also, R831 and R832 (on the negative driver) should be getting the same current as R829 and R830 and should also be overheating.
 
fred soop said:
Measure voltage between the negative side of R817 (end away from bias spreader) and collector of Q803. Turn the bias pot and see if the voltage varies smoothly or if it's erratic. If it is erratic then the bias pot(s) either need cleaning or replacement. Also, R831 and R832 (on the negative driver) should be getting the same current as R829 and R830 and should also be overheating.
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I checked and it went up smoothly, from 11.8V to 12.3V on the right channel and 11.2V to 12.3V on the left.

I can also confirm that neither R831 or R832 are overheating.

Edit: I should note that the matching transistor that was also super hot were Q806 and Q805, not Q809 and Q810
 
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That would indicate that there is no dead or open spot in the bias pots. Voltages appear to be reasonable. Voltage across R829 and R830 should be less than 1 volt. This would point to a shorted Q805 and Q806. The fuses would also need to be open for a high voltage on the resistors unless the upper output transistors are open emitter to base. Strange that both channels would have the same problem.
 
fred soop said:
That would indicate that there is no dead or open spot in the bias pots. Voltages appear to be reasonable. Voltage across R829 and R830 should be less than 1 volt. This would point to a shorted Q805 and Q806. The fuses would also need to be open for a high voltage on the resistors unless the upper output transistors are open emitter to base. Strange that both channels would have the same problem.
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So I wrote out a long reply explaining some more symptoms and asking some questions when I realized my lizard brain overlooked the fuses. F801 and F802 are blown, F803 and F804 are fine. I don't know if that's the root of the issue or if that's just a symptom. I'll hit up my local electronics store tomorrow AM and see if they have fuses.
 
It appears that the upper drivers are then being forced to carry current for the lower outputs. Before replacing the fuses, check the upper output transistors.

Another test would be to remove the lower fuses so that all output transistors are effectively out of the circuit. This will allow the upper and lower sections to balance out. With this particular circuit configuration, you would also be able to get output audio but the power available would be very low. Should be able to drive headphones with no problem. This would help determine if there are additional problems.
 
fred soop said:
It appears that the upper drivers are then being forced to carry current for the lower outputs. Before replacing the fuses, check the upper output transistors.

Another test would be to remove the lower fuses so that all output transistors are effectively out of the circuit. This will allow the upper and lower sections to balance out. With this particular circuit configuration, you would also be able to get output audio but the power available would be very low. Should be able to drive headphones with no problem. This would help determine if there are additional problems.
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When you say upper and lower, output etc, which do you mean? I don't know enough about circuit layouts to figure this out by the diagram alone, unfortunately. But to take a stab, are you saying to remove the leads with the fuse for Q811 and Q812 in order to push the entire workload onto Q805-Q808?

Also the transistors on the rear panel were good when I checked them, but I have replacements should they now test bad. I'll be running to my local store after work to see if they have what I need.
 
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Ok so i'm starting to get very confused.

I swapped in new fuses and the right channel is now buzzing very loudly at any volume, and it also started crackling and popping a scary amount after a minute or two.

Also the center voltage on the right channel was almost zero, while the left was 18 Volts!!!

I adjusted the bias for the left channel just fine. No reading on the right channel.

Starting to get frustrated, my limited electronics knowledge is really limiting my troubleshooting. Thank you @fred soop for your continued help.
 
Unfortunately, I may be at the limit for what I can do on this type of forum. What happens when all 4 fuses are removed and the drivers are working alone?
 
On the emitter lead of all 4 output transistors is a PICOFUSE of 3.5A rating(F801 thru F804) disconnect the emitter lead at the board or the fuse if it's a removable one, mine aren't). Pull all 4 fuses, is what Fred is saying and start it up using the drivers, and a pair of headphones. It will have enough power to run a pair of headphones. If everything is kosher, and you can adjust center voltage and bias to spec and it holds, then the outputs are most likely bad. (you can also pull each output in turn and do a 6 way test on each one as a backup.) It sounds like from your description that the output's are probably either dying or shorted completely (the 18V).

MJF15030 and MJF15031 would probably work fine for outputs. This is my guess as the originals are TO-127's and these are plastic coated TO-220, which are fine, just don't tweak the screw down hard and dig into the plastic.

863-MJF15030G $1.52ea Mouser.
863-MJF15031G $1.74ea Mouser
 
Q805도 같은 증상으로 단락된 것을 발견했습니다. Q805를 대체할 수 있는 트랜지스터는 무엇일까요?
 
larryderouin said:
On the emitter lead of all 4 output transistors is a PICOFUSE of 3.5A rating(F801 thru F804) disconnect the emitter lead at the board or the fuse if it's a removable one, mine aren't). Pull all 4 fuses, is what Fred is saying and start it up using the drivers, and a pair of headphones. It will have enough power to run a pair of headphones. If everything is kosher, and you can adjust center voltage and bias to spec and it holds, then the outputs are most likely bad. (you can also pull each output in turn and do a 6 way test on each one as a backup.) It sounds like from your description that the output's are probably either dying or shorted completely (the 18V).

MJF15030 and MJF15031 would probably work fine for outputs. This is my guess as the originals are TO-127's and these are plastic coated TO-220, which are fine, just don't tweak the screw down hard and dig into the plastic.

863-MJF15030G $1.52ea Mouser.
863-MJF15031G $1.74ea Mouser
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I found Q805 shorted with the same symptoms. What transistor can replace Q805?
 
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