Fisher 800t recap/repair questions

Vince1966

Vince1966

Super Member
I came across this 800t and a Pair of XP-10 speakers. First off the unit works with no huge issues. But after closer inspection I found a burnt resistor (180ohm 2watt) on the power supply board. I read through a few post on the 800t/500tx and resident expert Larryderouin’s repair advice. Hopefully he will jump in and offer advice.

I made a list from the 500tx recap and ordered the caps and diodes. First big question is on the power supply board 2000uf caps. I see 3 legs but I’m assuming from the schematic the 3rd top leg is for support? Also 3300 would work here once I determine correct placement for +- terminals. Is it better to go with a 3watt 180ohm for the burned R983? Also is the resistor overheating somewhat normal? I’m new to Fishers, mostly recapped/ repairs Sansui and Pioneer.

here is a few pictures.


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R983 should have 15 volts across it and would be dissipating 1.25 watts. For smaller resistors such as this, I would use at least a 3 watt replacement. This resistor feeds power to the selector switch and then to the AM and FM front ends.

As for the capacitors, it appears from the schematic that there are only 2 connections. Look at the back side of the circuit board to see if the 2 pins on one end use a common connection.

These capacitors are the main filters for the power amplifier and everything downstream. I would replace with the largest value that will physically fit in the space. Up to 10,000 µF would not be overkill for a unit with this power.
 
I have some 3 watters on order. I have the service manual and enlarged it, found the correct placement for the filter capacitors. I need to see if some 6800’s I have will fit but ordered some 3300’s. I’m sure there will be many more questions as I move to the other boards. Thank you
 
As you surmised the 3rd leg on those Main filter caps is just for support. Use hot glue on the new ones, lay them down and THEN solder the leads.

3 watter is in mine from years ago.
 
3 watt installed along with diodes and caps, power supply is done and working good. I pulled the main amp board to start next. A few things I noticed was the Bias Diodes were just pointing in the air, I’m assuming they should be thermally connected to Q805-806 transistors, I see a smudge of old thermal grease on the heat sinks. Also is there a mica insulator under the main output transistors? I wanted to pull those and regrease them with some thermal compound. I did not want to break anything I don’t have on hand. Any other problematic parts there that should be replaced? Also noticed a hot zener, 27 volt 3 watt. Suggestion for replacement, 1N5935BRLG possibly?
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1N5935BRLG is fine. Leave the bias diodes hang in space. Yes the output's have mica's behind them. The six white electrolytics replace. put it back in and test.
 
Everthing was going great, recapped the pre amp, amp and power supply, testing on dim bulb after each board and light would go out no smoke, well I replaced the bulbs for the dial, reinstalled face plate, dim bulb was good so went to main power and smoke. R829 on the main amp board fried. Had my finger on the switch to turn it off was only on 2 sec. Pulled the output and did not find a short. bummer. So I’m done for today…..
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Tested the output with the transistor tester, it’s toast, the driver is still good. Also the inline fuse off the emitter lead is blown. Not seeing what could have caused this. The resistor is still reading good 116 ohm’s, just burned. im Stumped at this latest development. This unit worked ok when I got it, was hoping to add longevity to it not blow it up. Oh well.
 
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Double checked everything installed, no issues there. Cut out the inline fuses and put in two 5 amp in the holders As per the update. Removed all the outputs, tested everything on the driver board in the pathway, no issues there.

powered up on Dim with outputs removed.

Q809 (the one that blew)
B. 35.22vdc.
C. -29.9 mV.
E. 36.20 vdc

Q811 (still good).
B. -34.99 vdc.
C. -27.8 mV.
E. -36.17 vdc.

Q812 (tested good).
B. -35.35 vdc.
C. 96.2 mV.
E. -36.02 vdc.

Q810 (tested good).
B. 35.03 vdc.
C. 106mV.
E. 35.77 vdc.
 
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My transistor tester is telling me I have 3 bad transistors, but only one fails the 6 way test. New transistors shipped would be $51.00. Inflation is sucking.

more voltage readings on main power transistors removed.

Q809
B 33.34 vdc
C 380mV.
E 40.50 vdc

Q811
B -36.34 vdc
C 96mV
E -40.99 vdc

Q812
B -32.03 vdc
C 357 mV
E -40.99 vdc

Q810
B 37.53 vdc
C 541 mV
E 40.74 vdc
 
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had time to get back to this unit. Replaced the outputs, performed the service bulletin and replaced the burned resistor along with the other 120ohm resistors with 1 watt. I carefully went over everything again, no broken leads, nothing disconnected, I did fix a few cold solder joints on the amp board. Tested on dim bulb, looks good.

went to main power and had my test speakers hooked up. There is a hum at no volume, increase in volume increased the hum. After about a minute and a half it got a little louder and then got very quiet. I let it sit for 30 min and adjusted the Idle current to 15 mV. Holding stable there. Connected turntable and aux sources. The both sound good.

now there is no AM/FM. The tune o matic meter works good, i get no static, no sound whatsoever. I rechecked the connections and they were correct With my pictures and notes.

the power supply was fully recapped, replaced almost all diodes and the overheated resistor.

im looking over the schematic to understand the power flow to the AM/FM tuners, none of those were recapped. I have read the .33 capacitor on the back of the FM tuner can go bad. What’s a good starting point to straighten this issue out? is it possible I have a weak transistor on the Amp board coursing the hum until it warms up? All grounds look good. Also to note the hum is present in all functions until roughly 90 sec and then all is quiet.
 
I recapped my first Fisher 500tx, and yes it does have a hum!!! It starts out a little loud, then slowly goes away as it warms up. I always thought it was because of the larger capacitors i used on the power supply board. I think i used 3200uf instead of the 2000uf. Maybe that is my problem, im not sure... i've learned to accept it. I do not like taking the power supply board in and out. It's fragile.

the AM/FM circuit issues you have could be the muting circuit. Mine had that issue. It was clearly receiving the signal, but not playing them. I pulled the FM board to recap everything and it fixed it.
 
Thanks for the info, next time I pull it out I will look into the mute board. Probably where the issue is. As far as the hum goes, it’s humming all the time now. Goes up with volume.
 
I’m still very new to diagnosing and repairing, but if it changes with the volume, does that mean the issue is in the preamp board?
 
Well I’m back to this 800t, not giving up and I have several days to go through it again. I left off with no fm and a bit of a hum, good bias and both channels working.

starts with testing voltage from the power supply. pin 9EE was missing the +12v. That feeds the Am/Fm, muting and auto scan. Tracing 9EE back I found the I was losing all the voltage at R983, I replaced this because it was burned. I found that I put in a 180k instead of 180ohm 3watt. Swapped it out and now I have the tuner and auto scan back.

But, I lost most of the right channel. I have almost no right channel in all functions including mono, only a faint sound can be heard. So I spent 2 hours rechecking every thing. I have 1.2 mV for the left and 92 mV right at the speaker terminals, bias is still good and there is no Hum in the left. the only boards not recapped is the muting, Audio control amp and Am/fm.

any suggestions? Maybe one of the caps in the audio control amp gave up. I see several 1 and 4 uf caps there for both left and right channels.
 
Well that was a snapped ground causing the hum. The lost right channel was a bad cap on the Audio control board.

sounds pretty good on the cornwalls in the shop.
 
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