Fisher 690A and 690T

Can you post the whole schematic? Bypassing the speaker selector switch should have no effect on the output. If it has hum, it has a problem.
 
Its kinda hard to see it, but where the little red circles are, are the positive connections to the speakers. The negative connections are the chassis.

FISHER20AMP20FULL1.jpg
 
I will, and I have them here already, but shouldn't I be getting audio with hum as opposed to no audio and only hum if the caps are all that's wrong?
 
I myself, do not own a hybrid amp, but I would go through it all. Check all diodes, output transistors, solder connections. Are those ELL80 tubes good? Are they glowing? How about the 12AX7 tubes?

I don't have the schematic so this is just off the top of my head, but someone else here owned the non-hybrid Fisher amplifier that looked like your amplifier, except it had the 7591 tubes, along with a couple of 12AX7 tubes, where the filiment voltage of the 12AX7 tubes were taken from the 7591 tubes cathodes.
 
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Right - I think there was some discussion of adding a 250 ohm resistor to the yellow wire off the tuner to ground, but in that thread it seemed that the yellow wire on that amp was more directly in line with the tubes...the yellow wire on my tuner plug is #12, so it's connected directly to a 220 mfd cap.
 
After again letting this sit for months and months, I fired it up again today and am back where I started...loud hum (I haven't replaced the caps) and no audio. I haven't taken the transistors out to check; am I right in understanding that any one of them failing on either channel could cause this exact scenario?
 
If you have a loud hum through the speakers, (which it sounds like you are describing), then the output transistors are probably ok. They are wired in series/parallel. So if one goes, it's gonna take out the other one in that particular section. Then NO SOUND/NO HUM.

change the CAPS! They are causing the HUM. CHANGE ALL OF THEM. Start with the BIG FILTER CAPS. and then work on down the line. Oh yeah. Don't mess with the 4 pots in the amp. They adjust the transistor bias and are extremely touchy and need to be adjusted in pairs together. Takes 5 hands, 3 pair of eyes on the pots, and meters, someone to read the manual and direct the adjustment, and 3 kibbutzing friends to make sure someone screws it up so they all get paid for doing it twice. :D :D :D Seriously, it's very involved and best left to a tech.

Larry
 
Thanks Larry...I have the replacement caps but was trying to get sound out if it before I got into that. The interesting thing I noticed yesterday was that the 12AX7, which tests great and lights up on the tester, isn't glowing in the amp.
 
check the AC voltages on pins 4 & 5 on that particular tube. If it's not glowing (and they don't glow all that bright), turn down the lights and check. If not check the voltages, if it is, then your problem is probably not with the tube.
 
Hello. I have a 690t amp and mine has a 120hz hum on both channels until the ell80 s warm up. Then quiet after that and it sounds good. I noticed someone else mentioned that his did it as well even after a re cap. Any ideas? Thanks, Paul James. Radiopj@att.net.
 
Radio -- Pull both ELL80s and turn the unit on. If there is still hum, then either the power supply filter caps for the transistor output stages are bad, or the output stage is unbalanced. If there is no hum then, then about the only thing that could cause 120 Hz hum when the tubes are in (but not yet warmed up) is the tubes themselves, due to significant heater cathode leakage.

Let us know!

Dave
 
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