Fisher 400, slight distortion after rebuild in one channel

[getone]

With the kind guidance of this community, I have rebuilt a Fisher 400 13 years ago:

Thread 'Fisher 400 restoration question'

Mar 9, 2013

Dear Members,

I was given a Fisher 400 receiver from a colleague of mine. According to him it worked 20 years ago. After reading many threads here and elsewhere and before plugging the amp in, I started a restoration project with the kit from Metalbone.

I've done the first round of replacements. Please look at the pictures and let me know if you see something very wrong before I slowly bring her up with a variac (after I put back all the tubes)

Also, a switch in the front is missing. Can I still evaluate if the amp is working roughly.

Thanks!

• getone
• Replies: 17
• Forum: Fisher

The unit served well, still goes strong, except an issue I just noticed: when playing a CD of a piano music, for the quiet parts I hear a slight distortion in one channel. Here are the things I have tried:

multiple CDs, same issue
not a problem when listening to rock or other "heavy" music
Same issue when connecting the CD player to "TAPE MON" input and having the "TAPE MON" on
Same issue when playing a record
Moved the 12AX7 tubes around, problem is on the same side always
Same issue for the headphones

Please note that I did not replace the can capacitor 13 years ago.

I have a soldering iron and a multimeter, do not have (nor would know how to use) an oscilloscope.

Any suggestion what might be an issue?

 

[RS Steve]

I remember Dave Gillespie telling me to run a jumper wire from one signal wire at the phase invertor to the other signal wire at the other phase invertor. If the distortion goes away then the problem is in the preamp section, if it stays it's in the amp section.

 

[grindfix]

Do you have output tube bias sensing resistors installed between cathodes and ground? What are the voltages across them in mV?
Distortion at low volume only may just be a worn out and weak output tubes.

 

[getone]

Do you have output tube bias sensing resistors installed between cathodes and ground? What are the voltages across them in mV?
Distortion at low volume only may just be a worn out and weak output tubes.

Thanks. Switching the L/R output tubes gave the same result in the same channel.

 

[grindfix]

So we know it's probably not the output tubes. Still, the individual output tube cathode current is needed. It may reveal the issue.
A leaking coupling cap will affect the bias the opposite way; the affected tube will draw too much current and possibly redplate at some point.

 

[CCook]

First thing I would do is to check all DC voltages at all tubes. Compare right channel to left channel tubes. Should be same. Remember that heaters are AC volts, except for preamp tubes V8-V11 fed with DC volts (to minimize any AC hum in small signals). Not sure that will find the problem, but is a good starting point to see if there is a wiring or component issue.

Check the voltages to V8,9,10,11 heaters. They are fed by a DC supply. That supply may use an old selenium rectifier bridge that degrades voltages over time. Or one of the filter capacitors could be failing in the circuit. I had a Fisher amp with a similar distortion issue. When the DC heater voltage is low, sometimes affects only one tube and one channel. My case it was a weak tube and a leaky electrolytic. Likely not a tube for you, as you swapped them around, so could be a related component or dirty/loose tube socket. A bit of a long shot, but worth a few minutes.

Someone appears to have replaced capacitors, the 'Orange Drops'. Look around at the solder connections. An accident or sloppy solder work could have dinged a wire or damaged a socket pin.

If things OK, I would *guess* a leaking coupling capacitor between stages. They all should be changed anyway. Other possibility is a dirty contact somewhere in switching circuits.

Lots of experts here on the site, I am not one of them. So post progress and I am sure help will come.