1971 Fisher model 301 Receiver Questions.

[Arty30]

Hello all, been a while since I've posted, bit I recently picked up a 1971 Fisher 301 Receiver for $7 at my local junk shop. It's a decent old thing, has some lettering worn off the face and plenty of scratches in the veneer, but I was surprised at how it sounded, even with my cheap test speakers. It seems to function well, but has a few issues, I always test DC on the terminals first and I saw that one channel would be up at 5v on startup and drift down to 150mv in about 30-45 seconds. I would guess leaky caps that take a while to charge? It also has noticeable hiss which may also be caps? finally I want to know if anyone has any good ways to clean slide potentiometers, the volume on the 301 is a slide and is very flaky in my unit. I tried De-Oxit but didn't make much difference other than to wash away the lubrication so it's less smooth now. if anyone has suggestions for this project I would appreciate it, caps probably wont be a very expensive place to start but I wanted to know what other people might think, thanks.

Arty.

[TimFox]

Is that a vacuum-tube unit?
Normally, the output transformer would prevent DC on the output, but often the secondary is connected (as a feedback path) through a resistor to the cathode of the input tube in the power amp section.
Your DC transient might be "normal", while the tubes warm up.

[Arty30]

oh sorry, I meant to mention it's solid state. I have schematics but the file is too big to attach.

[floobydust]

For this type of amplifier, it is normal to read some DC at the loudspeaker terminals when there is no load. Use a dummy load of 1k resistor for testing or a loudspeaker.
The amplifier uses a large output coupling capacitor C819/C820 2,000uF 75V that has enough leakage current to give ghost readings to a multimeter when no speaker is connected.
It's also a problem these caps have to charge up through the loudspeaker which gives a loud thump on power on/off.
Dynaco ST-120 is the same sort of design, single-supply so the amplifier has 1/2 the PSU voltage at its output and then the output coupling cap to get rid of the DC.
Careful this cap can discharge back into the power amp and blow the output transistors even when the unit is off. I blew up a ST-120 that way, monkeying with speaker wires when the amp was off and I shorted one channel and the charged cap dumped back into the amp and killed it.

To troubleshoot the receiver, just take and record voltage readings at the transistors and compare to the schematic or a working channel. At this age, many electrolytic capacitors will have dried out and failed.

[Arty30]

Thanks for the tips, I've yet to test any caps, but I wouldn't be surprised to find ones that have dried out.

[floobydust]

Also note that bad coupling capacitors can make potentiometers noisy. A scratchy volume control can be due to DC current flowing through and no amount of cleaning the pot will fix that.
For the control amplifier , input C201/C202 0.33uF 35V and then the three caps just around the tone controls C203/C204 1uF 35V, C205/C206 4uF 35V, C207/C208 1uF 35V and the output caps C213/C214 4uF 35V to the balance control and volume control. Total 5 caps/channel.
On the power amp, input cap C801/C802 1uF 35V could also cause a scratchy volume control.

For a re-cap project, I take pictures of the board, make the list and then replace all of them. Low value 2.2uF and less electrolytics I replace with film caps. You might have an old noisy transistor as well that needs replacing.

For its day this receiver looks pretty high end. I was surprised to see +61VDC rail in the power amp is high power for the day.

[Arty30]

Interesting, I wouldn't have considered that. hopefully I don't have any noisy transistors to deal with, I'm still recovering from my Sanyo Plus 75 project where I replaced a bunch of transistors, I'll ket back to it someday. Thanks.

[Arty30]

I've replaced the electrolytic caps now and it sounds great. I also was happy to find the volume slider to be surprisingly serviceable, it was a bit grimy inside so a fiberglass pen and de-oxit fixed it right up and it works a charm. There is just a small pop on startup just to be expected and the hiss is negligible. I ran it for over an hour with no issues, so I'm confident I've done it justice.