Assist With AMP 197-20 Please

[Hi-Fi-Mogul]

I have pulled this little stereo amp from a 1959 Aristocrat
that a nice young couple want serviced.

For doing the check out, I need to jumper across
the record changer input plug.

I want to be sure where the jumper wire will go.

I believe per the power supply, that I would place
the wire from the far left pin to the middle pin of the
3-pin plug.

Thanks !

[electra225]

That would be correct.

[TC Chris]

Correct for running the amp without the changer.

Chris Campbell

[electra225]

I believe his intention is to rig it up so he can work on the amp without the changer in circuit.

[Hi-Fi-Mogul]

So, I am slowly proceeding with this amp checkout.

I was looking at where I'll need to tape off the
areas, so I can cut off the multi-section capacitor can
with a hack-saw.

I noticed that the rectifier tube engraved number
on the chassis was 5U4.

The schematic says 5Y3GT, and the Magnavox branded
tube is a 5Y3GT, 43rd week of 1959.

There is also "Hum Bal" engraved next to the
5Y3 position, but there is no Hum Bal control,
only a hole in the chassis.

This seems to me a case of using the same physical chassis for
several variations of amplifiers.

[William]

I would agree, Mr. Mogul. I have not run into the tube difference but have run into the hum balance not being there and also the extra treble control that is sometimes found in Magnavox chassis not being in the chassis when it is clearly stamped in the chassis. I guess stamping out a chassis that can work for multiple amps was a bean counter thing.

Bill

[William]

Mr. Mogul. Do you have the full schematic for this amp? I would be great if we could see it.

Bill

[Hi-Fi-Mogul]

I hope to load it in Downloads soon.

[William]

Thank you!

Bill

[Hi-Fi-Mogul]

I cut off the multi-section electrolytic capacitor can top today.

I cannot get the paper/electrolyte guts out, though
they are still plenty wet.
They are rolled very tightly in the can.

There's no tar on the inside sides of the can that
would be holding the electrolyte paper in.

Did they put tar at the top of the can ?

If so, I can heat that and maybe get the guts to
release.

What ideas might work ?

[TC Chris]

I cut off the multi-section electrolytic capacitor can top today.

I cannot get the paper/electrolyte guts out, though
they are still plenty wet.


What ideas might work ?

Brute force works.

Chris Campbell

[electra225]

I have a concrete drill bit I use. One with those little wings on it. I drill two holes with the concrete bit, then use big needle nose pliers to twist and pull on the guts. Once you get the contacts broken, they seem to come out better. I occasionally heat the can with a hair dryer so the tar is more pliable and lets the guts come out in one chunk. Good luck....

[Hi-Fi-Mogul]

I had another idea from a friend.

I will twist a large eye screw bolt into the center of
the electrolyte folds.

I will use his vise to secure the screw, and then
heat the can a bit, and pull the can off the guts.

I've done this w/o a vise before, but this can's
guts are really tight.

[Hi-Fi-Mogul]

Another question, please.

What purpose do capacitors C-9 and C-16 serve ?

Thank you.

[electra225]

Looks like part of the feedback circuit. Those caps, and the 100 ohm resistors they are in series with, control the tone of feedback. You may see that a part of the signal from the output transformer is being fed back to the grids and cathodes of the 12ax7 audio tubes. This circuit is a tad unusual, to me at least, in that it provides bias for the cathode circuit of the 12AX7's. Magnavox used a similar feedback circuit in their bi-amps. You typically see the cathodes grounded, providing bias to the tubes. If you change the value of the caps in question, you can change the amount of feedback. That cap shorting or going open could also cause oscillation in the audio circuit.

[William]

To prevent feedback?

Bill

[electra225]

Bill, I may have been typing when you were. They are not there to prevent feedback, but to control feedback. Read the post right above your last one and if you have more questions, we can take it from there. I edited it and may have messed you up. Sorry.....

[electra225]

I found an Uncle Doug video on negative feedback loops. I really appreciate his work, since he talks in plain English and is an excellent teacher. He is mainly into guitar amps, but the same principles apply to both guitar amps and stereo amps. The video is only nine minutes long, and may help explain feedback circuits better than I can certainly.

https://www.youtube.com/watch?v=H5fFb5_9xyA

[Hi-Fi-Mogul]

I viewed Uncle Doug's video; very interesting.

So, in this amp, instead of using the 0.033 mF cap value
printed on the schematic, it uses a 0.068 mF value.

Sam's has a notation symbol indicating a different value
of the component was used, and to check the parts list.

Interestingly, Sams did not include any other alternate
cap value for the 0.033 mF in the parts list.

If I understand the video, using these lower value caps
will attenuate the bass levels ?

[electra225]

I don't understand all I know about feedback circuits. Feedback circuits for hi-fi are a bit different than in a guitar amp. I would think that decreasing the capacity SHOULD allow more of the treble to pass, or, in this case, go to ground. The way that feedback circuit is drawn makes it appear more complex than it actually is.