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This is on a Eico ST-70. The yellow circled ones I know are carbon comp. But what are the green circled resistors?
Also you can see a sprague .015 ceramic disk that has bubbled up. Why would that happen? Both ceramic dick caps on the tone control's 12AX7s did this. They only have 1 volt DC on them.

The green circled ones look like carbon film.

The yellow circled ones are carbon composition.

Typical to use carbon film upon replacement.

Cheers!

As far as the capacitors are concerned, replace all of them.

The bubbled-up surface on the ceramic disk caps is fairly normal, just an effect of too many years gone by.
It's a good idea to replace all ceramic disk caps, and the old paper ones, they're already gone bad, or will go bad, and the ceramics have terrible sonic qualities, their piezoelectric effect creates its own unique form of distortion...

The old (yellow circled) carbon film resistors can drift far out of their 10% - 20% tolerance, after 40-50-60 years.

/ed B

Speaking of caps.... I see "Black Beauties", "mustard" and a Tropicap.

Pull them and put them on Ebay... guitarists pay far too much money for them

Didn't know they had carbon film back then. Wonder whey they mixed the film and comp together?

I'm replacing all the caps and testing them as I go with a Jackson 112 cap checker. So far all the black beauties have been good. On the top of the chassis there are alot of ceramic disks in the low and high filters. I'm going to try to get the filters down from a 2 stage each to a one stage each.
I'm also replacing most of the carbon comp, gonna leave the film.

The ceramic discs should be OK - not necessary to replace.

Well they shouldn't have changed, but like Ed said they aren't very good for audio, and not very linear. I usually replace them with mica, but these are so big that I can replace them with film. From what I've read any cap is better than a ceramic disk in audio.

DeathRex wrote:This is on a Eico ST-70. The yellow circled ones I know are carbon comp. But what are the green circled resistors?
Also you can see a sprague .015 ceramic disk that has bubbled up. Why would that happen? Both ceramic dick caps on the tone control's 12AX7s did this. They only have 1 volt DC on them.

On a second look, the top green circled resistor looks like one of the plate resistors for the 6SN7 tube with value printed on it (28.75K?). I don't know what kind it is tho.

The bubbling on the ceramic disc is normal to see. It's only in there to quite the Lo Filter switch (click/pop).

hawaii.ken wrote:[On a second look, the top green circled resistor looks like one of the plate resistors for the 6SN7 tube with value printed on it (28.75K?). I don't know what kind it is tho.

Yep that's it. I was thinking since it's the same color as the other it is too a carbon film, but precision, so no color bands. Kinda like "A brother from a nother mother".

hawaii.ken wrote:The bubbling on the ceramic disc is normal to see. It's only in there to quite the Lo Filter switch (click/pop).

The bubbling quites the click/pop?

I had to change all the bigger carbon film resistors because I needed to change the values. I need to check them for drift.

DeathRex wrote:The bubbling quites the click/pop?

lol

The bubbling is normal to see. I sometimes see it on NOS ceramics.

Those ceramic caps are there to help to reduce the noise you hear when operating a switch.

Also, I think I can see that the 3 section cap has one of the legs touching the chassis.

The two that are bubbled are the cathode bypass caps for the preamp 12AX7s. I don't know why you would use a 0.015 as a bypass cap, unless you wanna get rid of alot of bass. The hi filter and low filter are ceramics too.

The cap is a JJ 50/50 500V. The center lug is ground. I still have to solder it to the chassis.

You can see the modifications and schematic here

Yes, I see now, C31 and C32 are explained at the top of page 6.

I thought he was talking about the feedback capacitor for some reason.
I have a 0.015 film to put in, but Ima experiment with it when I get it running.