Cool.  I love an inexpensive solution.

From the above it sounds like the jacks are fine and C39 and C40 are correct.  The voltages are pretty close to the schematic, except for the 3+ volts DC on the speaker.  I'm think we keep coming back to the chip. 

C38 per schematic, should be like c40, with zero VDC on the positive end of the cap and minus 15 VDC on the negative end of the cap.  Double check this - if the cap is original its not likely to be inserted backwards.

Regarding the jack, viewed from the top, the back-left lug is the tip of the guitar cable.  If you look closely, it is a spring and it grounds itself on the righthand side of the jack.  Sometimes the spring gets weak and doesn't make contact, which can hum.  With no plug inserted, that back left lug should be continuous to ground.  With a (good) cable inserted, it should isolated from ground and continuous with the tip of the cable.  The schematic shows this relationship with the little "arrow" connections that go to ground on the two jacks.  If you experiment with your multimeter on continuity (amp off), one end to ground, one end to probe with you should be able to see if the switch/spring isn't making contact.

One afterthought, the schematic gives "TP" locations, i.e., test points, presumably marked on the board, for you to  measure VDC to ground.  Those are meant to give you convenient locations to test DC. iF you get a the appropriate reading there, you don't have to really worry, you can assume the cap is in correct place.  Personally, I always write these measurements on a copy of the schematic as I work (in pencil).  IF they (TP 3,4,5,6) are good, you can continue on in your fact-finding.  Also, later if something changes, you have a baseline to work from.

As I read the schematic, you should have 0 zero volts on the positive side of C40 cap and minus 13V on the negative side of the cap, which it sounds like what you reported, so no problem in that case.  Zero is more positive than minus 13, so that works.

Some of the caps are in rectangular white "blocks".  Look at the top and see if it doesn't say something like ".1u" on top.

The 3 volts DC on the speaker is still a problem.  Somehow you have DC on the speaker and that shouldn't be.  The likely path is through your output device.  Maybe C46? Gurus help here.

I missed the part about your input jack.  A bad ground there can cause hum.  If you fixed it, check that the ground lug is really continuous to ground and that the "switch" lug is grounding the input lug when no plug is inserted.

Are you sure those two points aren't on the same trace?  Is that the ground end of the cap??

Thanks!

Hello Gurus:

I have a rickenbacker tr-14. The reverb pan is an accutronics 1FC2C1B (9", 2-springs).  One of the springs has detached itself and on one end the small ferrite bead and little attaching wire have gone awol.  Basically, I don't think it's fixable.  Looking around at AES and so forth, I don't see many pans with "FC" specs (1475 ohms input imped, 10,000 ohms out). How critical is it that a replacement pan have exactly the same specs? This is all-transistor amp from 1980's, I think.  I don't have schematic.

 

I think you are right.  The position it sounds best in is sitting on a cardboard box outside the cabinet.  If I flip the output side away from the transformer it substantially helps the noise.  I guess I could make longer leads and but it in a bag at the bottom of the cabinet.

You had me at the beverage holder (and bottle-cap opener).  "Wile-E-Coyote...Sup-er genius."

Okay.  So I have this Earth Sound Research transistor amp.  I got it up and running by replacing all electrolytic caps and the output devices (I used 2n3055 from mouser).   I don't have a schematic, but it looks very similar to peavey pacer in jack darr's book p. 263.

My issue is hum in the reverb section.  Amp is fairly quiet and normal with reverb on 1, if you turn it up to 10, it reverbs, but with hum.  I tried replacing the reverb wires and soldering to the inside of the rca jacks.  This helped a bit.  Oddly, when the pan is not bolted in, with chassis on my bench, it has less hum than when bolted in the amp.  The pan is not in  a bag, but is screwed to the chassis through rubber grommets.  It doesn't touch the chassis, but it is close to the transformer and everything else.  What else should i be thinking about?  I want to sell this amp and I want it to be quieter before i do.

Personally, I was always fascinated by the "rivets".  I wasn't sure if it was supposed to look like fancy upholstery or kind of medieval theme, but distinctive, nonetheless.

Quote from: J M Fahey on June 16, 2011, 04:28:15 AM
...I have built over 8000 amplifiers with TO3 2N3055 transistors, along 40 years.
Had to stop using them because nowadays all available ones are counterfeit...

Just a question about your comment about 2n3055s:  They still sell them at Radio Shack here (California) and places likes mouser... are those not real ones?

Good work!  Thanks for sharing.  The half-consumed pint of ale is also a nice touch!  Ha ha!

I forgot to add the most hilarious quote:  "Service manager of the Year Award - I ended up firing that guy!"

Thanks guys!  Great info.  I have TIP41s on hand.  Fortunately, the orginals still work, so I'll levae those inplace until they die, I guess.

I am looking at a couple of transistors labelled "RCA 7155" that I can't find info about in my (admittedly old RCA manual) nor online.

Anyone have any info on what these are and what they could be replaced with?  It is NPN power transistor, but must be fairly low power, has about 17.5 DC volts from C to E.  They are in a Kalamazoo model 4, for which I don't have a schemo.  I found a Kalamazoo 3 schemo online which listed it as Motorola SJ 3405 or 3408 (I can't read the last digit).

The good news is, these work for now, but I am always curous if I had to replace something what I might use.