One thing you can try is to, one piece at a time, transplant parts from the good channel to the bad channel, until you have found out which part it takes to fix it.
+1
stereo is great for checking measurements from good side to bad side, exchanging parts is a recipe for blowing more stuff on a once working side.
Besides being an excellent way of introducing new problems which didn't exist before, it's not even guaranteed to work.
With due respect, that might look reasonable to a "builder" , definitely not so for a *service* Tech with at least minimal experience.
Many times (as much as 50% of the time), "BAD PARTS" ARE NOT THE PROBLEM
LOTS of problems are caused by poor/bad connectivity, such as :
* bad/weak/dirty/corroded connectors
* not fully seated or slanted or replugged the wrong way connectors (includes socketed ICs)
* cracked/broken tracks
* cracked/broken/cold solder
* already dirty/corroded parts legs *before* soldering.
As in: flux is not enough to clean the grime/oxidation, solder does not "wet" the joint, cooling solder forms a ring around the leg, which crimps it and the fresh apparatus works, sort of; a few years later that connection becomes very poor or opens.
* corroded wires.
I often find wires which seem to be connected, but on close inspection have a greenish or whitish surface , you can break them with your pinky finger or simply vibration
* dirty/weak/corroded contacts in jacks or switches.
Including the typical : "amp is muted or sounds very weak, distorted o like a mosquito, but works well with a good guitar cable plugged from send to receive jacks"
* dirty trimmer pots, specially the vital bias ones
and so on and on and on.
People used to fresh builds and in general newish stuff won't see much of it, unless amp is seaside but on 30/40/50 y.o. Guitar amps such as this one, it's almost a given, doubly so because they might have spent *years* unused in damp basements or storage rooms.
I have old stashes of NOS RC4558, TI58 Fets, Germanium transistors, etc. , much loved by Vintage pedal makers, which are unusable because "solder does not stick" to their legs, surface tin absorbs oxygen, water or sulfur from the air and the resulting microns thick layer can not be dissolved by regular flux.
I had a Chemist friend mix me a special water based flux specially for that, also to be able to resolder some old amp parts, go figure.
So in a nutshell, even replacing *all* parts with new ones does not guarantee repair at all.
One reasonable approach is:
1) plug the amp and try to play, we want to define what are we actually trying to solve
2) under good light, do a close up visual search.
Most parts look good even if dead
, but at least we can catch the most obvious ones, such as cracked semiconductors, toasted resistors, etc.3) turn amp on and start measuring DC voltages, a grossly off one will probably mean that part or block will also fail Audio tests.
4) start thinking/analyzing why that wrong voltage is there.
An annotated schematic is best as a guide, one not showing voltages is still some help, you'll have to figure what's a reasonable value, and even if NO schematic is available, there are some general rules which apply to most semiconductors:
a) all NPN transistors have the collector voltage more positive than the emitter ; PNP ones are the exact contrary
b) if VCE (collector emitter) is 0 or VERY close to a diode drop (0.7V for silicon transistors) the transistor is either shorted or saturated or its path to the +V rail is open (i.e.there is not current passing through it), again switch expected polarity for PNP transistors
c) VBE (base emitter) must be again about one diode drop (0.7V) ; if higher than that then the transistor is open, if negative (say, -5V) then it *might* be fine but there some gross problem somewhere else, and the circuit is trying to compensate for that, unsuccessfully
and so on.
There's a lot of testing to be done even without a schematic
Including presence/absence of power rails, continuity (is the speaker really connected to the PCB speaker out rail?), etc.
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tiefbassuebertr, perhaps you can find some information or advertisement from here:
ELECTRONICS: Electronics Engineering magazine from the 30s to early 40s
ELECTRONICS: Electronics Engineering magazine from the 30s to early 40s
Attachments
I've pointed out before on these threads that a good 2N3055 built on epitaxial technology should have BVceo >100V. This has to be so in most cases to meet the original BVcbo spec. of 100V. This is not a guarantee that ALL manufacturers' 2N3055's will meet this spec, but all of the ST and ON Semi devices I have had seem to. Though ST have obsoleted their 2N3055 now.
If you said it to my post, studio2. No its stereo and its good.
Its bulgarian power, I am bulgarian.
That thread gave me an idea. I am very inspired from this studio2 and i will buy some 2n3055's since they are 0.5$ pcs. here in bg.
I will build an amp with them. I have 4 big heatsinks for that package and im very inspired.
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DJK- the original 2N3055, now long gone, only managed 70V with Rb of 100ohms. The epi 3055's will do 100V. What we don't get to see is the second breakdown over 60V, the original BVceo. It is very likely to be built on an epi line where all the transistors have BVceo over 100V, and modern ring emitter devices may have BVceo of 250V, which is also pretty irrelevant as many amps only need them to have good s/b at voltages up to say 120v, except for those who want lots more power...
