The dreaded Acoustic G100/60T channel switching problem

[km6xz]

This might not be a problem, the idle current of the amp would represent a pretty low series resistance to the bulb.
Measure the line voltage when it is on the bulb and you will see that the mains across the transformer will be almost half what it is expecting. Dropping the bias will pull more current also.
What you are seeing is what would be expected, time to remove the bulb limiter and try it.

You posted the same diagram that I have and my library has every model Acoustic I have ever heard of, any other differences between your unit and the print?

[Zouto]

This might not be a problem, the idle current of the amp would represent a pretty low series resistance to the bulb.
Measure the line voltage when it is on the bulb and you will see that the mains across the transformer will be almost half what it is expecting. Dropping the bias will pull more current also.
What you are seeing is what would be expected, time to remove the bulb limiter and try it.

You posted the same diagram that I have and my library has every model Acoustic I have ever heard of, any other differences between your unit and the print?

THANKS A LOT, you're right: I removed the bulb limiter, turned on the amp and all voltages were right, the amp works without blowing a fuse, no need to get a new power transformer. BUT: next task is to find out where's my input signal. Channel switching, volume and tone controls work fine, as I can hear, but there's some interruption in the input stage. No signal from input jack, only increased hum when touching the tone controls metal shaft. So it's signal tracing time, i guess.

At the preamp page 2 of the G100T service manual the 2N4248 transistor is mistakenly marked as 2N6474 in the switching circuit.

[Zouto]

Swapping corroded input jacks and broken cables, resoldering cables at the control pcb etc.brought that amp finally back to sound.

[km6xz]

Congratulations! It is always a nice feeling to figure something out and resolve it.That is why I like to repair things, each is little detective story of gathering evidence, putting easily jumped-to-conclusions out of mind and form a hypothesis that fits ALL the evidence, if one thing does not fit, the hypothesis is wrong.
I caution people from jumping in and doing common repairs in circuits the evidence indicates are perfectly good or where there is no evidence. But that is the way the internet works, everyone throwing suggestions that have worked in one unit in the past for them.
A benefit of not scattering efforts into many possible but not likely solutions, it can waste a lot of time, money and lowers the value of the unit by having a lot of parts changed so appears to a potential buyer to be a lemon by needing so much work, when in the vast majority of cases a single failure occurred. There are symptom defective components in things that generate a lot of current like power amplifiers or switching power supplies but the cause of the failure which must be isolated and identified is usually one simple part. Contrary to the internet "wisdom" of resoldering a board, it seldom fixes it for sure. You never know if you resolved it or not if it starts working, unless you isolate the exact solder joint that caused the problem. Besides, most people just reheat joints but that does not make a good contact, reflowing old oxidized solder is not a solution. The proven defective joint needs to be cleaned of old solder, fluxed and soldered with fresh solder.
Another suggestion on the internet that is predictable is to replace all a certain class of parts such as capacitors. Well, maybe if they have a fully functioning unit and they want to restore it but that sort of advice is counter productive in repairs, which is trying to eliminate as many variables as possible, not introduce more, when trying to isolate the one cause of all the problems. Anything that introduces more uncontrolled variables muddies the waters and makes a concise diagnosis longer to arrive at or reduce certainty that the cause has been found. That is the difference between an electronics technician and a electronic mechanic. The latter performs routine labor tasks such as preventative maintenance and the former mostly works with thought exercises after initial base line evidence is gathered. By far the majority of amp repairers are in the electronics mechanics job.

[52 Bill]

And to finish off the thread, did you rebuild the optoisolator? What did you use?

[Zouto]

First of all: THANK YOU ALL FOR YOUR GREAT ASSISTANCE.
I used a 3mm red LED with 390 Ohms serial resistor and two same type garden-variation cells I had, all fit well under the thimble. Works great and seems to be a durable solution. I used heat resistant glue to fix the thimble to the socket. I also rebuilt the other optoisolator, replacing only the lamp with a LED/resistor.