Hello, #12, and thank you for your reply! The "pop" is heard immediately after i press the power button. Almost instantaneous i might add, so yes, it pops before the tube gets warm.

As for my concerns regarding this problem, the past weeks have been very annoying since the constant volume dropping made the amp almost unplayable. Now the situation is better, since the amp is in either the problematic "low volume" mode, or the normal volume mode, which means i get to play it either way and this relaxes me a lot.

 

looks like its got a reverb unit built in,with valves,so lots of thermal drift.i would get another amp say from a pc sound system and use that as your sound tracer and probe the input of the master amp at point C first, to see if the sound is going into it.

 

A symptom that goes between 2 steady states; normal sound level and quiet sound level is very common symptom caused by a dry solder joint or other bad connection, ie a connection somewhere going from closed circuit to a faulty open circuit.

It might even be a switched pot or switched jack socket.

It can also happen from a resistor or a cap failure.

Start with the usual wiggling/tapping tests and then if that does not find it go to spraying some freezer spray around PCBs or sockets, or components etc.

 

Ok, so i thought about the dry solder too and i tried re soldering about 20 connections that seems to have something to do with the problem to no success. If it is, i must have missed it, next time i go in i will look at it again. I don't think it is a pot because the problem happens on both channels with 2 pots and there's no crackling sound when turning the pot.

This week i will try bypassing that resistor Mr Chips was taking about and taking some voltmeter tests.

In the meantime i uploaded some pictures of the board. Some are more clear than others, i can upload some better ones if they are required.

 

Ok, both MrChips and Dodgydave agree that i should inject a sound in the input of the amp then probe for it using a sound amplifier starting at point "C". I would like to give it a try this week, but since i don't have much experience with this, i would like to ask if someone could be a bit more specific as to how exactly can i probe for the signal with another amp without using an oscilloscope.... Do i connect a wire from point c to the amp? I'm sorry if this sound silly, but i would like to understand this better.

 

I'm still here with you on the side line.
Your photos are not sharp enough. We need to be able to read the writing and component ids on the board. From here we will be able to guide you on where to connect your audio probes.

Make sure you use a tripod or a rigid support to hold the camera steady.

 

C18 (or C48?) near the second large green resistor is heat damaged, it needs replacing. All 4 large green resistors have heat signs on the PCB so need resoldering.

Also the power amp chip is a likely place for a dry joint on one of it's pins on the bottom of that tiny PCB. If you don't have a microscope just resolder all the pins and apply fresh solder.

 

Thank you! Will post some better pictures this evening! Luckily, i have a tripod!

 

Sorry for the delay in posting, i've been out of town for a few days. Anyway, i returned with higher quality pics of the board. I pictured the whole board from left to right (8071-8074) and took separate pictures of the chip and the power source. For a picture of the board as a whole, please check out my other post with pictures.

Also, i'm starting to suspect the pots after all. I'll try replacing them and see what happens. Also, if you spot any other components that look damaged or might need replacing, please let me know.

So far i understand i need to change C18 and resolder all the 5 large green resistors and the chip.

 

Actually I think the cap's number is C37, it is near the 3rd green power resistor in your 2nd photo there. It looks as though heat from the resistor has caused it's outer wrapping to shrink which is a good sign to replace an electro.

 

Hello!

I just wanted to say that i think the problem is solved Apparently it was from an external speaker jack at the back of the amp which was malfunctioning. I plugged a guitar jack in it while the amp was showing symptoms and the problem went away immediately and it's still gone after several weeks. If it does any more trouble i'll just remove it since i don't use it anyway

I've waited this long before posting because i wanted to make sure the problem is truly gone before announcing it

Anyway, i would like to thank everyone here for your advice and interest into my problem! You guys are great!

 

There is one more thing you can go ahead and try.

When the amp has warmed up and the fault appears,
connect a jumper wire to short out LT9914, (exactly where you see those dotted lines,
i.e. a wire from the R41, LT9914 node to LT9914, R42 node
and see if the volume comes back up.

 

This is an old thread the the problem has been resolved.

 

There is one more thing you can go ahead and try.

When the amp has warmed up and the fault appears,
connect a jumper wire to short out LT9914, (exactly where you see those dotted lines,
i.e. a wire from the R41, LT9914 node to LT9914, R42 node
and see if the volume comes back up.

I was just curious to were i can find vtc5l3 octocoupling. search brings me back vox problems ?