marshall G50 R CD repair..?

[sonate]

Hi ..I am new here so i hope this is in the right section.

My gr50 stopped working...I opened him up and noticed a BA4558 on the input circuit..So i did the wet finger test and it all started working again. Switched off/on and it was not working again.

I have replaced the BA4558 with another 4558...But it's not solved the problem.
Is the BA different than the normal 4558's...?

I can't find any schems...So could someone help here who has experience on these...

Cheers....

[joecool85]

What is the "wet finger test"?  I don't dare google that one lol.

[sonate]

What is the "wet finger test"?  I don't dare google that one lol.

It's nothing kinky...If you wet your finger and place it on the opamp on the input stage you get a buzz that can indicate the stage is working...However it started working instead..
The BA before the number just indicates the maker of the chip.
To answer my own question.

[joecool85]

I knew it wasn't anything kinky, but I know google would think it is.  Thanks for the explaination though, I'd never heard of that.

[LJ King]

I knew it wasn't anything kinky, but I know google would think it is.  Thanks for the explaination though, I'd never heard of that.

You don't really need to wet the finger either. Try it on your input cable. (duh)

[sonate]

I knew it wasn't anything kinky, but I know google would think it is.  Thanks for the explaination though, I'd never heard of that.

You don't really need to wet the finger either. Try it on your input cable. (duh)

I didn't have the input cable around as i was looking at the circuit board...

Anyway i went over the board for bad joints ect....As it happens it all started working again.

But after a week it's stopped again...So what's going on.

I need some schematics or advice on where to look next...?

[sonate]

Update...It seems that if i leave it on for half an hour..It works again.

[teemuk]

All that intermittent and temperature-controlled behaviour sounds pretty much like you still have cold solder joints. Get some cold spray, let the amp heat until it works and start spraying. Work in logical order so you can locate the intermittent stage.

[joecool85]

All that intermittent and temperature-controlled behaviour sounds pretty much like you still have cold solder joints. Get some cold spray, let the amp heat until it works and start spraying. Work in logical order so you can locate the intermittent stage.

Sounds like a great way of trouble shooting.  But, what is "cold spray"?

[teemuk]

Cold spray is mainly used for localizing electronic interruptions like cold solder joints or cracked PCB tracks. It can also protect components during soldering. As far as I know it is some kind of freon gas (i.e. environmentally safe HFC134A) that (when sprayed) changes its phase from compressed liquid to gas and therefore produces a cooling effect. I don't really understand the physics behind this completely but it's pretty much the same process that powers your refridgerator. There are many kinds of these cold spray products, some of them, for example, are used in medical purposes as emergency treatment for muscle and tendon injuries. In electronic applications they are generally substances that are non-conductive, non-flammable and leave zero residue.

[teemuk]

Sonate, if you still haven't found the schematic PM me your email and I'll send it.

[sonate]

Thank you for the good advice..

I will try to locate some of that spray.

I did go over the board for cold joints but I must have missed something.

Sam.