See if the input jack has any effect on the noise (wiggling the male 1/4 jack).

nosaj

At first the yamaha piano wouldn't turn on But after resoldering the chip back in I was greeted with piano noise.  When i would play it loud it would cut out and sometimes reboot. 
After lifting the other circuit board  I found the metal dimpled upwards making slight contact with the circuit board. After hammering it down and reinstalling everything. It doesn't cut out or reboot anymore.
Someone had put a bolt in that was too long and it bent the metal towards the board.

Thanks,
nosaj

I get about 3megaohms one way and OL with the leads reversed  for both outputs. So I'd say it's not shorted.

Nosaj

I have a ST4151V  chip  I'd like to know how to go about testing it for shorted outputs.

I have the datasheet but not sure I quite get it.

Thanks, nosaj

I will check those as soon as I can.  Been a lot of Rain here this weekend So theres a river between Shed and here.

Will reply back soon.

Thanks so much,
nosaj

Q1 and Q2 removed  I'm getting about 32 volts into a 4.2 ohm load  so looking at around 250watts  Which should be roughly right seeing as it says a 300 watt amp(though we all know amp wattages are like politicians).

Q1 and Q2 are j111  (a npn chopper transistor)  What would be a downside to permantly leaving these out?

Thanks,
nosaj

When I main out into another amps line in. It drives it normally. So I'd say power amp issue. But what would cause it to not push more than 12 watts?

Thanks,
Nosaj

Still haven't gotten a chance to check voltages, but was still curious that since I get sound.  Would it more helpful to follow gain through the stages
Starting with Q1 through Q21

Thanks,
nosaj

No matter what input  I input into  I cannot max more than about 12volts.  If any of the outputs were shorted It would't produce sound correct?

What I will try to do tomorrow is to check voltages on the output transistors.

Thanks,
nosaj

304v is what I'm getting.  So all the rails look good now I have to hit the output section, because 1k .5 v input  into a 8ohm load I only get about 10v on the output  which I believe is around 12.5watts  definately not even close to what it should be doing.

Input was into CH1  I realize now I should try the aux and other inputs  to narrow some down.  In the power amp input  about what voltage should I be inputting there( I have an hp200 signal generator I think it will put out at least 20v)  For preamps I normally use about half a volt.


Thanks,

Nosaj

This is still the original unit.  My mistake in stating U3 which is a 5v as you stated.( I have not actually touched U3) It was U4 the 7815.  I apologize for not proof reading my post.

I am using a B&K 1655  Isolated power supply with a variac.

I included a picture showing what I understand your saying when you say
You need 300v with respect to R37.

Is that correct?

Thanks,
nosaj

Going back to R5 and R41   I'm getting about 58-60vac  it varied each time I turn it on.  Guess I'll go back further to c40.

nosaj

This one reared its head as u3 failed putting out only about 5v. I replaced it and 15v rail is good now.  What I'm missing now is the 300v rail.
q11 is getting 56vacI'm going to check voltage at c44 and D1 to see what I'm getting there.

D1-D4 what kind of circuit is that?  A rectifier or somekind of voltage doubler, trippler, etc?

Thanks,
nosaj

The load in this case would be a speaker or a dummy load.



nosaj

"quote"I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.

This thread folks recommend replacing the power caps and the transformer.

http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279"quote"

This is how your guessing.   Goober over at this other site said doing this fixed his amp what can I buy to fix my amp ?

Get voltages at your rails to really see if you have a power supply issue.  The transformer and caps are only a part of the power supply, just as oil and water and gas are only parts of a car engine.

People come here to learn and to hopefully pass it on. Maybe their cost benefit isn't worth your time.  Ask for help be ready to supply some answers, if its not worth your time why be here?
It's one of those days so excuse me
nosaj