Marshall Valvestate 15-Powers on..Low hum, no sound;sometimes amp works fine

[jck2v]

Hello,
I have two Marshall Valvestate 15 amps.

The first amp, the sound would cut out occasionally;but then the amp would begin working again in a few minuites.It would work for fine for 2 or 3 practice sessions which last 1-1.5 hours each without a problem.Also, sometimes the sound would sound weak, but this simptiom was rare, most of the time the sound would just go out and come back on in just a few minutes.

When the amp does not have sound, it is powered on(has a low hum from the speaker),the sound just goes out.
Sometimes, I would wiggle the guitar input cord and the sound would begin again and I would hear a scratchy sound.

However this does not help anymore.

I am not sure wiggling the cord helped to start with, it is just something I did naturally to make sure it was plugged in to the input jack. When on sound occurrs I hear a hum from the speaker.

The reason I bought the second vs 15 is because of the problem of no sound, from the first amp.......Well guess what?

The second amp has the exact same problem, no sound, a low hum from the speaker and sometimes the amp will come on and begin working! Can you believe it!

Is there anyone who can help me with this?
Thanks, James

[Enzo]

You will have to take the amps apart, expose the underside of the circuit boards and start repairing the variouos broken solder connections. Start at the input jacks.

[jck2v]

Possible TDA 2050 ? How to check to this point?

Hello,
Thanks for the quick reply!

I am trying to understand the inherant problems with this these valve stateamps?

It seems that valvestate amps have numrous sorder connection problems on the bottom of the circuit boards.

How can I tell if the sorder conection is bad ,will it be discolored or should I check the next part in the schematic for a certain voltage level?

Also, the TDA 2050 goes out frequently on these amps.(Does not mean this is the problem with either of these two amps).

How can I check backwards from the speaker for voltage drop levels so I can make sure thes part(TDA 2050) is not the culprit?

How can I get a schametic for this marshall valvestate 15 amp?

Thanks, James

[52 Bill]

In my experience, when the TDA2050 goes bad, it will short and cause the fuse to blow. I suppose that it could fail in some other way, but I honestly can't remember having one fail that didn't cause the fuse to blow.

Bad solder joints can look normal, but what you should look for are joints where the lead coming from the jack or other part is broken out of the solder blob that is on the copper pc board trace.

Check the solder joints on all of the larger items first, like the filter caps, any power resistors, the pots and especially the input jacks. If the guitar cord has be pulled out at an angle or has be stepped on while it was plugged into the amp, the force exerted on the jack will cause the solder joints to fail and cause the type of symptom that you are describing.

[unparalleled]

Pics of dry joints. You will need to re flow the solder.

[Enzo]

Good photo of CLASSIC cracked solder.

[jck2v]

Repaired- Cracked Solder Joints on input jack Marshall Valvestate 15

I took both amps apart and examined bottoms of boards on each at input connections.It was exactley as Bill 52 and Enzo said, the input jacks solder joints were cracked and not making connection when the guitar cord was plugged in.I touched these solder joints up and also the solder joints of the larger parts.The small fuse blew during testing, I had to purchase another at Radio Shack.
It looks as if someone had tried to repair this amp before.Two of the stand offs were missing from the circuit board. I made a couple by using the plastic expansion piece from some sheetrock expansion screws.Now both amps work!! Thanks for the excellent advice!!

Thanks again,
James