Peavey 5150 Loud Static when switching standby on/off

[versital]

I have a 5150 that makes a lot of radio like static when you flip standby on to play. After 20 seconds or so the static fades away but occasionally comes and goes at a lower amount while playing. When you flip back to standby, the static comes back and fades away. When you turn off the power(while in standby), the static comes back again with a pop and fades off.

While playing the tone of the amp sounds great but the overall volume is extremely low. at 1/4 volume (#3) amp sounds like a 15 watt combo, at 1/2 volume (#5) the amp sounds like room volume like most 100w amps sound at #2.

I've swapped power tubes and pre-amp tubes and had no effect. Bias readings are ok. I have some suspects but since the 5150 is so cheaply made and impossible to work on live without complete dis-assembly unlike SLO clones. I wanted to get some better ideas before I take on the project.

[Enzo]

Cheaply made?

I get the idea you don't have a 5150, but maybe you have a 5150 Combo? They are different amps, different circuits.

Isolate the problem. PLug the guitar into the FX return jack. Does it play strong and clean that way? Zero the volume on the guitar. Does the standby switch still make its noise?

Likewise, plug the guitar into the regular input and run a cord from FX send to some other amp. How does the FX send signal sound over there?

In my experience these are reliable amps, but when any of the 5150 series have low output levels, my first suspect has become the mute transistor. In your case Q1. I always change Q1 AND Q4 if there is a problem. I just snip out Q1 and see if it clears up the problem. Then I replace the part. If removing it helped, then I needed to replace the old part anyway. If it makes no differencee, then the cost of the little transistor is cheaper than spening a lot of time wondering. It is a J174 JFET.

[1ampman]

I had the same problem. It needs a total recap. You need to measure the size of the caps to make sure they fit. The amp I fixed made the channel volume controls not function properly. You could turn them all the way down but sound would still come out the speaker (bleed through)..... Dont' for get to check the 5 watt screen resistors.....

[Jazz P Bass]

[QUOTE=1ampman;283428]IThe amp I fixed made the channel volume controls not function properly. You could turn them all the way down but sound would still come out the speaker (bleed through)QUOTE]
Just had an EVH 5150 with the same symptom.
Post gain pots acted funny & would not turn off the signal.
C37 (22uf/450V) was bad.

[1ampman]

The caps just don't hold up in these amps. And they dry out losing capacitense which cuts their filtering letting noise, signals bleed thru to the next stage and this amp MANY stages. I'm used to seeing caps that short out with age and drawing too much current. The stock Ruby tubes take a beating but hold up great....