EQ changing amp volume

[Steve Blackdog]

Hi folks

I took a week or two away from my Proamp Viper 60 210/ KMD gv60 restoration to give my brain a rest.

I did another guitar plugged in test. Two things are obvious problems:-

1) Volume coming and going. If you play a 2 octave scale, it will start clear (although a bit distorted) and the die away ( same sound as when you switch off an amp and keep strumming using up the charge in the capacitors), then the sound will pick up again. Some notes (frequencies?) are quiet, others are really loud. A hard strummed chord will sound loud, albeit distorted. When it’s loud it is incredibly loud.

2). The second issue is the tone control. Dial all tone pots to zero and the volume also turns down to zero.

Before I start swapping pots, is there anything about these symptoms that points at a particular issue or set of issues?

Cheers

steve

[The Dude]

With tone controls all down, Most signal is grounded, so that "problem" would be normal operation. I'd start by using the line out jack to determine if the other problem is a preamp or power amp problem. Run line out into another amp and see if the problem still exists to check preamp. You can also insert a signal at C31 to see if the power amp is ok.

[Steve Blackdog]

With tone controls all down, Most signal is grounded, so that "problem" would be normal operation. I'd start by using the line out jack to determine if the other problem is a preamp or power amp problem. Run line out into another amp and see if the problem still exists to check preamp. You can also insert a signal at C31 to see if the power amp is ok.

Of course, I did wonder if I was being thick. I can’t believe I have never noticed that with any other amp I have used. I’ve only been playing for around 40 years. The problem arises from my “everything at 11” mentality

I will try your other suggestions.

cheers

steve

[Mick Bailey]

I've come across amps with the symptoms you describe in point 2 a few times. Sometimes a cracked solder joint around a component leg can do it and particularly with circuit board amps. Inserting a signal and chopsticking the board can often give a clue to whether this is the case. Inspecting the solder side under magnification can show a circular crack around the leg. Other times there's been a failure with a resistor or capacitor - quite likely a mechanical breakdown. A bad switch or relay contact can do this also. And of course, tubes can have intermittent connections inside the electrode assembly.