Trouble with my Music Man amp

[Mr. Air]

I got a Music Man HD-130 amp and I'm having a bit of trouble. There seems to be something wrong with the amp's 1st. channel. It plays very low and gets distorted when the volume is turned up. This channel shouldn't distort at all as it's meant to be used for bass as well as guitar.

To me there's no problem with the amp's 2nd. channel which is the only one I use.

I know nothing about amps so the problem of channel 1 might affect the whole amp without my knowledge.

Anyone who has an idea about what could be wrong?

[Tellboy]

I think the HD-130 had a solid state preamp and valve power amp. Sounds as if it's likely to be a preamp problem so it's not as easy as testing/replacing some 12AX7 preamp valves. Looks like a visit to a tech  :(

[Mr. Air]

I think the HD-130 had a solid state preamp and valve power amp. Sounds as if it's likely to be a preamp problem so it's not as easy as testing/replacing some 12AX7 preamp valves. Looks like a visit to a tech  :(

The HD-130 has one 12ax7 preamp tube and I've changed this without any result.

[Tellboy]

The HD-130 has one 12ax7 preamp tube and I've changed this without any result.

That's the phase inverter - the preamp itself is solid state, so no (easy) user replacement parts.

Here's a schematic of the HD-130:- http://www.music-man.com/techinfo/old_amps/2475-130_&_2275-130.pdf

[HTH AMPS]

looking at the schematic, if you only use channel 2, then don't worry about it.

channel 1 is pretty simple, could be a leaky cap or a bad opamp - tough to say without seeing the amp.  you could approach it by replacing all the caps in channel 1 and see where that gets you as the problem can only be with that small part of the circuit otherwise channel 2 would also display the same symptoms.

[jpfamps]

I would check that the power supply to the errant channel is OK (probably is as the other channel works).

Op amps in older kit are a good starting point as these often fail for no apparent reason. When replacing ICs I usually put an IC socket in first and then insert the IC into the socket.

Solid state kit is low impedance so any decoupling caps tend to be electrolytic as you need relative high cap values and these are prone to failure.

This should be a fairly easy repair.

[Mr. Air]

Thanks for the answers, guys. I know nothing about amps and the technology behind so I don't understand anything from the schematics. But if the problem shouldn't have any overall affect on the amp then I'll just leave it as it is for now. Maybe some day I'll have someone take a look at it. If I tried myself I'll probably just blow up the amp or get myself electrocuted!

[jpfamps]

Very wise.

The problem with your amp is almost certainly with the pre-amp of the channel you don't use, and as such will be self contained.

Frank