Carvin Legacy 212 problem

[guitardog]

Be careful with freeze around tubes. They will shatter. A short piece of 1/2 inch heat shrink or something similar is helpful. You never said if you deoxidized and jumpered the effects loop and/or pre out - main amp in jacks? AND how you did it. I recommend a plug and some Craig.

Thanks for the insight. I used deoxit on all tube sockets, input jack, effects loops jacks, foot switch and all switches. I jumpered the effects loop with a patch cable and the noise is present when patched through. The noise goes away if I leave a cable in one side or the other. (i.e. send or return). The noise is present in both channels at all times, with all controls at zero, with or without a guitar connected. I disconnected the reverb as well. The only time the noise goes away is if there is a cable plugged into one of the effects loop jacks. I connected from the pre-amp out of another amp to the return on this amp and it worked as it should, so I'm pretty sure the problem is in the pre-amp.

[Mick Bailey]

He did say he jumpered the loop.

[guitardog]

Just to update....I haven't had the chance to get any freeze spray so I removed Q5 from the board and it tested ok with a basic ohmmeter test.

That points towards a thermal fault. My favourite go-to for these is a source of heat (hairdryer will do) and a can of freeze spray. My gut feeling is to start with Q5 and then the associated components. With the amp running, warm up the board in that area until the noise worsens and then target the freeze spray on individual components and see which responds to chilling. You may need to heat/freeze a few times to pinpoint the fault.

[Mick Bailey]

Very often a transistor will check out OK with a DMM. The instrument doesn't test the transistor under operating conditions and in any case if it's working but noisy you won't 'see' the noise with your meter - it can be less than a single digit resolution. Given that you've already removed it you may be better off replacing it with a new part anyhow to save further disturbance if it does turn out to be that transistor.

[m1989jmp]

Just to update....I haven't had the chance to get any freeze spray so I removed Q5 from the board and it tested ok with a basic ohmmeter test.

I was checking out the schematic again, and I've noticed B1 connection that's bypassed when jack is plugged into either SEND or RETURN. Have you ruled out that piece of wire/track and associated joints are fine?

While that transistor's out, bypass from (right side of) C66 to (left side of) C67 with some alligator clips, that should give you an idea if it's (or the associated circuit) causing any problems.

Also, is that static noise constant or intermittent? If it's constant, I'd say one of the coupling capacitors might be leaking DC into the circuit.

[guitardog]

I was planning on replacing Q5 regardless. Unfortunately it's obsolete and not readily available. I am sure there is an equivalent available, I just haven't found it yet.

Very often a transistor will check out OK with a DMM. The instrument doesn't test the transistor under operating conditions and in any case if it's working but noisy you won't 'see' the noise with your meter - it can be less than a single digit resolution. Given that you've already removed it you may be better off replacing it with a new part anyhow to save further disturbance if it does turn out to be that transistor.

[guitardog]

I was checking out the schematic again, and I've noticed B1 connection that's bypassed when jack is plugged into either SEND or RETURN. Have you ruled out that piece of wire/track and associated joints are fine?

While that transistor's out, bypass from (right side of) C66 to (left side of) C67 with some alligator clips, that should give you an idea if it's (or the associated circuit) causing any problems.

Also, is that static noise constant or intermittent? If it's constant, I'd say one of the coupling capacitors might be leaking DC into the circuit.

There is a ribbon cable that connects the send/return board to the main board. I inspected and resoldered all joints associated with it. The noise is constant, so I am thinking a leaky cap or shorted transistor? I will try to get to it this evening and check on C66 and C67.

[Enzo]

MPSW42 is a 300v part. I list them i nmy stock, I'll look.

But wouldn't an MJE15034 work? Yes it is a larger package, 350v part.

https://www.mouser.com/datasheet/2/3...4-D-112132.pdf

[guitardog]

MPSW42 is a 300v part. I list them i nmy stock, I'll look.

But wouldn't an MJE15034 work? Yes it is a larger package, 350v part.

https://www.mouser.com/datasheet/2/3...4-D-112132.pdf

Hey Enzo, long time no speak! How about the NTE255? http://www.nteinc.com/specs/200to299/pdf/nte255.pdf

[Enzo]

I saw that. I am no fan of NTE, but it ought to work OK. It isn't like it has to match something else in there. The MJE part is something I already have in stock.

[guitardog]

Update: With Q5 removed, the noise is gone and I have input on both channels. I will replace Q5 and was thinking I should go ahead and replace C66 and C67 while I am at it. What do you guys think? Should I go ahead and replace the main filter caps while I have it apart?

Thanks and Happy Father's Day to all the Dads here!

P.S. I found what appears to be a suitable replacement for Q5. It's readily available and inexpensive. https://www.fairchildsemi.com/datasheets/KS/KSP42.pdf

[Mick Bailey]

I'm against shotgunning, but if a part costs pennies and is suspect I'll replace it as a matter of course to save time. When it comes to main filter caps, how old is the amp and are they leaking or bulging? Bear in mind that anything you do to an amp increases the risk of something going wrong. With repairs I tend to concentrate on fixing the principal fault and making sure the amp is working before touching anything else. That way I know the last-known good configuration if something is then wrong. Let's say you replace the transistor, the two caps and the filter caps but you then have a hum that wasn't there. Was the transistor faulty from new? are the replacement caps good? did I mis-solder a joint? did I splash some solder somewhere? Did I disturb something? Is one cap in backwards?

My advice is always to minimize the amount of repair work to that which is necessary, baseline the repair and then carry out additional work afterwards.

[guitardog]

I'm against shotgunning, but if a part costs pennies and is suspect I'll replace it as a matter of course to save time. When it comes to main filter caps, how old is the amp and are they leaking or bulging? Bear in mind that anything you do to an amp increases the risk of something going wrong. With repairs I tend to concentrate on fixing the principal fault and making sure the amp is working before touching anything else. That way I know the last-known good configuration if something is then wrong. Let's say you replace the transistor, the two caps and the filter caps but you then have a hum that wasn't there. Was the transistor faulty from new? are the replacement caps good? did I mis-solder a joint? did I splash some solder somewhere? Did I disturb something? Is one cap in backwards?

My advice is always to minimize the amount of repair work to that which is necessary, baseline the repair and then carry out additional work afterwards.

That makes sense. The parts are cheap so I figured I would replace the caps that were in the circuit that had the problem. The filter caps appear to be ok, but the amp is close to 20 years old.

[guitardog]

The latest: I replaced Q5, C66 and C67 today. I put it all back together, and powered it up. I let it warm up for about a minute, then flipped it off standby. No noise. All controls at zero. I grabbed a guitar and started playing on the lead channel. It sounded good. I pushed the button to try the clean channel, there was a slight "pop" sound and bam, the static was back. So now I'm pretty much back where I started. Defective transistor from the factory? I did get 2 of them in the event that happened.

[The Dude]

A few questions:

1) When you switched to the clean channel, the noise started. When you switch back to the lead channel, does the noise go away again?
2) What are the DC voltages on Q5 (E,B,C)?
3) Do you have a scope?