Gibson Amp Filter Cap Polarity

[bobloblaws]

I would unsolder and lift C3, OR short across that transformer primary, whichever is more convenient. Does that stop the hum or does it remain?

Ok, I have to pull the PCB out to do that, been trying to avoid until now, oh well.

[bobloblaws]

My general approach follows a path. Yes, power supply ripple will be 120Hz, unless one of the rectifier diodes is open. But nonetheless, we always ought to start with power supply.

Divide and conquer...another way of saying isolate the problem. Q20-21-22 are hte preamp, and they drive the power amp through the primary of the transformer through C3. I would unsolder and lift C3, OR short across that transformer primary, whichever is more convenient. Does that stop the hum or does it remain?

I forget, did we determine there is no DC offset at the speaker?

Ok, I temporarily snipped one of the leads on C3 (that was the most convenient in my case ) and the nasty hum is gone. So we know it's not the power amp. Given that it's OK for 2 minutes while cold and then something pops and it starts humming, I'm thinking I need to check Q20-21-22. Or lift C2 before I do that?

[Enzo]

Of course it could be anything, but I tend to doubt the transistors are making 60Hz hum on their own.

R5,6,7,8,9 all go to preamp ground. Is there low resistance continuity to chassis from their common? How about the input jack, is it well grounded to rust free chassis surface? Is there good continuity to ground over in the power amp?

[bobloblaws]

Of course it could be anything, but I tend to doubt the transistors are making 60Hz hum on their own.

R5,6,7,8,9 all go to preamp ground. Is there low resistance continuity to chassis from their common? How about the input jack, is it well grounded to rust free chassis surface? Is there good continuity to ground over in the power amp?

I can certainly check those things, but I don't see how they are consistent with the pattern I've described, i.e. no hum when first turning on amp (after powered off for a period of time) then roughly 2 minutes later ,"pop", and it's humming again. It's doing that consistently.

[Enzo]

I don't know either, but it might help establish a baseline. For example a poor ground connection could allow some circuit element to charge up then finally reach some voltage threshold that pops into evidence. Also, 60Hz hum doesn't really evoke thoughts of bad transistors. I am looking for things that could inject 60Hz into things. Grounds are always first on the list. When it is cold, it works for two minutes, I expect good readings, but after two monutes, SOMETHING lets go smewher, so maybe those ground checks will change.

Do you have any circuit chiller spray? In a pinch that spray air you can buy for cleaning computer keyboards can work, hold the can upside down the the stuff come out cold. Once it pops into the mode, chill the preamp area and see if it pops back out.

The three preamp transistors are direct coupled, the first two providing some gain and the third is an emitter follower to drive the transformer. Use a scope or a circuit tracer (listening probe) at each stage to see if the hum is throughout or coming in at the second or third stage.

[bobloblaws]

Picking up from a few weeks ago, I'm near my wits end with this thing. There seem to be intermittent sources of 60 cycle hum, in the preamp for instance , that have led me down the garden path. Anyway, right now I am focused on one aspect of the the power board. The way it is mounted in the chassis is quite asinine in my opinion, not conducive to servicing at all (see attached photo). On top of the fact that it is almost impossible to access the top of the board, it needs to be grounded to the chassis via the visible metal braces that are screwed into the chassis. If it is not secured in this way the amp oscillates like crazy. Even if removed the screws and tried to extract the board, it is hindered on the other side by about 25 wires that are connected to it (see other photo). So here's the thing. When I turn it on there is a significant buzz along with the guitar signal. If I take my trusty chopstick and place it at the top of the power board, in the middle, and push the middle of the board (where it is most flexible) about a quarter inch away toward the near wall (away from the pair of transformers), the hum goes away and the guitar signal is loud and clear. I would think this suggests a cold solder joint somewhere, but no amount of poking around on either side of the board reveals anything suspect.

Anyone?

[The Dude]

I'm not sure exactly what you're dealing with, but it's often easier to just get out the iron and resolder the entire board, or at least the suspect area (if you suspect a solder issue). Often, it takes less time than trying to hunt down a particular joint.

[bobloblaws]

I'm not sure exactly what you're dealing with, but it's often easier to just get out the iron and resolder the entire board, or at least the suspect area (if you suspect a solder issue). Often, it takes less time than trying to hunt down a particular joint.

You're probably right, it's just that I was hoping to avoid un-soldering and re-soldering all 26 wires that attach to it at the bottom. There just isn't enough slack in those wires to be able to pull the board up and out to where it would be practical. Unless you think it might be doable from the trace side alone. There might be enough room in the chasm to get it done that way. But everything seems fine on that side from a visual and chopstick test perspective.

[g1]

Any chance some of the screws holding the board are used for grounding?
If so, you may be completing an iffy ground when you flex the board.

[Enzo]

When the screws carry the ground, and I have to demount the assembly, I use a clip wire from chassis to assembly frame.

[bobloblaws]

Any chance some of the screws holding the board are used for grounding?
If so, you may be completing an iffy ground when you flex the board.

Yes, the screws and braces are used for grounding purposes. But the screws are screwed down real tight and the further away a given point on the board is from where I am pushing on it, the less movement that occurs at that point so the amount of movement at or near the screws would be virtually zero or negligible at best. But, it might be worth double checking that just to be able to rule it out absolutely.

[bobloblaws]

Any chance some of the screws holding the board are used for grounding?
If so, you may be completing an iffy ground when you flex the board.

Yeah I double checked the screws/braces, it's all in good order. I did just notice that when it is in the noisy state the output is louder overall than when I manipulate into the quieter state. So it could be that in the latter case I'm actually causing it to complete one less gain stage by shorting something as opposed to improving a bad connection.

[bobloblaws]

When the screws carry the ground, and I have to demount the assembly, I use a clip wire from chassis to assembly frame.

Yes, I tried that at one point but it wasn't sufficient, it still oscillated.

[bobloblaws]

OK, there was enough space to get the iron in there and reflow most of the solder joints and traces and...it worked! The loud hum is gone. Still some issues remaining but there may be hope for this beast yet. Thanks a lot guys!