Nice picture, teemu

Let me ask you this - and this is really a pretty important question.

Are you REALLY sure the compressor pops every time it kicks in?   COuld it be that the amp pops, and every time it does, the compressor reacts to it?

Once triggered, it is normal for the compressor to slowly release.

SO is it working now, with the right parts in it?

Isn't there a nicer pdf of that same drawing in your thread over at Music Electronics Forum.  I think that is where I grabbed it.   I have a scan of the real drawing, but due to size of my scanner it is two parts.  Yours was nicer.

Oh wait, it'll fit here.

Here is a pdf version.

Well what was that same reading, and from which terminals to which?   AFter all four burned out light bulbs all will read the same too.  I am looking for collector to emitter shorts mainly.

Might as well check the main rectifier diodes as well.

Well, they won't LOOK any different.  One gets out an ohm meter, and measures between terminals of a transistor, looking for low resistance where it doesn;t belong.

Normal, rapid, same thing... as opposed to slow blow or timed fuse.

You have power transistors inside on a heat sink.  You need to check them to see if any are shorted.  I'd wager my lunch money they are.

Can we come up with a schematic?  it doesn;t seem to have an FX loop, so that won't be the trouble.  Inspect your controls closely - are any cracked across their wafer?  And check the input jack solder.  DO the volume and EQ controls and channel switching affect the SOUND of the background noise at all?  If so, that tells us the noise signal is passing through the amp, not coming from the end of the processs.


I would point out to you that soldering is not a very difficult skill to master, in particular if that is what you do all day.  Korean people are just as capable of it as someone on this side of the ocean.  Even in this country, production solderers are not hired for their electronic knowledge.  it is a repetitive task, like sewing jackets together, or putting bags of potato chips into shipping boxes.

Furthermore, when things are on circuit boards, they are not usually hand soldered anyway.  Boards are usually soldered all at once on a wave soldering machine.  And to be honest, the wave machine will run about the same whether the guy running it is named Wong or Bubba.

Geez, my first reaction was that this sounds like the classic FX loop return jack problem, or a power amp in jack problem.  Electrically dirty cutout contact on one of those jacks can cause exactly this symptom.

One cheap source of heatsinks is dead home stereo amplifiers.  FInd an amplifier or receiver someone is discarding and strip it for the parts.

Thermal grease is NOT an insulator.

If i recall without looking it up, the LM3886 with the insulated plastic tab is the LM3886TF.  Correct me if I am wrong please.

Indeed.

SOlid state amps have no inherent load impedance, they are rated to a minimum impedance.  I have never seen an amp with a minimum impedance load remotely as high as 25 ohms.  I also cannot imagine a tube amp with a 25 ohm output winding.

What I CAN imagine is a PA amp with 70v and 25v outputs along with the 8 ohms and whatever.  In those cases - like old Bogen amps - that 25v is intended for a constant voltage wiring scheme.  It is not 25 ohms though.

Just what on earth do you have there?

Well that was just a simple thought.

Just to verify - plug a guitar cord from FX send right back to FX return.  Does that restore the sound?  If there is a pair of jacks marked preamp out/power amp in, try the same thing there.

Failing any of those, flip the main board over and carefully inspect the solder.  IN particular anything large like those 5 or 10 watt rectangular cement power resistors, but also all the jacks and all the controls.  Ir just reflow all thos ejoints and be done with it.  Various of those connections can result in your symptom.

Your line out worked, so apply a test signal to the power amp in or the FX return, wherever you can to test the circuit after the point of line out.  Does the power amp alone exhibit the symptom?

As on any amp, the effects return jacks and the power amp in jacks, and insert jacks all have cutout contacts that interrupt hte signal path.  When you are NOT using these features, those conatcs must close to carry the signal past the jack.  If those contacts get dirty, the signal gets blocked or attenuated.  Clean the cutout contacts on any such jacks in your amp.

Isolate the problem.

ANy control that affects the sound of the noise is AFTER the source of the noise.  SO if a tone control affects the tone of it,then it came in before the control.  COntrols that do not affect the sound are BEFORE its source.

It sounds like it is in your power amp.  PLug a guitar into the POWER AMP IN jack and dial it down to zero volume.  If teh noise remains, then it is indeed in the power amp.  If that kils it, then the noise is coming from the preamp.

You could also connect a cord from the PREAMP OUT jack to the input of some other amp to see if the noise is on the preamp output.  In fact, do both.

If the noise is on the preamp side, look at your schematic.  Between the MV, which you report doesn't affect the noise, and the preamp out is source follower Q101, which could easily be noisy.  Chip A111 could as well.  And output cap C138 could be involved.

If the noise is in the power amp, my inclination is to suspect the input differential pair or Q304.

For crackles I usually expect a noisy semiconductor, though resistors get noisy too.  A cap is less likely - they make their own kinds of trouble.

I would suspect those two little low value electrolytics on either side of the preamp out transistor Q105.  Looks like 2uf/50v.

Apply a sine wave to the input and follow it through the preamp with a scope or signal tracer.  Don;t have those?  At least monitor DC voltages throughout looking for some skewed funny.  Make sure both +25 and -15 rails are available at the preamp stages, not just at the supply.