This amp is called Combo Head II, it is in a small 12 inch combo, model CMD121P Will study the  SMPS fundamentals.  Will need it, I have established that there is 120v going in, but no power being supplied to fan, preamp, etc,. 

Has anyone looked at or attempted repair on these types of amps? I bought one non working for parts for a song... but now I see the complexity of such a device .   It seems that they are stingy with schematics and all roads lead to approved repair shops. This one won't even power up... no power LED, no fan, no life at all.   Just learning about class D, fascinating stuff!

Just bought a loupe... great tip thank you.   I also believe I may have solved my issue! I reflowed all the joints on amp board just in case... and the transistors that were held on by screws I removed and cleaned up the connections.   And cleaned the hell out of the "Mute" switch... could not play too loud last night due to sleeping house but did not hear any popping and sounded good... so I believe my problems were the bad power amp connections combined with the Zener diode in the preamp.   Thanks to the forum... what a feeling.

I Deoxit all outlets and switches, and bypassed the effects loops with cords... still popping and noise. 

I tapped around with a chopstick on the power amp board and that seemed to make the most noise/popping so I will pull that board and look for dry joints/etc. 

I'll definitely check those out... I'm building.  I'm getting to the point that I might be ready for that learning a lot but slowly.   

And, the new Zener diode did the trick!  No more distorted weak output. 

BUT... my fun isnt over yet.  I also have an intermittent cutting out and popping.  I have not found any bad solder joints, nothing looks loose... nothing real obvious yet but I will keep going.   

Any pro tricks for chasing down that bad joint/ground/whatever would cause that?

All of that was in the same SM we were both reading and had no idea what it meant.   Leveled up today! Thanks for the help. Will report back on if the diode does the trick

TassieViking, thank you for the that advice I will try that.   I saw that on the SM, but I could not figure out how to do that, it is beyond my understanding.   

I do have a scope.  How would I input .22 volts?  With a bench type power supply and then read it with the scope?  I am trying hard to learn here but have a LONG way to go.

I'm about 99% sure that I am there... I found that I'm supposed to have +45 and -45 going into preamp... then 16V at the test points.. but I was not getting -16 after the zener diode, pulled diode and it tested as a short on the tester... new Zener diode on order and we'll see.  Exciting. 

You gotta love good advice! 

So, my troubleshooting job just got 50% easier thanks to the advice on this forum. 

1K sine wave direct into POWER amp plug, results in a perfect waveform on my scope... perfect. 

So now I know it is the preamp.   Im new to reading schematics, any good general advice on finding the likely problem on this preamp board?  Can it be traced in such a way that I work through the signal path to find where it gets ugly?

I input a nice clean 1k sine wave, and here is the result from both the preamp out and from the power amp out... both appear to be wonky. 

I guess this tells me at least that the distortion is starting in the preamp section at least?  But it still seems that the power amp should be louder.

I guess I also need to understand the function of Gain and Master...Gain is there for crunch, but doesnt control volume?

First time using this scope, it is really cool to use such a powerful tool for diagnosis, cant wait to use to greater potential and understanding.

An awesome start! Did not know about the effect loop in back I will start.

Forgot to mention the output is distorted and kind of farty.


Will report back with results.  I will post a pic of the scope.  It input a sine wave to input but the output was far from clean, but never worked on a guitar amp so didn't know if that was normal

I have very little experience with electronics repair, about a year now of getting a few vintage stereos back to working.   I have the basic tools of the trade (solder station, DMM, O-scope), nothing pro or too fancy.  Has been a LOT of fun making things work that were trash.   

I recently picked up a really cool B15T, which was "working" in that it makes sound.  But at full volume, doesnt make enough output to be uncomfortable, its more like watching TV.   The amp will also cut in and out with a slap.    I dont SEE any solder joints or anything obvious (to my novice eyes). Can anyone advise on a good place to start?  It's too cool of an amp to be broken