Ok,, Now you had some trouble reading diodes before so maybe go back and recheck the main diodes in the power supply,
that's CR22,23,24,25.

It is the most obvious point of hum and you only need one to fail and induce hum through the whole circuit.
If you are not sure just replace them anyway as they are not expensive parts.
Phil.

Measuring CP1&2 at this point is not going to tell you where the issue is.
 Divide and Conquer is how you find the problem.
So go lift R82 FIRST
If no change then you move on to the next step.

So lift the top end of R82 as that will isolate the WHOLE switching system.
(top end as drwan in schematic is the AC input from CP5 which powers the switching)

Then power up
If no signal passes through the preamp then try inserting guitar into poweramp as it is separate from the preamp and switching so it should still pass signal.
If you can get a clean signal with no buzz then it's a good bet the switching circuit is bleeding AC back into the circuit somewhere.

If that kills the hum/buzz then you at least know the switching circuit is likely at fault.  I'm assuming it could be any place but seems obvious that Q3,4,5 or 6 are possible suspect points for AC to bleed through.
If they are working then maybe C44, C45 or C46 might be bad.
As I said better minds may know more.
Phil.

Yep diodes look fine,, moving on,,,,
You mentioned in the first post that you replaced u4 which fixed the channel switching.

well the switching circuit power comes from the main AC at CP5.  As u4 was dead there maybe other damaged parts causing ac to bleed through to the audio path.

Switching circuits always confuse me and Others here will likely know more but if it was me I would lift the top end of R82 which would remove power to the whole switching circuit and see if the buzz goes away.

If it does kill the buzz then I'd be checking for dead parts around U4. maybe one of those diodes are fried.
your amp won't work per normal but it may help to isolate where the buzz is actually coming from.
maybe check C44 ,C51,C52
Phil.

Yes TassieVikingman,, fp is short for Flame Proof.

@ Scooby,
Those resistors often run warm but as they drop a fair amount of voltage they cop heat, if the 130-Ohm resistor has burnt then the resistance may have dropped and then  C285 has likely been running over the voltage rating and died, as you have found. I'd replace C286 while you are there. Even better replace with 35volt caps.

If those 2 resistors are running very hot that can shorten there life.
If excessively hot in normal use you can use 2 watt resistors which will run cooler and hence last longer.
Phil.

I get the impression you are not clear on diode testing, If so then
You check diodes with power OFF and lift one end of the diode.
Meter set to diode test.

If you get a reading both ways it's dud.
To save writing it all,, go here;

https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes
HTH,Phil.

Quote from: Loudthud on July 18, 2022, 04:04:16 AMThis effect is not what I would call "Diode hash". It's not any fault with the diodes. The circuit is doing exactly what the math says it will do. To get rid of the "1+" effect, use the inverting form of feedback. See attached. The one disadvantage is the low input impedance, so you really need an input buffer like the TS pedal.
You cannot view this attachment.

Hi , thanks for that, yes I worked out years back that using inverting circuits tends to produce a better result, but was never quite sure why, so ta that makes sense.
I think I got slewed with the term crossover and Expansion at zero crossing. I'm Still not totally clear on that point and also, if I remove the diodes in the sim then the jagged edge on peaks is gone. Of course it just square waves but the flat top peaks show no hash. A bit hard to compare because it hits the rails so early.
It's certainly and interesting subject.

Hey it's not too bad but likely to struggle if you want distortion.

Hey you have a BBoard,,Maybe test some of the Marshall circuits that *TassieViking* mentioned on first page?
FWIW,
     In My 30 plus years of building guitar circuits I can say that if you just want to strum a few clean chords then bandwidth and tone shaping will not be much of an issue but if you want OD/Distortion, crunch, big fat sweet singing leed guitar tones then you will spend years building land fill if you don't research the bandwidth issues of Distortion,, an Why do I know this,,Well I have draws full of land fill circuits that went nowhere. In all those years I've only had a few success circuits that actually worked well. It's a long journey but I'm glad I persevered in spite of all my failures as those fails taught me stuff that I may never have known otherwise.
Phil.

Yes, maybe check them.
Phil

Yes I have often wished I had more bench gear but at older age and
eyes failing It becomes harder to work on fine detail.
I'm happy to have been able learn enough to build a whole rig that I'm
very happy with and I'd rather sing and play music nowadays than to try
and reinvent the fuzz box.

FWIW,
My understanding about this quirk is, it's just diode hash (often mentioned in pedal forums) as Grit,Fizz,Hash and like terms.
I think you will find that what you are hearing is diode switching.
Using this circuit as example (See Screen shot) you will find at levels below ~200mV you get a smooth flattening of the signal But once you increase the input signal those diodes obviously struggle. Hence the peaks display a jagged edge.
This is high freq hash from the diodes switching state.
And Yes as you have noticed when you increase the input it sounds like Gravel.
the bigger the input the worse it gets.
If the bandwidth is not limited at higher frequencies it can sound gawd damned awful, no longer musical.

I get around this by using 3 OD pedals but Never turn the gain/dist knob too high on any of them.
This issue is not limited to TS circuits it seems to happen to most  Dirt circuits that use diodes in this manner.
Phil. You cannot view this attachment.

Quote from: Loudthud on July 14, 2022, 02:56:00 PM

Quote from: phatt on July 14, 2022, 07:51:00 AMHi Loudthudman,
Just to clarify, D9&D10 are surely a Limiter?   
I can't see how they could work to impart any kind of Xover dist
on the signal. They would need to be back to back in series
with the signal to cause Xover. Maybe that is what was meant?

You know how the equation for gain of non-inverting opamp is 1+(Rf/Ri). Well that "1+" means the input signal is added to whatever distortion the diodes create. If you look at the output simulation @Carriage posted in reply #2, you see the top of the wave is just the input signal added to a more of less square wave, so it looks like soft clipping. If the input signal gets bigger, the top and bottom of the output just get bigger. This is not the kind of "dead zone" crossover distortion you are used to seeing, it's kind of the opposite where the area near zero crossing is expanded. Run a Clean Boost pedal into a TS and it just sounds louder and cleaner than it did without the boost, but with a gravely undertone. Not what I want my guitar to sound like.

Thanks for the explanation,, sadly I'm still miffed.
I appreciate these things can be hard to explain in a few words but I'm having trouble working out what you mean by "expansion near the zero crossing", because all traces still cross zero at the same time point. I hope that makes sense.
Although I'm not trained I've spent years learning how audio stuff works and never come across this one, no doubt there are still holes in my understanding.
Is there any literature you can point me to that covers this in depth?
I've read many books on electronics one being The Art of Electronics as well as building and repairing heaps of gear in the last 30+ years.
Cheers, Phil.

Quote from: Loudthud on July 12, 2022, 04:46:59 AMThe diodes D9 and D10 really don't clip the signal, they just add what looks like crossover distortion. The Tube Screamer really doesn't make a good preamp.

Hi Loudthudman,
Just to clarify, D9&D10 are surely a Limiter?   
I can't see how they could work to impart any kind of Xover dist
on the signal. They would need to be back to back in series
with the signal to cause Xover. Maybe that is what was meant?

@ Carriage,
As for smart design the tube Screamer is actually good design practice. It has a Buffer stage in front of the first opamp.Early ones were just a BJT follower but later ones used an opamp stage.
They knew quite well that *NOISE is the enemy of high gain circuits* and that is why they used it.

I'll use your Schematic as reference, If R1 is 1Meg and R28 is 500k then you will pay a noise penalty, at high volume it can become unusable.

Any opamp with HiZ input and high gain (such as yours) will be noise prone.
The TS input buffer takes care of the HiZ input and then the gain stage sees a much lower input sensitivity.
The Equivalent R1 in a TS circuit is usually 10k. Not as much Gain but far less noise and a far more usable range of Distortion.

If you care to note, look at many of the so called bootweak guru designs of similar circuitry and they remove the buffer simply to get more gain, dirt or grit. Yes of course it's MORE BUT the noise floor will make that extra dirt unuseable,,
meantime the xperts tell you you need a noise gate.

In reality it's just a poor understanding of the art of smart design. Yes in some circumstances N-gates might help but logic suggests, tiss far better to design out problems rather than bandaid the flaws.
Most of the pro circuit use a hiZ buffer front end if you look around.

As to distortion before tone, that can go either way. With these guitar audio paths it can often come down to personal taste. experiment and find what works for you.

Note;
If your power amp is only 1 Watt then you will have little clean head room for any decent clean sound. Which is likely what Loudthud was eluding to.
Plus; I urge anyone wishing to build stuff, go get a Bread board and test it all first. You will learn a heck of a lot more than just poking resistors into a pcb. That's a 90% chance you will be building land fill.
Keep going,, be warned this stuff is very addictive.
HIH,, Phil.

Most likely the pot has failed, you might need to replace it.
The wiper which is the center pin can grind away the carbon track or just get dirty and looses contact with the track.
Also look for hairline solder cracks around the pins on the PCB.
You could try circuit cleaner spray into the pot but a new pot might be cheaper than the can of spray.  
Phil.

I've not worked on this amp but have worked on similar small amps with some good results with improving tone efficiency.
Assuming I'm reading the right schematic you can raise the value of R14 which is 100k, up to 500k or even higher. (At pin3 of U2A,, after tone stack)
It should open up the tone a little better making it more effective at Boost and Cut.
100k might be loading down the tone section.
Hope it helps, Phil.

Hi Ben welcome
Regards to IMG_4223.1.JPG
My guess is that green wire you mentioned goes to the Chassis.

The idea is to re-route how the ground paths return to the main Common/Ground point,, which is the Chassis. I'm not sure about the 2 dots?

It would be best if you PM "bajaman" as he will be able to explain it to you.

Yes it must be hard when you have to translate.
hope it helps, Phil.

Yes could be the socket but more likely the plug is faulty.
If the speaker cable has a metal shroud then unscrew it and check the connections.
If you have another cable around try that.
Phil.