Although a schematic can help you can still do some testing to work out what has failed.
Maybe give us some more details of what is wrong and folks here maybe able to help.
BTW,,Close up, Clear & well lit pictures can be a big help. (natural sunlight is best) Phil.

Basically you divide it up.
If you have a line out then connect that to another amplifier as that will establish if the preamp is working.
If the preamp is working then you know it's likely the power stage has issues.

If the unit has sat for a long time then insert plugs many times into input sockets as they maybe corroded. you can use WD40 or similar to get them working.
If there is no luck then you will need a DMM to test for voltages inside.
Phil.

Sorry to hear that.
Ok I'm gonna take a stab in the dark and asking better minds to look at this.

Look at J1,J2 & J5 as they are all interconnected to the low voltage rails via R1 & R49 as well as CR32, C54 & C55.

Now IF??? one of those switching circuits inside J1,J2 or J5 was faulty then it could cause a big DC offset. yes/no?

I've never liked those encapsulated plastic sockets as they are rather fragile inside.

My thoughts,,If the input side of C33 was lifted that would completely isolate the power amp input from any interference.

If that resolves the Buzz then I'd hunting down the offending socket,, or just replace all 3.
Phil.

Any lights come on?
You really need to add a bit more info.

Please connect speaker to speaker output,, line out is not for speaker.
Phil.

As mentioned yes check for DC on output but tiss a fair bet that now that the circuit is correct it maybe well be just drawing more idle power.
I would suggest, try a speaker  (with bulb still connected)
If the bulb glows very bright at idle (no signal passing) then there is still an issue.
Phil.

Quote from: joecool85 on May 05, 2022, 03:08:51 PM

Quote from: phatt on May 04, 2022, 08:26:15 PMFor me personally I tend to like Sealed Cabinets for single speaker combos.

Phil.

I was just talking about this with a co-worker.  I believe combos should be open-back, the way God intended.  Just kidding, though I am an open-back believer.

Yes open back delivers a more spatial sound hence fuller effect especially in small spaces but can present weird sonic issues in larger venues.
You may notice that a lot of big pro gigs tend to use closed back cabinets.

A player friend noticed this weird effect at an old wooden school gig where his open back rig was giving him a crossover of out of phase in the position he was limited to in the building. It can mess with your brain and the sonic result can be off putting.

That does not happen as much with a closed back cabinet.
Although it can still effect a closed back cab it is not as bad.
Venues with big Glass windows can have really bad reflections.
Phil.

Speakers are a rabbit hole but a general rule for balance is; If the Amp is dark ,,use a bright speaker.
If the Amp is bright use a dark speaker.

Compare the cost;
You just altered the Amp and it cost you 50cents for a cap.
So even if you have to try several caps to find the right tone it's still only costing pennys.

While speaker swaps may cost you hundreds before you find the right one.
Then there is the Cabinet the speaker is mounted in,, another rabbit hole.

Be warned some of the fancy painted magnets will cost.
For me personally I tend to like Sealed Cabinets for single speaker combos.

Phil.

A whole lot of muckin about that may well cause complications.
As Enzo & G1 have said just unplug the speaker. A lot less work and no chance of stuff-ups.

There is no issue with the speaker unplugged on SS amps ,, Valve Amps Always need a load on the output, while SS amps Do Not.

Hard to read the blur but IC111 looks like a protection circuit,, don't mess with it.

Phil.

Yes it's possible when the power Tr's blew it also effected U1.

U1 is the power stage input, those 4 Pwr Tr's boost the current needed to drive the speaker so both U1 and Pwr tr's could have failed.
The other 2 tr's (Q8 & Q9 I think) are part of the over current protection. 

Now assuming none of the passives or Q8&Q9 are at fault then replacing U1 Should fix the problem.

Hopefully others more qualified will chime in,, if I've missed something.

As you have noted that plugging into the Pwr Amp input still sounds distorted then the Power stage is most likely at fault.

If you want to check if the preamp is working normally then take the Preamp output to another amp (if you have one) if that produces the same Buzz then the problem maybe further up stream in the signal path.
Phil.

Oh dear, Reset,, that noise is not ground hum/buzz that sounds more like a blown poweramp.
I asked if the amp is working and passing signal except for some hum/buzz,, if so you may have a ground issue.

Obviously this is NOT the case.

In your fist post you said you replaced the power transistors but there might still be a problem in the power amp.
My best guess is the opamp at the input of power amp stage might be stuffed.
Check the DC test points on U1. (pin7&4)
Phil.

Good to hear it's worked for you and having the feedback may help others who struggle with these Amps.
A lot of SS Fenders from this era tend to suffer from excess treble response.
Phil.

Just turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.

A scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?

Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.

Often components like input jacks, FX loops and speaker outputs,  even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.

Tiss easy to loose these little isolation washers or forget to reinstall them.

If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.

*I would be researching ground path issues long before replacing parts in hope.*

I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner.
Phil.

Quote from: SemiConductive on April 19, 2022, 06:20:38 PMThanks, @phatt. I am studying that and will compare to the sound samples I'm getting. Seems to make sense.

Couple more questions: To shift that mid dip... to 400hz as you suggest... is that a one or two component swap, or a rearrangement of the whole tone stack? I've played with stacks a bit in Duncan's Tone Simulator, but this thing doesn't really fit in there.

You can change R15 to 10k which will move the mid dip a bit lower but you will loose a little bass.

Quote from: SemiConductive on April 19, 2022, 06:20:38 PMBack on the capacitor C6, which is effectively filtering out treble: I've installed a switch and a pot for the diodes and played with that in conjunction with the overdrive (IC1-B) switching. It really is cutting a lot of treble. I think they were going for that dark boost tone. Misses the mark.

I know I can change the value of C6 to get more treble. Is it reasonable to just life C6 and see what it's like? Just "eliminate" the bypass? I can also try some lighter values but I'm wondering if it's safe to go to infinity.

Removing C6 will give more treble but it also might oscillate at high gain levels. Maybe try 100pF see what happens?

Phil.

These are small practice amps and In My Experience a waste of time changing every cap that will likely not make any difference to the sound these cheap amps produce.
Re scratchy pots
I just spray the pot guts with RP7 or WD40 and unless the pots are totally stuffed they usually come back to life.
As example, I scored a home Hifi years ago that had a scratchy volume pot, a squirt of WD40 and still going fine, that was 10 years back.

Note; some tecks will frown on using RP7 or WD40 but Cricky it's a cheap guitar amp.

If you want the best pots then track down the conductive plastic units as they last much much longer.
Phil.