If you used thermal paste on the transistor insulators, make sure that you are using non-conducting paste.
It is very common to use conductive thermal paste in computers and other places where there is no need for any insulation between the component and the heatsink.
Thermal heatsink paste is a big no-no in amps, I refuse to have any at home just in case I make a mistake one day.
Good quality paste and just a very thin film of it is all you need, if you have the cheaper stuff it will dry up and become useless faster the the good stuff.

Could it be the TO-3 insulators, maybe you have a short from the power transistors to the heatsink.

The 1N4744 diodes are ok at up to 175C (347F) temperature, but the power handling will be severely reduced.
At 100C (212F) the power rating drops in half to 0.5 watts (boiling water hot).
At 100F they are still working at 100% capacity, that's not even warm for a semiconductor.

The X in VX means nothing as far as I know, maybe the designer picked X because it feeds the effects (FX in shorthand) circuit. (Noisegate)
I'm guessing the CD4013 flip flop chip might be noisy when operating so they made a separate supply for it to reduce noise in the rest of the amp.
I think when the signal into U8 drops below a certain level Q4 and Q7 turn on, Q4 shorts the signal going into the octave unit and Q7 drops the gain in U6:B, ;that drops the signal right off.

Play with the clean channel gain when using the distortion channel and see if it affects the cut out level.
If I am wrong about that then I hope someone will jump in and correct me.
(I'm old school, I learn better if I get a clip over the ears so don't hold back).

I am wondering where you are picking up the test points, I can't see them on the layout.
Test point TP17 to TP19 voltage don't exactly match R42 voltage, but it should.
If you are measuring TP17 far away from R42 then I guess you could loose some voltage in the tracks but with only 34mA it seems strange to me.
Could there be some bad solder joints on JW7 or JW32 ? Maybe it is just the track loss but I would want to know if it was my amp. Measure voltage drop from D14 to R42 leads to find out.

G1 will lead you in the right path as I think he knows a lot more then me.

Cheers
Mick

I can't see TP1, but I think I have the correct schematic.

The noise gate is shown on page 7 of the user manual in the block diagram, I'm not smart enough to know how that works. =)

 Good photos Gonflable, I like being able to see what it looks like and sometimes a photo shows bad solder joints that we miss and others notice.

Something is not adding up with the voltages, TP17=21v, Vdrop on R42=8.6v, TP19=11v.
We seem to have lost 1.4v in those measurements.

Could you please measure TP18, TP20, Vdrop on R43, R46, R85  just for reference.
It would be interesting to compare the positive rail and the negative rail current drain.

I just noticed that the tuner and foot-switch circuit are all run of the +15v rail as well, this would add further load on the +15v rail compared to the -15v rail.

Just the distortion red LED would add more voltage drop to the +15v compared to the -15v rail.
Led = approx 8.3mA, 0.0083mA x 220R = 1.826v in R42, 8.3mA 47R = 0.39v in R45.
Surely the engineers who designed the amp would have realized this and made slight differences to the design.
Maybe I'm on the wrong path there but I would be wondering about tweaking the values of R42 to maybe 150R, but I would do it in small steps.
D18 is a 15v 1W zener diode, but it can't do anything if the voltage drop across R42 is less then 6v, it needs more then 15v to start conducting as far as I know.

Measure the -15v rail first and we will start to work it out, after all I could be completely wrong.
 

I can't find any pictures of that amp on the net, any chance of taking some photos of the insides while you have it open ?

There is apparently a noise  gate circuit in the distortion channel that needs the clean channels Level set to more then 0 to operate according to the user manual.

I use a very strong magnifying lens to look for bad solder joints on older amps, look for tiny cracks on every solder joint and then I re-flow any bad or suspicious one.
Intermittent faults are often caused by bad solder joints on older amps.

You would have 45.45mA going through R42 to drop 10 volts between TP17 & TP19.
Where is it being drained too ?
Measure the voltage drop across R45 and R82 to see how much current is going through them to find out. (amps = volts / resistance)
If the current through R45 and R82 adds up to 45.45mA then the circuit is using it somewhere and you have to follow the path it takes.
If the current does not add up to 45.45mA then I would suspect the zener diode D18 is faulty.

I can't see anything wrong with those readings on Q5 if your 15v has dropped to 11v, TP15 will be low if the supply rail has dropped.

It sounds like the OP made his PCB's by using a printout of the original layout drawing and transferring the tracks to a PCB that he then etched himself.
If that is the case then he would end up with a fairly close copy of the original amp dimensions.
The original amp was 26 inches wide so the original PCB must have been over 24 inches wide (610mm), no wonder he made the PCB in 3 sections.

AIONFX has kits and bare PCB's to make a full L5 pedal minus the reverb, they also have kits for just the clean channel and just the drive channel.
If you don't want a pedal then it would be best to make a new PCB that incorporates the mod that AION made to change from the CA3080 and CA3094 IC's to a single LM13700 IC

I resized the original PCB's drawings to 150DPI approximately and ended up with this:

It appears to be 100 ohms
More manuals : https://www.audioservicemanuals.com/e/epiphone/epiphone-triggerman/

I have been given some cheaper practice amps and beginner amps that needed some small repairs over the years, usually just a good clean of the pots and jacks got them working.
I ordered new pots and new jacks for these to make them reliable.

I think most local schools would love them for the young kids in the music department.

In Australia there would be 2 x SPDT switches and the last switch is a special 4 terminal switch called an intermediate switch made for this purpose.

If you are using normal toggle switches then you would need 2 x SPDT and 1 x DPDT switches for the 3 switches and one light.

Just imagine a really long hallway with a door in the middle and doors at each end of the hallway.
You would want to be able to turn the light on/off whichever door you go through.

I have to agree with Phil, too much theory can get in the way if you don't do a lot of practice with real circuits.

I think what would be good for you is to get a breadboard and start building small circuits like stomp-boxes and even small SS amps, and then you can modify them to see what happens in real life with the signal.

You would not need to spend a fortune on parts because you will be able to re-use all the components, just build up a common variety of parts and off you go.

A bread board needs to be a good size to start with, I would buy one with at least 800 points on it.
You can always get another one and use several breadboards for a single build if the circuit is big.
Here is a link to a well priced one: https://www.taydaelectronics.com/hardware/breadboards/830-point-solder-less-plug-in-breadboard.html

I get most of my components from Tayda, if I order resistors I get 50 at a go and then slowly order more values as I need them.
any components that are regularly used I order multiples of them, and if there are better ones for the same price I get the better ones.
One part comes to mind, 1N4001, 1N4004 diodes are used a lot, I get the 1N4007 instead as they have a higher voltage rating but are the same size and price.

Cheers
Mick

All I have is the power amp schematic, I have never seen the pre-amp schematic.
It shows there is a send and return jack so that might be good news.

If the noise disappears when you plug a patch cable from the line out to the line in jacks as Loudthud suggested it means the line in jack is dirty.

The line in jack has a switch in it that disconnects it from the line out jack when a plug is inserted into it, no plug and it is connected to the line in jack.

A quick spray with Deoxit and then insert a plug several times might dislodge the dirt and restore the amp to normal.

You cannot view this attachment.

It looks like the Blue and Yellow wires are reversed, as long as the wires are connected correct on the PCB.

Check the colour of the 2 wires that are closest to the headphone jack, directly behind the jack on the PCB.
They should be the positive wires, the 2 wires closer to the back of the amp are the negative wires.
Cheers
Mick

I have come across many instances where thermal grease has dried up and is no longer performing well, most of these instances were DC drives for very large motors.(200kw+)
I used to remove all components on the heat-sink and clean the old thermal paste away and apply a very thin coat of new paste before re-assembling the units.
I have also seen this in many computers on the CPU, clean and reassemble.

Unless there was a fault I would not replace any components, just fresh thermal paste applied very thin and fully covering the transistor and heatsink.

Make sure you use good quality insulating thermal paste, there are thermal paste that conducts electricity, they should not be used in amplifiers.
Cheers
Mick