Confirm that CR14 cathode (or CR13 anode) is still 0VDC with no load connected.
If so, connect load and recheck.
Also check DC volts at pin6 of U1.

So you now have no significant DC on the output, and the bulb does not get any brighter with the load connected.  That is good so far.

Connect your guitar to the 'power amp in' jack.  Do you get the same distorted sound?

Quote from: A7 on May 21, 2022, 07:16:41 PMI'm sorry when I said line out, I meant the line to the speaker.

So what did you mean by 'moved' in the first post?  If you mean just wiggling the wire going to the speaker makes a buzz come and go, then that sounds like a bad connection.

Perhaps it accentuates the advantageousness of the 'high headroom' speaker? 

Less than a hundred millivolts DC at output is fine.  You have about 50mV so no problem.
On the bulb limiter with power on, connect the speaker.
Some amps won't start up right with load connected when on the bulb, so that's why I said connect speaker while power on.

Is there a jack for PEQ footswitch?  I think you may be able to turn it off and on so you can use it like a lead boost etc.

Quote from: SemiConductive on May 04, 2022, 03:58:20 PMWhat's the function of IC111 i.e. what fault is it protecting against?

It senses DC at the output and kicks out the relay to protect the speaker.  It's also supposed to be able to prevent over-current but I'm not sure if that function is being used.  (see datasheet attached)

Quote from: SemiConductive on May 04, 2022, 09:26:37 PMSlightly OT: For you guys who can't read the schematic I posted... did you click on the graphic once you got there to zoom it? Inquiring minds want to know (so I can plan better next time if needed).

My computer is old and my eyes are too.   The picture of the schematic only offered me one level of zoom and it still was too small for me.  Probably the zoom function was not working right with my old browser.

Enzo's approach would be the simplest. 
But if you really want to automate it, there is already a relay engaging the speakers, you could just toggle that.
The type of switching jack you mean is probably with 'isolated' switch.
The switch could replace the connection between D112/C183 junction and R1998/R198 junction.
I believe that would leave the protection function via pin2 of IC111 enabled.
Full service manual attached.

As real MC1436 are hard to find, and I think the circuit is picky about the exact part, post your DC voltages for U1, pins 2,3, and 6.

The pot brackets in these amps are used to carry ground connections.  I've circled one example in your photo below.  I would just resolder them all.
Suggest you deal with the obvious audio problems first (ch.1 distorted), maybe you will get lucky and fix the hum while you are at it.


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Here you go.

R4 and R5 run in parallel from circuit ground to chassis (pot brackets).
As they are each 47 ohms, 24 ohms seems the correct reading from CP4 to chassis.
It's possible there is another connection to chassis somewhere but I don't see anything on the schematic.
You could try shorting across R4 or R5 as a test and see if it affects the hum/buzz. 

Check supplies for ripple.  You don't have a scope so you can measure AC volts at TP's 3,4,5,6

It's a very legitimate concern and I don't think you can over-state it.
If you really want to be fail-safe, isolate your string ground (isolation transformer at amp input) or go wireless.

Play safe and don't count on anything.  If you are holding strings and going to be touching a mic, or another player, do a shock test first (with something other than self).  Same if you play barefoot or standing in water.

It takes multiple failures at the same time or catastrophic single failure to put you at risk of serious harm.  Otherwise, like loudthud said, it's just really annoying (like the static zap I get everytime I open a cooler door at the grocery store).

Outlet polarity checkers will not show you a 'bootleg ground' so they are not failsafe.
With double insulated equipment, 3rd prong ground is not required to go direct to chassis so you can't necessarily count on that either.
The double insulation is deemed safe and if they are not, I guess the lawsuits are being settled out of court.
Know the risks and act accordingly.

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