Marshall Transistor Amp Power Switch Problem

[BullyBeef]

Yes, this is another switch pop question.  I have looked at other forums for switch popping and found a few possible solutions that worked, but none that worked for me. 

I bought a Marshall 5120 guitar transistor amp that had a few issues, hence the 100 euro asking price.  This is an old amp, it was actually made in Dec 1984 by Sue W. in Marshall's Bletchley factory.  I fixed the performance issues with some new caps, a couple of new pots,  twisted the mains wires and transformer secondary wires, relocated the diode bridge to the chassis to eliminate any AC wires going near the output transistors to reduce the hum that the seller said the amp had since he bought it new.  That lot fixed those problems.  The amp works well now and sounds great with a Les Paul.

The only problem remaining was/is a loud pop on switch off - even with all the controls zeroed. The seller said this had also been a problem for years and he used to just unplug it from the wall.  This shouldn't be too tough to solve , but after a new switch I still get a big pop on switch off.  Switch the power off at the outlet, or pull out the mains lead and it shuts off normally.  OK, then that must be switch arcing - I tried a cap across the switch, no go.  I read that a resistor would help as the current would just be reduced and not shut off - NOT a good idea as it would be live to some point and a trial revealed the resistors get hot - no go.  Maybe the transformer was creating high volt surge on power down, so I put a MOV surge arrestor across the primary - no change.  I even tried putting a grounded ali shield to screen off the PCB from the mains area.  No change.  Anybody have a suggestion?

[sluckey]

Schematic?

[shooter]

without a schematic...;


here's an example of bleeder resistors, 4 total.  I built this supply and used it in a homebrew "brick amp"

[PRR]

...here's an example of bleeder resistors, ...

Probably nothing to do with Beef's issue. Those resistor will bleed the cap slow, but not before the POP happens.

It IS odd that breaking at the far end of the cord does not pop.

As for cap and resistors across the switch: values and quantities matter. Caps typically used here won't pass much heat to typical resistors..... but who is typical??

[BullyBeef]

This is the only schematic available - it's not very clear as it has probably been photocopied a million times. PSU is CT transformer with full wave bridge and 4700mF Caps to give  +- 25V   for output push pull stages, dropped with series resistors and zeners to  +- 15V for the ICs.  I have seen photos of other 5120 PCBs and Marshall has changed the layout since mine came out. 

The thump comes immediately at switch off and then the PSU caps drain pretty quickly, so I don't think they are to blame. Something is kicking the ass out of the output transistors the instant the switch opens, but I can't work out what. 

Thanks for your replies so far!

[shooter]

Re-measure the rail +/- using the PT CT as reference.  As close as you can get physically.
(guessing.....a miss-match Plus to Minus)


didn't see, is there a MUTE?  If so, with it muted, get the POP?


WAG - POP with AND without signal IN?

[PRR]

Nothing on the plan is suggestive. Well-lit gut pictures of your amplifier?

[BullyBeef]

Pics attached - not much attention paid to safety in 1984 - not an insulating boot in sight.   Power rails +25.23 and -25.07.   No mute - the pop happens when the amp is switched off only by the chassis switch, even with all controls set to minimum and no input.  I'm thinking of some kind of RF noise when the switch breaks and arcs - this will account for the small pop on switch on from the closing arc and the bigger pop from the opening arc.  Maybe if I change the fuse to a panel type and try an ali shield between the PCB and all the mains side?   

[BullyBeef]

I forgot to mention - the original reservoir caps were axial leads and large cans - the only ones I could get locally are radial leads and smaller cans - I bought four just in case and soldered them all in because I thought the extra capacitance might calm the pop. It didn't.   The original caps showed signs of internal pressure , is why I changed them.

[sluckey]

Try this... Disconnect the switch wires and remove the switch from chassis. Use some two or three foot jumpers to reconnect the switch wires outside the chassis. Tape the switch for safety. Does the amp still pop when the switch is operated? Try switching while holding the switch far away from the amp as well as holding it inside the chassis. Still pop?

[PRR]

Black from transformer is spliced to green from PCB near C47. Is it convenient to try breaking that connection?

I suspect black is an internal shield. Normally best to ground that. But the ground point is very near a tone pot? And the shield may have a leakage issue.

[BullyBeef]

The transformer centre tap tag was broken and the wire was hanging on the winding ends - I glued the tag back and used a scrap of wire to extend to the board as the 0V connection as the original was a bit tight.

I tried the remote switch option and it drastically reduced the pop - so it is switch arcing.  I found this page about arc suppression - https://testguy.net/content/362-Arc-suppression-circuits      I worked out .047uF  & 39R and wonder of wonders, I have those in my parts box.  I am being dragged out shopping now (as pack horse)  but I will try this when I get back.  Some women do not understand the sense of urgency  a man feels when poised at the brink of solving a difficult electronic problem, but I understand the need for a peaceful life at my age.

[BullyBeef]

The cap & resistor across the transformer primary reduced the ON pop a lot, but did nothing to the OFF pop.  I dug in my parts box and found an EMI suppressor that I bought years back to stop the noise from a central heating pump (must have been just after I was married in 1976 - I never used it cos we moved house and I left the problem behind).  This is a LCR network tied across the live and neutral with a ground wire.  I tacked that across the primary as well, thinking the extra capacitance might help. and it did. Now I get a slight pop on switch on - nothing unusual, but on switch off, the loudest noise is the click of the switch. I think that is a success.

Thanks for you help, folks.  You gave me a couple of things to check and made me use my brain for what it was built for.