Marshall 1959 SL RI Phase Inverter Noise

[andYz00m]

Hey there, I have on the bench our Marshall plexi reissue from the studio. This has been modded with a PPI master volume and variable NFB. Amp was working and then magic smoke escaped.

The component that blew was R1 on the V1b cathode bypass.

Replaced tubes (output tubes were fried). Amp sounds correct and has the right tone again but...

Issue now is there is a huge amount of noise coming from PI. With all knobs up and no signal injecting its about 14VAC coming out of the PI plates and about .7VAC going into the grids. Im testing without power tubes installed.

Seems like there is a ground issue somewhere but I can find it and not sure where to look. Ive chop sticked around and reflowed joints where I can access them, etc. Tried bypassing the dumb effect loop card but that wasnt it either.

One of the challenges is these amps are half hand wired and half PCB with the soldering under the PCB and the components on top. Not ideal for servicing. So to service the PCB solder joints you need to remove one half of the leads to the sockets or pots to be able to get the PCB up. Very frustrating. Also the coupling caps are through hole tight to the PCB so no leads to solder on to for replacements.

This was the other thing I thought might be the issue is the coupling caps on the PI could be leaking but I dont know if that would explain the massive amount of noise. Also this amp is not that old. The PPI master volume mod is... after the PI so its not the issue, unless its the source of some weird grounding issue.

I hate working on amps with PCBs and I hate even more working on amps modded by someone else!

Any ideas welcome.

Scheme:
https://www.drtube.com/schematics/marshall/1959-01-60-02.pdf

 

[CJ]

Spend two weeks fixing the problem only to have more bs down the line or spend two or three days ripping out the board and going point to point for a trouble free better sounding amp that is easy to service, if you own the amp.

 

[Tubetec]

Think about what your doing Andy ,
without power tubes in place HT voltages are going to be way high ,
also without the power tubes there is no negative feedback from the transformer secondary to the phase invertor ,that will make a difference to the amount of noise .
Having an attenuator after the pi reduces the amount of feedback so thing probably wont work right

My advice , carefully remove all mods and return it to factory . .

 

[andYz00m]

Think about what your doing Andy ,
without power tubes in place HT voltages are going to be way high ,
also without the power tubes there is no negative feedback from the transformer secondary to the phase invertor ,that will make a difference to the amount of noise .
Having an attenuator after the pi reduces the amount of feedback so thing probably wont work right

My advice , carefully remove all mods and return it to factory . .

I hear ya, it had this crazy noise with all the components and master volume at max (stock ish). The guy who owns this (my studio partner) likes the master volume mod so I’m gunna leave it up to him as what to do.

I got a million other projects right now so I don’t want to rip out the board and rebuild it PTP.

Thanks for the advice dudes.

 

[musipol]

So the resistor that burned up was the 2k7? I don't think I have ever seen one of those fail. Did you troubleshoot and fix it, or was it someone else? Is the replacement correct?

So when you say noise from the phase inv, you are measuring this with the input cap (.022u) shorted to gnd? What sort of noise, humming/buzzing, or white noise kind of thing?

Some good hi res pictures of the board, which include showing the repair and the PPIMV mod, might help, unless the problem is underneath.

 

[andYz00m]

Photos and video. The resistor that blew was R1 and it was replaced. 820R

 

[andYz00m]

Here’s some video of the noise it makes.

We found the tech who did the mods and probably going to send it to him. I’m done with this thing

 

[musipol]

Here's what I see. It looks like both 1st stage Rk's are 820, but maybe one appears to be only 82 ohms?? Maybe not. You might ohm it out. The treb cap was replaced with a mica cap?? The FB resistor mod looks ok if foil shield coax was used since it's running across the preamp section. The larr-marr MV looks ok except I can't tell where he's referencing the shield for that.

None of those mods "should" cause the noise you're getting. The noise sounds, to me, like you lost a gnd reference, somewhere. Sounds like sending it back is the right move at this point.

 

[andYz00m]

Here's what I see. It looks like both 1st stage Rk's are 820, but maybe one appears to be only 82 ohms?? Maybe not. You might ohm it out. The treb cap was replaced with a mica cap?? The FB resistor mod looks ok if foil shield coax was used since it's running across the preamp section. The larr-marr MV looks ok except I can't tell where he's referencing the shield for that.

None of those mods "should" cause the noise you're getting. The noise sounds, to me, like you lost a gnd reference, somewhere. Sounds like sending it back is the right move at this point.

Treb cap is apparently a common mod. Resistors are correct but you nailed the analysis, PPI mod "shouldn't" do this and there's likely a ground issue somewhere.

 

[abbey road d enfer]

there's likely a ground issue somewhere.

Not enough data.
What is the character of the noise? Hum, buzz, random, hiss?
Can you describe the master volume mod?

 

[Brian Roth]

Abbey, post #7 above has the Nasty Noise <g>. I am NOT a Marshall Mechanic....but it sounds like a VERY bad ground issue.

Bri

 

[abbey road d enfer]

Abbey, post #7 above has the Nasty Noise <g>.

Yes, my bad.

I am NOT a Marshall Mechanic....but it sounds like a VERY bad ground issue.

Indeed. More reason why I would want to see how the master volume mod has been inplemented.

 

[abbey road d enfer]

I hear ya, it had this crazy noise with all the components and master volume at max (stock ish).

Does the noise follow the master volume?

 

[heatwalk]

Master volume wired incorrect? Wonky DC ref / bias if incorrect. Caps needed after wipers if lug 1 to gnd

 

[andYz00m]

Does the noise follow the master volume?

Yeah the noise follows master volume as in it gets quieter when you turn it down. You can read the noise as AC voltage on the PI. The grids of the PI look clean with a scope and then the plates have the noise.

Master volume wired incorrect? Wonky DC ref / bias if no caps after the wipers

The amp was working well for years with this mod and it was done cleanly. Its possible something happened with the mod but it doesnt seem like its the source of the issue.

 

[abbey road d enfer]

Yeah the noise follows master volume as in it gets quieter when you turn it down. You can read the noise as AC voltage on the PI. The grids of the PI look clean with a scope and then the plates have the noise.

Is B+ clean at R22/25?

 

[andYz00m]

Yeah. 475VDC looking clean on the scope.

 

[abbey road d enfer]

B+ at R22/R25/R15 cannot be 475V.

 

[andYz00m]

B+ at R22/R25/R15 cannot be 475V.

You’re right of course. My mistake, I did not have a photo of that reading. Don’t have the amp here on the bench at the moment.

Will check it when I’m done with another amp.

 

[GermanTech]

Hey there, I have on the bench our Marshall plexi reissue from the studio. This has been modded with a PPI master volume and variable NFB. Amp was working and then magic smoke escaped.

The component that blew was R1 on the V1b cathode bypass.

Replaced tubes (output tubes were fried). Amp sounds correct and has the right tone again but...

Issue now is there is a huge amount of noise coming from PI. With all knobs up and no signal injecting its about 14VAC coming out of the PI plates and about .7VAC going into the grids. Im testing without power tubes installed.

Seems like there is a ground issue somewhere but I can find it and not sure where to look. Ive chop sticked around and reflowed joints where I can access them, etc. Tried bypassing the dumb effect loop card but that wasnt it either.

One of the challenges is these amps are half hand wired and half PCB with the soldering under the PCB and the components on top. Not ideal for servicing. So to service the PCB solder joints you need to remove one half of the leads to the sockets or pots to be able to get the PCB up. Very frustrating. Also the coupling caps are through hole tight to the PCB so no leads to solder on to for replacements.

This was the other thing I thought might be the issue is the coupling caps on the PI could be leaking but I dont know if that would explain the massive amount of noise. Also this amp is not that old. The PPI master volume mod is... after the PI so its not the issue, unless its the source of some weird grounding issue.

I hate working on amps with PCBs and I hate even more working on amps modded by someone else!

Any ideas welcome.

Scheme:
https://www.drtube.com/schematics/marshall/1959-01-60-02.pdf

Pls check the R31 (100R) it looks like it burned off. If yes , the hum balance of all heaters is wrong and you will have hum in the circuit. Maybe the reason why R1 and R31 failed .Could be a short between Heater of V1a/b and its cathode.

B+ at R22/R25/R15 cannot be 475V