1973 Marshall Lead 50W up and running but no sound

[Pave Dog]

Hey,

I posted earlier about needing to replace a 1202-164 Power Transformer, I took the advice got a Heyboer and the install went great. Everything powered up, Tubes all glow, voltages were all correct, filter caps are good, but no sound, not even a hiss.

I figured it would be the HT fuse but that checked fine, I even checked the continuity of the fuse holder to make sure it was not bad but that is good too. Checked the speaker cable, and speaker too.

I am thinking since the original power transformer failed, maybe it is the OT, or Choke? I need to check the tubes, but I lent out the tester and won't have it back until early next week.

It has the Drake 784-139 OT, what would be the best way to test the OT and choke in the chassis or should I be looking elsewhere in the amp for the issue?

 

[Matthews Guitars]

Do you have a signal generator and an oscilloscope? With those it's pretty easy to trace the signal through every stage.

Don't forget to check tubes for bent pins and check tube sockets for bad contacts. I've seen just a worn out tube socket cause no audio output at all due to a socket contact that was so worn out it no longer was touching the tube pin.


Also check all the input jacks and all the speaker jacks.

 

[Spanngitter]

Erything powered up, Tubes all glow, voltages were all correct, filter caps are good, but no sound, not even a hiss.

Where's your Voltage chart? List all tubes/pins with resp voltages achieved, be aware that Heaters run on AC, not DC when measuring.
Any pictures of the new installed transformer?
Where did which wire end up?
Without this info everything is pure speculation and the majority of knowledgeable people here have issues with their crystal ball, it went dull due to overusage whilst attempting to answer such global questions....(SCNR)...

 

[Purgasound]

testing the PT is simple. Check the AC on the secondaries. Should have something above 300V across the high voltage taps and of course 6.3v on the heaters. There are only the two secondaries on a 50W I believe. The 100W models had their own bias tap. If there is an extra wire then it should read something under 100V.
The OT can be tested with a multimeter but I use low voltage from a Variac. There are multiple videos on youtube detailing the process.
Check continuity to ground throughout the amp too. Make sure something hasn't come undone. Definitely check the cathode to ground on the output tube sockets

 

[Pave Dog]

testing the PT is simple. Check the AC on the secondaries. Should have something above 300V across the high voltage taps and of course 6.3v on the heaters. There are only the two secondaries on a 50W I believe. The 100W models had their own bias tap. If there is an extra wire then it should read something under 100V.
The OT can be tested with a multimeter but I use low voltage from a Variac. There are multiple videos on youtube detailing the process.
Check continuity to ground throughout the amp too. Make sure something hasn't come undone. Definitely check the cathode to ground on the output tube sockets

Thanks, yes I checked all the leads from the new power transformer and all of them are producing correctly, I checked continuity to ground before I fired it up and then again when I had no sound. I checked the voltage across the pins of the socket all are within Marshall's specs. I do have a signal generator and scope will trace the signal, will look up the OT videos your talking about.

 

[AndyD]

Hi, with all of the mods you have in your amp, you might have a hell of a time working out the issue. To test the OT, measure the resistance between the OT centre tap and pin 3 of your output valves. You should have approx 40-50 ohms resistance. Beyond that, you could do with a schematic from the modder, or set about rewiring it to stock.......or pass it on to a tech.

 

[Pave Dog]

Where's your Voltage chart? List all tubes/pins with resp voltages achieved, be aware that Heaters run on AC, not DC when measuring.
Any pictures of the new installed transformer?
Where did which wire end up?
Without this info everything is pure speculation and the majority of knowledgeable people here have issues with their crystal ball, it went dull due to overusage whilst attempting to answer such global questions....(SCNR)

Everything matched the schematics and voltages from Marshall/Heyboer after the PT change. All wires went to the same point as the original 1202-164. This amp has sat dormant for over 10 years and I did not own it when the PT fried so it is possible that there are more issues down stream from the transformer, will have to break out the signal generator and scope.

 

[Pave Dog]

Hi, with all of the mods you have in your amp, you might have a hell of a time working out the issue. To test the OT, measure the resistance between the OT centre tap and pin 3 of your output valves. You should have approx 40-50 ohms resistance. Beyond that, you could do with a schematic from the modder, or set about rewiring it to stock.......or pass it on to a tech.

Andy, Yep exactly my thoughts with all the mods. I was able to get the info on the mods, now I just need to fully understand the signal chain. There is foot switch that even switches between high and low outputs which you adjust on the back of the amp. I am going to start with signal generator and scope today and go slow. Thanks for idea for testing the resistance, was thinking something similar, figured if it indicated open there was a fried winding.

 

[Pave Dog]

Do you have a signal generator and an oscilloscope? With those it's pretty easy to trace the signal through every stage.

Don't forget to check tubes for bent pins and check tube sockets for bad contacts. I've seen just a worn out tube socket cause no audio output at all due to a socket contact that was so worn out it no longer was touching the tube pin.


Also check all the input jacks and all the speaker jacks.

Going to take your advice and use my scope and signal generator, thanks. All jacks were checked and confirmed good. Will go back through and check the sockets.

 

[Pete Farrington]

Safety first! It looks like the mains switch is on the mains live feed, whilst the mains fuse is on the neutral feed.
That’s poor practice, should be live in to mains fuse, then to switch then PT primary.
Neutral direct to the other end of the primary, preferably via a 2nd pole on the mains switch.
Consider that if there’s a live to chassis earth short, a fuse in the neutral side won’t blow.

Yes older North American amps used a fuse on one side and switch on the other, but that was in the bad old days of 2 pin, non earthed, unpolarised mains connectors, where it was complete chance which side was which, and a short to earth was unlikely anyway, as so little was earthed.

 

[Pave Dog]

Safety first! It looks like the mains switch is on the mains live feed, whilst the mains fuse is on the neutral feed.
That’s poor practice, should be live in to mains fuse, then to switch then PT primary.
Neutral direct to the other end of the primary, preferably via a 2nd pole on the mains switch.
Consider that if there’s a live to chassis earth short, a fuse in the neutral side won’t blow.

Yes older North American amps used a fuse on one side and switch on the other, but that was in the bad old days of 2 pin, non earthed, unpolarised mains connectors, where it was complete chance which side was which, and a short to earth was unlikely anyway, as so little was earthed.

Thanks, that is an easy fix and your right should have been done when it was upgraded to grounded power cord.

 

[Pave Dog]

I haven't had a chance to signal trace yet, but after work I did go back with the LCR and look at the voltages. I did remember if the tubes are subjected to extreme negative voltages they will shutdown completely. Based on what Spangitter recommended here is what I have by pin for both EL34s

Pin V4 V5
1. 0 , 0
2. 3.14, 3.15
3. 467, 466.2
4. 465.3, 464.5
5. -366.7, -365.1
6. 465, 465
7. 2.55, 2.56
8. 0, 0

This is on a variac @ 120V.

 

[Chris-in-LA]

Yeah, pin 5 should be in the neighborhood of -43 or something. Sounds like something is wrong in the bias circuit. Your OT might be just fine.

 

[Purgasound]

Holy smokes there's some wild stuff going on in there. First thing that comes to mind when seeing those gigantic resistors on the side is a dummy load. Those resistors mounted to the side of the chassis are likely 25W each. The only reason someone would put something that big in there is to make a dummy load, possibly for silent recording. Although that's not a good method of installation. If used as a dummy load and without a proper heatsink those resistors will get extremely hot and the chassis is not a sufficient enough sink.
Can you upload more pictures of the internals of the amplifier? Also those bias caps are ancient. They need to be replaced. Also looks like your filter caps are original too. I don't know the methodology used to verify that they were good. If I work on an amp with electrolytic caps older than me I won't even troubleshoot it until they're replaced. It's a complete waste of time in my opinion.
The bias circuit is extremely important and if you've got -366 volts on those pins that's really out of wack. Change those 10uF 160V caps in the bias circuit asap. I would also take these readings with the tubes out until you get this thing reconditioned.

 

[paul-e-mann]

Hey,

I posted earlier about needing to replace a 1202-164 Power Transformer, I took the advice got a Heyboer and the install went great. Everything powered up, Tubes all glow, voltages were all correct, filter caps are good, but no sound, not even a hiss.

I figured it would be the HT fuse but that checked fine, I even checked the continuity of the fuse holder to make sure it was not bad but that is good too. Checked the speaker cable, and speaker too.

I am thinking since the original power transformer failed, maybe it is the OT, or Choke? I need to check the tubes, but I lent out the tester and won't have it back until early next week.

It has the Drake 784-139 OT, what would be the best way to test the OT and choke in the chassis or should I be looking elsewhere in the amp for the issue?

Im no tech but I found the Rob Robinette startup procedures very helpful in troubleshooting a tube amp to help isolate where the problem is: https://robrobinette.com/Tube_Amp_Startup.htm

 

[Pave Dog]

Yeah, pin 5 should be in the neighborhood of -43 or something. Sounds like something is wrong in the bias circuit. Your OT might be just fine.

Chris, Yep I agree. I was able to check the OT and it seems to be fine. I am hoping I can get some work done it on this weekend.

 

[Pave Dog]

Holy smokes there's some wild stuff going on in there. First thing that comes to mind when seeing those gigantic resistors on the side is a dummy load. Those resistors mounted to the side of the chassis are likely 25W each. The only reason someone would put something that big in there is to make a dummy load, possibly for silent recording. Although that's not a good method of installation. If used as a dummy load and without a proper heatsink those resistors will get extremely hot and the chassis is not a sufficient enough sink.
Can you upload more pictures of the internals of the amplifier? Also those bias caps are ancient. They need to be replaced. Also looks like your filter caps are original too. I don't know the methodology used to verify that they were good. If I work on an amp with electrolytic caps older than me I won't even troubleshoot it until they're replaced. It's a complete waste of time in my opinion.
The bias circuit is extremely important and if you've got -366 volts on those pins that's really out of wack. Change those 10uF 160V caps in the bias circuit asap. I would also take these readings with the tubes out until you get this thing reconditioned.

Yep, it is a bit of a fixer uper....... I am considering returning it back to stock, but before I do anything I need to work on the bias circuit. I have an ESR meter coming this weekend, but I did a reform of the main filter caps and checked the uf, voltage, and as I was reforming monitored the current and I believe they are all good. Will need to go through the entire board but with all the mods on the board it makes lifting the PCB a huge pain in the ass. I really appreciate the advice, here are the pics.

 

[Pave Dog]

Im no tech but I found the Rob Robinette startup procedures very helpful in troubleshooting a tube amp to help isolate where the problem is: https://robrobinette.com/Tube_Amp_Startup.htm

Hey Paul, Thanks, I actually used these procedures during start up, I agree good resource.

 

[Gunner64]

I would check the components in the bias circuit, the dropping resistor, and change the bias caps. It may be as easy as that to get it running, and see what you've got.

But I have to agree with you. I would take it back to stock, and start over. That's a lot of stuff in there..lol

Is It possible the thing never worked after the mods and the previous owner gave up?

 

[Pave Dog]

I would check the components in the bias circuit, the dropping resistor, and change the bias caps. It may be as easy as that to get it running, and see what you've got.

But I have to agree with you. I would take it back to stock, and start over. That's a lot of stuff in there..lol

Is It possible the thing never worked after the mods and the previous owner gave up?

According to him he loved the sound, I agree this has the most mods I have seen in a Marshall. Interesting how there are two output controls on the back so you can switch between them? I know he was pursuing EVH's early tone and it was modified with this in mind.