Uneven plate voltages? - now a Marshall troubleshooting thread
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Re: Uneven plate voltages? - now a Marshall troubleshooting thread
Here are the tube sockets:
I still need some help here. I just replaced the cap cans and the bias supply electrolytic caps as well as the bias resistor which was incorrect from the factory.
EDIT: Disregard previous comments. I was able to get the amp to turn on now without the current limiter in place. Bias is turned down all the way until tomorrow when I have tine to adjust it but for now the bass seems to be back as well as some volume. I’ll report back with any other problems.
I still need to take care of replacing the rest of the electrolytic caps on the preamp board. I am going through this amp and giving it its first servicing in 30 years.
The schematic for it is the early 2100, not the SL-X. The only schematics I’ve found are very blurry but they are for circuit boards JMP50C and JMP51C.
I still need some help here. I just replaced the cap cans and the bias supply electrolytic caps as well as the bias resistor which was incorrect from the factory.
EDIT: Disregard previous comments. I was able to get the amp to turn on now without the current limiter in place. Bias is turned down all the way until tomorrow when I have tine to adjust it but for now the bass seems to be back as well as some volume. I’ll report back with any other problems.
I still need to take care of replacing the rest of the electrolytic caps on the preamp board. I am going through this amp and giving it its first servicing in 30 years.
The schematic for it is the early 2100, not the SL-X. The only schematics I’ve found are very blurry but they are for circuit boards JMP50C and JMP51C.
Last edited by Yoda on Sat Mar 13, 2021 6:59 am, edited 6 times in total.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
This is the schematic for my amp, the JCM 900 2100 (non SL-X):
https://el34world.com/charts/Schematics ... x_100w.pdf
https://el34world.com/charts/Schematics ... x_100w.pdf
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
R12 on preamp board has a black spot. Burned, or just dirty?
I build and repair tube amps. http://amps.monkeymatic.com
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
It’s a little bit of black silicone from the nearby caps.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
I also found the new F&T filter caps to be quite underwhelming in size to the original LCR caps they replaced. The F&Ts look like Legos.
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Re: Uneven plate voltages? - now a Marshall troubleshooting thread
so at least some result!
Modern lytics are small and efficient. Be thankful they didn't get bigger in 30 years.
Modern lytics are small and efficient. Be thankful they didn't get bigger in 30 years.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
I got the amp turned on, both power and standby switches turned on, without the current limiter. Then it blew the fuse after warming up for a minute. Still scratching my head on this one.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
The OT might be shorted and drawing excessive current.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
I can't think how any type of short could cause these symptoms, which winding do you think is shorted to what?
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Re: Uneven plate voltages? - now a Marshall troubleshooting thread
It actually can be the PT as well.. still dont get how a piece of wire shows two voltages at opposite ends though..
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
Probing power valve anodes, especially true pentodes, can invoke oscillation, thereby making a nonsense of the meter reading.Reeltarded wrote: ↑Sat Mar 13, 2021 7:15 pm It actually can be the PT as well.. still dont get how a piece of wire shows two voltages at opposite ends though..
If it really needs doing, best remove the LTP valve first.
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Re: Uneven plate voltages? - now a Marshall troubleshooting thread
A short in the primary that measures fine with multimeter but fails under high voltage. Wild guess, but I want in on the lotto.
I build and repair tube amps. http://amps.monkeymatic.com
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
Hey, that’s my special ’bete noir’, go find your own
But seriously, as it happened whilst idling, that seems a really long shot
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Re: Uneven plate voltages? - now a Marshall troubleshooting thread
I need to pick up some new fuses before I revisit the issue of unequal voltages on the plates, that happened before I replaced some parts.
If there’s a short in one of the transformers is it still possible for the amp to make sound? It was making sound if hooked up to the current limiter.
If there’s a short in one of the transformers is it still possible for the amp to make sound? It was making sound if hooked up to the current limiter.
Re: Uneven plate voltages? - now a Marshall troubleshooting thread
New fuse is in. I powered up the amp with no current limiter and it turned on again. This time I measured 400-402v on all four power tube plates.
Now I have my two bias probes hooked up, I have the bias trimpot set in the middle of its range. Upon flipping the standby switch I watch the bias climb to around 22-24mA on one meter and roughly stabilize while the other meter continued to climb slowly. I shut the amp off when it got to 50mA on only one side before it blew a fuse again. Something obviously still not right here. I switched the probes to the other output tubes (still one meter per push-pull pair) and the fuse blew when I flipped the power switch without the current limiter.
Now I have my two bias probes hooked up, I have the bias trimpot set in the middle of its range. Upon flipping the standby switch I watch the bias climb to around 22-24mA on one meter and roughly stabilize while the other meter continued to climb slowly. I shut the amp off when it got to 50mA on only one side before it blew a fuse again. Something obviously still not right here. I switched the probes to the other output tubes (still one meter per push-pull pair) and the fuse blew when I flipped the power switch without the current limiter.