Another Marshall SLX issue - no heater voltage on one socket?

ivan H said:
Ok, firstly I'll explain why you need to use "timed" (= Slo-Blo) fuses here;
When cold, the tube filaments have notably higher resistance than they do when hot. So at initial power-up the filament current draw surges briefly. Ever seen that nice bright flash the old production Mullard 12AX7's exhibit at power-up, this is what causes it.
So with that out of the way, I'd best go back & read through the thread properly so I can give correct advice.
So, here i the JCM900 SLX2500 Power Supply Unit's schematic
View attachment 86295
From this, it appears that the power amp tubes (PI included) are AC heater'd, while the pre-amp tubes are DC heater'd. Further, it shows the power amp filament supply is applied to the PI (V4) tube socket & from there connects to the power tube sockets (V5 then V6). Being that I'm not familiar with "post JCM800" amps, I checked a few 2500 SLX gutshots to confirm this, & here's a gutshot where this can be seen.
View attachment 86297
So we can see the red & black filament leads coming from the 'board to the PI (V4), from there on to the first power tube socket (V5) & then the next power tube socket (V6), in the normal "twisted leads" daisy chain fashion.
So V5 is the socket reading 1VAC, while the V4 PI socket & the V6 socket have 7VAC(?) & their tubes light up.
The thing is, for the filament current to get from V4 to V6, it has to come via the V5 socket, as their filament leads are linked there (at V5).
So there's a few possibilities as to what the problem may be, but, you say you previously created a short that blew the filament fuses,,, can you take a close look at the V5 socket (well lit & possibly using something like a jewelers loupe or similar magnifier). Look say, between pins 2 & 1, or pins 7 & 8, looking for "carbon tracking" between them. I'll wait to hear the results. Cheers
Click to expand...
Cool info! I saw the same pic you posted a couple days ago, while I was trying to find out what all the fuss is about regarding "BR3". WTF is BR3??? It's a rectifier for the amp's heater circuit that converts the AC filament voltage to DC for the first 3 preamp tubes. Apparently the rectifier was speced with a 1 amp rating. Three 12AX7's will draw about 900 mA of current. Initial power surge at startup was too much for a rectifier rated for 1 amp. Part of the fix is to replace the puny stock rectifier with a more robust rectifier. Most pics of this mod usually shows a new rectifier occupying the same location on the board where the old rectifier was. But going back to the Marshall Amp Forum where I saw the above pic, there's another pic of the same amp that shows the new rectifier was chassis mounted! Which I think is a good method to do this mod, especially if the board and its traces got toasted and crusty:
20160622_212126 copy.jpg
 
AndyK said:
I checked both sides of the sockets, they look brand new. No burning or damage that I can see.
Click to expand...
While in bad cases burning etc might be visible, carbon tracking can also resemble a light, barely visible pencil line drawn between the pins.
That said, Mr AMS is on the money, so
1) reflow the solder on V5 pins 2 & 7 (tube removed from socket).
2) replace the V5 socket. Cheers
 
syscokid said:
Cool info! I saw the same pic you posted a couple days ago, while I was trying to find out what all the fuss is about regarding "BR3". WTF is BR3??? It's a rectifier for the amp's heater circuit that converts the AC filament voltage to DC for the first 3 preamp tubes. Apparently the rectifier was speced with a 1 amp rating. Three 12AX7's will draw about 900 mA of current. Initial power surge at startup was too much for a rectifier rated for 1 amp. Part of the fix is to replace the puny stock rectifier with a more robust rectifier. Most pics of this mod usually shows a new rectifier occupying the same location on the board where the old rectifier was. But going back to the Marshall Amp Forum where I saw the above pic, there's another pic of the same amp that shows the new rectifier was chassis mounted! Which I think is a good method to do this mod, especially if the board and its traces got tasted and crusty:
View attachment 86359
Click to expand...
Yep! I'm sure I got a picture of this amp floating back over there too over this same issue somewhere lol. Those original rectifiers were abysmally small and mounted flush against the board. Most toasted the the boards as a result and damaged the traces but others were way luckier and just burned out the rectifier itself.

Upgrading to a bigger rectifier fixed this and like the photo you got; a lot chassis mounted it. I actually mounted mine with the leads left about 1 inch above the board to create an air gap and a bit of heat dissipation since the board and traces were uncharred or damaged.

Another incidental victim in this was the C8 capacitor which is located directly ahead of the BR3 in the heater circuit. It's the one labeled 10,000uF 15w. They would swell up like a watermelon and caused some charring of their own. Mine was one of these too
 
Good news and bad news.
I replaced the fuses with the proper ones, and now all tubes are lighting up nicely.
Bad news is the bias is all wacky. Only showing 13ma, and turning the trimmer pot does nothing. Yet, with no tubes, I see -41 to -48 v at pin 5 on both sockets.
More bad news, now that V5 tube is dark again, and so are all the preamp tubes, and the fuses are fine.
Off to a tech it will go!
Worst Marshall design ever! My '98 DSL was more reliable, and didn't burn up traces on the board.
 
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AndyK said:
Good news and bad news.
I replaced the fuses with the proper ones, and now all tubes are lighting up nicely.
Bad news is the bias is all wacky. Only showing 13ma, and turning the trimmer pot does nothing. Yet, with no tubes, I see -41 to -48 v at pin 5 on both sockets.
More bad news, now that V5 tube is dark again, and so are all the preamp tubes, and the fuses are fine.
Off to a tech it will go!
Worst Marshall design ever! My '98 DSL was more reliable, and didn't burn up traces on the board.
Click to expand...
Do you want to do some tests?

The DC bias voltage on pin 5 of output tube socket sounds correct...
But I think you need to test the screen resistors and some other stuff.

DSL is not very reliable, has it's own issues with biasing.
 
syscokid said:
Cool info! I saw the same pic you posted a couple days ago, while I was trying to find out what all the fuss is about regarding "BR3". WTF is BR3??? It's a rectifier for the amp's heater circuit that converts the AC filament voltage to DC for the first 3 preamp tubes. Apparently the rectifier was speced with a 1 amp rating. Three 12AX7's will draw about 900 mA of current. Initial power surge at startup was too much for a rectifier rated for 1 amp. Part of the fix is to replace the puny stock rectifier with a more robust rectifier. Most pics of this mod usually shows a new rectifier occupying the same location on the board where the old rectifier was. But going back to the Marshall Amp Forum where I saw the above pic, there's another pic of the same amp that shows the new rectifier was chassis mounted! Which I think is a good method to do this mod, especially if the board and its traces got toasted and crusty:
View attachment 86359
Click to expand...
BR3
the actual problem is the filter caps for the DC filament voltage.
The filter caps get old, short out, and BR3 overheats from too much load current.

BR3 does not need to be upgraded, or mounted externally.
The stock rectifier has no problem.
The filter caps should be changed and that will solve the rectifier from overheating.

I see lots people try to modify BR 3 for no reason; and it's pointless.
 
Amp Mad Scientist said:
BR3
the actual problem is the filter caps for the DC filament voltage.
The filter caps get old, short out, and BR3 overheats from too much load current.

BR3 does not need to be upgraded, or mounted externally.
The stock rectifier has no problem.
The filter caps should be changed and that will solve the rectifier from overheating.

I see lots people try to modify BR 3 for no reason; and it's pointless.
Click to expand...
Is it a chicken or the egg thing? I've read how closely spec'd the BR3 is and that it can't handle spikes over it's value - so it fails. But if the caps are replaced first, the BR3 issue wouldn't happen?
 
AndyK said:
Is it a chicken or the egg thing? I've read how closely spec'd the BR3 is and that it can't handle spikes over it's value - so it fails. But if the caps are replaced first, the BR3 issue wouldn't happen?
Click to expand...
The people who tell you BR3 needs to be modified:
do not realize why it overheats.
BR3 will last if the filter caps are good.
"can't handle spikes..."
Then how did it keep working for the previous 25 years?
It has nothing to do with "spikes,"
everything to do with underrated filter capacitors....they get old and short out.

Replace the filament filter caps with a higher voltage rating, higher current, higher temperature rating, longer life rating.
10,000 hour life rating minimum, 105C temp rating minimum, 16 volt (not 6 volt)!

Replace the filter caps for the plus/minus 16 volt low voltage supplies also.

The factory used under rated caps.

We can make these amps sound quite killer when the caps are good...
 
Amp Mad Scientist said:
The people who tell you BR3 needs to be modified:
do not realize why it overheats.
BR3 will last if the filter caps are good.
"can't handle spikes..."
Then how did it keep working for the previous 25 years?
It has nothing to do with "spikes,"
everything to do with underrated filter capacitors....they get old and short out.

Replace the filament filter caps with a higher voltage rating, higher current, higher temperature rating, longer life rating.
10,000 hour life rating minimum, 105C temp rating minimum, 16 volt (not 6 volt)!

Replace the filter caps for the plus/minus 16 volt low voltage supplies also.

The factory used under rated caps.

We can make these amps sound quite killer when the caps are good...
Click to expand...
AndyK said:
Is it a chicken or the egg thing? I've read how closely spec'd the BR3 is and that it can't handle spikes over it's value - so it fails. But if the caps are replaced first, the BR3 issue wouldn't happen?
Click to expand...
This info would be very interesting to discuss with Andy Fuchs... :hmmm::hmmm:
 
syscokid said:
This info would be very interesting to discuss with Andy Fuchs... :hmmm::hmmm:
Click to expand...
I see people racing to modify what they don't understand....

The rectifier (BR3) gets hot for a reason...what is the real reason?
Why does the original part (un modified) work for 25 years with no problem?
Do you really think Marshall made a mistake in the design?

I think the fact is:
after replacing the filter caps, original BR3 is usually still good.
But circuit tracks have been over heated due to shorted filter capacitors...
 
Amp Mad Scientist said:
But circuit tracks have been over heated due to shorted filter capacitors...
Click to expand...
Are you referring to the C8 10,000 uF/15 vdc filter capacitor? You don't think that the BR3 mini rectifier can heat up enough to affect the circuit tracks, even if the stock filament rectifier fails on its own.

Do you know if Marshall ever offered a service bulletin for what seems to be a common with these JCM900's?
 
syscokid said:
Are you referring to the C8 10,000 uF/15 vdc filter capacitor? You don't think that the BR3 mini rectifier can heat up enough to affect the circuit tracks, even if the stock filament rectifier fails on its own.

Do you know if Marshall ever offered a service bulletin for what seems to be a common with these JCM900's?
Click to expand...
They damn well oughta lol it was a common issue in this amp like the bias drift was from the 100w JCM2000 DSL/TSL amps ( 50watters SOMEHOW dodged this and rarely caught it, yet to this day I dunno why or how).

Marshall TRIED to remedy the JCM 2000s with that Revision #20 board but Ive heard even that wasnt always a safe bet and often it came down to a better potentiometer replacement ( @Amp Mad Scientist knows this issue well and even helped originate the solution). But with the SLXs, specifically the 50watters, that damn BR3/C8 failure happened 10:1 against even the 100watters
 
Got the amp back from Fuchs! He did the following:

Re-solder heater wires to tube socket due to cracked solder joint(s)
Re-solder and clean impedance switch (it was cutting in and out)
Re-bias

The amp works perfectly now! I guess if I had better analytic/testing skills, I could have done all this for free, but it was worth it to have it gone over and fixed by a pro for peace of mind.

Glad to know it WASN'T one of my repairs that caused the issues!
 
syscokid said:
Are you referring to the C8 10,000 uF/15 vdc filter capacitor? You don't think that the BR3 mini rectifier can heat up enough to affect the circuit tracks, even if the stock filament rectifier fails on its own.

Do you know if Marshall ever offered a service bulletin for what seems to be a common with these JCM900's?
Click to expand...
The problems crop up in older units.
No service bulletins as far as I know....

Cooking the circuit tracks from excess heat is a really common problem in PCB amplifiers.
It happens over many years usually , but not right away.
When it happens, the tracks need to be repaired.

The amp plays for 25 years until the filter caps start to go bad.
Then the rectifier starts to run hot...then the circuit tracks are also getting hot.
Eventually the circuit tracks are damaged from the excess heat.

But it worked for 25 years because:
The rectifier was properly sized to start with.
The circuit tracks were properly sized to start with.
The filter caps were never designed to last more than 10 years.
 
Amp Mad Scientist said:
I see people racing to modify what they don't understand....

The rectifier (BR3) gets hot for a reason...what is the real reason?
Why does the original part (un modified) work for 25 years with no problem?
Do you really think Marshall made a mistake in the design?

I think the fact is:
after replacing the filter caps, original BR3 is usually still good.
But circuit tracks have been over heated due to shorted filter capacitors...
Click to expand...
As I previously said, I'm unfamiliar with these amps, but this here makes perfect sense. As electrolytic capacitors age, the electrolyte dries out, ESR, leakage current etc goes up, so that bridge rectifier has to work harder. Spikes aren't the problem, it's a 1A rectifier so has been surge tested at about twice that. It can safely supply 1A, so supplying 900mA for the filaments isn't a problem. The extra current continuously drawn by an old, out of spec electrolytic capacitor would be a problem. Cheers
 
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