JCM 2000 DSL 50 Low output

[theholyweed]

Hey guys, got an issue with a dsl 50 that I have on my bench just now, it came in with a blown internal fuse on one of the output tubes, Replaced fuse, installed a full new set of valves and biased the amp, what im left with is a working an amp with a low output and a clipped sound, Tried plugging another preamp into the return jack so im assuming its an issue with power amp section, all voltage on the valve sockets checked out, all the big power resistors check out too... any ideas? or a starting point? any help would be very much appreciated!

[Jazz P Bass]

Check the phase inverter tube plate resistors for an open.

[TimmyP1955]

See if the amp has C46 installed - a 22p between one of the plates and ground. They used a 500V cap there and they tend to blow short-ish, in which case the amp works, but not very well.

[theholyweed]

C46 seems fine and not shorted, Voltages seem to check out too....

PI
1 - 286VDC
2 - 32VDC
3 - 52VDC
4 - 3.3VAC
5 - 3.3VAC
6 - 296VDC
7 - 29VDC
8 - 51.9VDC
9 - 3.3VAC

power tubes
1 - -
2 - 3.3VAC
3 - 510VDC
4 - 507VDC
5 - -43VDC
6 - -
7 - 3.3VAC
8 - -

[theholyweed]

Also PI Plate resistors seem fine

[The Dude]

Your plate voltage seems a bit high. Makes me wonder if the tubes are conducting. Have you checked bias to see if it will adjust within spec?

[g1]

Your plate voltage seems a bit high. Makes me wonder if the tubes are conducting. Have you checked bias to see if it will adjust within spec?

I was thinking the same about the high plate voltage. He said he checked the bias, but it uses test points that are dependent on the 1 ohm cathode resistors being good (R6 & R9). Perhaps they have drifted.
Also I'm wondering exactly which fuse blew.
Schematic attached.

[The Dude]

Oh, yep. I missed the part where the amp was biased. I'd still be curious as to what the bias measurements are for both sides. And, yes- it wouldn't hurt to check the cathode resistors. Knowing which fuse blew would indeed help. IMO, at any rate, there should be a reason why B+ is that high. Seems like a good clue.

[m1989jmp]

Hey guys, got an issue with a dsl 50 that I have on my bench just now, it came in with a blown internal fuse on one of the output tubes, Replaced fuse, installed a full new set of valves and biased the amp, what im left with is a working an amp with a low output and a clipped sound, Tried plugging another preamp into the return jack so im assuming its an issue with power amp section, all voltage on the valve sockets checked out, all the big power resistors check out too... any ideas? or a starting point? any help would be very much appreciated!

From what I've read, output transformers on these amps are a bit flimsy.

With the amp off and filter caps discharged, measure DC resistances from both output tube pins 3 to output transformer CT.

Also, check this thread too:

http://music-electronics-forum.com/t29029/

Is this the right schematic? I can't locate HT and mains fuse on this schematic, maybe because it's early

[theholyweed]

Hey guys, Thanks for all the replies, Turns out it was the OT, Swapped it out with a DSL 100 OT and the amp works perfectly...Customers not gonna be too happy to learn that he needs a new OT when he thought it just needed a re-valve..I measured the plate without the valves in which is why it looks a little high, the internal fuse that went was one of the 6.3a fuses for the heater on one of the output valves, also I confirmed the bias test points reading on the amp with a separate bias tester as well... Im just in the process of tracking down and ordering a new OT, im curious as to what was the main cause here though? the amp only a year or 2 old, could it be that he was using the wrong impedance? what would cause the heater fuse to blow? although I cant seem to find anything wrong that would be the underlying cause im a little worried about just chucking in a brand new OT for it to happen again!

[Leo_Gnardo]

im curious as to what was the main cause here though? the amp only a year or 2 old, could it be that he was using the wrong impedance? what would cause the heater fuse to blow? although I cant seem to find anything wrong that would be the underlying cause im a little worried about just chucking in a brand new OT for it to happen again!

Couple years ago I had a similar case with a JCM 900. The transformers in those look much like the ones in JCM2000 series, maybe they are the same, I dunno. In any case this particular 100W amp passed thru a couple owners, looked showroom new, I guess nobody was satisfied. As a distortion amp, it sure sounded terrific but . . . never got very loud. When I scoped it, turns out it delivered about a quarter of a watt at full crank. Uh-oh! Bad OT, maybe straight from the factory. A short in the speaker winding I expect, as the primary read normal resistance.

Bad from the factory isn't the only reason an OT could be defective in a 2000. There's a trick circuit in the speaker jacks. The ground/speaker negative has to pass thru a switch built into the 16 ohm jack in order to reach the 4/8 ohm jacks. If that switch becomes intermittent or opens up, that disconnects your 4/8 ohm speaker and can damage output tubes and/or OT. I've made it a habit when servicing 2000's for any reason, to wire a jumper across the 16 ohm output negative terminals then this condition can't happen. Marshall wanted to goof-proof the amp so nobody could run signal out of 16 and 4/8 output jacks at the same time but their "solution" has turned into a problem itself, easily fixed.

Heater fuses sometimes let go for seemingly no particular reason. They handle a lot of current, and the current inrush when the amp is switched on is a particular stress, it can be multiples of the steady state current when the tube is warmed up. Also, if the metal at the fuse ends and the metal of the fixtures that hold the fuse are not super clean and making good contact, they start to heat up which corrodes the metal further, causing more heat, until there's enough to melt the metal in the fuse element. I'm not in favor of heater fuses. You may jumper them or replace the fuses as you see fit.

[m1989jmp]

Hey guys, Thanks for all the replies, Turns out it was the OT, Swapped it out with a DSL 100 OT and the amp works perfectly...Customers not gonna be too happy to learn that he needs a new OT when he thought it just needed a re-valve..I measured the plate without the valves in which is why it looks a little high, the internal fuse that went was one of the 6.3a fuses for the heater on one of the output valves, also I confirmed the bias test points reading on the amp with a separate bias tester as well... Im just in the process of tracking down and ordering a new OT, im curious as to what was the main cause here though? the amp only a year or 2 old, could it be that he was using the wrong impedance? what would cause the heater fuse to blow? although I cant seem to find anything wrong that would be the underlying cause im a little worried about just chucking in a brand new OT for it to happen again!

Is the OT connected to the PCB via spade connectors?

You can look up online, there seems to be an issue with these connectors corroding/getting loose so the OT makes intermittent contact.

[g1]

Not sure how you wired up the replacement, so just in case:
If you wired the replacement OT direct to the load, you should double check the 2 other possibilities mentioned above, 16 ohm jack and spade connectors.
Or try the original bad OT wired directly if that's how you tried the replacement.