JCM2000 OT question

[glebert]

I've got this JCM2000 TSL100 head that had very, very low output when I got it. In doing some troubleshooting I tested the OT and it seemed to have an internal short (tested with variac and LBL) I got a "deal" on a replacement off Reverb. I did the variac and LBL test and measured DCRs on primary and secondary and everything seemed good. I put it in and the output was higher but still way low for such a monster amp. I figured there was something else wrong with this box of spaghetti (crazy wiring harnesses) and set it aside for a year and a half. Pulled it back out to either fix it, sell it as-is, or set it on fire, and wondered if the OT was hooked up wrong, so I measured DCRs again but noticed that on the primary side I get 16-18 ohms from the center to the outside taps (which seems reasonable to me) but between the outside taps I only have 2.5 ohm, when I was expecting about 34. I doubt that I measured that resistance the first time. Is this what is (maybe) causing my low output? Did I buy a lemon?

If this OT is crap, the question is what to replace it with. If I am 0-2 with the original Dagnall transformers I don't know I want another, if I can even get one. I'm not willing to pay Mercury Magnetics money, Classictone's replacement is out of stock. This is a laydown style, so most of the other replacements aren't going to be drop-in.

Or maybe I should just set it on fire

[Jon Snell]

Unplug / disconnect the primary of the output transformer. W2, W3 & W4.
Check again.
Very unusual measurement. Is it original wiring? Anyone been in there playing?

[Pedro Vecino]

Original wiring:

[Helmholtz]

I tested the OT and it seemed to have an internal short (tested with variac and LBL) I got a "deal" on a replacement off Reverb. I did the variac and LBL test

Not clear how you tesed the OT with an LBL.
Shorted turns in an OT don't increase idle consumption, so won't show with an LBL between mains and amp.
The only OT faults that increase mains current draw are either a short/leakage between primary and core/endbell/chassis - or a short between primary and secondary (which is grounded).
That's easy to test with an Ohmmeter.

and measured DCRs on primary and secondary and everything seemed good. so I measured DCRs again but noticed that on the primary side I get 16-18 ohms from the center to the outside taps (which seems reasonable to me) but between the outside taps I only have 2.5 ohm, when I was expecting about 34.

Ohmmeters often give wrong readings when measuring the full primary, because of the high inductance involved.
To avoid measuring errors, short one of the secondaries while measuring the DCR of the primary.
A DCR of around 17R per side is typical with a 100W Marshall OT.

In case of doubt, use the neon flash OT test.

Would this Hammond transformer fit your amp?:
https://www.hammfg.com/files/parts/pdf/1750X.pdf

[glebert]

Not clear how you tesed the OT with an LBL.
Shorted turns in an OT don't increase idle consumption, so won't show with an LBL between mains and amp.
The only OT fault that increases mains current draw is a short/leakage between primary and core/endbell/chassis.That's easy to test with an Ohmmeter.




Ohmmeters often give wrong readings when measuring the full primary, because of the high inductance involved.
To avoid measuring errors, short one of the secondaries while measuring the DCR of the primary.
A DCR of around 17R per side is typical with a 100W Marshall OT.

In case of doubt, use the neon flash OT test.

Would this Hammond transformer fit your amp?:
https://www.hammfg.com/files/parts/pdf/1750X.pdf

Using Pedro's picture above Blue wire=terminal 2, Violet=terminal 4, White=terminal 6

To double check this I remeasured with a different meter, with secondary shorted, and shorted one side of the primary to the center tap (terminal 4 shorted to terminal 6) to eliminate the effect of high inductance. When doing this I get 2.5 ohm from center to the unshorted primary (terminal 4 to terminal 2). Transformer is completely disconnected from amp for these measurements.

I may be misremembering using the LBL and variac on this. I had two completely different amps with bad OTs at the time, it may be that I didn't test this one.

Thanks for the link to the Hammond 1750x. For some reason using I was only finding the 1750U which is a standup. I also found that Mojotone has a JCM900 laydown version, I think their transformers are made by Heyboer.

[glebert]

OK, I think I am figuring out what is going on here, at least a bit. I had the terminals screwed up for the DCR measurements (not the wiring to the amp itself though). I thought that terminals 2,4,6 on the transfomer were P, CT, P but it is really CT, P, P. There are no markings on the transfomer or the schematic about the transformer terminals, nor have I been able to find a datasheet for the Dagnall part. W3, W4, W5 are only marked on the board. So I am getting 16 ohms from 2-4, which is probably OK as a CT-to-leg measurement, but only 2 ohms from 2-6 CT-to-leg, obviously that is an issue. The 18 ohms I see from 4-6 is across the full primary (16+2).

I have not been able to find a Dagnall datasheet for this transformer, but the one for the Hammond 1750x looks like it is the same terminals. I have found evidence of the Dagnalls being the same terminal assignment. http://www.hammondmfg.com/pdf/EDB1750X.pdf

I think I am going to hack in different transformer to see if the amp works better. It won't be a perfect match but better than this one seems to be.

edit: I also noticed that Pedro's diagram is different than than the schematic I have. His has board terminals W2,W3,W4 hooked up to the transformer but the schematic I have is W3,W4,W5. My transformer was hooked up correctly according to the schematic I have, even though I had the terminals confused on the DCR measurement.

[glebert]

Had a discovery on this amp today. Seems as though the very weak output is due to a problem in the VPR or mute circuit. If I disconnect the cable that goes to those buttons I get good signal to the tube grids, otherwise it is super low. The mute button definitely killed the weak output, so it was doing something, but not clear exactly what is going on there. So the low output I have now is probably not related to the OT, but I guess I will have to replace it anyway since it appears 1/3 shorted.

Thanks to those who weighed in on my windmill tilting.

[Pedro Vecino]

In that circuit SW3 should be the VPR (virtual power reduction). In normal mode it bridges the 680K resistors and in reduction mode it leaves them online.
SW4 must be the mute. It bridges the EL34 grids (like a cross master volume set to zero).
CON19 is connected towards the line out circuit.