Marshall JCM2000 DSL Superlead 100 DSL - PWR Transformer Shorted?

[Jecarroll7]

Hope everyone is off to a nice start on their 2014.

Have a 100w head that came in with one of the 5W 1K resistors shorted & cracked.

The HT fuse is blown (but not the 4A mains - unless it was replaced before I got it)

My question is what should these power transformers read at the windings. I've got about 14 ohms at the heater winding which doesn't sound bad, but the HT winding (disconnected) is showing 0.2, and the two primaries (connected, switch off) are showing about 1.4 ohms each. Seems fried to me, but I'm not familiar with this transformer, don't have a reference and hate to take something for granted.

This is a first issue JCM2-60-00.DGM (ISS-1) amp with the original Dagnall D2105 120V tranformer in it.

Appreciate your help.

[Jazz P Bass]

The power transformer readings sound about right.

The 1K 5 watter probably blew from a bad tube.

And make sure to check that funky ass C46 for a short.

[Jecarroll7]

Hey Jazz,

Oh yeah, C46 is toast. Tubes are all good though.

Just want to confirm that 0.2 ohms is a good reading for the HT secondary winding? Man that seems low.

[Mark Black]

That does seem low, and the heater secondary seems high at 14 ohms. Any chance you got them mixed? Or somebody pre-mixed them for you in a previous repair attempt?

[J M Fahey]

Why do you think the PT is fried?
What makes you suspect that?

[nevetslab]

Just following this thread. I have a JCM 900 on the bench at the moment, and it DOES sound like you've confused the readings on the HT winding with the Heater winding. 14 ohms is in the right ballpark for the DCR of the HT winding, and less than 0.2 ohms for the Heater winding. Good call by Jazz P Bass on C46. They are prone to fail, and make recovery from the burnt PCB in that area messy, when they flame out. I assume the cracked 1k 5W screen resistor was on that tube (V8)?

While you're in there, check for solder fractures on the little Bias Pot board. When they have any discontinuity, you have problems. Also check the tube socket pins. Where I ALWAY find solder fractues on JCM2000's is on the rear panel PCB. Output jacks, Impedance Switch, etc. That board is being supported by the solder connections of the jacks, which ALWAYS leads to fractures. Better also check the AC Mains PCB, as the fuse holders' solder joints are prone to the same fate. De-solder and then Re-solder where you find the fractures.

Rare to find a shorted power transformer.