Marshall 1923C ( DSL 50) blows HT fuse

[jvm]

Marshall 85th Special Edition 1923C (DSL 50). Blows HT fuse at turn on. I have lifted R63 to unhook preamp filters, fuse still blows. Lift R71, fuse does not blow. I have replaced C34, C35. Both 220k resistors are good. I have lifted R65 and R74, fuse still blows with R71 installed. What am I missing? Anybody have any ideas? http://www.drtube.com/schematics/mar...03)%20iss7.pdf

[Jazz P Bass]

What fuse are you blowing?
I don't see anything marked 'HT'.

[Mark Black]

Bad power tubes?

Also check screen resistor installation - I've seen some where the metal of the flattened "standoff" part of the lead comes really close to other traces.

[jvm]

Sorry, the HT fuse is not shown on the schematic. It is inline with W7 at D4 on one side of the HT winding of the power transformer. Mark, it blows fuse without tubes installed. I also checked the screen resistors. All appears to be ok.

[Mark Black]

Sorry to ask, but are you using slo-blo for the HT fuse (it should be).

Not shown on the referenced schematic, but is there a little ceramic disc capacitor (the infamous C46 on other schematics) present across of the screen resistors. If so they are known to short.

[jvm]

yes, I'm using slow blow fuses. Made that mistake once before! I don't see any cap across the screens.

[Enzo]

The HT fuse is not on THAT schematic, but it is on the page with the mains wiring.

If it blows fuses with the tubes removed, then disconnect the center tap of the output transformer primary from the B+. while you are at it, measure from pin 3 to ground from each power tube socket to look for shorts to ground. Check the transformer primary for shorts to ground or frame or secondary. Look closely at the power tube sockets both above and below the chassis, looking for signs of arcing.

[jvm]

OT center tap unhooked-blows
1K screens unhooked-blows
R63 unhooked-blows
Points to the second filter stage does it not? New caps, resistors test fine and I don't measure any shorts to ground. I know I am missing something. I just can't figure out what it is.

[Enzo]

You may never measure a short to ground, but that doesn't mean there is not one. Caps operate at 450v or whatever. Your hand meter uses maybe a volt. Caps that do not leak at one volt can leak like a screen door at 20v or 200v or whatever. And that can easily blow fuses.

If the problem is at the second filter node or further, then disconnect stuff. How many paths are there? If lifting R71 stops the fuse blows, then we have C34/35, we have two screen pins on tube sockets, and we have R63 to the later B+ nodes. R63 is 4.7k, so even if the low end of that is hard to ground, we are only going to draw about 100ma, so I doubt that is the route. SO disconnect C35. Still blows? If so, the cap was not likely the thing. If it now holds, then the caps are the issue. Not the cap? YOu have two tube sockets. Look top and bottom around pins 4 for signs of arcs. Is either pin 4 shorted to ground? or to pin 2? Or to pin 1? You have R65 and R74, the4 two screen resistors. You could remove one at a time to see if either affect this.

Thyen you have things that are not on the schematic. COpper traces on the board involving this portion of the B+ might be running right next to some other trace that is not involved, but might be at ground. And if somehow the two adjacent traces become shorted together, like by a blob of solder or a bit of wire, then fuses blow.

[jvm]

UPDATE- I was not giving the fuse enough time to blow. Fuse blows with R71 unhooked. Therefore, the problem must be in the FIRST filter stage not the second. I unhooked C39. Fuse held. I replaced C38, C39, R73, R75, D3,4,5,6, C40 and C41. Output tranny CT is unhooked, R71 is unhooked, R65 and 74 are unhooked. Still blows HT fuse after a few seconds.

[Mark Black]

So the fuse blows over some period of time when the 1st stage filter is hooked up - how long does it take?

Also, any chance you got a bad or mis-marked batch of fuses? And they are 500mA right?

[jvm]

Takes about two seconds for the fuse to blow. It's fairly quick but not instantly. I wondered the same thing about the fuses so went to the shack and bought 500mA slo-blow. They look exactly like what I have. The fuses I am using are marked WT500mA 250V. What I don't understand is that I have replaced every component that is still in circuit and still blows fuses. I have had to replace pc boards in DSL's before due to the bias drift issue. I wonder if this could be a faulty board. I doubt it but still wondering.

[jvm]

Customer called for an update. He apparently didn't like the fact that I was actually unhooking components in his amp as this would affect originality. ???? He wants to take it to a Marshall Authorized Service Center. I agreed that this would be the best course of action.

[Enzo]

Go ahead and ditch a problem customer. Don't let him scold you, how the hell else can we test parts without removing them from the circuit? Parallel paths through other components routinely confuse meter readings. It blows fuses, did he think none of the parts would be bad?

[jvm]

Quite honestly, Enzo, I'm not sure what this guy is thinking. Regardless, I am much better off without him.