CRATE V18

[livewire]

This amp came into shop, would not power on. Found D103 shorted. Replaced it, and the amp came on, but when you turn stby on it went out right away. I found that D103 had once again shorted. In following this back, I found an arc around one of the output tube sockets, I took care of that, then went back and replaced D103, and 105 again. I also disconnected the yellow/orange wires which are the high voltage from the supply also. Same scenario. Has anyone played with one of these? Some of these switchmodes can be a bear, I have done tons of them , but sometimes you get a real dog. Many thanks..

[The Dude]

http://www.blueguitar.org/new/schem/...rawings_v2.pdf

D103 is on the secondary of the supply. I would think then that the primary side of the switch mode is working. Look for shorts or near shorts on the 320V line and don't power up until you don't measure any shorts there. From what you say, you've disconnected that line from the amp? If so, that leaves C106, C108, and C119.

[Enzo]

"Took care of" the arc'ed socket? Or replaced it. Once something arcs, it is false economy to "fix" it.

But your issue seems to be on the SMPS itself? Still blows diodes with the wires to the tube circuit disconnected? Then I am with the Dude, look at those caps.

[livewire]

Well Enzo, the arc was not from the socket itself, it was on the circuit board around it. The caps, I have checked. Tomorrow I will get back on this, and just replace them, yes. I did unhook those wires. which led me back to the SMPS . Thanks for the replies.

[Enzo]

Not to dwell on it, but I am a dwell kind of guy.

By checking those caps, did you put full voltage across each? meter may not read them as shorted, and an ESR might read them as fine, but a cap that meters well at the couple of volts a hand tool puts out may leak like a sieve at 200v.

So if we are popping fuses with the SMPS only, I might replace or at least remove those caps just as a test. Just my guts, but I;d suspect the cap across the first shorted rectifier. A shorted rectifier puts AC right across the cap, and caps don't like that.

If the wires are off the output of that supply, and the diode is before the loading, there are not many places left other than the caps. I assume the SMPS board has copper to and bottom, I haven;t seen one in a while. Are there layers within? I am guessing at some possibility that an arc between layers could occur on the SMPS. The inductor doesn;t weem like a place to short, but we could eliminate it simply.


And as a service hint, that HV supply is really two in series. They used the same board with three of those in series for the 50 watt amps. That explains the gap on the drawing between the two, where R111 would have gone. We ought to be able to run those supplies independently for testing. You wouldn;'t likely have R111 on th board, but There is probably a short wire jumper in its place or in place of C107. Remove that jumper and we have a couple of lower voltage supplies. I tend to think we don;t need to go that far, but if we do, there it is.

[livewire]

Thanks Enzo..