Peavey PT secondary fuse blown causes?

[erkalump01]

I have an older (2012) Peavey Valveking VK112 (50W combo) which suddenly stopped working while playing at low volume. Just totally went silent. No warnings, weird noises or smoke.

I removed the chassis and quickly found that one of the three secondary fuses on the PT had blown. Fuse F203. This PT has three secondary windings. The one that has the blown fuse generates 18VAC into a diode bridge rectifier which serves the tube heaters, the +14V, and the pilot (on LED). The fuse is soldered in place (no holder) making sourcing a new one and replacing it a bit more complicated.

All three secondary windings still output voltage, just the one has the blown fuse between the secondary and the rectifier.

This concludes my amplifier troubleshooting capability. I am hoping the group can give me some ideas of where to look for the fault which caused the fuse to blow. I bought this amp used and as far as I can tell by looking inside, it doesn't look like anyone was messing around in there.

El34world has the full schematic posted for the 100W 212 version. The PT section is the same as the VK112.

For a cheap amp, it's pretty good, and I've grown fond of it. Hoping to resurrect it without too much cash outlay.

Thanks in advance for any advice you can give.

-erkalump01

 

[corliss1]

Does a new fuse blow? Sometimes they really do just blow. If a second one blows, it's likely related to a tube issue.

 

[Jon Snell]

I would say the fuse blew because of excessive load ... probably one of the 1N4003 diodes has shorted or something further down the line like one or more 1000uF capacitors but, that is where I would start.
It is unlikely the heaters themselves are short circuit.

 

[erkalump01]

Does a new fuse blow? Sometimes they really do just blow. If a second one blows, it's likely related to a tube issue.

The fuse is soldered to the board like a resistor. Any idea where to get fuses like that? I could do a wired fuse holder and use replaceable fuses in it, I suppose.

 

[erkalump01]

I would say the fuse blew because of excessive load ... probably one of the 1N4003 diodes has shorted or something further down the line like one or more 1000uF capacitors but, that is where I would start.
It is unlikely the heaters themselves are short circuit.

Thanks!

 

[corliss1]

Most electronics supply places (Mouser, Digikey) will have that fuse.

 

[Mowgli]

First, find out for certain which fuse is supposed to be used along with its current rating. If an incorrect fuse was used previously, check its current rating. A higher or equal fuse current rating usually means excessive current is being drawn through that circuit.

Second, most fuses blow because of shorts if I recall Jack Darr’s book correctly. Here’s where the utility of a variable isolation power supply used along with a lamp current limiter is useful diagnostically. A short usually draws enough current at low values of AC voltage to light a high watt bulb in series with the AC power cable such that you can readily diagnose a short.

Hope this helps.