Jsx212 blowing preamp fuse

This forum is for talking about all kinds of Peavey guitar amplifiers.
Post Reply
DEC
New Member
Posts: 1
Joined: Wed Oct 20, 2021 4:10 pm

Jsx212 blowing preamp fuse

Post by DEC » Wed Oct 20, 2021 4:22 pm

Hello, my 1st post.
My JSX212 is blowing the F2A fuse, located under the chassis.
It is on the preamp board,, the lower fast blow next to the connector with 2 red and 1 red/yellow wires.
I removed all tubes, think bad preamp tube.
No luck, still blows.
Speaker is connected, all volumes at zero.
Short hum, then it fails.
Any ideas, direction, I can diagnose if lead to where to look.
I have done some research, but cannot find anything about this.
Thank you for any help :)

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: Jsx212 blowing preamp fuse

Post by Enzo » Wed Oct 20, 2021 9:22 pm

High voltage fuse. Very common in tube amps. When they fail at least.

I doubt it is a preamp tube, they really have a hard time blowing fuses. Most common reason is bad power tube. Second most common reason is a shorted flyback diode. Less likely is shorted rectifier diode in power supply and even a shorted filter cap. Really odd ideas? Shorted standby switch to frame. LAst thing on the list is the output transformer.

Power tubes removed and stil blows? OK, you still could have a bad power tube, but the amp still has issues. SO the two diodes I mentioned are on the power tube board. Check for shorts. EAsy way? Disconnect the three wires connector on the end of the power tube board. Red, blue, brown wires. NOTE WHICH WAY IT CONNECTS. Does the amp now still blow fuses? Another way, measure resistance to ground from pin 3 of any power tube.It should NOT measure shorted.

User avatar
JamesPaul
Member
Posts: 2477
Joined: Fri Sep 12, 2008 7:28 pm
Location: Moving Towards Adam Brown's State

Re: Jsx212 blowing preamp fuse

Post by JamesPaul » Wed Oct 20, 2021 9:34 pm

If the blows with all the tubes out, I would think likely shorted flyback diodes or something shorted in the power supply circuit. I would still have to chase it with my meter and possibly light bulb current limiter.

Enzo will have the best idea of what it is.

Here is the schematic if you do not already have it.

JSX 212 Combo schematic.pdf
JSX 212 Combo Schematic
(2.46 MiB) Downloaded 88 times


Edit: See, Enzo beat me to it while I was finding my schematic!
Enzo wrote:I find if the amp is working, that is a good point to stop fixing it.
James Paul's Peaveys
Decade, Classics, Ecoustic, Windsors, VYPYR, Triple XXX, XXL, VKs, Bandit, JSXs, VIP, Piranha and a Penta.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: Jsx212 blowing preamp fuse

Post by Enzo » Wed Oct 20, 2021 11:10 pm

Well, we seem to agree on the analysis.

Post Reply