Fender EVH5150-III Screen shorting on unknown small pcb

[christarak]

I have a EVH5150-III that comes up as a short on current limiter. I have traced this issue to a little PCB that connects to the Reverb and Power Level pots. The short is on this little PCB. When I bypass the little PCB by linking the Yellow and Red leads between B+ and Screen the birds sing and the sun shines. I assume the issue has to do with the Power Level feature. I can't find a schematic for it. Most likely suspects are these 2 zeners (with cathodes connected) or the MOSFET, but rather than just replacing blindly, I wonder if anyone has come across a similar issue or even has the schem for this little "afterthought" PCB. Thanks fellas.

[Jon Snell]

The only power brake switched EVH I can find is this;

It's not a Fender.

[Enzo]

Photo?

[christarak]

The only power brake switched EVH I can find is this;

It's not a Fender.

Its seems like there are SO many revisions of the Fender era of 5150. The schematic is the closets I could find but it doesn't have this Reverb DSP & Power PCB.

[christarak]

Photo?

Thank you for your help. I have attached photos of the PCB. The PCB from 2013 is stamped 0096002000 RevA. I couldn’t find anything about it on Google.
The 2 zeners Z300 & Z301 are 1n5370b 56v 5w
The MOSFET on the heatsink is a 7N60Z: 600V 7A N-channel
I know I can just change parts, but its good to have an understanding of how it works. When it works, I suppose it manipulates the screen voltage. The short disappears when I bypass the board and connect the Red and Yellow wires directly, either side of the first 390R/10W dropping resistor between the B+ (RED) and the screen (YELLOW) instead of the fancy-schmancy MOSFET circuit. (I used a 470R/10w as I didn't have a 390R in stock) . fender_evh-5150-iii_50w_rev.e_sch (1).pdf

[jack briggs]

The only power brake switched EVH I can find is this;

It's not a Fender.

Jon - check the schematic

[g1]

Any chance the mosfet insulator is bad and the drain tab is shorting to the heatsink?
Not sure about this model, but in other amps failure of the screen regulator fet just causes the amp to stay stuck on full power mode.

[christarak]

Any chance the mosfet insulator is bad and the drain tab is shorting to the heatsink?
Not sure about this model, but in other amps failure of the screen regulator fet just causes the amp to stay stuck on full power mode.

Thanks for that suggestion G1. I'll check that. I am not familiar with this method of screen voltage control to control power, and I can't find a schematic to aid in my learning. I'll let you know.

[Jon Snell]

Jon - check the schematic

I did. The schematic is for a Fender EVH 5150 III but there is no power break switch or FET.
The amps 'made'for Eddie have no schematics available at least not that I can find.
As Enzo suggested, a detailed photo would be helpfull as it is easy enough to reverse engineer with sight of both sides of the pcb in question.

[christarak]

Any chance the mosfet insulator is bad and the drain tab is shorting to the heatsink?
Not sure about this model, but in other amps failure of the screen regulator fet just causes the amp to stay stuck on full power mode.

OK G1. Once again, I bow deeply before you. You were right. Have I ever said that I love Canadians? They are the closest people to Aussies (other than Kiwis) ... and that's a good thing.

The Drain was indeed shorted to the heatsink. It looked physically sound ... but more fool me for testing with my eyes instead of a multimeter. I lifted the MOSFET off the heat sink and powered the amp up (sans the power tubes) ... NO SHORT. The Screen voltage even went up & down with the Power pot. I didn't want to push my luck with no heatsink. I will install a new insulation kit tomorrow and complete my checks of the amp.

WOOO HOO!

[Enzo]

And remember, the amp must be upside down to operate properly down under.

[christarak]

I did. The schematic is for a Fender EVH 5150 III but there is no power break switch or FET.
The amps 'made'for Eddie have no schematics available at least not that I can find.
As Enzo suggested, a detailed photo would be helpfull as it is easy enough to reverse engineer with sight of both sides of the pcb in question.

AHA! That would explain why I couldn't find a schem for the Screen Bias PCB. By the way, I posted some photos in reply to Enzo's post.
The solution was suggested by G1 who correctly asked if there was a short between the Drain (connected to the TO220 tab) and the heatsink. I looked at it, but I have no good reason for not checking for continuity. I never charge the owner for my "Dumb Time", so this will be a pretty cheap repair!

[christarak]

And remember, the amp must be upside down to operate properly down under.

I don't care how many times you use that gag, Enzo. It ALWAYS makes me laugh.