Ashdown Evo II 500 Blowing FS1 & FS2

[BassRepair]

Hello everyone!
I have been searching this forum as well as other places but long and short, I have an old (hand numbered) Evo II 500 that has recently started popping the fuses in the FS1 & FS2 positions. I've found the schematics here and have looked at them but, frankly, I am unsure of where to even start. Any suggestions for directions onto the correct path would be appreciated.

Thanks for your help!

[The Dude]

Schematic link:

https://music-electronics-forum.com/...8&d=1562022646


I'd start by checking to see if any of the ouptut transistors are shorted.

[BassRepair]

Thanks for the schematic but I'm not sure that's the correct one. Mainly because in the board, fs1 and fs2 are directly next to one another and that schematic shows fs2 & fs3 next to one another.

Regardless, I'll start to look for the output transistors. The schematic I found on this forum seem to indicate they are part of the power supply.

[The Dude]

If that's not the correct schematic, possibly you can point us to the correct one?

[BassRepair]

The .zip file at the bottom of this post.

https://music-electronics-forum.com/...ad.php?t=19369

It's the first page listed as ACD010 1 of 2

[The Dude]

Got it. That is a different animal. Those fuses are on the low voltage supply, so not likely an output transistor problem. I'd start by checking D1,2, 3, and 8 rectifiers and the two zener diodes Z1 and Z2. That supply also supplies all of the op amps in the unit, so you could have a shorted op amp or bypass cap. If you don't have one, you should build a light bulb limiter for troubleshooting purposes. If you check continuity of the + and -15V supplies to ground, that might give you a clue as to where to start looking.

Edit: Copying the link for those who want to play along.

https://music-electronics-forum.com/...2&d=1443279508

[BassRepair]

Thanks! I'll start checking those diodes.

I'm having problems finding Z1 and Z2. Can you give me a little more info on what parts I'm trying to find?

Thanks again!

[BassRepair]

Using the diode feature on my multimeter, D1 & D2 measure as .074V and D3 & D8 measure as .037V.

[The Dude]

Those diodes should measure around .7V voltage drop. One or more of them might be shorted. If you measured them in circuit, you'll have to unsolder at least one leg of each one and check them out of circuit

Also, a good diode measures that voltage drop only in the forward biased direction. If you swap the test leads on the diode, it should read open/no voltage drop the other direction. If you get a low voltage drop both directions, the diode is shorted.

[BassRepair]

I did measure in circuit but on the legs and not the solder points. However, I'll desolder them and test again.

Thank you for your help!

[BassRepair]

Alright, I have desolder 1 leg from each of the diodes mentioned and tested again. They all read .54-.55V.

I also noticed that there were some burnt traces on the bottom of that board. I drilled a small hole in a blank space of the board next to them and added a jumper from one side of the burn to the other. So whatever overpowered this thing has caused some traces to have a bad time too.

If they're supposed to read .7V, should I replace them?

Also, what kind of completed is Z1 & Z2?

[The Dude]

.7V is approximate. If they are .5ish, that is close enough as long as it's only in one direction. There is probably something downstream of the rectifier shorted. I would find and check the zener diodes next. There is a 1W/100 ohm resistor and a 1W/150 ohm resistor after the rectifier. I'd expect the zeners to be near those. There is also a 12V regulator (IC5) after the rectifier that supplies DC filament to V1. That could be shorted. There are filter caps that could be shorted. Shorts on power supplies can be difficult to track down. You just have to divide and conquer. Again, I'd check the zener diodes next. Also, you found burnt traces. Where do they go?- from what to what? That might lead you to the short.

[BassRepair]

Well, that's the fun part. I have checked this board from end-to-end and have only found one 1 watt resistor on it, R59. I saw R58 was connected in line with Z1 and thought it would be easy to track down from there. Jokes on me!

[The Dude]

If you can post a picture of the board, I might be able to spot them.

[BassRepair]

I have uploaded photos of the boards here. The first few are mainly focused on cabling in case I unplug them and need to replace them later.

https://imgur.com/a/iqfGVm6

I hope that helps!