Ampeg SVT4 pro blown power channel

[phydauex]

Good Day All,
I'm going thru an Ampeg SVT 4 Pro head that came in with very low output (per the owner), and I noticed a couple burnt resisters. Wasn't till I got the poard all out that I found that all of the .47/5 w, 47 ohm & 1.5k resistors on the A Hi side were shot. To get at these I had to unsolder all the devices on that heatsink and pull the whole thing out (sink + devices). I had to do that anyway, so... All five of the mosfets (IRFP240) on that side show continuity between sourrce, drain and gate when I use my tester set to diode/continuity. Of course the B side on that same heat sink show some resistance but no continuity.
Also seems that the A Lo side is bad also, but the bias resistors and the .47's test ok.

I guess my question is, if I'm getting continuity on both the D/S & G/S & D/G on a mosfet, then it's toast? I just want to make sure as Mouser has the IRF240's but not the IRFP9240's. The only place I can find those is from Loud Tech, everyone else (Mouser, Digikey, etc) are looking at 26-30 weeks backorder, but Loud also wants $11+ / device, making it $58 bucks just for the one side.

I'm still a novice with solid state amps, and this one seems kind of complicated.

I know when I do get the devices back in, I'll need to set the bias. Anything else I should test or look for? I also have replaced R118 and Q102 in the 'A' driver section as they were bad also. I'm kind of suprised that with all the baked devices and resistors that the fuses didn't blow.

any advice would be welcome.

Matt

[Jazz P Bass]

The Mosfets that are shorted D/S & G are toast.
The ballast resistors being open are a good clue.
I would go with the parts from Loud Tech.
They will be matched for current draw.
One less thing to worry about.
These amps are a PITA to get apart.
Which you have already found out

[MarkusBass]

You haven't mention where you are located. In Europe you can buy these transistors for $2 without any problems. But they have to be matched, so you need to buy 10 to get 5-6 that you can use in one amp.

Mark

[phydauex]

Thanks for the confirmation All,
Mark, I'm on the West coast of US, Loud Tech is just up north of us here. I'm aware that they should be matched, but when I can get the part (if it's in stock) for $4.60 ea and they sell them for $11.50 each, it bugs me. Although, they have them in stock so there you go.
This was a pain to get apart, but the bigger issue is not being able to test it with the board out. Replace parts, reassemble then check. But thats the nature of it. As far as I can tell all the questionable parts are in the 'A' section, and I've replaced all drivers/transistors that test funny. I still don't know what could have caused this meltdown, I'm hoping it was just a heat related event, and not somthing like voltage anomalies in the regulaters, as that is a bit over my head at this stage. If it's a heat issue, then I can wire the fan to run constantly.

This thing was bashed up a bit though, he didn't have it in a case and the thing was heavily coated with dust inside and had been dropped enough to have bent up both the rack handles. Also, the main rectifier bridge was undone from the heatsink, so overheating, and stress in the power supply could be a source of trouble.

Thanks again for the information.
Matt

[Jazz P Bass]

It sounds to me like it failed from over abuse.
I would let the fan circuit stock.
If the fan runs slow it pulls less dirt into the amp.

[MarkusBass]

... but when I can get the part (if it's in stock) for $4.60 ea and they sell them for $11.50 each, it bugs me.

In Europe you can buy them for $2, so I would never pay $11.5 for matched transistors. In another thread about SVT4Pro you can see that factory matched transistors are far from being matched correctly. It's hard to believe that in States you have to pay almost $5. How far is Silicon Valley from you? Have you checked ebay: irfp240 | eBay ?

This was a pain to get apart, but the bigger issue is not being able to test it with the board out. Replace parts, reassemble then check.

I think that you can test the amp without reassembling it if you put the amp enclosure vertically (or unscrew the transformer).

I still don't know what could have caused this meltdown

You're kidding me, right? . And who wrote this ->

This thing was bashed up a bit though, he didn't have it in a case and the thing was heavily coated with dust inside and had been dropped enough to have bent up both the rack handles. Also, the main rectifier bridge was undone from the heatsink, so overheating, and stress in the power supply could be a source of trouble.

Could it be you ?

Mark

[phydauex]

Originally Posted by phydauex
"This thing was bashed up a bit though, he didn't have it in a case and the thing was heavily coated with dust inside and had been dropped enough to have bent up both the rack handles. Also, the main rectifier bridge was undone from the heatsink, so overheating, and stress in the power supply could be a source of trouble."

"Could it be you ?"


The amp was brought to me to repair, because it "wasn't working right". Do I suspect abuse, You bet! I knew nothing about the type of use/abuse or the user other than what I saw when I opened it up and that the owner is a friend of a friend.

I think it's probably wise to try and determine the actual cause of a meltdown so I'm not just replacing parts that failed and assume it's fixed. Do I think overheating and abuse could account for this kind of failure.... maybe. But since I haven't been inside one of these or used one myself, it's nice to get some input here from others who have.

And yes Mark, Mouser, Digikey and other major suppliers all sell the IRFP240/ IRFP9240's's for $4 and change. I did find a matched set of 12 (6 ea) on Ebay for $89.95 + 10 for SH, but it's coming from China and delivery will be about a month. So that really didn't help much price wise (@ what I ended up paying) and certainly not time wise as the owner would like to get it back this week not 4 weeks from now. I don't like it either, but didn't really see any other options in a resonable time frame.
It must be nice to have something cheaper in Europe for once.

Matt