Dreaded SVT3 Output Mistake....

[Mr_bibbles]

So I did a boo-boo...

Successfully corrected bias on SVT3-pro, was measuring MV ratings across .047 resistors with alligator clipped probes. One of the alligator clips "snapped" off the resistor's lead and I saw a spark on a transistor where it shorted to the heat sink. R27 smoked before I could do anything.

Measuring diode checks across each of the 8 OP trannys measure 0.000 except for one or two that measure .030... I'm guessing now I have made a very, very expensive mistake. Should I shotgun replace every transistor in the power section at this point? Any tips?

-Thanks,

A sad, sad man.

[Mr_bibbles]

I also found this online: Ampeg 2041114 00 Power Amp PCB for SVT 3PRO | Full Compass


I'm wondering if it's the best solution.

Here's the schem since I forgodPro SVT-3 Pwr.pdf

[The Dude]

Just repair it like you would anything else. Troubleshoot it. Shotgunning is never a good idea regardless of the cause of the failure. I'm not sure why you'd replace the entire module before you even find out what's wrong with the one you have. It could be as simple as one shorted transistor and a burnt resistor.

[Mr_bibbles]

Yeah, well, you know, that’s just, like, my opinion, man.

I guess the reason I was asking is that each of the 8 OP transistors measured 00.00 from all legs to all legs with the exception of two, that measured .030. I've heard this thing is a beast and thought It might be easier to replace all the transistors since it seems almost all are fried. I will take your advice though and measure them each independently and replace what needs replacing. Thanks for your time dude, always appreciate the input!

-Matt

[MarkusBass]

Did you desolder the transistors from the board? If not, you have to know that one shorted transistor will cause three other to appear as they are shorted. Fixing the amp will be much cheaper than $230 for the new module (+delivery fee).

Mark

[Mr_bibbles]

This is new information to me, which is exactly why I love you guys and this site. Thanks for the excellent starting point! SS amps are still a mystery to me.


-Matt

[Enzo]

Do you realize that the row of transistors is in parallel? So if one is shorted source to drain, they will all appear so. Start pulling them off and testing them, you might find they are not all shot.

[MarkusBass]

Of course, even if you find that only one, or two transistors are shorted, it is advised that you replace all the transistors. The reason for this is that they should be matched (in order to have the same quiescent current). I usually buy 10 transistors of one type and select 4 for an amp. The other 6 are left for other amps. This still be much cheaper (maybe 10 or 15$) than the new module.

[Mr_bibbles]

Yes, when I flipped that board I understood exactly what Markus meant . Never tested MOSFETS beyond using a diode checker, does this page seem pretty on the level? Any tips or tricks from the Pros? I saw another good thread here that involves hooking up voltage sources to each MOSFET to test.

[J M Fahey]

Absolute worst case, a full set of transistors plus resistors plus micas/goop should cost less than $30 at Mouser. Not sure about shipping but in any case shoud be less than for a full module.

Personally I would replace just the dead transistors and not worry that much about old (but good) ones or matching because the large 0.47 ohm emitter resistors voltage drop under load will *swamp* minute Vgs differences.

We are talking 2.5V@5Apk compared to differences in the order of, at most, tens of millivolts.

[Leo_Gnardo]

Absolute worst case, a full set of transistors plus resistors plus micas/goop should cost less than $30 at Mouser.

Last time I looked Mouser didn't have these output transistors. In fact no USA distributor did*. Maybe I'm looking in the wrong places? An outfit in China / Hong Kong offered a set of the same transistors aimed at kit builders but I'm reluctant to buy what could possibly be counterfeit parts.

* except possibly at exorbitant rates from outfits that specialize in out-of-production parts or have absurd minimum order rules.

I'd be delighted to find 'em at a reasonable price in USA, I have three SVT-3's here, dead in the water.

[Enzo]

SVT3Pro uses IRFP240, yes?

http://www.mouser.com/ProductDetail/...uHmAlkPuz34%3d

Mouser shows 4000 of them in stock at $2 each. $1.74 each if you buy 10.

And IRFP9240
http://www.mouser.com/ProductDetail/...8rpRYOMim0Q%3d

Mouser only has 800 of those at $2.50

[Leo_Gnardo]

SVT3Pro uses IRFP240, yes?

Thanks so much for the lead Enzo. Last time I went to Mouser to find these, well maybe they had 'em but dam' if I could navigate their search. Now they're in my shopping cart & soon to be 3 happy(er) bass players. I'm getting a dozen each.

[Mr_bibbles]

OK I have an order pending at Mouser. When you say ALL transistors, you mean ALLLL transistors right including the smaller non-jfets on the board? And then power up with a limiter for the first time just in case right?

EDIT:
4 of each of the JFETS:
IRFP240
IRFP9240

Little trannys:
TIP-50
2N5415
MPSA06 X3
MPSA56
2N3440

[MarkusBass]

The output transistors (IRFP....) are MOSFET transistors and not JFETs. When I said "replace all transistors", we were talking about output transistors and I meant exactly output transistors (power MOSFETs). But... any of the other transistors could have failed as well. It's your responsibility which of the other transistors require to be replaced.

Mark