Transistor substitution guide...where can I find one?

[mike80]

I'm repairing a solid state amp which has a burnt transistor. I can't seem to find a supplier who has it in stock. It's a 2sc3478. I found one place that can get it from a manufacturer in 67 days :puh.

So, I'd like to know if there's a transistor that I could use in it's place. Does anyone know of a site that has a substitution chart or guide? That would be great.

[rog951]

Is this the part?

http://www.mcmelectronics.com/product/2SC3478&cid=prodCrossSell

If so, it seems to be in stock.

[donnyjaguar]

This is a small, Japanese, high-gain, low-power NPN transistor designed for high-voltage use. My thoughts are three-fold. Replace it with a higher-spec'd unit; fix the problem that allowed it to blow in the first place*; consider re-designing the part of the circuit its in to improve performance.

If you have a schematic I can add more value here.

*many solid state failures are catastrophic. Meaning when one component blows it can blow, or fatigue, other supporting components. Replacement of only one failed component will often be a short-lived fix. There are no lack of stories of musicians taking their solid state gear to be fixed and the repair not lasting as long as it did from new. This tells me the person repairing it wasn't fully competent.

[skipm45]

*many solid state failures are catastrophic. Meaning when one component blows it can blow, or fatigue, other supporting components. Replacement of only one failed component will often be a short-lived fix. There are no lack of stories of musicians taking their solid state gear to be fixed and the repair not lasting as long as it did from new. This tells me the person repairing it wasn't fully competent.

+1 donnyjaguar
Most solid state amp circuitry is direct coupled. When a part fails, think of a "string of firecrackers"


Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

[mike80]

Is this the part?

http://www.mcmelectronics.com/product/2SC3478&cid=prodCrossSell

If so, it seems to be in stock.

That's the one. Thanks!

This is a small, Japanese, high-gain, low-power NPN transistor designed for high-voltage use. My thoughts are three-fold. Replace it with a higher-spec'd unit; fix the problem that allowed it to blow in the first place*; consider re-designing the part of the circuit its in to improve performance.

If you have a schematic I can add more value here.

*many solid state failures are catastrophic. Meaning when one component blows it can blow, or fatigue, other supporting components. Replacement of only one failed component will often be a short-lived fix. There are no lack of stories of musicians taking their solid state gear to be fixed and the repair not lasting as long as it did from new. This tells me the person repairing it wasn't fully competent.

I have the schematic, but it's in PDF and I'm not sure how to convert it. The amp is a Gallien-Krueger Backline 250. So far, I've found the burnt transistor, and a burnt 1/8 watt 1K resistor that was connected to it.

[mike80]

I found the schematic online. Go to http://www.about-guitar-amps.com/free_guitar_amplifier_schematics.html

Down to the Gallien-Krueger section and click it. When prompted, enter the username "amp", and password "schematics". It's then under the Backline series. BL250+115 poweramp. It's on the first page of the document, on the right side. The transistor is Q11. It and R37 were toasted, so I'm going to replace them.

[Rosewood]

This is a small, Japanese, high-gain, low-power NPN transistor designed for high-voltage use. My thoughts are three-fold. Replace it with a higher-spec'd unit; fix the problem that allowed it to blow in the first place*; consider re-designing the part of the circuit its in to improve performance.

If you have a schematic I can add more value here.

*many solid state failures are catastrophic. Meaning when one component blows it can blow, or fatigue, other supporting components. Replacement of only one failed component will often be a short-lived fix. There are no lack of stories of musicians taking their solid state gear to be fixed and the repair not lasting as long as it did from new. This tells me the person repairing it wasn't fully competent.
Yes, and this is why I stopped working on solid state amps a few years ago. People would not pay for the time it took to make sure the amp was fully repaired. Tube amps are so much easier to repair.

[donnyjaguar]

I tried the login and password and they were rejected. Can you send the PDF to my email account? Same name as here add gmail dot com for domain.

Solid state can be a quick fix too!! I think some guys get confused with the much higher number of components but the circuit logic is generally the same - although on first blush it looks totally alien to vacuum tube techs.

[mark norwine]

There are no lack of stories of musicians taking their solid state gear to be fixed and the repair not lasting as long as it did from new. This tells me the person repairing it wasn't fully competent.

Sadly......Same thing with tube gear...

[mike80]

Oops! I had the username wrong. It's "amp", not "amps". Sorry about that.

I went ahead and emailed it anyway. I found another burnt resistor. R22. Looks like R41 at least got a little hot too.

[donnyjaguar]

Got it- reviewing now!

[donnyjaguar]

Okay, I've had a look at this one. Given the location of the burnt transistor (muting/warning circuit) it was probably caused by a malfunction of the main power amplifier section.

Questions:
Is the amplifier working right now - does it produce *any* sound?

I'm guessing not otherwise you wouldn't have opened it up.

Basically the Q11/R37 is blown because too much voltage appeared across it. I'm guessing (test will need to be performed) that one or more of the resistors R43, R44, R45, R46 is open. This will happen under one of two conditions: one of the output devices is shorted (blown); the driver circuit provided contradictory signals to each half of the output section. R22 (bias circuit) is blown for the same reason, but being upstream indicates that the problem may have started before it. Because there's a closed loop in this amplifier (two actually), its difficult to go much further without getting into a lot of suggestions.

You could just replace the blow transistor and resistors and get lucky, but I don't think that will solve the problem. Do check R43-46 for opens and also output devices Q17, 20, 21, 22 for shorts and replace accordingly. I'm pretty sure any 200V NPN transistor can be subbed into Q11.

[mike80]

Thank you very much.

IIRC, it didn't make any sound at all, except for a hum. It's been a year or so since I messed around with it last. I acquired it in non-working condition from a friend several years ago and I'm just now getting around to tearing into it.

It says "minimum load 8 ohms" on the back, so knowing who I got it from, I figured he plugged it into a 4 ohm cab and that is what fried it.

I'll check those components tomorrow and post my findings.