Somewhat OT: Troubleshooting a solid-state amplifier

[greenacarina]

Hello all. I have a Rosac 66G guitar amp that has suddenly gone "quiet". Can only get sound out of it if the volume pot is cranked to the max...resulting output is faint, but there.
I have built some pedals successfully, but I scarcely understand circuit design. Troubleshooting this with an audio probe, starting from the input leads me to a transistor that is not passing my input signal...so my inclination is to replace it. Here is where I'm stuck-
Transistor is not marked. Is a TO92 package. If it's toast, I won't have a way to tell if it's PNP or NPN. How should I go about replacing it?

Thanks,
Chris

[jmth]

Yeah, I know nearly noting but the basics says the transistor doesn't gets the activation intensity if it's not in saturation...
Are you sure it has no mark??? try looking it with a magnifying glass...
You can measure the gain, the voltage, the intensity... all the values and compare. The transistors have a nomenclature based on their values.

[Kesh]

Transistor is not marked. Is a TO92 package. If it's toast, I won't have a way to tell if it's PNP or NPN. How should I go about replacing it?

You also need to know it's pin out. And determine which is collector emitter etc. You should be able to do this by following the PCB, or at least narrow down the possibilities.

I would get 4 run of the mill small signal transistors. 2 NPN and 2 PNP. One of each with ECB pinout and CBE pinout. Then see what works.

Then again, it could be a FET.

Then again it could be fine and it isn't passing signal because some support component is dead.

So test it when you take it out.

[R.G.]

I did a little research on Rosac amps. They are old (early 70s, I think), ideosyncratic, and possibly variable from unit to unit.

What you really need to do is trace out the schematic. Once you have that, the identity of the device will probably be obvious. However: it is entirely possible that it's not the device in question that is the problem, and that there may not be a "the problem", there may be several. The amp almost certainly needs an entire new set of electrolytic capacitors. A failing electro could well have caused a biasing problem that makes the audio stop at this device.

If you're not so good at tracing it out, good, high resolution photos of the entire circuit will probably make this possible for someone on the net.