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Carlsbro Wasp Lead 20 watt 1980's solid state electric guitar amplifier

Started by franblanc, December 30, 2023, 06:23:44 PM

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franblanc

I'm restoring a 1980's solid state Carlsbro Wasp Lead guitar amp and have run out of luck finding schematics. Any assistance would be gratefully received. I have ordered new Nichicon Gold Audio capactitors and am considering replacing an NPN transistor that is giving anomolous hFE readings. It's an RCA1C03-C (619) T0220 power amplifier for audio transistor. Compatible replacements seem to be the MJE15032 or the 2SC2810 but I am unsure.

I plan on fitting a new Jensen C8R 4 Ohms, 8", 25 Watts, speaker which is on order from Thomann. I have a suspicion that the basic design of these solid state amps is sound but they suffered from cheap components and cheaper car speakers. (I found an old car door speaker in the last Japanese 1980's amp that I restored. It had been factory fitted.) Any assistance would be gratefully received.

g1

Here's a Carlsbro schem that uses the 1C03 in TR6 position.
Unless you are getting very strange HFE readings out of circuit, I would be hesitant to replace it just for that.
There is a 1C04 acting as it's complement in TR7 position, does yours have that?  If so, how does the HFE reading on that one compare?

Was the amp working, or did it have a fault you are trying to fix?


franblanc

Thanks very much for your prompt reply. I received the amp without the speaker so I have not heard it working but the previous owner says it is working but "might need new filters". So I have not tested it, but I ordered new caps regardless. The attached images show the RCA1C03 highlighted in red. There does not seem to be a companion RCA1C04, which you mentioned and was evident in other schematics. I might need some help spotting it in the photograph. The other problem I have is that the two terminals between the filter caps are speaker outputs but I don't know which one is ground. I think it is marked 4 but it could be 5. This is an old amp, the RCA1C03 is a legacy component and I am not sure if I should replace it for that reason. Thanks for your advice.

g1

I would not replace the 1C03 transistor unless it is proven to be defective.  Solid-state components do not wear out or degrade, like electrolytic capacitors eventually do, so they are not normally replaced if they are working.

One of the speaker connections will be routed through the 'phones' jack (so the amp is quiet when using headphones). 
If it is through the 'tip' connections, that would go to the speaker (+) terminal.  If it is the 'sleeve' connections that disconnect the speaker, then that would be for the speaker (-).
If you can post a picture of the trace side of the board in that area, it should be easy enough to figure out.

franblanc

Thanks for your help G1 I appreciate it. I will leave the transistors alone. It looks like the trace from the pad marked "4" goes into a resistor (R28) and then into the tip and ring of the phone jack. So I believe 4 is positive speaker wire and 5 must be negative. This is a restoration project for my brother in law who is a gifted guitar player. I am trying to cheer him up as he is going through a bad break up.

g1

Yes, that should be correct.
It really shouldn't matter anyway, as it is a single speaker unit and you are not concerned with phasing.
The only other time it can be a problem is if the negative terminal on the speaker is connected to the speaker frame, and the frame is grounded.  (ex. some Fender Mustang amps)