Carver Preamp and the Toshiba 2SC2878 transistor

[EchoWars]

I have a Carver preamp/tuner with about 250mV of DC riding on the output of one channel, and is causing thumps when switching sources. It uses a Toshiba 2SC2878 transistor on each channel to mute the output, and I seem to have narrowed down the problem to one of the transistors.

For a situation where low C-E sat is necessary and high gain, I usually pop in a Zetex ZTX1056A, and I did so here. However, after subbing the Zetex trannie for the Toshiba, there is still voltage on the output...less, but still there. I pulled out the Zetex transistor, and moved the Toshiba transistors from L to R channels and soldered 'em back in...the DC followed the transistor, so I know that's the problem.

Why can't I just swap out the C2878 with any decent high-gain transistor? I found the data sheet pretty easy, and unless I'm missing something there doesn't seem to be anything real special about it...

[amplifierguru]

Why is there DC on one channel???

When you not mute with 2SC2878 she come back. Must fix DC.

[EchoWars]

DC follows the transistor when swapped from one channel to the next...hence the transistor is the source of the DC.

There is an IC which buffers the output that is just before the circuit that you see. I can remove the IC so that the output leads are only connected to the transistors, and the DC is there.

Two ways of confirming the same thing...that the source of the DC is the transistor. So what's so special about the C2878 that I can't replace it with about any decent transistor?

[amplifierguru]

Why not just buy one and put it in - they're only cheap < $1. Perhaps you'll never know.

[amplifierguru]

Perhaps it"s a dry joint toying with you.

[EchoWars]

You are correct...but I must spend at least $25 to place an order, even if it's a $0.50 part.

I just pulled out both transistors...no DC at output. Also, I checked the transisors with the diode tester. Odd readings on both.

The good transistor reads .72V from B to C and B to E, but my meter 'beeps' for a proper diode drop, and I get no 'beep' with these transistors, so this usually means some resistive element in there somewhere.

The bad transistor also measures the same, but I also measure a diode drop when reverse biasing the E to C diode...about .58V, and I get the 'beep' too .

[amplifierguru]

Try cleaning all the residual solder from the area and inserting 2 substitute Zetex (which you usually rely on) and see what eventuates - whether you get some DC on both channels or NONE! The cct shouldn't be that sensitive.

[mirlo]

Probably the special thing about the 2SC2878 is the high reverse beta.

That way when it saturates hard during muting, it doesn't cause a lot of base current to end up on the output.

[EchoWars]

That must be it, I suppose.

When I put in a pair of Zetex transistors, I had offset on both channels... -25mV on one channel, -55mV on the other. With the C2878 transistor in place, offset is 0.1mV.

Looks like I need to order more parts. Again...a case of needing one transistor that is gonna cost $25.

[AndrewT]

Hi,
is a fet used as the mute a solution?