:Help: Are these transistors interchangeable?

[Eva]

2SB1186A is not a good replacement at all.

There are some standard models that should fit your needs, though:

2N5401 (TO-92)

MPSA93 or MPSA92 (TO-92)
MPSW93 or MPSW92 (same as previous but in a big TO-92 case rated at 1W)

BF422 or BF420 (TO-92)
BF472 or BF470 (same as previous but with TO-126 case)
BF872 or BF870 (same as previous but with Philips propietary case)

Be careful with pinouts

[tarnationsauce]

Thanks, Eva. I'm going to look into those transistors.

However, I replaced the transistor from one from a working channel of the amp. And it still is distorted andshuts down. So my problem is somewhere else. But I checked every component except the caps. But usually caps dont fry when a transistor's junction melts. I suppose maybe it could be an opamp.

[tarnationsauce]

Alright. I hooked the output to an o-scope. It plays really distorted audio, then ramps up to +20vdc, then 60hz oscillation. Then the amp turns off.
I have a feeling the problem is in the preamp circuit. Because the power amp's rail voltage is quite a bit higher than 20v. But the preamp/protection circuit appears to run off +/- 15v.

[anatech]

Hi tarnationsauce,
It sounds like you may have other problems there. Can you post a schematic or at least a make and model (in case I missed it).
-Chris

[tarnationsauce]

Originally posted by anatech
Hi tarnationsauce,
It sounds like you may have other problems there. Can you post a schematic or at least a make and model (in case I missed it).
-Chris

Chris, My amp is an Alesis RA500. I looked high and low for a schematic. On Alesis's website they claim the amp is "non-servicable" and must be repaired by Alesis. So I'm sure they're going to be unwilling to give schematics.

The good thing about the amp is it's extremely modular. The PSU / power amp / preamp& protect are all on seperate boards. But to remove the preamp from the power amp would be pretty hard as it's soldered/glued into place.

It uses three each, 2sc5200 / 2sa1943 with one c4793 / a18c7 drivers respectively.

When the transistors blew, the 5w/ .22 ohm Emitter resistors blew. I replaced them with .25 ohm resistors as I couldn't find 5w .22 ohm resistors any where. But I doubt that affects the amp this way.
THere are 2 original power transistors (2sc5200) that survived, but maybe they are bad but read good on a meter. My next test is to borrow 2 known good transistors from the working channel.

All in all this is a verry simple design and it's frusterating thateverything checks good, but it don't function properly.

Anybody know a good way to test zeners PIV without building a test circuit? All my zeners forward voltages read good ~.53v

[tarnationsauce]

BTW I forgot to mention the KTA1024 is one of the differential input transistors.

[anatech]

Hi tarnationsauce,

Okay, I couldn't find where they say it can't be serviced. I did find a description that gives you an idea of what the circuits generally look like.

Replace all outputs with new. Do not use parts from the other channel to find out what's going on. The other channel works, it's important you leave it alone. Without a schematic this is crucial!

-Chris

[Tony]

hmm...output trannies survived is a miracle! make sure you get all busted parts found out and replaced before attempting to power up! i would check around the bias circuit up to the input ltp.

[tarnationsauce]

Originally posted by Eva
2SB1186A is not a good replacement at all.

2N5401 (TO-92)

That transistor worked perfectly, thanks for the recommendation.

Originally posted by anatech
Replace all outputs with new. Do not use parts from the other channel to find out what's going on. The other channel works, it's important you leave it alone. Without a schematic this is crucial!

I replaced all the transistors and it still is did the exact same thing.
So I went through and replaced all four of the zeners (1N4148) and voila! It works perfectly now!

Thanks a bunch everybody for helping.

[Eva]

It's not desirable to mix different transistor models in differential pairs since that would degrade performance. It would be a good idea to replace the other KTA in case you haven't done it yet.