9090 Thermal Run-away

Have a 9090 here that the bias would not adjust and causing one channel of outputs to instantly get grilling hot...The thermal compensating transistor was acting like a 1.5K ohm resistor between the emitter and base. Looked all over for a 2SC984..I didn't have much luck.. Sooo.plan B. I picked up a Fairchild BD137-pretty close to the same specs.. Bent the little clip just a tad bit so it would hold the TO-126 transistor firmly- flat against the clip.Man it works great...Bias current now aligns " dead nuts on target" at 50ma . Offset < 1 mv. Sweet..We is Rock'n and a Roll'n .:thmbsp:

That's interesting, I had a bias adjustment problem on my AU-9900, I wondered about what to replace the equivalent transistor with (also a 2SC984). The main problem was finding a suitable 'isolated collector' T0-1 style transistor - now you have shown the way !

I didn't need to replace the transistor in the end - so I'm keeping tabs on info like this just in case.

Did you need to use an insulating washer to isolate the transistor (underneath) from the clip?

Thanks for sharing this nugget of information:thmbsp:

FYI, TO-92L's fit nicely where those C984's were removed. Lots of good trannies in that case style.

You always get me thinking...

TO-92L - do you think there is enough heat coupling with the spring clip to use a plastic case transistor?

Thinks...
I never even considered the possibility of using a plastic case transistor for that position...

Thinks some more...
I suppose as the original is 'isolated collector' a T0-92 might even be better !

Note to self... THINK !

Hyperion said:

That's interesting, I had a bias adjustment problem on my AU-9900, I wondered about what to replace the equivalent transistor with (also a 2SC984). The main problem was finding a suitable 'isolated collector' T0-1 style transistor - now you have shown the way !

I didn't need to replace the transistor in the end - so I'm keeping tabs on info like this just in case.

Did you need to use an insulating washer to isolate the transistor (underneath) from the clip?

Thanks for sharing this nugget of information:thmbsp:

Click to expand...

The BD137 is a plastic cased tranny..no insulator required.

OK great - I thought it had a rectangular metal pad underneath it... :thmbsp: