Sbf3bp ic ?

[sp10mk11]

Hello
The above IC is the pre driver stage of a small Philips stereo amplifier in a record player.
The problem is no sound, and I mean absolutely no sound, no hum, no switch on/off thumps nothing.
There is 14volts on the collectors of each of the left and right transistors.
It is the same with this IC in or out of circuit absolute silence when switched on.
I am guessing this is more than just an op-amp IC as there are no transistor pre-drivers.
Anyone any thoughts?
Thanks
Gary

[Herald1360]

Looks like one of those rare types only found on sites trying to sell obsolete parts.

There's one on ebay.it

http://www.ebay.it/itm/Circuito-Inte...3D301830340056

Reckon it might be easier to replace whole amp with something cheap off the shelf.

Found the attached datasheet circuit which may help.....

And a record player circuit.....

https://onedrive.live.com/view.aspx?...by&app=WordPdf

[sp10mk11]

Hello Chris
Thanks very much for that, armed with information I will have a look today.

Gary

[sp10mk11]

hmmm the plot thickens

There should be 7v coming from both emitters but there isn't.
so I took out the power transistor to check them and noticed there were no insulating sheets, all the transistors checked out, so I quickly put them back on the board but didn't fix them to the heatsink, switched on and there was the 7volts going to the output caps, speakers in and both channels working!
but if I connect the output transistors to the heatsink it all goes off no sound no 7volts ?

So I am going to find some mica and make some insulators

Gary

[Herald1360]

What are the power transistors? If they're AC1xxK types in rectangular blocks, they may have succumbed to tin whiskers. Insulation will only be a temporary fix.

At least you don't need any unobtanium to fix this now!

[sp10mk11]

Hello Chris
Yes they are AC188/187 pairs, it is all working at present with the transistors as you describe rectangular blocks screwed to the heat sink.
Yes the immense relief to think I do not have to obtain the IC

Can I ask what is happening to the transistors (tin whiskers)

Gary

[Herald1360]

Tin whiskers is a problem which afflicts some transistors which have tin plated cans. (And it's a problem in other areas too- just do a google search on it!) AF11x devices are notorious, but as devices get older, more types seem to be succumbing.

In brief, little whiskers of tin literally grow from the inside tin surface until eventually they hit a sensitive point on the transistor chip and short it to the can. They can mostly be cleared by splatting them using a PSU and/or charged capacitor connected between bce legs in parallel and the case. How long before the next one that hasn't quite got there yet makes it is a random period between hours and years.

Insulating your devices from the heatsink will work for a random time period.

[sp10mk11]

Hello Chris
I read up on "tin whiskers" and am thinking about doing the BCE voltage to see if I can get rid of them, as I say now it is all working with the transistors bolted to the heat sink.
I have a variable power supply that can deliver up to 3 amps, so from I have read it is a tread carefully approach.
Thanks very much for your help on this, after all my years this is the first time I have encountered this problem.
Gary

[sp10mk11]

I ran 30v DC through each transistor, connected up as described and nothing so for the time being I presume them to be ok.

And it is all working except, yes another deteriate with time item, The ceramic cartridge is playing up on one channel, oh I love old stuff and again the price of replacements (about £18 for a GP215) is off putting

Ah well soldier on tomorrow

[Herald1360]

Originally Posted by sp10mk11

I read up on "tin whiskers" and am thinking about doing the BCE voltage to see if I can get rid of them, as I say now it is all working with the transistors bolted to the heat sink.

Sounds good.

If you found mhennessy's write up on clearing tin whiskers, I think that's pretty much the last word on the subject.