R200 current consumption

[kestrelmusic]

I am working, with some reluctance as I have previously worked only sets with glass bottles in them, on a Roberts R200.

It's producing an output of sorts, but very distorted. The overall current consumption is around 100mA which I have a notion is much too high, and the output transistors are getting hot.

Any advice gratefully received!

[G6Tanuki]

If the output transistors are the metal cased TO1 type (AC127/128) fitted in copper clips and bolted to a heatsink, try pushing the transistors out of the heatsinks.

Some of the TO1 types transistors develop the same tin whiskers as the OC170/OC171 and the AF114-117 variants, if they are fitted in heatsink clips then mounted on an earthed heatsink it can really cause crazy excessive current drain when the whiskers do their thing.

Also, if there are any of the Plessey electrolytic capacitors with red and black plastic ends and a yellow label on the body, these are replace-on-sight. They lose capacity and also pretend to be low value resistors.

[Station X]

The service manual for the OC** transistor version quotes a current of 8mA under no signal conditions.

[paulsherwin]

Indeed, that's ridiculously high current consumption so you have a significant fault somewhere, either a failed decoupling electrolytic or a shorted transistor. The quiescent current of a 60s AM 9V radio will almost always be less than 15mA unless something is amiss.

[kestrelmusic]

Many thanks for these comments. Is it normal for output transistors to run hot? Sorry to pose such an elementary question, but I've never worked on a transistor set before!

[McMurdo]

No, I've never had them running hot, try what G6TAnuki says, it's a common problem.
I should add that lifting them out of the heatsink is a fault finding method, not a permanent solution!

[paulsherwin]

They certainly shouldn't be getting hot when quiescent.

[kestrelmusic]

Solved! I took out the chassis to see if I could see anything, and it turned out that the diode in parallel with the bias adjust preset was open circuit due to a broken lead.

The overall current consumption is around 15mA now. I still get nothing but noise, but since its got a load of those red-and-black caps I suspect I'll need to change most of them. Heigh-ho!

Funny thing, though... each time I touched anything on the chassis I found myself automatically looking at the mains plug (I was using a 9V power supply rather than a battery) to check that it was disconnected. One thing with working on transistor sets - no nasty voltages waiting to jump out and bite you!

Oh... and the first three transistors are the dreaded AF117...

[Techman]

Originally Posted by kestrelmusic

Oh... and the first three transistors are the dreaded AF117...

That'll likely be the main reason it's not working, rather than those capacitors in this case.