The Workbench

Re: BM 8000 CPU: cut traces?

Premiumverum — Tue, 17 May 2011 02:34:52 GMT

A fellow member reminded me I never posted the solution to my problem.+

Replacing U264B did nothing, it turned out to be fine.

The tuning problem came from inductor L2, near the U264B. One wire was broken off, was real hard to see. When I resoldered the wire the tuner worked again!

It gave the same symptoms as a broken U264B chip. Worth checking out before ordering one!

Re: BM 8000 CPU: cut traces?

Premiumverum — Fri, 07 Jan 2011 02:52:09 GMT

Hey Richard,

Thanks for this hint! Unfortunately time is still not on my side but I hope to have some spare time soon, I would love to tackle this problem.

Thanks!

Re: BM 8000 CPU: cut traces?

richtoy — Thu, 06 Jan 2011 16:37:27 GMT

I have seen this fault on a couple of the BM8000's I have restored and in both cases it turned out to be the U264B frequency divider. The frequency would appear fixed at one end of the frequency range or the other (87.50 or 108.0) but if you spin the tuning dial quite quickly the frequency would start to move and then go all the way to the other end of the frequency range. With a scope all looks ok on the input to the divider but there is just noise and the occasional blip out the other side. The chips are hide to find but Martin can probably help you out. Eliminate dry joints and dirty connectors first (use a fibreglass pen to clean the oxide off every single pin...) and try to confirm with a scope if you have one.

Richard

Re: BM 8000 CPU: cut traces?

Premiumverum — Wed, 29 Dec 2010 02:48:27 GMT

Thanks Martin! When I have spare time I'll check it out.

Re: BM 8000 CPU: cut traces?

Dillen — Tue, 28 Dec 2010 16:30:23 GMT

Check the opto readers on the tuning wheel and the related connectors and their solder joints
on the processor board.

Martin

Re: BM 8000 CPU: cut traces?

Premiumverum — Tue, 28 Dec 2010 14:25:56 GMT

Thanks for the explanation and hints gentlemen! Indeed I have the piggyback IC installed.

Unfortunately my tuner problem is a bit more complicated then just no sound: It does not tune up nor down with the tuner dial, although the frequency counter moves. Then, out of the blue, it starts tuning either all the way op or all the way down the tuning scale, passing all the stations until it can't go any further. Probably pretty complicated problem I guess, and I don't think I have the time these days to start puzzling it out. If anyone has any hints on it would be always welcome though!

Re: BM 8000 CPU: cut traces?

Blackrix — Sat, 25 Dec 2010 17:18:53 GMT

I once had the shield of the shielded cable disconnected at the connector, so the connector was connected in the right way, but the tuner did not work. So when checking, also have a look at that!

Re: BM 8000 CPU: cut traces?

Dillen — Sat, 25 Dec 2010 15:33:17 GMT

(IC6 is only a 16 pin chip so I take it that must be a typo, IC4 makes more sense).
According to the schematics, IC4 p21 should connect to IC8 p13
to give a pre-scaled signal to the frequency readout.
But that refers only to the unmodified (early) circuit.

Most BM8000's have an extra little piggyback board with a 4013 IC on
the processor board, using flip-flops to further scale down
the frequency to the counter because it turned out the processor had
trouble counting the fast pulses, thus displaying wrong frequencies.
The embedded code in the processor was changed accordingly (making the
processors not interchangable).
If you have this board installed, the signal from IC 8 will be fed
to this board instead of directly to IC4. The output from the piggyback
board will then go to IC4.
I haven't seen this modification mentioned in any service manuals.

The shielded cable that runs from the tuner to the processor is very short and
will easily pull out from its connector on the tuner board unnoticed when the
processor board is swung up to service position.
Pulled out, the radio will be silent so I suggest you check this.

Martin

BM 8000 CPU: cut traces?

Premiumverum — Sat, 25 Dec 2010 08:28:58 GMT

Hi all,

First of all, a merry christmas to all of you! And may Santa bring you all a pair of BL5's!

I was bored this christmas morning, so I opened up my BM8000, on which the radio does not work anymore after I did the fix introducing new connections on the CPU board (the rest worked fine again though, and I can do without a radio). Having a fresh look at the board, I spotted a cut trace between IC6 p21 and IC8 p13, labelled 'frequency counter' in the service manual, last page.

I'm sure I did not cut that trace. Is this normal, done in the factory for some reason? It puzzles me.

Who knows more?