The Workbench

Re: BM4000 FM issue

Dillen — Mon, 14 Jun 2010 07:52:26 GMT

The design of the Beomaster 4000 and 4400 makes them look strongly related on the outside but they have practically nothing in common inside.
Circuitwise, the Beomaster 4000 is a larger 3000 where the 4400 has more in common with the later 6000.

Martin

Re: BM4000 FM issue

yachadm — Mon, 14 Jun 2010 07:33:58 GMT

I don't have the BM4000 circuit with me right now, but quoting from the BM4400 circuit (which is very similar IIRC), on the +35V and -35V circuits are 2 0.1uf caps, 1 between the AC and +35 rail, and 1 between the AC and -35 rail.

These are B40C800 (40V 800mA) or B80C800 (80V 800mA), which can easily be replaced with the 11DQ10 (100V 1100mA)

On the 15V circuit, those decoupling caps don't exist.

I'm working on a restoration writeup of the BM4000, and it'll go onto my website, but not just yet.

Menahem

Re: BM4000 FM issue

Dillen — Mon, 14 Jun 2010 06:42:00 GMT

I am not sure which film caps you are referring to.

The rectifier in the corner for the 15V supply is a known troublemaker.
Slightly under-dimensioned and generally of a bad construction. Sometimes coming apart physically when touched.
I like to replace it on sight.
If the tuner itself is in good shape, it has excellent stability and will only drift if the above mentioned diode isn't
adequately thermically decoupled. It's a fine circuit.
Problems with the rectifier most often shows as hum on the Tape - and especially the Phono inputs whereas the tuner
seem to run fine even with only 3 of the 4 diodes still present in the rectifier. It's a nice circuit.
The axial 470uF cap (usually blue) nearby will also need attention, a 470uF 25V or even 40V will be nice here.

Martin

Re: BM4000 FM issue

yachadm — Mon, 14 Jun 2010 04:18:47 GMT

I replace the FM section bridge-rectifier with 4 11DQ10 Schottky diodes for much more stable performance. If this is done, then the film capacitors (part of the old regular bridge-rectifier) must be removed.

Menahem

Re: BM4000 FM issue

burantek — Wed, 19 May 2010 07:48:54 GMT

UPDATE

It ran for over 8 hours last night...

Re: BM4000 FM issue

burantek — Tue, 18 May 2010 19:11:43 GMT

Martin,

Many thanks!

Dillen:
There's a diode mounted in a little bracket on the outside of the FM frontend casing.
It's a capacitive diode and very sensitive to temperature changes which also explains why
it is mounted to the metal housing using heatsink compound.

The diode in question: 138 / E3 / 8300038 on the IF DECODER BOARD - PC8002058? It is compounded to the case of the TUNER / FRONT END - 8050054 as you describe.

Dillen:
Next thing would probably be the rectifier in the corner of the tuner board.

Is the rectifier located at E1 on the same board, PC8002058? I do not see one on the TUNER / FRONT END.

In the meantime, I decided to fire it up and get a better feel for just what it was doing ~2 years ago...

Of course, it hasn't burped in ~2 hours of playing!

Re: BM4000 FM issue

Dillen — Tue, 18 May 2010 15:38:38 GMT

There's a diode mounted in a little bracket on the outside of the FM frontend casing.
It's a capacitive diode and very sensitive to temperature changes which also explains why
it is mounted to the metal housing using heatsink compound.
As the compound dries the thermal connection is broken and the diode is free to heat up and
change its charateristics.
I suggest you check it and renew the heatsink compound.

Next thing would probably be the rectifier in the corner of the tuner board.
I like to replace it on every Beomaster as a matter of cause.

Still no luck, check the stability of the tuning voltage and let us know.

Martin

BM4000 FM issue

burantek — Tue, 18 May 2010 15:20:32 GMT

Hi all,

In the midst of some gear re-arranging, I have decided to bring a 4000 back into active service.

I haven't touched it in ~2 years... last time I did, everything worked beautifully (it is mint) but, I did have an issue w/ FM stations.

As best I can recall (I will test again when I have more time) it would do something like this: Tuned into ANY station, it would play along fine, and then suddenly drop the station... not allowing to retune it at all! If I pulled it from the mains, it seemed to recover and then after a while -do it again.

I assume something is heating up and loosing connectivity... broken solder? Any ideas or perhaps a pointer as to where to look?

Thx in advance!