BM4400 FM Stereo Decoder

5 replies
B(
Offline
Bristol603 (archived2)
Bronze
Posted August 30, 2015 at 18:20 GMT
#2203177

Dear All,

I have a newly acquired Beomaster 4400 that has a problem. The radio isn't giving a stereo output (it is otherwise sonically and cosmetically excellent). With a good strong FM signal (4.5 on the signal strength meter), the stereo light (LED) doesn't shine and I cannot detect any audible difference switching the Mono control on/off. The mono FM sound quality is very good, but I do want stereo.

Looking at the Service Manual, this suggests an adjustment of 3C8 to obtain a 19kHz signal at pin 10 (TP7) on the decoder IC. One problem is that the manual says to do this when tuned to a mono FM station. I don't have any mono FM stations in range in the UK. I do have a DFM and so I set the tuner between frequencies and adjusted 3C8 to get 19kHz at pin 10. When I then tune to an FM stereo radio station, the signal at pin 10 locks to a very stable and accurate 19.0000kHz. However, the stereo lamp still doesn't light and I don't hear stereo audio.

I have also tested pin 6 (connects to stereo LED) and this shows 10.0V whether tuned to a strong FM stereo signal or off signal, i.e. no difference.

Is my improvised adjustment method for 3C8 to get the19kHz reference workable (I don't have a dual input oscilloscope). Does anyone have any suggestion for what to check next?

Thank you.

DI
Offline
Dillen
BronzeModeratorFounder
Posted August 30, 2015 at 18:32 GMT
#2203178

The stereo decoder is a piggyback-board.
Before adjusting anything check its solder joints to the main board.
And any other joints where connectors are soldered to boards in general.

Martin

C(
Offline
chartz (archived2)
Bronze
Posted August 30, 2015 at 20:14 GMT
#2203179

Another possibility is a defective decoder IC. I had the info from Tim Jarman : see

B(
Offline
Bristol603 (archived2)
Bronze
Posted August 31, 2015 at 19:24 GMT
#2203180

Dear Martin and Jacques,

Thank you for your quick and helpful suggestions. I tried Martin's advice first and remade all the connections on the pig-back board - no change, but a really good idea as I even in my very limited experience I have seen bad connections result in some weird behaviour.

By shear luck I had an MC1310P stereo decoder IC in my parts box (I bought it  from EBay for a couple of pounds to repair a Quad FM3 a couple of years ago, but it turned out not to be the problem for the FM3 and so was spare and known good). I swapped this in for the IC in my BM4400 (marked 17891 274 723) and the stereo LED and proper stereo decoding have both been restored.

Thank you for your help in returning a lovely old receiver back for full health.

Best Wishes,

Nigel.

C(
Offline
chartz (archived2)
Bronze
Posted August 31, 2015 at 21:33 GMT
#2203181

Here you go!

Well done Nigel! 🙂

or
BeoWorld Gold Membership card
Join BeoWorld

Register to access the forums, reference material, and exclusive member benefits — including our monthly prize draw.

Become a Member

December 2026 Prize Draw

Gold Members Only

As a Gold member you're automatically entered into our exclusive prize draw — your chance to win iconic Bang & Olufsen products. A unique benefit of BeoWorld Gold membership, every draw brings a new opportunity to win something special.

View Prize Go For Gold