I have just acquired a Decoder which has just been repaired, as the volume level was down on one channel. Audio wise it now sounds fine, but the green STEREO indicator does not illuminate.(it lights up OK on Tape and Phone).

Can anyone offer any advice please?

One of the two IF-transistors are bad and/or the decoder hasn't been aligned.

Martin

Thank you Martin, how do we check and rectify these please?

Regards

David

There is a useful description of the decoder,and a diagram in the service manual on site.

When the decoder is fitted,all audio passes via it including mono.If the signal strength meter is indicating a strong signal,and there is no hiss in the background,you can reasonably assume that the if stages are ok.

However,for correct stereo operation it's crucial that the stereo pilot tone is intact,and the other switching frequency (36Khz) too.A 'scope of course will help here,but if not available,check the voltages around the decoder transistors for clue's.Electrolytic's are suspect at this age of course,but in the case of the "repaired" decoder,this may have been attended to?

Don't be tempted to twiddle anything!

Other enthusiasts have discovered poor tuning capacitors in the various tuned networks too!

I have a BM900 in the "round tuit" corner!It has the decoder fitted,so I can compare notes if it will help?

Nick

Thanks for that, Nick. Unfortunately I don't have the equipment to check the tuning.

Should it still pick up "stereo" with a small indoor aerial or would I need an outdoor aerial?

These Beomaster's have quite sensitive tuners normally,so although an outdoor aerial is ideal,you should be able to use an indoor one too,depending upon local reception conditions of course.

As mentioned earlier,if the signal strength meter indicates a strong signal,and there is no hiss in the background when listening,the tuner and i.f stages are probably ok.

You say that it sound's "fine",but can you tell if it's actually stereo.You can compare by pressing the mono/stereo button

Nick

Note that the signal strength meter is at max when not powered.
It is forced down by voltage derived from interstation noise - not up.

In most areas the built-in aerial should provide fine signal strength for stereo reception, but stereo will be next to impossible if one IF section is bad even with a good outdoor aerial.
The AGC will not function correctly without both IF sections working, so don't blindly trust the amount of noise.

About 50-60% of the Beomaster 900s I see have IF troubles, and the number is increasing.
The dreadful AF116 transistors suffer from the tin whisker syndrome and should ideally be replaced on sight.
AF116 (and several others) will develop faults even if never used, so I suggest using something different;First IF replace with f.e. AF179
Second IF replace with f.e. AF109r  - If fitted upside down, the pinout is perfect.
Normally, replacing the IF transistors alone won't require a complete tuner realignment, but the stereo decoder may need aligning if serviced, depending on what was done.

Martin

All valid point's of course Martin,but I'm still intrigued by the "repaired" decoder,and we need more info too on whether in fact the BM is producing stereo?

The indicator lamp issue may be due to simply to having the wrong bulb fitted! It's not unknown to come across all sorts of odd torch etc bulb's fitted,as they "looked the same" as the original,and would work ok in the direct switched supply situation such as indicating in phono and tape mode.

Over to you David?

Ben's reception problems do seem to warrant more in depth investigation though,and I think that some more info on voltages and possibly waveforms(?)encountered would be useful please

Nick

Thanks for your help guys. Bulb is OK as it lights up on Phono or Tape.

They friend who has taken a look has replaced the transistors and a diode which was cracked.

At least there is some sort of sound coming out of the radio now, but it doesn't sound any different when pressing Mono and back again