I have a problem on both my 900K and my BM1000. They are both fitted with stereo decoders which work (FM is definately in stereo and I have opened them so know that they are actually there) yet when using the radio the "stereo" light fails to light even when tuned to a really strong signal. The light lights up like normal when using a different source. Its strange that its the same problem on both units, does any one have any ideas.

Or should that read "Martin, whats happening!"

beaker:

Or should that read "Martin, whats happening!"

Probably, but I'll try in the meantime

The first question is, are you really receiving in stereo? Flick the mono switch off & on and try to tell if you can hear a difference. If the sound stays mono, the decoder isn't working, or the signal isn't strong enough after all.

If you definitely hear the program in stereo but the light doesn't come on, it is an electronic problem with the light control. On FM stereo, a transistor (the one mounted separately for cooling) on the decoder board feeds the voltage to the bulb, on other sources it comes simply through a switch. The transistor may have failed, I seem to recall it runs rather hot for a small-signal germanium type.

The decoder can actually play in stereo and still not even try to turn the light on, there's a separate subcarrier detector (a tuned LC circuit) for the light. It might just be out of adjustment, but don't go blindly turning anything, it is unlikely to happen in practice. One of mine is just slow to light the bulb!

I have repaired some bad solders on these. If there's really no stereo, it is a little hard to diagnose without an oscilloscope to follow the 19/38 kHz signals.

Its definitely in stereo. You press the mono button and the change is distinct. 

Oh yes sorry, you pretty much said that in your first post.

Must be the light control then. All the schematics I rechecked (old & new BM1000, BM900) show a sensitivity control trimpot for the indicator light, but I don't remember actually seeing it in any of mine... never adjusted it anyway.

I guess it's best to start by measuring the indication voltage (pin 7 on the Noval plug) from the stereo decoder itself, and if it is "on", tracing where it ends. It goes through a number of switch contacts before reaching the bulb.

There's also the two electrolytic caps right at the detector outputs.
4,7uF or so.
They are often bad and will easily cause sensitivity problems with regards to stereo switching.
And of course the caps on the MPX board itself.
It's rarely the coil adjustments, so I suggest you leave those alone.

Martin

The trimpot for adjusting the lamp sensitivity can be adjusted through a hole in the rear of the chassis on the BM1000(at least on mine anyway).A good start would be to clean it(or replace it)and give it a tweak.Those caps that Martin mentioned are 5uf,I replaced them with a pair of new 4.7uf and heard a marked improvement in stereo separation.The old ones were obviously bad, they could also cause the symptom you have. 

Paul