ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
I have recently installed a stereo decoder in my Beomaster 900M.
It fits into the socket correctly but the problem is that the stereo signal is very intermittent.
Both the stereo light flashes and it switches from mono to stereo as it drifts in and out of stereo on FM. The signal is not the problem as it is picking up a strong enough signal but it almost seems like a loose connection. I have reconnected the decoder into the socket without success.
Any ideas?
Anyone had issues with the stereo decoder before?
Ben
It could be the decoder in need of alignment - depending on the type it is not too uncommon.Are you sure, that both IF-stages are working correctly ?
Martin
That model is not the worst when it comes to the need for alignment.Alignment is calibration of the tuned circuits (adjusting of the coils) on the module as mentioned in the servicemanual.
My memory is a bit hazy on this issue,but wasn't there a filter network that had to be removed when retro-fitting the decoder?
Did you have any fitting notes with your kit.
I'll go through my "archive" of Beonotes.
Was your kit "new old stock" and as such previously unused?
If the coil cores are not sealed,the "phantom twiddler"may have visited at some stage,and you may indeed have to realign the decoder.
Regards,
Nick
There is a de-emphasis plug, but it will have to be removed because the MPX module plugs into the same socket.Both cannot be plugged in at the same time.
The coil cores are locked, that's the yellow stuff you can see in the photo. Looks undisturbed as far as I can tell, but they couldhave been fiddled with from the other side.Most sensitivity problems in these receivers are caused by bad IF-transistors.
Also the stereo indicator light is switched by a dedicated resonator circuit that needs to be in tune as well. So the receiver might actually be playing in stereo even though the lamp goes on and off, or doesn't light at all. Of course, with most music material you would hear the difference.
--mika
Martin is right, the plug has to be removed to plug the stereo decoder in.
I confirm that I have got stereo playback on FM (sometimes!) and as you say Mika you can tell the difference.
I will have a look at the IF transistors and let you know if this cures it.
Thanks all
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.
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?
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
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. AF179Second 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.
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
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