Beomaster 3000-2

[Krolroger]

Good morning,

I am tantalisingly close to solving an FM reception problem on a Beomaster 3000-2 IF board. The symptoms are heavily distorted sound, lack of stereo, and fade from the moment any station pushbutton is depressed.

However I have full functionality of stereo and discriminator lamps and sig strength meter.

I have recapped the board entirely and changed several zeners and the rectifier and all the trimmers. I have also changed the switching diodes and the associated transistors.

However I have discovered is that if I connect a DVM on DC volts setting to the collector of TR21, to the adjacent test point or to the TR21 side of C197, I get full stereo sound. In other words, by briefly loading the 8012051 IF transformer circuit, normal service is resumed. Voltages appear to be in spec.

Am I right in thinking that this points to a problem with 2.2nF styroflex cap C197?

I don't think anyone has twiddled with the alignment, by the way.

(Ron Bryan's earlier post on how the switching matrix works was most helpful).

Thanks all.

[cathoderay57]

Hi, I restored one of these about a year ago. The faults were all attributable to bad preset potentiometers. The one that was fouling up FM reception causing frequency instability and distortion was one in the AVC circuit but I would check all of them on the board. The failure mode was not just dirty contacts but broken ones. Hope that helps. Cheers, Jerry

[ronbryan]

It does seem as if 8012051 is off tune, and it might be the 2.2nF tuning capacitor C197 as you suggest, or it might need re-tuning.

Can you 'scope the 38kHz waveform on TR21 collector and see if it is anything like the 10V pk-pk sine-wave shown on the service sheet?

Ron

[Krolroger]

Hi Jerry, I have already replaced all bar one of the trimmers (which was ok) including the two tuning voltage potis. Now that was a task.. Simon

[Krolroger]

Hello Ron,

Please see attached for what's going on at the collector of TR21 when the set is in underwater mode; i.e. not behaving.

Regards,

Simon

[Krolroger]

...And when it is behaving (after loading circuit briefly with DVM).

Still haven't got 10v pp but more like 1.2v. Sounds absolutely fine though.

Simon

[Krolroger]

Went back and rechecked that 38kHz sine wave; this time getting 4.5v which is better.

Touching the TR21 test point with a disconnected DVM probe also has same beneficial effect so I am presumably introducing some extra capacitance into the circuit in question.

[ronbryan]

When the fault is present, the frequency of the signal at TR21 collector is approximately three times the correct value, as the timebase setting appears to be the same for both photos. Polystyrene capacitors have a habit of going open circuit, so changing the 2.2nF capacitor tuning the 38kHz coil must surely be your next step.

Ron

[cathoderay57]

Maybe a bit obvious, but worth checking continuity for cracked circuit strips and dry joints in the area where putting the probes on brings back the signal.

[Krolroger]

Ron and Jerry, thanks. Waiting for some styroflex caps in the post. In the meantime will check for dry joints/continuity.

Simon

[Krolroger]

Gents,

C197 was indeed open circuit. Normal service is now resumed.

I take it that styroflex caps can be mounted in either direction in this application. There is no marking on the cap body to indicate one end from the other.

Now onto reworking the output and pre-amp boards.

Thank you very much for your help, Ron.

Simon

[Krolroger]

Hello again,

Just a quick postscript. I've pretty much completed a full recap on this very nice Beomaster and it sounds excellent with full functionality.

However, I'm getting noticeable sibilance on FM speech.

Some relevant threads on here suggest this might be due to multipath signal reception. I'm inclined to discount this as I have a clear line of sight to two local transmitters, and the IF trimmers haven't been touched though they could be off slightly.

Just wondering if this could be due to a faulty de-emphasis network component (I haven't yet changed C123 in the screened discriminator module 8002957 or anything in the front end) though I am really just guessing. I wouldn't recognise a de-emphasis network if it bit me.

Any possible solutions would be most welcome.

Thanks,

Simon

[SteveCG]

Have you tried tuning into different stations? Does the sibilance vary? Different broadcasters apply different levels of 'audio signal processing'. My suggestion would be to listen to BBC Radio 3 - it's probably got the 'cleanest' audio. If the sibilance is still noticeable on R3 then perhaps your set does need a little realignment. However are you sure your FM signals are fairly multi-path free? - being close to the transmitter does not necessarily mean that there is not a strong ghost signal from a nearby building.

[Krolroger]

I will try attenuating the signal in to see if that makes a difference.

Yes, sibilance is on BBC R3 as well; rather too much speech on that station these days!

Thanks.

[SteveCG]

By the way, have you tried a different tuner connected to the same aerial to see if it is sibilant as well? This is a belt-and-braces way of eliminating multi-path as the cause of the sibilance...