Hi All,
I decided to finally fix a strongly humming Beomaster 3000-2, which I was given about a year ago. It looks pretty good on the outside and even has the little presets plastic cover. So far so good. I opened it up and replaced the three big capacitors for output stages and power supply (they were leaking and probably causing the humming) and then started it up. It came on, and spontaneously gave me some decent FM reception. All presets function and the tuning meter works, and its readings nicely coincide with the perceived station strength etc...Maybe the output volume is a bit weak (but this may have to do with the 6 Ohm speakers I have on my work bench...)
However, the "S lights" (arrow lights) do not work, and neither does the 'stereo' indicator. The lamps are o.k. I cannot figure out what is wrong, but then this is the first tuner I am working on...so far all my B&O experiences were rather focused on the power supplies and amplifiers (seems the tuners work most of the time), i.e. my conclusions may be of poor judgement...;-).
Anyway, here is what I checked so far around the S-lamps:
There is zero voltage at the collector of TR9 at all times independent on the field strength reading on the meter, which explains why the lamps are off. The base voltage of the TR9 is just below the turn on point (emitter is at 15.2V, and the base is at 14.7V, i.e. close to the voltage differential one would expect). If I understand the service manual correctly, the transistor is turned on when there is enough field strength, which pulls the base of TR9 down a bit via the RF signal coming out of the filter(?) (at measurement point D) fed in between the two Ge diodes D1/D2, which act as a rectifier/voltage doubler generating a negative voltage:
I hooked up my oscilloscope to D, and all I get there is a constant DC bias (independent of the field strength on the meter), i.e. the capacitor has nothing to transmit into the diodes.
After this I did a little experiment: I put a 1k resistor across the 4.7k base resistor of TR9 to pull the voltage a bit towards the 14V on the other side of the resistor. This produced a collector voltage of ~10V and one of the S-lams came on. I guess this suggests that TR9 and the rest of the S-lamp circuit may work.
Since the tuner is basically working the absence of a signal at D really baffled me...I played a bit more, and tried to follow the 19 and 38kHz signals in the decoder section. The amazing thing: I only saw the circuit diagram-prescribed DC biases on the scope, but nothing of the pilot signals. While this would explain why there is no stereo indicator, but how can the tuner work in absence of these signals?
What am I missing here? Am I making these measurements the wrong way (I connected the oscilloscope between GND and the measurement points)? Any input would be appreciated. I guess I need to brush up a bit on my RF knowledge...;-). Thanks much in advance!
