Beomaster 2400 (Type 2902): Repair of Permanently Illuminated FM Stereo Indicator

After repairing the weak FM reception issue of a Beomaster 2400 that I am currently restoring, I had to accept that the unit had yet another issue: The FM stereo indicator was permanently on whether an FM station was dialed in or not, or even whether FM was selected at all. The indicator was on all the time! If everything works well, the green indicator only comes on when good FM reception is achieved.

A glance into the circuit diagram revealed that the stereo light is directly controlled by pin 6 of the monolithic MC1310 FM Stereo Demodulator IC that extracts the audio signal from the FM broadcast signal.

This left little room for ambiguity: This clearly meant that the bulb driver integrated into the IC had failed. The good news here is that this IC was pretty popular in the 1970s and 80s and so it can still be found as NOS. I ordered a bunch from a vintage IC vendor in China. I received the package a couple weeks later. 

I put the Beomaster back on the bench. This shows the original MC1310 soldered in place:

It is implemented in a strange way with the pins bent in an alternating way. This shows the solder side of the board with the pins already unsoldered:

I removed the IC and it looked like a little cockroach with its bent pins:

Buying NOS ICs is always a bit risky, and I expected having to do a bit of triage to find a properly functioning IC among the ones I received. So I desired having an IC socket implanted that would allow me to insert a few of the chips until I found one that would work properly. I had a few 14-pin IC sockets in my stash with long pins. This shows one of them next to the original IC:

I cut and bent the pins to match the pattern of the original IC. It turned out that the best way to achieve a good fit is to try inserting the socket from the solder side first to make sure the pin pattern is matching the solder points:

This shows the socket with the pins bent into a machining pattern:

I inserted the socket from the component side and soldered it into place.

Then I thoroughly cleaned the pins of one of the NOS ICs, coated them with De-oxit, and stuck it into the socket:

I plugged the Beomaster in and fired it up. The stereo light was off! Yes!! But my euphoria subsided quickly: It turned out that only one or two stations across the dial would achieve stereo reception. So something was wrong. Normally, most local stations in Albuquerque will come in with a nice, strong stereo reception on a Beomaster 2400!. 

The most likely culprit was the VCO (Voltage Controlled Oscillator) frequency. The 1310P uses a Phase-Locked Loop (PLL) to lock onto the 19kHz pilot tone. If the VCO frequency of the 1310 is off, it becomes impossible for the PLL to achieve a lock. I measured the VCO signal on 2TP4 with my oscilloscope:

 

The trace yielded a frequency of 19.8 kHz, i.e. a bit too high. The frequency of the VCO can be adjusted by tuning 2C24 as shown here:

Turning 2C24 a bit resulted in a clean 19kHz signal:

And this fixed the issue! Now the stereo light behaved as it should: Strong stations bring it on and weak stations do not. It looks like the tuner department is finally sorted out! This project may finally come to a successful end! Stay tuned for the full restoration report!