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!

Beomaster 2400 (Type 2902): Repair of Weak Tuner Reception

I am currently restoring a Beomaster 2400 (Type 2902) for a customer from Virginia. After doing all the standard restoration tasks (replacing all the electrolytic capacitors, implanting new quiescent trimmers and installation of the Beolover LED Replacements for Beomaster 1900 and 2400 Indicator Light Bulbs I fired the unit up and it worked beautifully with a Beogram 4002 plugged in.

After enjoying a couple of records I noticed I got only static from the FM tuner no matter where I set the dial. Normally, the 2400 produces a pretty strong stereo signal from local stations without even plugging in an antenna. I plugged in an antenna and now I was able to pull in some stations, but with a loud hum in-between them and in poor sound quality. So something was definitely wrong with this tuner!

I pulled out the circuit diagram and started scratching my head. I measured the voltages around the JFET transistors in the front end cascode. The circuit diagram prescribes 3.6V at the gate of 1TR1 but I got only about 440mV. Way too low!

This shows the relevant circuit diagram section (click on it to see a magnified version):

The "1" section is the tuner 'front end' that is contained in the shield can next to the antenna connector of the Beogram. TR1/TR2 are its cascode amplifier stage. This design essentially results in a very high input impedance 'transconductance' amplifier that also has a low capacitance to be able to work at high frequencies. The gate of 1TR1 is connected via two resistors 330k and 1M to the emitter of 2TR1 in the IF stage of the tuner (yellow wire that goes into the box). This connection is essentially the Automatic Gain Control (AGC), which changes the gain of the cascode stage to keep the signal at a similar level for strong as well as weak stations to prevent overloading of the IF stage. So the 3.6V shown in the circuit diagram is for a specific signal strength coming in at the antenna port, "500uV with pilot signal" as specified in the manual and will vary with the signal strength of the station that the tuner is set to.

After measuring the too low gate voltage I initially thought the cascode transistors had a problem (they can easily get damaged since they are not protected from static charges coming in through the antenna port). So I ordered a few BF256 JFETs that can still be found new and that have similar parameters like the original TIS88A that have been long obsolete. But installing them brought no change. The gate voltage remained at 440mV.

This made me think that 2TR1 in the IF stage on the main PCB ("2") may have an issue since its emitter essentially defines the gate voltage of 1TR1 (yellow wire that goes into the front end can). I measured the DC voltages around it, and indeed I found only about 500mV at the emitter instead of the prescribed 4.6V in the circuit diagram.

This transistor is originally a BF240 which is also obsolete these days. The good news is that it can be replaced with still available MPSH10 types. I put in a new MPSH10

and this fixed the problem. Now the tuner pulled in local stations perfectly without antenna and the loud hum between stations was gone, too! I was now able to measure voltages between 2.1V and 4.1V at the 1TR1 gate depending on what stations I tuned in and whether I had the antenna plugged in or not. The lower voltage corresponded to stronger stations and the higher to weaker ones. The red voltages in the above diagram were measured with the BF256 and MPSH10 in place. 

There is one remaining issue with this Beomaster 2400: The tuner stereo light is permanently on, even when Phono or Tape is selected. Since the light is directly connected to pin 6 of 2IC2 via a resistor I am afraid the IC has an issue. Luckily the MC1310 is a popular monolithic FM stereo demodulator used in many tuner designs of the 1970s and 80s, and it can still be found at NOS dealers. I ordered a few. Stay tuned! (pun intended!...;-).