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!...;-).