Beomaster 4000 (2406): Exchanging the Electrolytic Capacitors and Adjustment of the Quiescent Current

After testing the new toroid transformer for a while (and coming to the conclusion that it works very nicely - no hum in the amplification chain whatsoever) it was time to complete the restoration of this Beomaster 4000 and replace all electrolytic capacitors on the various PCBs with new 105C grade major Japanese manufacturer units. While time consuming, this is not too difficult to do, but a few of the capacitors are challenging to access since the 4000 is from a time when board-to-wire connectors were not commonly used in consumer electronics. In other words, it is difficult to take out boards since most connections are soldered. So it is best to just leave everything in and work your way around a few obstacles like the power switch that obscures a solder point or the vertically installed loudness board. But with a bit of patience, a few tricks and a steady hand it is a straight forward process. Here are a few impressions. The first step is usually to take out the preamplifier board that is directly bolted to the frame with four screws next to the DIN jacks on the back. This picture shows the board in its original state:

And after exchanging the capacitors:

Then it was time to work on the two main boards. Unfortunately the preamp board does not disconnect like in the Beomaster 4400 where a rare wire-to-board header allows this convenience. In the case of the 4000 the board needs to be left connected and dangling from the frame while one works on the main PCBs. But this is not such an issue if one props up the Beomaster vertically using a couple of carpenter clamps affixed to the heatsinks (use cardboard shields to prevent scratches).

This shows the output amplifier board in its original condition:

The four red resistors at the bottom are the emitter resistors that can be used to adjust the quiescent current. In between them are the associated trimmers. I replaced these with modern encapsulated units along with the capacitors. This shows the board after restoration:

Next up was the FM tuner and 15V power supply board. this shows the original condition:

And after rebuilding it:

This board has two EMI cans. On the right is the FM 'front end' that selects the right carrier frequency and on the left is the 'detector' that converts the frequency modulation audio information into amplitude change that can be amplified and analyzed into an audible stereo signal. Unfortunately there is one electrolytic capacitor in each can and so one needs to get in there. The easy one is the front end, where the top and bottom covers come off easily. Note that the bottom one has a soldered ground connection which needs to be unsoldered before it can be taken off:

This shows the solder side of the PCB after taking the bottom cover off:

And here is a peek into the top:

The capacitor is in the top part, a brown/black tantalum type. This shows it replaced:

Care needs to be taken to not disturb the inductors. If they get accidentally 'adjusted' one may need a FM signal generator to recover. After putting the covers back on I turned my attention to the detector. Here the board is piggybacked on top of the main FM board and so one has to unsolder it first before the bottom of the PCB can be reached for soldering. The top cover can simply be pulled off. This is how it looks after it is off:

Then I took the board out:

The capacitor is up front center (also brown/black; 10uF). Before one can unsolder it the bottom shield needs to be taken off. One of the pins is soldered into the PCB, making the ground connection, i.e. it needs to be unsoldered first. Then the shield comes off:

And then I was able to put the new capacitor in:

After putting everything back together in reverse I took this picture:

On to the loudness board that is right behind the loudness switch on the front panel. There are two capacitors (red):

I should have replaced them while the transformer was out, but it did not occur to me. So I did some 'artistic soldering'. Copper brain is very useful for such situations when the desolder gun is too big to fit. After a bit of effort I had them out and replaced with new units:

Alright! Almost done. On to the FM preset capacitors that are soldered directly to the preset switches and a ground wire:

I also replaced them with new units:

The next step was to adjust the quiescent current with the new trimmers:

I elected to ignore the prescription of the service manual to measure the current in the output transistors directly with an ampere meter connected into the circuit. The setup allows for that via two pin headers that are not soldered. However, I did to like the idea to connect a multimeter with an internal resistance of about 5 Ohm in mA mode into the circuit and thereby altering the setup. I instead calculated the current that corresponds to the prescribed 80mA quiet current in the 0.15 Ohm emitter resistors in each of the B-style push-pull output branches (12mV), and hooked up my multimeter across these resistors and adjusted the trimmers to get the 12mV. Now this Beomaster is running cool. When it arrived one of the channels got pretty warm indicating trimmer issues, as they are frequently found in this vintage of B&O.