Beogram 4004: Repair of Seized Platter Motor and Its Drive Circuit, and a Few Adjustments to Get it Going Again

Recently I received a Beogram 4004 from the UK for repair. The unit did not drive the platter anymore. I The first step was to extract the DC platter motor for restoration:

I tried to run it with my bench supply, but the rotor would not turn. In fact I was not even able to turn it by hand. It was completely seized. I pulled the pulley off and extracted the motor from its outer housing. Then I put a drop of WD40 on the shaft where it meets the top bearing and let it sit for a while. After the WD40 had done its job, I was able to move the shaft and extract the rotor from the housing when I took the motor apart. This shows the disassembled motor with the bearings extracted for oil infusion:

I submerged the bearings in motor oil and pulled a vacuum:

Immediately, vigorous bubbling started indicating that the vacuum drew the air from the empty pores of the Oilite bearing material. This created room for oil to replenish the bearing. After about 72 hrs the process finally stopped and it was time to extract the bearings:

I reassembled the motor and put it back into the Beogram 4004 for testing. I pressed start: And nothing happened! The motor did not turn. Since it ran nicely from the bench supply, there had to be something wrong with the motor drive circuit. I checked the voltage at the emitter of 1TR2, and it was zero. Normally it should be about 8-9V. TR2 supplies the motor circuit with a stabilized current, but not now. This suggested that TR2 had a problem. I extracted it and checked it with a transistor tester:

The transistor tester saw it as a diode, i.e. one of the junctions was open circuit. I replaced it with a TIP120 transistor which is a bit stronger (5A) than the original TIP31 3A transistor. Using another TIP was also convenient to maintain the same package layout, so it would fit into the same spot on the circuit board (it's heatsink is soldered to the board with a feedthrough plug). My theory is that this transistor died due to the seized motor drawing maximum current for a while at some point.

After installing the TIP120 the motor started running and I moved on to replace the RPM relay and the RPM trimmers, frequent sources of RPM instabilities in DC motor Beograms. This shows the original setup of the RPM section of the PCB:

And after installing the new relay and trimmers:

After this I installed the board again, and checked the sensor signal to make sure the record detection mechanism was up to snuff:

This signal looked good, so it was time to measure the RPM stability over a 24 hrs period with the BeoloverRPM device:

The BeoloverRPM allows recording the RPM for extended periods of time in 10s intervals. This is the curve I measured:

This measurement is as good as it gets with Beogram DC platter motors.

My next focus was the arm lowering mechanism, which was sluggish. This was apparent when I played a record and the needle dragged over the platter when the carriage returned. This usually indicates that the lubricants are hardened and that the mechanism needs to be taken apart for cleaning and lubrication. This shows the mechanism:

I disassembled it

and cleaned everything in an ultrasonic bath. Then I put everything back together and the the arm lowered and raised smoothly again.

Now it was time to adjust the tracking weight. It turned to that the adjustment mechanism was seized. I was not able to turn the adjustment wheel at all. I took it off and it turned out that the secondary pinion was stuck in its orifice in the arm base:

I put a drop of oil on the shaft and then it started moving again. I put the adjustment wheel back on and calibrated the counterweight to make the scale accurate between 1 and 1.2g which is the range that counts for B&O cartridges:

Then I adjusted the arm lowering limit to prevent future damage to a cartridge, and then it was time to play a record. It performed admirably and if it can repeat the task a few more times it will be time to send this Beogram back to the UK.