This is a follow up to my recent post about the redesigned Beogram Commander remote control board, which now works in both (DC-motor) Beogr...
Sunday, May 24, 2020
Beogram 4002 (5513): Restoration of PCBs and RPM Adjustment Panel
A while ago I restored a Beogram 4002 DC platter motor for customer in California. Unfortunately, there were still noticeable RPM fluctuations at 45 RPM after re-installation of the motor. This usually points towards an oxidized RPM relay. They tend to go bad on 45 first, since in most Beogram they stay put at 33 RPM and are only rarely switched to 45. This allows oxide to grow on the 45 RPM contact terminals. My customer decided to let me restore the rest of the 'RPM chain', and I recently received the PCBs and RPM adjustment panel of this Beogram.
I started with the main PCB:
I replaced all electrolytic capacitors and then installed a new RPM relay and new RPM trimmers. This shows the original parts:
And the new ones:
Then I installed the board and tested the record detection sensor signal at the collector of TR4 with an empty platter turning:
This was a pretty weak signal, an indication that TR4 had lost some of its gain (Hfe). This shows the original TR4, a BC138C with its 1MOhm biasing resistor in the back:
I installed a new 2N5089 high-gain transistor with an Hfe of about 750. Correct biasing of this transistor requires replacing the 1MOhm biasing resistor with a multi-turn 5MOhm trimmer, which I first installed on the solder side, that I could adjust the collector voltage to the specced 4V:
Then I put it 'below deck' in the original place of the resistor:
This is the sensor signal I measured on the empty platter after this procedure:
The valleys (that correspond to the black ribs on the platter) now go close to 0V, which is what the service manual prescribes.
This concluded the main PCB restoration:
On to the output PCB, which also has an often oxidized relay. this shows the original board:
I installed a new relay, replaced the capacitor that is responsible for the time delay for actuating this relay after the needle sets down on the platter, and I added a switch that allows the convenient connection of system and signal grounds in case there is a hum issue:
The remaining task was replacing the incandescent light bulbs that illuminate the RPM panel scales with LED fixtures. This alleviates the remaining cause for RPM fluctuations which can also be caused by thermal changes due to the heat emanating from the light bulbs. This shows the original bulbs installed:
I replaced them with LED boards:
and put the light shields back on:
Now it was time to install everything in one of my 4002s for testing. This shows the 33 RPM LED in action:
The red-green LEDs that I use for these panels allow to give them a realistic incandescent-like sheen, and also properly illuminate the red indicator. Beolovely!
On to testing the RPM performance with the BeoloverRPM device:
The BeoloverRPM device is able to log the RPM over extended periods of time. This is the curve I measured over about 24 hrs at 45 RPM:
This is as good as it gets with the DC motors, i.e. this Beogram should be back in business!