Dear forum members,
I am restoring a Beogram 4002 and would appreciate some help with a platter motor speed issue.
So far, I have done the following work:
- Relubricated the mechanical parts
- Re-infused the motor bearings with fresh oil
- Re-veneered the wooden trim (not relevant here, but still cool 😀)
- Replaced the burned-out carriage motor H-bridge components
- Replaced electrolytic capacitors
- Replaced the 33/45 speed adjustment potentiometers with new 5 kΩ parts
The unit now works and looks almost perfect, but the platter motor speed is far too high.
Current measured speeds:
- 33 RPM setting: around 55 RPM
- 45 RPM setting: around 67 RPM
Adjusting the speed potentiometers does not bring the speed anywhere close to correct.
I have checked the motor feedback coils during reassembly, and checked them again today. Both measure around 18 Ω, so they appear to be intact.
One strange observation:
If I touch/bridge R18 or R19 to what I believe is ground — a large solder pad on the main board — using my finger, the platter speed drops. If I do this to both R18 and R19 at the same time, the speed drops even more. If I make the same bridge with metal, the motor stops completely.
This makes me suspect that the speed control circuit is not receiving or processing the feedback signal correctly, but I am running out of ideas on where to look next.
Could anyone suggest what I should test next? In particular, I would be grateful for advice on which transistors, resistors, capacitors, or IC pins in the platter motor speed control circuit are worth checking.
Thanks in advance for any guidance.
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