Beogram 4002 platter motor running too fast — 55 RPM at 33 setting

[Nik]

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.

[Mark-sf]

First, I would check all of the voltages around the 1IC3 and the 8.5V at P1 Pin 3. While you’ve replaced the electrolytics, sometimes the tantalums can also fail. If you have a scope or frequency meter you can check the feedback frequency of P1-1 which should be 125Hz for 33 and 167 for 45. Now all of this pertains to the DC gold motor.

If you have the older AC silver motor then the frequencies are 42.3 and 57.3 respectively at 5V AC. You should have also changed the 4000mf motor cap. You should also get 11.4VDC on its transistor side.

[Nik]

Hi, thank you for your advice.
I’ve already measured the IC3, got following measurements:

• leg 1 = 6.06V
• leg 2 = 5.4V
• leg3 = 1.27V
• leg 4 = 5V
• leg 5 = 7.75V
• leg 6 = 6.77V
• leg 7 = 7.35V
• leg 8 = 8.56V

so quite close to the service manual.
I did replace most of the tantalum capacitors as well (all but 2.2µF 35V, they were not in stock unfortunately)
Unfortunately I don’t have a frequency meter, just a multimeter (also not the best one)
I will check the transistor voltage tomorrow.

What I did as a temporary fix – I installed 50kOhm potentiometers, so I have wider adjustment range for the speed. With this I was able to drop the speed to correct values, but the W/F is still around 1% (measured with RPM app on iOS) which is of course not great.
And also I don’t like using not spec parts and would love to find the actual reason for this behavior and not just apply bandaid on it.

[Nik]

Also just measured the P1 to ground:
Pin 1: 8.63V
Pin 2: 5.9V
Pin 3: 8.62V

 

[Author]

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