Beogram 4002 (5523): Rebuilding the Control System and RPM Stability Qualification with the BeoloverRPM Device

After fixing the basics (tracking system and platter motor) in the Beogram 4002 (5523) that I am rebuilding right now it was time to get the electronic control system up to spec. An important part is to replace the electrolytic capacitors, replace the baseline RPM trimmers and the RPM and output relays. I also installed a grounding switch that allows to connect system and signal grounds, which is often helpful if there are ground loop/humming issues.

Here are a few impressions: 

This shows the main PCB before rebuilding:

A detail shot of the RPM relay and trimmer section before restoration:

These were the days when they still made relays in Germany...;-). Anyway, after 40 years even a Siemens relay has corroded contacts, and it is a great idea to replace it with a modern encapsulated Japanese quality relay. Another important upgrade for these turntables is to replace the single turn RPM trimmers with 25x multi turn units. Only such trimmers allow a precise RPM calibration. This shows the new relay and the two 33 and 45 trimmers:

They are implanted in a way that their trimmer screws are accessible from the other side of the PCB, that one can adjust them while the PCB is installed.

This shows the main PCB after rebuilding together with the exchanged parts:

I am always a bit sad when the colorful Ta capacitors are replaced by drab modern black aluminum cans. But it is a good idea to exchange the Ta units with modern Japanese 105C grade components since they can literally go up in smoke if their age related failure mode is a short circuit between their electrodes.

After doing the main PCB it was time to focus on the output board that carries the output relay. this shows the original board. It has the same Siemens relay like on the main PCB:

This shows the board after replacing the relay and putting in a new delay capacitor, and doing the ground switch upgrade:

The last step of the overhaul of the electronic system was to deoxidize the contact tabs in the keyboard switches:

This deck has unadulterated tabs, so I did not have to take them out for straightening. I pulled 2000 grit sand paper through the contact areas to clean them from oxidation. Then I coated the tabs with DeoxIT D100 which I applied using a cardboard strip coated with it.

After this it was time to put everything back together and check the motor RPM and do an adjustment of the baseline RPM settings with my recently developed BeoloverRPM device:

After adjusting the 33 and 45 RPM setting to spec it was time to do a RPM stability test. The BeoloverRPM device allows to log the RPM over many hours in 10 sec steps. It is also sensitive to sudden deviations and registers them at any time they occur via an analysis of the standard deviation of the measurement. Here is a graph of the RPM performance of this unit over about 8 hrs:

This is a pretty good result as far as I can tell. There are some small variations in the 0.05% range, which is about the 'wow and flutter' spec stated in the service manual. There is also a slow superimposed RPM drift of a similar magnitude, which may be temperature related.

Since I currently do not have any way to qualify the measurement error of the BeoloverRPM device other than putting it into my Beogram 8000 with uProcessor controlled RPM, I am thinking that the actual variations are probably a bit smaller than what is shown here since any measurement errors should add to the actual variations since they are independent.

Anyway, after this test, I plugged it into my Beomaster 6000 4-Channel and listened to my latest vinyl acquisition, "Aqualung" by Jethro Tull (Steven Wilson Remix), and thought that this deck sounds pretty lovely! This is Beolove!