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Sunday, May 15, 2016

Beogram 4002 (5524): Too Early Arm Lowering and RPM Change During START

The PCBs of the Beogram 4002 (5524) that I just restored came with an indication of 'arm does not lower at 30 cm set-down during START' and that sometimes also the RPM would immediately switch to 45 after pressing START.
This was a difficult problem to trouble-shoot due to its intermittence. After replacing all the electrolytic capacitors and RPM relay and trimmers, I installed the boards in my own 4002 (5513) for testing. At first everything was good and I successfully played a few records. Then out of a sudden the arm would set-down before reaching the lead-in groove of the record. After a few repeat attempts it also started switching the RPM from 33 to 45 immediately after the carriage started moving after pressing START, while continuing setting down too early.

I started doing measurements to verify that the record detection circuit was working properly. My measurements indicated that it did. Then I turned my attention towards the position sensor circuit and then it became apparent that the issue was caused by a strange problem in that circuit. 

This shows the relevant part of the circuit diagram (from my increasingly annotated 4002 diagram..;-):

Here is how this works: TR17 is controlled by the position indicator plexiglass bar with the black markings. Whenever a black marking passes between 4IC1 and the IR diode 4D1 the resistance of 4IC1 increases dramatically and the voltage at the base of TR17 drops, turning TR17 off. This causes the collector voltage to be pulled up to the 21V rail via R85. This in turn pulls up the base of TR7, which then enables the charging of C20 via D22 and D23, which are at about 15.5V from the collector of TR6 when a record is detected by the sensor arm.
However, charging of C20 causes a brief strong current through D22/23 and the voltage at the cathode of D22 drops close to zero for about 50 ms. This negative pulse causes the arm lowering circuit to activate and the arm drops when the bar indicating the 30 cm set-down passes between 4IC1 and 4D1.
When no black bar is present, TR17 is on, pulling the collector down to 0V and the arm lowering cannot happen.

It turned out that the too early arm set-down was caused by TR17 not turning on completely, causing an ambiguous 10-13V at the collector of TR17 instead of a clean 0V. When I measured the base voltage at TR17 while 4IC1 was covered by a black marker, I got about 0.63V, which is a bit too low. My own (properly working) PCB yielded 0.69-0.7V in comparison. This led me to the conclusion that there are some tolerances in the circuit that under some circumstances may yield an ambiguous signal at the base of TR17.
I fixed the issue by exchanging the pull-down resistor R86 from 4.7k to 10k, which caused 'less competition' with the EB-path in TR17, resulting in a stronger pull-up via 4IC1. Once the 10k resistor was implanted the system started working properly and reliably, and I measured a solid 0.7V on the base of TR17.

It is an interesting question to ask why sometimes the RPM also changed during this malfunction. This can be rationalized by the connection to IC2 via R46, which allows to turn on IC2 when no record is present but the wide 45 RPM bar passes through between 4IC1 and 4D1. In that case TR8 is off, allowing voltage build up at the base of IC2 if TR17 is off (20.5V at the collector). Apparently the presence of 10-13V at the TR17 collector is enough to trigger the RPM switch if no record is detected, which is the case before the detector arm reaches the record during the START process.

Ah, the pleasures of analog control systems!!...;-). All this would be so much more straight forward if they had had access to microcontrollers when they designed the 4002. On the other hand, the absence of microcontrollers ensures that the 4002 design can be maintained indefinitely, while a microcontroller based design (as in the Beogram 8000/8002) dies with the microcontroller that controls it due to the proprietary firmware that governs the controller.

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