Beogram 4004 Issue: STOP (>>) Function Permanently Engaged

A customer from Canada recently sent me the boards of his Beogram 4004 for trouble shooting. The fault the deck exhibited was this:

When the START (<<) button was pressed the deck very briefly came on, the carriage moved a few mm and then it immediately returned home and the deck shut off. This issue typically arises when the STOP (>>) function is permanently engaged. Normal examples would be a stuck actuator under the STOP key or a damaged OFF switch on the board under the carriage that contains the carriage position sensor. 

In this case these standard root causes did not check out. When I received the boards I first tested the main PCB in one of my Beograms together with a Beolover RIAA pre-amplifier board that I had already installed in this Beogram. The RIAA board works together with any 4002 or 4004 main PCB. When I plugged the Beogram in and pressed START everything worked normally. This immediately told me that most likely the 4004 output board was the trouble maker.

I replaced the RIAA board with the 4004 output board and tested again. This indeed brought about the described malfunction. I looked at the circuit diagram of the 4004 output board and identified two transistors that are able to initiate the STOP function, TR3 and TR4:

They essentially are used by the Beomaster 2400 remote control function to initiate STOP/OFF when a different source is selected on the Beomaster or it is turned off.

P10(8) connects directly to the OFF switches, i.e. the Beomaster simply 'simulates' the actuation of the OFF switch under the carriage when it wants to shut the Beogram off:

Since TR4 needs to produce a pulse for OFF initiation it was most likely not the culprit. Dead transistors usually do not make pulses, they are either permanently off or on. So I replaced TR3 with a new BC547B and put the board back in. I pressed START and the carriage moved normally to the LP setdown point and the solenoid actuated. Perfect! I tested all the other keypad functions and everything appeared to work normally. This Beogram 4004 appears to be back in business! 

Beogram 4002 (5501): Repair of a Strange Fault Caused by a Shorted Diode D6

After replacing the reservoir capacitors in the Beogram 4002 (5501) that I am restoring right now, a strange control system malfunction remained. Here is a description:

After pressing START, the carriage would start moving but then after about 20 mm it would stop and the lights would go out. Then after a few seconds, the lights would come back on and the motor would run on 45 RPM. Once on 45 RPM, I was able to drive the carriage with the >> << buttons, but arm lowering would not work anymore. Switching back to 33 RPM killed everything again, but lowering would briefly function before the lights went out. Pretty strange all this!

Luckily the root cause turned out to be an easy fix. Here is the relevant part from my annotated circuit diagram:

After a bit of head scratching and doing some measurements, it occurred to me that a short circuited D6 would kill the 22V rail if TR2 is on, as it is the case when 33 RPM is selected. This was quickly confirmed by a 0V diode tester reading across D6 in both polarities. After replacing D6 with a new 1N4148 signal diode everything was working again properly.

This is how this fault 'worked': When TR2 is on then its collector is on GND level. If D6 is making a short, GND is applied to R2/D1 (dashed red lines). This kills the voltage on the base of TR1 which then kills the voltage on the base of 0TR1. This turns the 22V rail off and the lights go off. The time delay that was inherent in this process is enabled by C1, which preserves the base voltage on 0TR1 for a couple seconds until the capacitor is empty. At this point TR2 turns off. This allows the voltage to recover since now the anode of D6 is no longer grounded. So it does not matter if the diode is shorting. The voltage rail recovers, and that turns TR3 on via L1 and R8, and the deck is now operating on 45 RPM. Ahhh, the mysteries of analog control systems! This is Beolove!

Beogram 4002 (5513): Sudden RPM Changes

I thought I was pretty much done with the Beogram 4002 (5513) that I recently restored and I started testing it by listening to my favorite vinyls. After a while I realized that the RPM drifted over time. This was cured by replacing the original RPM relay with a drop-in replacement based on a modern encapsulated signal relay and exchanging the original principal 5k RPM trimmers on the circuit board with modern encapsulated multi-turn precision trimmers. Here are a few impressions:

This shows the original RPM relay on the main PCB:

This relay is the same as used in the Beogram 4000, so my recent design of a replacement of the 4000 relay could be fitted here, too. This shows the drop-in replacement board:

And installed:

Above-left of the relay are the original 5k trimmers. I replaced them with these:

The lead-extensions were needed since I had to install them upside down to make sure that the set screws are accessible through the PCB from the solder side. Otherwise, it would be impossible to tune the RPM while the deck is running:

Here is a top-view:

This solved the RPM drift issue. However after a lot of listening to vinyls, most notably an original first pressing of a 12" single of Grace Jones' "Slave to the Rhythm", it became apparent that in addition to the drift phenomenon there was an other, more serious issue: Sudden sharp drops in RPM that would only occur on relatively rare but completely random occasions. Not so great! I decided to get to the bottom of this and I connect my oscilloscope to the signals around IC3 that controls the DC motor feedback control loop. To observe these signals while a record is playing one needs to solder jumper wires to the interesting signals of IC3:

This enabled to hook up my 4 channel oscilloscope:

Here are the traces that I measured during the occurrence of the phenomenon. Luckily once the issue started to happen it reoccurred a few times in a row, allowing me to capture traces with the single shot button. This shows the phenomenon at 33 RPM

and the much more frequently occurring analog at 45 RPM:

The traces are assigned as follows (see circuit diagram below): blue pin 1 (this is the AC induction signal from the DC motor feedback coils), yellow: pin 6 (this is the pulse width modulated output from the Schmitt trigger that corresponds to the amount of power the motor receives, green pin 7 (this is the RC network that determines the actual pulse width that the Schmitt trigger puts out, and red pin 4 (this voltage relative to the TR2 stabilized 8.5V output corresponds to the voltage the motor receives).

All this was pretty interesting: The DC motor of the Beogram 4002 (5513) is a 3-pole motor. This means that each of the three coils on the rotor is fully powered up every six commutator contact changes (see here for some awesome animations and basic info). This suggests that the observed aberration in the pin 1 traces are related to one specific cycle of the motor rotation since the 'missing valley' phenomenon occurs every 6 wave cycles. So I took the motor apart again and throughly cleaned the commutator and the brushes and put it back together. This made the phenomenon much rarer but did not alleviate the issue completely. I may go ahead and put the motor of a parts unit in for now until I figure out how to fully fix this. The issue with intermittent problems is that they are intermittent. So it is difficult to make sure that a fix 'sticks'  100%. All this shows the principal issue with DC motors in turntables: They need a precisely working feedback mechanism to keep the RPM constant. All this is a non-issue with AC synchronous motors that simply 'obey' a sine wave that is fed into them.