Beogram 4002 (5513): Restoration of the Platter Motor, PCBs, and RPM Panel

Recently, a customer in California sent me the DC platter motor, along with the PCBs and the keypad assembly, of a Beogram 4002 (Type 5513) for restoration. I was given the additional information that the Beogram would blow fuses whenever plugged in.

As usual, I began with the platter motor. The oil infusion of the bearings under vacuum can take up to 3 days, so it was the perfect starting point for this project. This shows the motor as received:

I took it apart to extract the shaft bearings:

The bearings are the two small donuts on the black pad upfront. I submerged them in synthetic oil and pulled a vacuum. Immediately, strong bubbling started:

The bubbling is indicative of air being drawn from the pores of the Oilite bearing material. As the air goes out, the oil goes in! When the bubbling stops, the bearings are replenished and can be used again. While this process was underway, I focused on the main PCB. This shows it in its as received condition:

Here is a detail shot of the 'RPM section' with the original Siemens RPM relay and the RPM trimmers:

Due to the 'blows the fuses' warning I was given, I installed the board in my bench 4002 to see what was going on. I hooked the board up to power using a bench supply instead of the Beogram transformer. This allows ramping up the voltage slowly while watching the current meter. Indeed, already at a voltage below one volt, the current of the bench supply was maxed out, and the current limiter prevented worse things from happening.

The culprit for the short circuit was found quickly: When I removed the screw that holds one of the two Darlington power transistors that are mounted on the solder side, it became apparent that the screw was missing its insulator sleeve and the mica sheet under the package was also absent:

The sleeve and mica sheet prevent contact between the collector of the transistor and ground (via the PCB mounting post that is used for heat dissipation from the transistor in this design). Without these insulators, there is a direct short circuit between the 21V power rail and ground, i.e., the fuses will blow immediately when the Beogram is plugged in. The other Darlington was also missing the bolt insulator:

Someone clearly did not appreciate Ohm's law!...;-).

While the board was still mounted, I replaced the two Darlingtons. This shows 1IC1, which regulates the 21V rail:

I usually replace the original TIP120s with their higher current cousin, TIP102. For some reason, modern TIP devices need some additional capacitance (the yellowish component in the above picture) at their emitters in this circuit configuration. Otherwise, they can develop a high-frequency oscillation superimposed on the 21V rail, which can fool the record detection circuit into believing there is no record on the platter. This subsequently disables the arm-lowering circuit even if there is a record on the platter.

This shows the new TIP107 that replaces the original TIP125 that serves as 1IC4 to control the arm-lowering solenoid:

Then I removed the board and inspected the component side. Immediately, I saw that one of the four H-bridge pnp transistor cans had been replaced with a non-spec type:

A closer look revealed it was a S9012, a pnp transistor rated for 500 mA collector current:

The original BC143 types are rated 1 A. 500 mA may work for some time if the carriage mechanism is in top shape and there is not much mechanical resistance. Then the carriage motor runs reliably below 500mA. 

I replaced all the electrolytic capacitors and the power transistors. This shows the restored board together with the extracted original components:

The Siemens relay was replaced with a Beolover Siemens Relay Replacement for Beogram 4000, 4002, and 4004, and the RPM trimmer received an upgrade with modern 25-turn precision encapsulated trimmers for more exact adjustment:

Next, I focused on the output PCB. This board had been modified by a previous owner of the Beogram:

The circuit on this board delays the output relay so it only opens after the needle has hit the groove:

I replaced the output relay and the electrolytic capacitor that determines the delay:

The picture below shows the RPM panel that is mounted above the keypad. It contains two incandescent bulbs that I usually replace with LEDs. The panel is shown flipped on its back, revealing the two bulb covers:

I removed the covers. This shows the bulbs still installed:

The two small green PCBs are the Beolover RPM Panel LED Backlights for Beogram 4002 and 4004 (Types 551x/552x). The boards are directly soldered to the terminals that connect the wires of the bulbs:

This shows one of the boards installed in detail:

The PCBs do not obstruct the bulb covers, which can be reinstalled after the boards are in place:

Next, I calibrated the DC bias of the sensor arm transistor (1TR3) to yield 4V at its collector:

Then I moved the bias trimmer to the component side:

Whenever work is done on the record detection circuit, it is a good idea to measure the sensor response. This oscilloscope trace was measured at the collector of 1TR3 with the sensor over the empty rotating platter:

Each dip corresponds to a platter rib passing under the sensor. The amplitude of the signal is about 6.3V, a perfect result. This record detection circuit is in good shape!

Meanwhile, after about 48 hrs the bubbling around the motor bearings had stopped. I extracted the bearings from the oil:

I reassembled the motor and installed all the components for testing in my bench Beogram 4002. I ran a 24-hour RPM stability test with the BeoloverRPM device:

The BeoloverRPM has two operational modes. In 'slow' mode, it measures the RPM in 10-second intervals and relays the measurement to a serial port of a computer. This allows graphing the RPM over long periods of time using Excel or similar software. This shows the result of a 24 hrs measurement:

This result is pretty much as good as it gets with the DC motor Beogram 4002. 

In the 'fast' mode, it transmits an RPM measurement every time a platter rib passes under the sensor. This yields high-resolution graphs that show short-term RPM changes ("wow and flutter") in detail. This graph shows a measurement covering about 35 turns of the platter, representing a run time of a little more than 1 min:

The zig-zag pattern is a measurement artifact that originates from small spacing variations between the platter ribs of my bench Beogram. All Beogram platters have such variations due to manufacturing imperfections. This generates a repeating pattern every 24 measurements (there are 24 ribs around the platter), which is superimposed on the real RPM changes that are introduced by the feedback system that keeps the motor RPM stable over time. This real RPM change is essentially the sine-wave-like pattern that modulates the zig-zag pattern. An evaluation of the wavy component yields a wow and flutter estimate of about 0.1%. This is 2x of the 0.05% stated in the specs list in the service manual.

    This difference is most likely systematic due to the entirely different way wow and flutter were measured in the 1970s when these Beograms were produced. Back then, the measurement was carried out with a 1 kHz tone on a test record. In these measurements, deviations from the 1kHz center were measured with an analog spectrum analyzer and then converted into a wow and flutter number. It should be pointed out that this discussion is pretty academic since humans typically start recognizing frequency fluctuations above the 0.7% threshold, i.e., the RPM fluctuations of this Beogram are well below this threshold, whether the number is 0.05% or 0.1%. This motor is definitely ready for duty again!

This concluded my work on the received parts, and I will soon send them back to my customer in California.

 

Beogram 4002: DC Platter Motor Restoration

I recently received the DC platter motor of a Beogram 4002 from a customer in Ohio. The deck showed the usual RPM variations indicative of dry motor bearings.

This shows the motor as received:

I disassembled the motor to extract the bearings:

The bearings are the two small donuts on the black pad. I immersed them in synthetic oil and pulled a vacuum. Immediately bubbling started around the bearings:

The bubbles represent air being drawn from the porous Oilite material. As the air leaves the bearings fresh oil can diffuse into the evacuated pores. The process took about 2 days until the bubbling stopped.

I reassembled the motor and then it was time for a 24 hrs RPM stability measurement with the BeoloverRPM device. It is able to log the RPM every 10 seconds into a serial port of a computer. This shows the BeoloverRPM in action:

This is the data I collected over 24 hrs:

There is still a degree of slow long-term RPM variations. These should go away after playing a number of records, which will polish the bearing surface in its new position. This motor is ready for duty again!

Beogram 4004: Restoration of the DC Platter Motor

I recently received the DC platter motor of a Beogram 4004 from a customer in Minnesota for an oil infusion of its bearings.  

This shows the motor as received:

I disassemble it to get the bearings out:

The bearings are the two small donuts on the black pad upfront. I immersed them in synthetic oil and pulled a vacuum. Immediately bubbling started:

The bubbling represents air being drawn from the porous Oilite bearing material. As the air leaves the material oil can diffuse into it. This process can take up to three days. After the bubbling stopped I extracted the bearings from the vacuum chamber:

Then I re-assembled the motor and installed it in my bench Beogram 4002 for a 24 hrs RPM stability test with my BeoloverRPM device:

In its 'slow' mode the BeoloverRPM is able to log the RPM in 10s intervals into a serial port on any PC. This is the curve I had measured after about 24 hrs:

This graph is about as good as it gets with Beogram DC platter motors after restoration. In my experience the slight choppiness will go away over time as the top bearing settles in. This motor is ready for another tour of duty!

Beogram 4004: Restoration of PCBs, DC Platter Motor and RPM Panel

I recently received the PCBs, the DC platter motor and the RPM panel from a Beogram 4004 for restoration. As usual I started with the platter motor since it can take a few days until the oil infusion of the bearings is complete. This shows the motor as received:

I disassembled the motor to extract the bearings:

The bearings are the two small donuts on the black pad upfront. I immersed them into synthetic oil and pulled a vacuum. Immediately strong bubbling started:

This bubbling represents air that is drawn from the empty pores of the Oilite bearing material, making room for oil to diffuse into the material.

While this process was going on, I focused on the other tasks of this project. First I restored the main PCB. This shows it in its original condition:

Here a close up shot of the RPM section consisting of a National brand RPM relay and the two RPM trimmers to its left:

I replaced all electrolytic capacitors, power transistors of the H-bridge and platter motor control and the RPM relay and trimmers:

This shows the rebuilt RPM section with a new Beolover National Relay Replacement for Beogram 4002 and 4004 and two encapsulated 25-turn 5kOhm RPM trimmers:

Next came the output PCB:

In the Beogram 4004 this PCB carries both the output relay delay circuit as well as the remote control circuitry that allows a Beomaster 2400 controlling start and stop of the Beogram via its remote control.

This shows the output relay circuit:

I replaced all the electrolytic capacitors and the output relay:

Here a detail shot of the output relay:

I also installed a (red) switch that allows connecting system and signal grounds in case there is a hum in the output signal.

Finally, I updated the RPM panel with LEDs. This shows the panel on its back which reveals the two bulb covers:

I removed the covers

and replaced the bulbs with Beolover RPM Panel LED Backlights for Beogram 4002 and 4004 (Types 551x/552x):

These little PCBs solder directly to the terminals where the original bulbs connected. They essentially become extensions of the original PCB:

The boards do not interfere with the bulb covers, which can be installed like before:

Now it was time to implant all the components into my bench Beogram. After bolting in the main PCB I replaced the two power Darlingtons mounted on the solder side of this board. It is best to replace these while the board is bolted in. This makes positioning a snap. This shows the original IC1, a TIP120, which is responsible for regulating the 21V rail:

I usually replace these ICs with stronger types. In this case a TIP102 is the perfect replacement:

In this circuit configuration the modern TIP packages need a 100nF capacitor soldered between the Emitter (output) and ground. This quenches a high-frequency oscillation that can occur after the replacement. I also replaced IC4, the solenoid transistor, with a TIP107 Darlington:

One more item was to be done on the main PCB: Adjusting the sensor transistor bias to yield 4V at its collector

then moving the trimmer to the component side

and verifying the sensor signal with my oscilloscope:

The signal was very good with an amplitude of ~5.7V. Each dip corresponds to a platter rib passing under the sensor. Of course this signal may be different in my customer's Beogram depending on how bright the light bulb still is and whether the photocell is o.k.. But the trace demonstrates that the circuit is up to the task.

In the meantime, the oil infusion of the motor bearings had come to an end and I extracted them from the vacuum chamber:

I reassembled the motor and implanted it along with the PCBs and RPM panel in my bench unit. Then it was time for a RPM stability test with these components. This shows the BeoloverRPM device in action:

In its 'slow' mode it outputs the RPM in 10s intervals. Using a terminal program on a PC or Mac the RPM can be logged for extended periods of time for generation of a RPM stability plot. This shows the 24 hrs RPM stability graph I measured for this motor:

It is slightly choppy. This is a result of this motor having been opened before without noting the original orientation of the top bearing. If the bearing is not installed in the same orientation there may be a period where the shaft polishes a new segment of the bearing as it is pulled towards the platter by the belt. In my experience these small variations slowly go away after playing the deck for a while. At any rate these fluctuations are much smaller (~0.3%) than what humans can typically discern when listening to music (>0.7%). So this motor is ready for duty again!

I also measured 'wow and flutter' (the short term RPM fluctuations introduced by the feedback speed control system) using the 'fast' mode of the BeoloverRPM device. In the fast mode it logs the RPM after every passing of a platter rib. This reveals a pattern that repeats every 24 measurements, i.e. after each turn of the platter. It is a result of minute spacing variations between the ribs due to manufacturing tolerances. The wow and flutter RPM changes are superimposed to this pattern as a sine-wave like feature, which is normal for analog feedback-based control systems:

Evaluation of this pattern yields a wow and flutter number that is smaller than 0.1%. This is slightly larger than what the manual specifies (<0.05%), but this may well be a result of the different way they measured wow and flutter in the 1970s, where they used a test record and an analog filter based 'frequency analyzer'. The joys of analog audio!...;-).

In summary, this was a successful restoration project and these components are ready for duty again!