Beogram 4002: Restoration of DC Platter Motor

I recently received a Beogram 4002 DC platter motor from a customer in France for restoration. His Beogram had the usual randomly occurring RPM variations. Something that will most likely be fixed by re-infusing the dry motor shaft bearings with oil under vacuum. Most of these motors have bearings that lost all of their stored oil, and this causes the shaft friction to increase significantly.

This shows the motor as received:

I disassembled it to extract the shaft bearings:

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

The bubbling indicates that air is being drawn from the pores of the Oilite-bearing material. The leaving air is successively supplanted by oil in a diffusion process. The process has completed when the bubbling stops. In this case, this happened after about two days. At that point, I extracted the bearings from the vacuum chamber:

Then I reassembled the motor with a new Beolover DC Platter Motor Pulley for Beogram 4002 and 4004 and installed it in my bench Beogram 4002 for testing with the BeoloverRPM device:

The BeoloverRPM is able to log the RPM in 10s intervals into a serial port of a computer over long periods of time. This allows precise detection of intermittent RPM deviations that otherwise would be difficult to detect. This is the graph I measured after about 24 hrs:

This flat curve is 'as good as it gets' with Beogram DC platter motors! This motor is ready for duty again! It is time to send this motor home to France for another tour of duty in its Beogram 4002!

 

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 DC Platter Motor Restoration Using Dksoundparts Bearings

I recently received the DC platter motor of a Beogram 4004 from a customer in the Netherlands. This customer wanted me to install new reproduction bearings he had purchased at the dksoundparts store.

Normally, I re-infuse the original bearings with oil under vacuum. So it was an interesting experiment for me to see if there is a difference in outcome. 

This shows the motor as received:

I took it apart as usual:

The original bearings are on the black pad up front.

The new dksoundparts bearings came in a small metal can:

I took them out of the can. They are oily since they were also infused with oil under vacuum:

This shows them installed in the top and bottom parts of the enclosure:

If you try this at home, make sure the larger bearing goes into the top part and the smaller into the bottom.

After closing the motor up it was time for a 24 hrs RPM stability test with the BeoloverRPM device:

In its 'slow' mode the BeoloverRPM sends an RPM measurement every 10 sec to a computer serial port.

This is the curve I measured after about 24 hrs:

This curve looks a bit 'wilder' than the curves that I get for most of my restorations using the original bearings (see for example this restoration project for a comparison). This does not really come as a surprise to me since over time I came to realize that it is important to re-install the original bearings in the same orientation as they were installed originally. This significantly shortens the time it takes to break in the bearing. It seems the bearing surface gets polished around the main contact area defined by the pull of the platter belt, which over time lessens the amount of RPM fluctuations. 

When installing new bearings there is no already polished area, so they always need to get broken in. I expect this motor will probably clam down after playing it for a few hundred hours. I suspect that new Beograms back in the 1970s had this issue, too, but no one noticed it since these variations are well below the threshold humans can detect.

I thought it would be interesting also measuring wow and flutter (short term RPM variations introduced (mainly) by the feedback loop that keeps the RPM constant in DC motor Beograms).

This can be done in 'fast' mode with the BeoloverRPM.

This is the curve I measured during about 60 platter rotations:

In fast mode the BeoloverRPM logs a RPM measurement every time a platter rib passes under the sensor. In other words one gets 24 measurements per rotation. The above graph looks 'complicated' since the actual RPM variation measurement is superimposed by a measurement artifact caused by minute variations in spacing between the platter ribs around the platter. Since the time between ribs passing is measured and then converted into an RPM value, changes in the spacing results in proportional changes of the calculated RPM. This causes the observed zig-zag pattern (the 'platter pattern'), which is added to the actual RPM variation. The true RPM variation in turn corresponds to the slower wave pattern that is superimposed onto the platter pattern. An evaluation of the graph seen here suggests an RPM variation of maybe 0.2%. 

When using the original bearings I usually get something around 0.1%, i.e. a slightly better performance. This probably also has to do with the rougher surface inside the new bearing that challenges the feedback loop a bit more causing more variation.

It would be very interesting to measure this motor again in a couple years after this Beogram played a few hundred records. I would expect to see a better performance.

After this experience, I think one can say that it may be better to use the original bearings if possible. But the new bearings available at the dksoundparts store are a viable alternative if a vacuum setup is not at hand. It would be interesting to know how the results with new bearings compare statistically, i.e. after doing a larger number of motors with them. What I know from my restorations with re-infused original bearings is that in a small number of cases (<5%) the restoration results can be less than satisfactory.

In general, this discussion is pretty academic since even 0.2% variations are much smaller than the 0.7% threshold where humans begin noticing pitch variations.

In summary it can be concluded that this motor is ready for duty again! Time to send it back to the Netherlands!

 

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!