Beogram 8002: Transformer Block - A Comparison Between 120V and 240V Versions

I just sold a Beogram 8002 (Type 5633) that I recently restored to a customer in the UK. Unfortunately, the Beogram was a 120V US version. This meant it came with a transformer block specifically designed for the US power grid, which offers 120V at a grid frequency of 60Hz.

The Beogram 8002 (and 8000) were designed in a way that they could be converted for different power grids by simply swapping out the transformer block. This enabled B&O to use the same hardware globally with only the transformers adapted to local requirements.

So before sending this Beogram to the UK, I wanted to convert it to 240V/50Hz so that my customer could directly plug it in without needing an external voltage transformer. 

It is interesting to note that while the external transformer approach works well with most devices, the Beogram 8002/8000 have a design quirk that makes this approach less than perfect: They use grid frequency AC to run the linear 2-phase AC motor that drives the platter. Since the second phase is shifted relative to the first using a phase capacitor, the capacitor value is specific to the AC frequency used. This means that when a US 120V/60Hz Beogram is hooked up in the UK via a 240-to-120V grid voltage transformer, it sees the proper 120V, but it gets only 50 Hz. This means its factory phase capacitor does not have the proper value. This is the reason why B&O put this phase capacitor into the transformer block, so it would match the specific grid frequency the block is specified for.

I was able to buy a 240V/50Hz UK transformer block from a fellow Beolover in the UK. When I received it, I decided to explore the differences between the 240V and the original 120V blocks. Most importantly, I wanted to measure the voltages of the secondary windings of the two blocks to make sure I could swap them without issues. I had never done this before, so caution was on order!...;-).

This shows the original 120V/60Hz block:

And opened up:

The big can is the phase capacitor for the motor. It has a 27uF value optimized for 60 Hz. The fuse is a 300mA slow-blow type:

I had the fantasy that one might be able to convert 120V blocks into 240V types by changing the primary winding connections, but a look at the wiring of the primary side suggests this is not possible. This shows the primary connections:

Each winding has a solid colored and a transparent wire coming out. On the top side of the terminal board, the transparent wires are connected via a solder terminal:

The grey and black wires go into the power cable (the black one via the fuse). This means the two coils are connected in series. Since the turns ratio formula for transformers states that the ratio between the primary and secondary voltages (Vp/Vs) is equal to the ratio between the number of windings on the primary and secondary sides (Np/Ns):

Vp​/Vs​​=Np​/Ns.

​This means the lower voltage bloc should have the windings connected in parallel, and the higher voltage bloc in series if conversion would be possible. This is how the grid-voltage-switchable earlier Beogram 400x do it. Their voltage switch basically re-arranges the way the primary coils are connected to adapt the number of windings on the primary side so that voltages between 100 and 240V can yield the same secondary side voltages.

Anyway, lets have a look at the 240/60Hz block that I just received:

This shows it opened up:

There are small differences to the 120V version: The phase capacitor is a bit longer, hinting at a larger capacitance, and the terminal plate looks different:

The first thing to note is that the fuse is a 160mA slow-blow type, i.e., has about half the current capacity of the 120V fuse. This makes sense since the same hardware is powered, i.e., the current on the secondary side is the same. Since the primary voltage is double, the primary current will only be half for the same power (P=V*I). 

A look under the plate reveals that the primary windings are connected in exactly the same way as in the 120V block:

The transparent wires are again connected in a series configuration. This means the 240V transformer indeed has a different design than the 120V version. It has different winding counts on the primary coils.

The phase capacitor indeed has a larger value, 39uF:

After this inspection of the two transformer blocks, I set out to measure the voltages that they produce on their secondaries when plugged into the specified mains voltage. For powering the 240V version, I used my big old 0-250V variac 

I measured peak-to-peak (Vpp) voltages with my oscilloscope. Here is an example of how I connected my probes (to measure the coils for the ±15V rails):

This is what I saw on the scope:

I made this table after I measured all secondaries on both blocks:

The calculated ratios are about ±10% around 1, which is 'good enough' considering the circuit design. Since there can be significant grid voltage fluctuations, the power supply of the Beogram 8002/8000 is pretty forgiving regarding the secondary voltages it expects to make the stabilized 5V and ±15V rails.

After this measurement, I felt confident that I could plug this 240V transformer block into the Beogram and give it a spin! But before I did that, I updated the phase capacitor with a Beolover Motor Capacitor for Beogram 8000 and 8002:

Note that for my initial test, I connected the 27uF capacitance for 60Hz grid frequency as shown above. My variac only changes the grid voltage but not the frequency. Ah, physics!...;-)

After installing the new phase capacitor, I plugged the 240V transformer block into the header on the main PCB of the Beogram and plugged it into the variac. Then I ramped the voltage to 240V, and the Beogram showed the usual red dot in the display! I pressed START, and it ran like before! Beolovely!

Of course, I was curious if the platter motor of this unit would still run properly if the 39uF/50Hz capacitance were used for the motor while running at 60Hz. Sort of a test of "can one use a standard grid 120V/240V transformer in a pinch if there is no proper transformer block available?".

I changed the capacitor setup to 39uF/50Hz:

I reinstalled the block and plugged it back into the variac. Then I pressed START. And everything seemed to work properly. The platter ran as usual, and the Beogram was able to lock onto "33.33" on the display, which means the microcontroller was measuring the proper speed and had no trouble maintaining it.

I played a few records, and it seems there is no practical penalty for using the 'wrong' phase capacitance.

So why use different capacitors for 50 and 60 Hz in the first place? I think it comes down to 'efficiency'. If the phase shift between the two motor coils is a bit off the optimal value, the motor consumes a little more current since it is less efficient. This does not matter much in this design since the RPM is feedback-controlled. So the control circuit simply compensates by giving the motor a bit more power to get the specified RPM.

I will now play this Beogram a bit more, and then it will be time to send it to its new owner in the UK!

Beogram 4004 (5526): Restoration of the DC Platter Motor and RPM Stability Test

After replacing the RPM trimmers, the RPM relay and the light bulbs in the RPM trimmer panel with LEDs, it was finally time to restore the DC platter motor of this Beogram 4004 (5526). This sequence is important, since one can only test the RPM stability performance of the motor properly after the entire RPM control circuit has been rebuilt. This shows the extracted motor:

The motor needs to be completely disassembled to get to the dried out Oilite bearings:

The bearings are the two small donuts on the black pad. I infused them with motor oil under vacuum:

The raising bubbles indicate that air is drawn from the porous brass bearing material. This creates space for oil to enter the bearing. After about 12-24 hours the process usually stops, at which point the bearings are again full of oil.

This shows the bearings after the infusion process:

After reassembling the motor I put it back into the deck and adjusted the RPM with the BeoloverRPM device:

Then I used the device to measure a RPM stability curve over 18 hours:

This motor showed an exceptionally stable RPM performance and can be considered ready for duty.

Beogram 4002 (5514): RPM Performance Measurement after Restoration of DC Motor and Control System

After rebuilding the DC motor and the control system of the Beogram 4002 (5514) that I am restoring right now, it was time to do a RPM performance test. I set up my BeoloverRPM device that allows the long-term logging of the platter RPM, and then did a 24 hrs test:

After the data was logged on my computer, I graphed it relative to a measurement that I took when I initially received the unit for restoration. This shows the two curves in comparison:

The red curve shows the performance before the restoration. The typical telltale sudden negative RPM spikes are visible that indicate failing brass sleeve bearings in the DC motor. This is very common for DC motor Beograms. In this case the spikes are still fairly moderate, but would have been audible since they are in the 1% range. The black curve shows the performance after rebuilding the motor and the installation of new RPM trimmers and RPM relay. The large spikes are gone and the variations are much smaller. There are still some variations, which are related to the intrinsically slightly erratic performance of the analog motor control system, which is affected by temperature changes and probably the moon phase (that was a joke...;-). Some Beograms show them others do not. At this point I do not have an answer why they sometimes occur. The good news is that they are fairly slow over time and also small enough that one cannot hear them. The short term 'wow and flutter' RPM changes (the high frequency 'noise' on these curves) are within the spec given in the service manual (0.05%).  So this Beogram is back in business.

BeoloverRPM: RPM Measurements on a Beogram 8002 and Comparison to a DC Motor Beogram 4002 (5513)

My recently developed BeoloverRPM device determines the measured RPM via the time it takes between the 'ribs' on the platter to cross the sensor field of vision. It is simple math to determine the RPM from that, and I was pretty confident that my measured absolute RPM numbers were accurate. However, there is always the possibility of human error, and so I was itching to test the BeoloverRPM on a microprocessor controlled Beogram 8002, which should have a very good RPM accuracy and stability.

So the Beogram 8002 that I just resurrected was in the 'right place at the right time' for this! I used my most recent version of the BeoloverRPM that can be set up on top of the aluminum plates, i.e. the Beogram does not need to be put into service position for a RPM measurement. I did two measurements, for 33.33 RPM

and for 45 RPM:

And it appears the Beogram 8002 and the BeoloverRPM microcontrollers are in happy agreement! The Beogram is spot on, and the BeoloverRPM appears to deliver a precise RPM measurement.

I measured a RPM performance curve for a few minutes while having my 7" test single (Donald Byrd: Black Byrd) on it. This graph shows the curve in comparison with a DC motor Beogram 4002 (5513) in perfect condition:

The red curve on top was measured on the Beogram 8002, while the blue curve below represents the performance of the 4002. We see that both units have a periodicity in their RPM, but the 8002 has a noticeably smaller variation than the 4002. The periodicity itself is unavoidable in a feedback-based control system since a PID mechanism needs some change to be able to operate. The art of designing such systems is to get the change as small as possible, while still being able to run the mechanism in a stable way.

But we clearly see here that a microcontroller based feedback system is superior to an analog system that depends on temperature, component tolerances and probably the moon phase (that was a joke...;-). In practical terms this difference is probably not perceivable by a human ear, but the 8002 definitely represents progress. I am curious to see how the AC motor Beograms perform in comparison...stay tuned for the next episode of this exciting unfolding story...;-)

Beogram 4002 (5513): More RPM Measurements

This is a follow up on yesterday's post about this Beogram 4002 (5513). I did an overnight RPM performance measurement with my BeoloverRPM device. This is the curve that I measured:

This result is pretty much as good as it gets for the DC motor versions. Minimal <0.05% variations over time satisfying the specifications stated in the service manual. The initial slope during the first couple hours is usually observed in the 551x and 552x models. I currently think it is a result of D13 (the Zener diode that stabilizes the DC motor supply) and R12 with which it forms a pretty stiff (i.e. high current) voltage divider to set the base voltage of TR2 heating up. After a while the temperature stabilizes at a certain value in equilibrium with the environment. The band gap change in the semiconductor of the Zener changes its Zener voltage and with that the motor slowly gets a bit more power until a sable condition is achieved. This 'ancient' voltage regulator design is a bit of a drawback of the DC motor versions of the 4002. But one should not get to worked up about it since the change is pretty slow, and only represents a total ~0.3% change, hardly noticeable by most listeners. I am thinking this phenomenon could possibly be alleviated via the adaption of the circuit to use a modern voltage regulator. 

Beogram 4002 (5513): Initial RPM Performance Characterization

This is a follow up to my initial post about this Beogram 4002 (5513). In the meantime I did a RPM performance evaluation of the unit using my recently developed BeoloverRPM device.

This measurement clearly demonstrated the suspected sudden speed variations. This graph shows the RPM vs. time over an approximate 24 hrs period:

The sudden negative spikes suggest that the DC motor control system and the motor itself needs an overhaul.

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.