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!