Redesign of the BeoloverRPM Device: Precisely Measure and Log the Platter RPM

I recently decided to redesign my BeoloverRPM device, which I started developing in 2015 for the detection of intermittent RPM variations mainly of DC platter motors. I thought after ~8 years it could use a little update in terms of ergonomics and performance. The original device was based on an Atmega 328P processor (aka 'Arduino') and this limited the performance a bit. I recently got into using the much more advanced ESP32 platform. This enabled a few new tricks due to the much higher speed.

The new BeoloverRPM can be placed on the aluminum panels for a quick and convenient RPM check

but it can also be configured for frame mounting when working on a Beogram:

This was achieved by a design where the frame adapters can be removed from the main device:

The BeoloverRPM comes with two frame adapters that have different spacing between the 'legs' that straddle the enclosure wall. B&O used slightly different enclosure designs with varying wall thickness.

Another significant update is that the BeoloverRPM now has two data acquisition modes:

The modes can be selected via buttons on the device. The 'slow' mode performs similar to the previous designs. It shows the RPM and min/max values on the display, and sends a RPM snapshot to the serial port every 10s (faster if the RPM changes quickly).

The 'fast' mode is new: It graphs all datapoints as they come in whenever a rib passes under the sensor. The display shows a rolling snapshot of the last 64 datapoints. The data is also sent out via the serial port. Here is an example measured on a Beogram 4004:

The main graph shows the data output from about 60 platter rotations. The graph shows a repeating pattern modulated by a sinusoidal envelope. Closer inspection yielded that the pattern repeats every 24 points, i.e. everytime the platter makes a full turn (there are 24 ribs on the platter). This can be explained by minute changes of the rib spacing (it varies by about 0.1%).

The sinusoidal modulation is a result of the feedback based RPM control of the DC platter motors of the later Beogram 4002 and 4004. Feedback control always results in an oscillation of the actual value around the set point. And that is what we see here. The analog feedback mechanism used by the later Beograms is based on time constants defined by capacitors and resistors, and this results in sinusoidal RPM variations.

Fascinating stuff!...;-).

I made a short video about the new BeoloverRPM. Enjoy!

Beogram 4002 (5501): Return of an Old Friend from Australia - Dead Solenoid Transistor and a Carriage Motor with a Corroded Lead

Sadly, recently I received a well-worn Beolover Beogram shipping container from Australia. Contents: A Beogram 4002 (5501) that I restored in 2016. Type 5501 Beograms are special since they represent a briefly made transitional version when B&O went from Beogram 4000 to 4002. While the 5501 already has the analog control system characteristic of the 4002 series (the 4000 had a more advanced hard wired digital logic system, which was unfortunately (IMO) downgraded for the 4002), it still has mechanical carriage position switches like the 4000. It also has an exciting rotary encoder for the detection of the end groove. You see the Beolover is a bit love with the 5501, but seeing it for a third time caused a bit of a heart ache...;-).

Anyway, the reason it needed to visit my bench again was that it stopped working. The indication was that upon plugging it in the solenoid immediately became activated, while the deck was unresponsive to any keypad entry except 33/45 RPM changes.

So I removed the hood and the aluminum panels to have a look:

After a bit of poking around I found that the TIP41 that drives the solenoid had a short circuit:

After replacing it with a new one, the solenoid started working properly and it again responded to pressing the arm lowering key. This shows the new type implemented:

I also replaced the browned 8.2 Ohms solenoid resistor (4R1)

With a modern 2W rated unit:

This shows the voltages measured across the activated solenoid

and the resistor:

This means that the resistor runs at about 1.2W. The original types are rated 1W, i.e. a bit too low.

After this repair, the deck still did not respond to any of the 4 keys that control the carriage motion. After checking some of the transistors in the electronic switch it finally occurred to me that the carriage motor itself did not work anymore. I extracted it and opened it up. I found that one of the flimsy wires had disconnected from its tab on the motor due to corrosion. The other wire also came right off when I moved it slightly:

I cleaned the terminals and soldered 22AWG wires to them. I also replaced the completely deteriorated sound proofing with a cut to size self-adhesive piece of neoprene rubber:

Then I installed the motor again:

and everything was fine. At this point the deck's functions had been restored. I decided to do some preventative maintenance since I learned since 2016 that the AC motor driver transistors (found underneath the main PCB) often have issues in the AC motor type Beograms. This shows the original transistors 0TR2 and 0TR3:

I also found a hood attachment bolt spring near the transistors (which explained the absence of said spring when I removed the hood..;-). This shows one of the original transistors, the PNP TIP32 (0TR3, on the right):

I replaced it with a modern BD438. Note the reversed emitter and base leads on these types:

I also replaced the NPN TIP31 with a complementary BD437 (also with reversed wiring). This shows both new transistors installed:

I will now test this Beogram for a while, and if nothing else comes up it will travel back across the big pond to Australia!

Beogram 4002 (5513): Restoration of PCBs and RPM Adjustment Panel

A while ago I restored a Beogram 4002 DC platter motor for customer in California. Unfortunately, there were still noticeable RPM fluctuations at 45 RPM after re-installation of the motor. This usually points towards an oxidized RPM relay. They tend to go bad on 45 first, since in most Beogram they stay put at 33 RPM and are only rarely switched to 45. This allows oxide to grow on the 45 RPM contact terminals. My customer decided to let me restore the rest of the 'RPM chain', and I recently received the PCBs and RPM adjustment panel of this Beogram.

I started with the main PCB:

I replaced all electrolytic capacitors and then installed a new RPM relay and new RPM trimmers. This shows the original parts:

And the new ones:

Then I installed the board and tested the record detection sensor signal at the collector of TR4 with an empty platter turning:

This was a pretty weak signal, an indication that TR4 had lost some of its gain (Hfe). This shows the original TR4, a BC138C with its 1MOhm biasing resistor in the back:

I installed a new 2N5089 high-gain transistor with an Hfe of about 750. Correct biasing of this transistor requires replacing the 1MOhm biasing resistor with a multi-turn 5MOhm trimmer, which I first installed on the solder side, that I could adjust the collector voltage to the specced 4V:

Then I put it 'below deck' in the original place of the resistor:

This is the sensor signal I measured on the empty platter after this procedure:

The valleys (that correspond to the black ribs on the platter) now go close to 0V, which is what the service manual prescribes. 

This concluded the main PCB restoration:

On to the output PCB, which also has an often oxidized relay. this shows the original board:

I installed a new relay, replaced the capacitor that is responsible for the time delay for actuating this relay after the needle sets down on the platter, and I added a switch that allows the convenient connection of system and signal grounds in case there is a hum issue:

The remaining task was replacing the incandescent light bulbs that illuminate the RPM panel scales with LED fixtures. This alleviates the remaining cause for RPM fluctuations which can also be caused by thermal changes due to the heat emanating from the light bulbs. This shows the original bulbs installed:

I replaced them with LED boards:

and put the light shields back on:

Now it was time to install everything in one of my 4002s for testing. This shows the 33 RPM LED in action:

The red-green LEDs that I use for these panels allow to give them a realistic incandescent-like sheen, and also properly illuminate the red indicator. Beolovely!

On to testing the RPM performance with the BeoloverRPM device:

The BeoloverRPM device is able to log the RPM over extended periods of time. This is the curve I measured over about 24 hrs at 45 RPM:

This is as good as it gets with the DC motors, i.e. this Beogram should be back in business!

Beogram 4004 (5526): Restoration of PCBs and Reservoir Capacitor

After the restoration of the arm lowering mechanism of the Beogram 4004 (5526) that I am restoring right now, it was time to look after the PCBs and the reservoir capacitor. This shows the main PCB in its original condition:

and a detail shot of the 'RPM control' section:

I replaced the electrolytic capacitors, the RPM relay and the RPM trimmers. The original electrolytic capacitors often fail at this point in time, and the relay/trimmers are often oxidized, which can cause platter RPM fluctuations. This shows the rebuilt board

and the RPM section:

I did the same to the output/remote control PCB:

This is a detail shot of the output section:

I replaced the capacitors and the relay, and also added a switch that allows connecting the system and signal grounds together if there is a hum issue (often an issue when using RCA adapters without ground breakout):

The final step of this part of the restoration was to replace the reservoir capacitor. This shows the original unit:

And the modern replacement:

On to the RPM LEDs and the platter motor restoration!

Beogram 4004 (5526): Restoration of the PCBs

In my last post about the Beogram 4004 (5526) that I am restoring right now, a bad capacitor turned out to be the reason why the arm would not drop at the LP setdown point. So we decided that the main board should be completely restored to prevent further capacitor issues. When one goes the others are probably not far behind. I replaced all electrolytic capacitors, as well as the RPM trimmers and relay:

This shows the 'RPM section' before

and after:

I always mount the RPM trimmers that the screws peek through the original adjustment openings so one can adjust the trimmer while the platter is turning:

While I had this board out, I also rebuilt the output board:

This shows the output relay section before

and after:

The red switch allows connecting signal and system grounds. A convenient feature in case the Beogram is used with a RCA adapter and there is some hum. Usually connecting the grounds cures that.

This concludes the restoration of the electronics...on to the DC motor, which made some nice loud shrieking noises when I tested it.