Beogram 4002 (Type 5504): Installation of Newly Adapted Beolover RIAA Pre-Amp

After recently redesigning the Beolover internal RIAA pre-amplifier for DC-motor Beogram 4002/4 turntables, and adapting the circuit for the Beogram 4000, it was finally time for the next chapter in this design process: Adaptation of the circuit for the AC motor Beogram 4002.

The owner of the 4002 (5504) that I just restored asked me to install the RIAA and so his Beogram will be the first AC-motor 4002 featuring a Beolover RIAA!

This shows the new board:

The input jack of the board connects to a replicated Beogram style plug via a multi-lead wiring harness. The output jack on the right matches the plug that is on the output cable of this type Beogram.

Due to the large footprint of the output jack, I decided to give this design bypass switches instead of the two completely separate signal paths via separate input and output jacks that I chose for the DC motor board. The switches allow connecting the Beogram to a standard Phono input without re-installing the original output board.

A look at the backside of the board reveals a separate grounding wire that allows connecting the board ground to the chassis of the turntable. This has shown in my experiments to help if there are any 50Hz hum issues (but in some cases it also causes hum, so this is something to be tried out in your particular setup):

Here is a look at the board next to the original board that I restored when I finished this turntable:

For installation of the Beolover RIAA board, all that is required is unplugging the board

and replacing it with the Beolover RIAA board:

The only difference is that the ground wire needs to be bolted to the 'cable clamp' bolt:

This shows the board after bolting the keypad back in:

It is designed that the control features are accessible with the keypad cluster installed. This allows setting the gain and switch between RIAA and non-amplified operational modes.

Beogram Commander Remote Control: Successful Test of New 4002 (550x) AC Motor Version

I received a few inquiries about making the Beolover Commander remote module work with AC motor Beogram 4002s. My recent update of the Commander for Beogram 4002 and 4004 only works in DC motor Beograms since AC motor versions have no keypad plug socket on the main PCB. Their keypads are hard wired to the PCB. So I thought it would be nice to offer an AC version. But this made a redesign of the Commander module necessary.

This shows the redesigned version:

The main difference between DC and the presented AC version is an additional small breakout PCB (the long and narrow item on the picture) that can be soldered to the keypad wire-to-board solder pads. It feeds the keypad signals into a small connector that then connects to a matching connector on the Commander board. That is what the green jumper is for. The final version will come with a custom made jumper that has an appropriate length.

I thought it would be perfect to celebrate the arrival of the AC Motor Commander by implementing it for the first time in a rare early 5501 version (the first 4002 series they ever made after discontinuing the original 4000). This 5501 Beogram recently revisited my work bench from Australia due to the malfunction of the solenoid transistor and a non-working carriage motor. My customer agreed to let me delay return shipment to try out the AC Commander on his precious deck! Thank you very much!

The first step of the installation was to solder the breakout to the keypad PCB. This shows the original PCB after extracting it from the keypad assembly:

The wire harness from the main PCB connects to the pads up front from below. The two resistors on top are 22k pull-ups for the 33 and 45 RPM transistors, which seem to have been added after the fact due to operability issues in this early version. In later AC motor versions the PCB was changed and the resistors are placed on dedicated solder pads on the central part of the PCB. 

Unfortunately, on the 5501 they are a bit in the way of installing the Commander breakout board. When I tried to push them slightly out of the way for removing some of the solder on the pads, one broke apart:

I replaced them both with 20k resistors that I had in my stash. The 10% different value is not important, they only pull up the RPM switch transistors to make their ON/OFF states more reliable.

This shows the two resistors in place. They now allow some room for Commander breakout PCB:

The next step was to solder the breakout across the pads, which was easily done by putting some flux on the pads and then touching their orthogonal contact area with the soldering tip carrying a bunch of solder:

This is how it looks when the PCB is inserted back into the keypad:

The next step was bolting the Commander onto the main PCB and connecting the two connectors with the green jumper:

Then came the moment of truth, when I powered the deck up. And hurray, no magic smoke arose!

So I flashed the chip. The next step was pairing the Apple remote to the Commander board.

On the DC version Commander this involves pressing two buttons for ~6 min until the 33 RPM light comes on and the platter moves very briefly, indicating the completion of the pairing process.

I did the same here: And nothing happened! Panic arose, but pressing the Start button on the remote set the carriage in motion! So it seemed to have paired properly! 

And then it dawned on me: The AC versions does not have the 'record cleaning function', which is activated by pressing the 33 RPM key with the carriage in its home position. On the DC versions, pressing 33 RPM simply sets the platter in motion until the key is let go again. This is very convenient for wiping the platter before playing the record.

B&O probably introduced this function with the DC motor since the DC motor has a much lower torque than the AC motors, causing strong RPM fluctuations during swiping the platter. On AC motor units one can wipe the platter while the music plays, and there is almost no RPM slowdown. But with the DC motor one has to do it before actually pressing START for listening.

Anyway, so far so good! V1.0 seemed to work!

But the above made me think: It would be nice to have a record swipe functionality like on the DC models! And of course the pairing process feedback needed to be changed.

So I changed the code:

• Instead of the 33RPM/platter twitch response to a completed pairing process, now the auto-repeat LED briefly lights up.
• Now one can press the 'select' button on the remote for platter swiping like on a DC Beogram. This was done by replacing the absent swipe circuitry by a combination of activating START and then immediately "<" to stop the motion of the carriage. So it only moves a few mm until the 'shut off (SO)' switch is cleared that the deck can run the motor. Subsequent release of the select button activates the STOP function and the carriage goes back home and the platter motor goes off. What would we do without microcontrollers these days!!...;-)

Beolovely!! I estimate the AC Motor Commander should become available in 2-3 weeks when I will hopefully have received the proper jumpers for connecting keypad breakout and Commander board. Stay tuned!

Beogram 4000: A Fun Time with a Suddenly Activating Solenoid, an Anemic Carriage Motor, and an AC Motor Driver with a Weak Driver Amplitude

I am making good progress with the Beogram 4000 that I recently started to restore. More about the standard restoration tasks in a following post. This post focuses on two non-standard issues with this Beogram that I encountered after rebuilding its boards and motors.

I started realizing there was something wrong when I adjusted the platter height relative to the arms. During this process I needed to drive the carriage, so I plugged the Beogram in and I realized that the carriage movement was a bit slow compared to other 4000s I had worked on previously. I measured the 24V rail, and it only showed 21V. A bit odd. Usually this is 25-26V due to a warm Schottky diode, or 23.8V in my case since I now always replace the original 1.3W diodes with a cool running modern 5W type, which brings the voltage down to 23.8V at the capacitor terminal. But in any way the presented voltage was way too low.

Then I accidentally touched the carriage in a way that it was lifted up in the back a bit. This briefly activated the arm lowering solenoid. Very strange!!

I was able to reproduce this and it dawned on me that there must be a major issue, somehow putting a significant voltage onto the carriage when it touches the floating chassis (the carriage is normally insulated due to the plastic components that hold it on the guiding rods. The solenoid issue even 'worked' with the Beogram turned off (but plugged in).

I put the multimeter to it, and after poking around for a while, I realized that the enclosure of the Beogram was at ~30V relative to the system ground. Not a good sign, I counted myself lucky that there had not already been some 'magic smoke' emitted from an unlucky component. I also realized at this point that the transformer got unnaturally warm. 

30V 'smelled' like the unwanted voltage originated from the 24V system, which is fed by about that voltage before the Schottky diode (OD2)/transistor (TR1) regulator stabilizes it to ~24V. 

I determined that there was a ~9 Ohm short between the positive end of the 24V 3000uF reservoir cap (0C4) and ground:

More precisely, the short was between the red lead connected to the cap and the chassis. I followed the red lead, which goes to the collector of 0TR1. So first I suspected 0TR1 might have an issue, but it turned out to be o.k..

But at the collector of the chassis-mounted 0TR1, the red wire from the cap meets a thin blue wire, which goes straight to the carriage circuit board, and from there to the right contact of the solenoid, supplying it with un-stabilized 30V for maximum action! The plot thickened. I successively unsoldered/measured/re-soldered all the connections as I 'moved away' from the collector of 0TR1 keeping an eye on the resistance to the chassis. I made it to the left solenoid contact, and then to the terminal where the solenoid connects to the collector of its driver, 0TR4. All the while the short remained, i.e. all that separated me from the chassis was now 0TR4. A closer look at 0TR4 revealed that previous 'creative human interaction' had caused the bolt insulator to go AWOL:

The way this transistor is bolted down, a direct short is made between its heat sink (=collector) and the chassis via the bolt. What was missing is a part like this, which I fortunately had in my stash:

I installed it

and this cured the spontaneous solenoid activation issue. Details, details!!...;-)

I measured the 24V rail again, but the voltage was still only at ~21V and the carriage still moved too slow. So there had to be a secondary issue draining power from the system. Also the transformer still ran pretty hot. The other 'power electronics' system of the Beogram 4000 is the AC platter motor. So I measured the AC platter motor signal just to make sure this was working properly. What I saw was a much too low signal, however:

The RMS voltage was only 2.5V with the motor pot adjusted to the maximum smooth waveform. It should be about 5.5-7V when everything is up to specs. This suggested the secondary issue was in the motor driver. I moved the main PCB out of the way and had a look at the push-pull output stage of the motor oscillator beneath it:

The left transistor is 0TR2 (NPN, push) and the right is 0TR3 (PNP, pull). Since the motor wave was about 50% of normal, I immediately suspected one of them being in trouble, causing the motor to run only on one 'cylinder' instead of two. I touched the transistors and I found the PNP to be pretty hot. I extracted it and measured it with my transistor tester, and it appeared to be in perfect working order. So it had to be the other one! I extracted it, too, and indeed, it had turned into a small resistor:

This basically caused the NPN transistor to carry a big DC component along with the AC fed to the motor, explaining the high heat load.

I replaced 0TR2 with a BD437G power transistor (45V, 4A - a bit stronger than the 3A original TIP32),

(this modern package does not need a bolt insulator since the heat sink is encased by plastic on all its bolt facing surfaces)

and that cooled down things while yielding a normal strong motor signal for 33 RPM

and 45 RPM:

After this the 24V rail also was fully restored to 23.8V, and the carriage now moved much more quickly. 

On to do the final adjustments and giving this baby its first restored spin!

UPDATE: 

My customer suggested: "You write that 0TR3 (PNP, pull) was hot but working.  Running transistors hot obviously shortens their lives and I’m just wondering, now that you know this circuit had a problem, would it be worthwhile to just replace 0TR3 pro-actively to make sure that there isn’t a latent issue that could lead to another failure downstream?"

I thought that made a lot of sense!...;-), and so I put in the complementary PNP (BD437) in, too:

If you do this at home, note that the BD437/438 SOT-32 packages have base and emitter reversed compared to the original TIP31/32 TO-220 types, i.e. the brown and blue/orange wires need to be reversed.

Beogram 4000: Restoration of the PCBs and the AC Platter Motor

After rebuilding the arm lowering and tracking systems of the Beogram 4000 that is on my bench right now, it was time to replace the electrolytic capacitors and the RPM control relay and trimmers as well as the AC platter motor. This picture shows the original reservoir and motor capacitor setup together with the AC platter motor:

I removed the caps and the motor,

and assembled the new capacitors into the 3D printed holder that I designed to fit the strap that held the original capacitors in place:

Then I took the motor apart for doing an oil infusion under vacuum. This shows the extracted motor:

and opened up after drilling out the two rivets that hold it together:

This shows the rotor with the spring and washers as they are placed on the shaft:

I put the enclosure parts including the bearings into motor oil and pulled a vacuum:

While I let this sit for 24 hrs I focused on rebuilding the PCBs. This shows the main PCB in its original condition flipped out that I was able to access the components:

I replaced all electrolytic capacitors as well as the RPM switch relay and the RPM trimmers that allow to tune the speed:

This shows the RPM section in detail:

The two blue 25 turn trimmers were installed in a way that their adjustment screws are accessible from the solder side of the board:

This is important that one can adjust the RPM while the board is installed. The next step was to rebuild the small power supply board. This shows the original condition:

The two red dots are the only electrolytic capacitors on this board. When I flipped the board around after taking out the three screws that hold it in place the fuse that is located in the yellow tubing in the back came apart:

This seems to be a frequent issue with these fuses, and I think it is time to add replacing them to the standard Beogram 4000 restoration procedure. I installed a new fuse and secured it with some shrink tubing and then installed the board again:

After the bubbling stopped in the vacuum jar I extracted the motor enclosure parts and put the motor back together. Since the rivets that need to be drilled out to open it up also doubled as M3 nuts to hold the tilt adjustment screws of the motor, I used 3D printed parts that I designed a while back to hold M3 nuts in place for the adjustment screws (which I usually replace with hex head stainless screws since they allow much easier adjustment when the platter is installed and running):

Then I implanted the motor together with the capacitor assembly:

This shows the 3D printed parts on the motor in some more detail:

The motor housing is held together by the other two screws that bolt it to the enclosure.

This shows the two 2200uF caps behind the main motor and reservoir caps:

I usually measure the big capacitors after I remove them. Indeed one of them was completely dead showing just a few pF instead of 2000uF:

It is always a good idea to replace all the electrolytic capacitors at this point in time since frequently they are dried out or leaking. Sometimes they even short circuit, which can cause them to burst...not a beolovely sight. After this measurement, I plugged the deck in and started it up. It showed life signs but there still seems to be an issue in the control system since the arm spontaneously lowered when pressing one of the << or >> keys.

Beogram 4000: Adjusting the AC Motor Voltage and Measuring the Voltage Differences Between the Phases

Today I adjusted the AC motor of the Beogram 4000 that I am currently restoring. The manual prescribes to adjust the main phase of the motor to 6V (RMS) and the frequency to 42.3 Hz for 33 RPM. This is easily done with an oscilloscope. To see both phases the probes can be simply connected to either side of the phase capacitor:

Adjustment procedure: First set the 33 RPM trimmer on the control panel to its null position (i.e. vertical). This ensures that the adjustment of the RPM offset potentiometer on the main PCB can now be adjusted to yield a 42.3 Hz motor signal when the control panel trimmer is nulled out. This ensures that the user can both trim the pitch for the same frequency deviations in + and - directions. Once the frequency is set the MOT potentiometer can be adjusted. It is important to do it in this order, since the frequency affects the amplitude of the signal. We are working with an analog oscillator here...;-).

Once everything is adjusted the oscilloscope should show about this:

The yellow trace is the main phase and the green trace is the 2nd phase. It is interesting to note that the 2nd phase has a smaller 5.3V (RMS) amplitude. This concludes the adjustment.

When switching the motor to 45 RPM one sees this:

Note that the 2nd phase is now much larger, about 8V (RMS), while the main phase dropped a bit to 5.6V (RMS). The manual also wants 6V (RMS) here, but there is nothing one can do about this since the MOT trimmer determined both frequencies, and 33 RPM is probably for most people the much more important speed. 

There is an interesting detail I noticed about the RPM trimmers on the control panel. The 45 RPM trimmer is nulled when the correct frequency of 57.3 Hz is adjusted with it:

When I took the control panel apart for converting it to LED illumination, I noticed that the red inserts that allow the adjustment of the trimmers with a screw driver are different for 33 RPM and 45 RPM: The driver slot is at an off-angle for the 45 potentiometer, while for the 33 trimmer it is symmetrical. Again, the designers gave the 33 speed priority and made a small compromise for 45: The consequence of this off-angle is that the 45 speed can only be adjusted by about a quarter turn to higher speeds, while one can go about a half turn to lower speeds. All this was probably dictated by the need for using standard resistors and trimmers in the Wien oscillator feedback circuits, which prevented them from designing the circuit in a way that the 45 trimmer would be exactly in its center position for an exact 57.3 Hz.

With modern resistor values one could easily fix this since nowadays one can buy resistors with almost any value. Apparently not so in 1973. Of course if one would now fix this by replacing the resistors, the trimer slot would be at an angle, which would not look good. What a conundrum! Will I be able to sleep tonight?? This is Beolove!..;-)

Beogram 4000: Reservoir and Motor Phase Capacitors Replacement

I recently supplied a customer with reservoir and motor phase capacitors together with my 3D printed adapter for his Beogram 4000.

He reported that he was only able to measure half waveforms at the AC motor of his newly acquired Beogram 4000. This indicated issues with the phase capacitor or the power supply reservoir capacitors. He installed my kit and the problem was fixed. Here is the link to my original post about this procedure. Another Beogram 4000 back in service! He sent me these great pictures:

This shows the new capacitors and the red 3D printed adapter that holds them in place using the original mounting strap of the original, much larger, capacitors:

And here some impressions of the deck back in service. What a beautiful design!