Beogram 4000: Replacement of Sensor Arm Bulb with an LED Assembly

The final light bulb to replace with an LED in this Beogram 4000 was the sensor arm bulb. I recently developed a flexPCB based assembly with a warm-white LED that folds into the bulb compartment of the sensor arm. This Beogram 4000 has a sensor arm where the bulb/sensor compartment connects via a socket. Therefore, one can simply pull out the entire assembly and perform the LED conversion on the bench:

This shows the LED assembly together with the extracted bulb compartment:

and with LED installed:

The red 3D printed wedge serves to position the LED in the same spot where the filament of the bulb previously resided. Before installing and testing I measured the voltage at the collector of TR14, which amplifies the sensor signal before it is fed into the record detection circuit. In the Beogram 4000 2.2V DC should be present at the collector without a sensor signal. This was the case for this Beogram,

and so the installation could proceed:

The warm-white LED has enough red photons to present the B&O logo in its original red color. The final step of any light source replacement in the sensor arm is to measure the sensor response when no record is present. This is also measured at the collector of TR14 and an amplitude of 2.2V should be achieved with the sensor over the spinning platter to ensure proper disabling of the arm lowering circuit if no record is present:

As you can see this Beogram 4000 passed with flying colors. Each dip represents a platter rib passing under the sensor. A 'real-life' test with and without record confirmed the proper working of the circuit.

Beogram 4000: A New DIN 5 Plug and Installation of a Grounding Switch

The Beogram 4000 that I am restoring right now had a badly corroded and poorly installed DIN 5 plug. Obviously not the original plug. Here are a couple pictures of the sorry state of this plug:

I think the dangling black lead may have been an attempt to reduce some humming issues (no surprise, considering the way this plug was installed...;-). Besides this strange construction, one of the signal leads was also disconnected due to a sad soldering job. 

I cut off the plug and prepared the leads for the new plug:

This picture shows the advantage of DIN connections over RCA: The entire cable and connectors are double shielded. Each lead is shielded by the signal ground lead and then there is an outer shield that connects the enclosures of amplifier and turntable. That way the shielding is complete, which gives the best defense against EMI. RCA usually only have a single shield via the signal ground. That is why there is usually a separate wire that connects the amplifier and turntable system/enclosure grounds.

Then I soldered it on:

And this is how it looks with the shield on:

Beogolden I'd say!

The final step of refurbishing the signal path was to install a switch that allows connecting signal and system grounds. This is a convenient features if there is a hum when connecting to RCA inputs via an adapter.

Switch to the left (as shown) and the grounds are connected. Switch to the right and the grounds are disconnected as in the original configuration. 

Beogram 4000: Restoration and Adjustment of AC Platter Motor

After rebuilding the electronics of the Beogram 4000 that I am restoring right now it was time to look into the AC platter motor. This particular Beogram 4000 has the old style pulley that does not have belt guards integrated, i.e. they added guards on the posts that hold the motor:

These guards are not really needed since the pulley is slightly crested which securely holds the belt on the pulley. The vertical position of the belt can be adjusted via the tilt of the motor.

Similar to the DC motors of the later Beogram 4002 and 4004, the AC motors also develop dry bearings. This motor made this clear via a noticeable 'knocking' noise when driving the platter that could not be adjusted away via the motor voltage trimmer or motor tilt (without platter some 'cogging' can be normal, since the motor is 'bored' without platter, hence the phase lag for smooth operation between rotor position and AC voltage is too small) . So I took the motor out

and disassembled it:

Disassembly requires drilling out the rivets that hold this motor together:

The standoffs (left most parts in the above picture) need to be reused once the motor is reassembled, i.e drilling should be done carefully in small steps to limit damage to the parts. Since they also served as rivets, they needed to be drilled out with a 3 mm bit to allow 25 mm M3 screws all the way through. 

Anyway, the bearings cannot be extracted without doing damage, so I usually put the entire motor housing in the oil and under vacuum:

After 24 hrs I reassembled the motor using 3D printed 'nut arresters' that allow the replacement of the rivets with M3 bolts and nuts:

the purpose of these parts is to hold the nuts in place under the motor that the tilt screws can be adjusted when the motor is installed. This is how the parts fit on the motor:

After re-installing and adjusting the vertical position and tilt of the motor, I adjusted the motor voltage. Per manual this is to be done at 33 RPM that the sine wave measured at the connection point between motor coupling and phase capacitors is maximized without showing distortion:

This motor could be adjusted to 6.5V RMS as specified in the manual. At 45 RPM is still had 6.0V:

There is no way to adjust 45 separately, i.e. one has to live with whatever 45 shows when 33 is properly adjusted. But 6.0V is plenty to drive the platter vigorously. This motor seems to be back in business!

Beogram 4000: Restoration of the Main and Power Supply PCBs

After replacing the reservoir and motor capacitors of the Beogram 4000 that I have on the bench right now, it was time to rebuild the main and power supply PCBs. It is a good idea to replace the RPM relay and the RPM trimmers along with the electrolytic capacitors since they are often corroded. This shows the main PCB in its original condition:

This is a detail show of the old Siemens relay and the RPM trimmers:

And here an impression after replacing the parts:

The Beolover replacement relay is a drop-in assembly that has the same footprint as the original non-standard Siemens design:

It uses a modern encapsulated relay that promises upward of 2 Min switch cycles before failure...that should last for a few extended vinyl listening sessions!...;-).

The remaining task here was to replace the two Ta capacitors on the power supply board. this shows the original:

The two brown-white units on the right side are the 1uF caps that need replacing:

And this concludes the restoration of the electronics. The next step is the rebuild of the motor.

Beogram 4000: Replacing the Reservoir and Motor Electrolytic Capacitors

After gold plating all of the switch terminals in the Beogram 4000 that I am restoring right now, it was time to rebuild the electronics. I usually start out by replacing the old reservoir and motor capacitors that are often out of spec and on the brink of failure. This shows the original units:

A while back I developed a 3D printed capacitor fixture that holds the (smaller) modern replacements neatly in place:

This assembly is a drop in replacement for the original capacitors since it uses the same retaining strip:

On to the circuit boards.

Beogram 4000: Gold Coating of Solenoid Activated Switch Terminals

The third group of switches that control the Beogram 4000 is comprised of four switches that are activated by the solenoid when the tone arm is lowered. Like the other switches in keypad and under the carriage these usually suffer from oxidation. One of the switches is particularly important, since it controls the solenoid current during lowering activation. If it is oxidized, the arm will not lower anymore. Therefore, we strongly recommend to also get these switches gold plated like the others. This shows the switch assembly next to the solenoid (this photo shows already the gold plated terminals - I forgot to take a picture of the original condition):

The switches are installed on the three small vertical PCBs that are soldered to horizontal base-PCBs. The left most switch activates the tracking system once the arm is down. The two switches on the center PCB are the output switches that ground the cartridge signal lines while the arm is up, and the right most switch (under the wide yellow conduit) reduces the solenoid current to a lower value once the arm has been lowered.

These switches are a bit more difficult to get to than the others since they are connected to many flimsy wires. The solenoid current switch terminal is the only one that can be removed directly after unbolting its base-PCB:

The other two vertical PCBs need to be removed from their base-PCB to access the switch terminals. The first step is to unbolt the base-PCB and turn the board around that the solder points for the vertical boards can be accessed:

Unsoldering requires three hands, but with a de-solder pump and a bit of persistence it can be done with only two...;-):

After removing the oxidized and deformed terminals

I electroplated them:

And then it was time to put them back in:

Nice! On to the PCBs!

Beogram 4000: Gold Plating of Carriage Position Switch Terminals

After restoring the command center (aka 'keypad'...;-) of this Beogram 4000, it was time to restore the carriage position switches. Their functioning is crucial for the Beogram 4000's digital control center and the malfunction of just one of these switches will make it impossible to use the Beogram. Like the keypad switches, they are susceptible to oxidation and gold coating is a reliable remedy for potential contact troubles down the road. This shows the PCB that features the switches after removal of the carriage position indicator assembly:

After removal of the servo motor leads and the two screws that hold it in place, the board can be flipped up:

While the contact tabs look shiny and happy on first glance, their removal reveals oxidation in the contact areas (small black dots):

Electrochemical processes enabled by the humidity in the air acting as electrolyte the terminal metal oxidizes where it is in contact with the opposing contact terminal. Coating with a noble metal like gold is a good idea since it dramatically slows down this process. This shows the terminals after the electroplating process:

And after installation:

All good in the carriage position switches department! On to rebuilding the main PCB and the reservoir and motor capacitors!

Beogram 4000: Restoration of the Keypad Assembly - Gold Plated Switch Terminals and Light Bulbs Replaced with LEDs

After restoring the arm lowering and tracking mechanisms, it was time to rebuild the command center of this Beogram 4000. The keypad assembly contains an early digital control system, which responds to inputs by the keys on the keypad. The mechanical switches associated with the keys are one of the most often malfunctioning features of this design. This is why we recommend to gold plate the terminals in these switches to ensure trouble-free long term performance after the restoration process.

This shows the keypad cluster extracted from the Beogram and opened up:

One immediately sees that one of the light bulbs of the position indicator scale is missing. The other, still present one was not working, too. The chips that are visible are early TTL gates that comprise the digital control system that orchestrates the functions of the Beogram. The keypad switches are located in the lower PCB:

The black spots are oxidation, which often renders these switches permanently open circuit. A quick fix is to pull some 2000 grit sand paper through the terminals and their counter electrodes. But this is only temporary since the oxidation quickly grows back. It is better to remove the terminals, clean them and then electroplate them with a layer of gold. This shows the terminals removed in their original condition:

And after the plating process:

After soldering them back in the OFF button started to work again. After restoring the switches I replaced the light bulbs with LEDs. For the position scale illumination I use red/green LED-based assemblies that are tuned to emit an incandescent-looking spectrum which contains also red. This is important since the position indicator is a red piece of plastic, which would not appear red without the presence of red photons in the LED light. This is the reason I do not use amber LEDs for such tasks. The position indicator would look grayish in the light of amber LEDs. This shows the LED boards:

The RPM indicator bulbs can be replaced with standard red LEDs and current limiting resistors (the bulbs run on 12 V):

This shows the LEDs installed and in action:

And this is how the LED keypad illumination appears when looking at the keypad:

Beolovely! On to the carriage position indicator switches (you guessed it! they also need to be gold plated...;-)!