Beogram 8000: Electrical Work Checkout and Forward/Reverse Voltage Adjustment

After testing some basic Beogram 8000 control functions I am satisfied that the electrical work is done. It is time to bundle up the microcomputer board box so I can move on to the service manual adjustments.

The microcomputer box lid contains a heatsink for the processor device. I had cleaned off the old thermal compound and applied new compound.

The lid attached securely and I can peak in the end and see the thermal compound is sandwiched nicely between the heatsink and processor.

While the main Beogram components are still out of the chassis this is a good time to do some of the service manual adjustments. There is still one that is related to possible electrical tasks and that is the adjustment of the forward and reverse voltages for the tangential arm transport.

There is a common lamp with two photo resistors on either side in the Beogram control panel. One photo resistor (R9) is for forward movement and the other (R10) is for reverse movement. The voltages on those two resistors should be 650mV when the Beogram 8000 has been on for at least five minutes and the Play button is activated (with the tangential arm assembly stationary).

I initially measured around 600mV for the reverse photo resistor (P5-6, R10) and 300mV for the forward photo resistor (P5-4, R9). Here are the measurements after the adjustment.

Here is what the related connector, P5 looks like on the Beogram 8000 control panel.

This picture shows the final position of the forward/reverse adjustment screws.

The relative positions are a little unbalanced to say the least. I will recheck those forward/reverse arm motor voltages again before wrapping up this project to verify the settings are stable.

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 4004: DC Platter Motor Restoration and RPM Performance Test

A Beogram 4004 DC platter motor arrived recently in the mail for some TLC. I bench-tested it and it ran quite rough. It seems these days there are not many Beogram DC motors that do not need their dry Oilite bearings infused with fresh oil. We make it a point in our restorations of the 4000 series Beograms to always rebuild the motor. Over time the oil that is in the pores of the Oilite brass material is drawn out into the motor and the bearings run dry.

Fresh oil can be infused under vacuum, but for this the bearings need to be extracted from the motor. Here are some impressions of the process in this 4004 motor. This shows the motor extracted from the Beogram as received:

After taking it apart and immersing the bearings into motor oil:

This shows the bubbles coming from the bearings after pulling the vacuum:

After about 24 hrs the bubbling stopped and I assembled the motor again. This shows the 3D printed tool I use for pressing the top bearing retainer back into the housing:

I tested the motor in one of my Beogram 4002s and measured a 24 hrs RPM stability curve with my BeoloverRPM tool (which is available to other aficionados and repair shops - just send a message using the form to the right or via email). This shows the curve:

A perfect result. This Beogram 4004 motor is back in business! 

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...;-)!

Beogram 8000: Completing the Capacitor Replacement and Checking the Work

Today I finished up the capacitor replacement by changing out the motor capacitor in the transformer box and the 5VDC power regulator capacitors on the chassis.

Here is the motor coil coupling capacitor located in the transformer box and its Beolover 3D printed part replacement.

Very nice and tidy (and secured).
Next were the capacitors for the chassis mounted 5V regulator.

Now for a quick test to see where this restoration is at. The first thing to check is the +5V power to the microcomputer. I measured the voltage across the new C28 capacitor in the microcomputer board. This is why I haven't close up that board assembly yet.

Processor supply power looks good and the Beogram 8000 red standby dot illuminated in the speed display window.

Now to check the platter speeds. When this turntable first arrived I did a quick check to see if there was anything noticeably wrong. I remember that the 45 RPM appeared to work but the Beogram was having problems locking in the 33.33 RPM speed.  After the capacitor rework I wanted to check the speeds again.

Both speeds lock right in when selected now.

Those were good test results after the capacitor replacements.  There are still quite a few tasks remaining though. I need to run through the mechanical adjustments and there are several electrical adjustments/checks to do.