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Saturday, May 29, 2021

Beogram 4004 (5526): Complete Restoration and Test Drive with Herbie Hancock

I just finished up the restoration of the Beogram 4004 from Pennsylvania (see initial assessment here). This post summarizes the work done. As usual I started out with the DC platter motor. They all need a re-infusion of their dried out Oilite bearings with fresh oil to run stable, and with minimal RPM variations. This shows the motor after extraction from the deck:

The upside-down installed pulley already indicated that there was probably already some trouble in the RPM department noticed at an earlier point. I disassembled it to extract the bearings (they are the two small donuts on the black pad upfront):
I immersed the bearings in motor oil and pulled a vacuum. Immediately bubbling started, which indicates that air is drawn from the porous bearing material. This makes room for oil diffusion into the material.
This process usually takes 1-3 days, and so I focused on the other restoration tasks. First, I focused on restoring the arm lowering mechanism :
I took it apart for cleaning and re-lubricating the moving parts:
The linkage that connects the damper back to the tonearm is part of the sensor arm assembly:
This means for lubricating its (often stuck) pivot point the arm needs to be taken out. Once it is out, the linkage can be removed:
After putting the lubricated arm lowering mechanism back together it was time to replace the tracking sensor light bulb with an LED based setup. This shows the original setup:
My replacement part has a small white LED at exactly the location of the filament in the original light bulb:

This shows the replacement implanted. The small blue box is a trimer allowing to tune the intensity of the LED. This is very helpful for precisely adjusting the tracking sensor feedback.
While working on the carriage, it is also a good idea to replace the usually cracked plastic pulley with a precision machined aluminum replacement and a new belt:

The next step was rebuilding the electronics. This shows the original main PCB.
I usually replace all the electrolytic capacitors and some more parts that have a tendency to be at the end of their service life at this point in time. An important set of components that should be replaced are the RPM adjustment trimmers and the RPM relay. This shows the modern replacements implemented: An encapsulated SMD relay replaces the often corroded original Siemens or National relay, and the original single turn trimmers were replaced with modern 25 turn precision trimmers. This allows a much better adjustment of the RPM.
On to the 'output PCB', which hosts the signal output circuit (muting relay and relay delay circuit)
I usually replace the relay and the electrolytic capacitor that defines the time constant of the mute relay delay after set-down of the needle. I also installed a switch (red component in front of the white header) that allows connecting signal and system grounds (helpful if there is a hum, as is often encountered when using RCA adapters):
In the Beogram 4004 this PCB also contains the remote control circuit that makes an early version of Beolink possible allowing control of the 4004 via the remote of the Beomaster 2400. This circuit has a few electrolytic capacitors that should also be replaced:
this shows the board with fresh capacitors installed:
The last capacitor needing replacement is the reservoir capacitor. They have a tendency to be out of spec or leaking:
I replaced it with a modern 105C type that I adapted with a 3D printed sheath to make it fit into the original mounting clamp:
At this point it was convenient to completely remove the floating chassis for replacing the transport lock bushings (and vacuuming out all the fragments of the decayed original bushings - they can sometimes keep the floating chassis from floating if they get stuck in the wrong places):
This shows one of the decayed original bushings:
This are the replacement bushings that I designed. They are printed with laser sintering for stability and thinness. They are designed in two parts that can be inserted from above and below the opening that holds the bushing:
This shows one of them installed:
and with all the parts of the lock put back into place:
At this point I re-installed the floating chassis and put the electronics back together. Then it was time to adjust the sensor arm record detection circuit, which had received a new transistor and a biasing trimmer. I adjusted the bias to yield 4V at the collector of the transistor as prescribed in the manual:
Then it was time to move the trimmer 'below deck' to join all the other components:
There are a few more incandescent light bulbs that needed to be replaced. Two bulbs are in the RPM panel where they illuminate the RPM adjustment scales. This shows the panel with my LED replacement boards installed:
Once the LEDs were in I tested them. This shows the one that illuminates the 33 RPM trimmer scale:
The final bulb to replace is the one in the sensor arm. It produces light that is bounced off the platter into a light sensor that is housed in the same assembly. This allows detecting if there is a record on the platter or not. I usually replace this bulb with a small circuit on a flex-PCB that folds into the bulb cavity and draws a current similar to the bulb to convince the bulb circuit that there is a working bulb in place (otherwise the arm lowering mechanism is disabled). This shows the original bulb still in place together with the LED replacement:
this shows the LED circuit in place:

and 'in action':
Whenever the sensor arm light source is being replaced it is always a good idea to check the sensor response. This shows the signal at the collector of the sensor amplifier with the empty platter spinning underneath the sensor:
A very good signal. Each dip corresponds to the passing of one of the black ribs on the platter. This is how the deck determines that there is no record on the platter. If there is a record, the reflection is constant, i.e. these dips are missing and that can be detected by a circuit that controls the arm lowering mechanism.

In the meantime the platter motor bearings had completed their oil infusion. I put the motor back together and installed it in the Beogram. Then I ran the deck for 24 hrs, while measuring the RPM in 10s intervals with my BeoloverRPM device:

This is the curve I measured after the 24 hrs:
This is as good as it gets with the Beogram DC platter motors, i.e. this motor is back in business!
The restoration was getting close to its completion at this point, but there were are a few more things to do before I could enjoy my first record on this deck. I realized it was very difficult to get the aluminum platters back onto the enclosure. After a bit of head scratching I realized that the enclosure was a bit bent near one of the plinth guidance washers. This kept the plinth from sliding smoothly for securing the panels:

I used a large adjustable wrench to bend the metal tab back in-line with the rest of the metal enclosure:

And that fixed the issue. Now it was time for some adjustments. After adjusting the platter and floating chassis to get everything horizontal and aligned in the proper ways I adjusted the tracking feedback:

Then I focused on the adjustment of the tonearm. My first act is usually to replace the flimsy circlip that holds the adjustment screw for the arm counterweight in place
with a M3 nut and a washer:
This allows locking the counterweight in place in a more permanent way, which is required for being able to ship the deck without loosing the tracking weight calibration. The next step was adjusting the tracking weight adjustment wheel to correspond to the actual weight (these wheels are notorious for being off) near the 1-1.2g range that is used for most B&O cartridges:
In general it is a good idea to not pay too much attention to the adjustment scale, but rather adjust the weight with a digital scale.
The other important arm adjustment is the arm lowering limit. This is important in case the record detection circuit fails and the arm is lowered on to a spinning platter without record present. The limit needs to be adjusted that the arm can put the needle lower than the top surface of a record, but not low enough to hit the ribs on the platter:
One more thing needed to be done before playing the deck for the first time: The corroded original DIN5 plug needed to be replaced:
The Beogram 4004 is actually fitted with a convertible DIN7 plug due to the remote control feature that uses the two additional pins. In most cases, however, the two pins that can be removed to make a DIN5 out of the DIN7 are lost. And they would be corroded anyway. But this poses an interesting challenge for a restoration that wants to keep the remote control functionality intact: These convertible DIN plugs are no longer available. My solution to this is to install a female DIN7 on a short cable. This allows connecting male-to-male DIN5 or DIN7 jumpers, or a DIN5 to RCA adapter cable, depending on what amplifier is being used. The issue with this approach is that it is difficult to find a nice female DIN7 connector with gold plated terminals. I finally found these very nice professional T3485-018 DIN7 connectors made by Amphenol (they are available at Mouser) 
They can be had with gold plated contact terminals (this is signified by the -018 ending of the part number). Here some impressions of the installation:
They have a very nicely machined internal shroud:
And this is the final result:
This picture shows a DIN5-to-RCA jumper plugged into the installed DIN7:
After giving the aluminum surfaces a deep clean and putting everything back together it was finally time for a test spin! I selected "man-child", an awesome album by Herbie Hancock from 1975. I especially like the track "Bubbles", which is perfect for challenging restoration sessions. 
Of course this album was cleaned ultrasonically with a CleanerVinyl ProXL setup before play! Here an impression:
I am happy to report that the deck is behaving perfectly! What a beautiful sight and sound! Beolovely! I will play this deck for a couple more weeks to make sure there are no intermittent issues, and then it will be time for it to travel back home to Pennsylvania!



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