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Sunday, February 28, 2021

Beogram 4000: Complete Restoration and a Test Spin with Oscar Peterson

A while ago I received a very corroded Beogram 4000 from Sweden. This post shows my initial assessment. Since the corrosion was very 'comprehensive', but the cosmetic condition of the aluminum surfaces and plinth was pretty good, we decided that it would be better to use an additional 4000 unit as donor of the 'under the hood' parts. This is the donor unit, which had a few parts missing, but no corrosion:

As usual I started out with rebuilding the carriage. First was the arm lowering system:
I removed all the linkages and the damper
and also took out the carriage rods and spindle for cleaning in an ultrasonic bath:
I also removed the sensor arm assembly to get to the pivot point of the damper-to-arm linkage:
As is often the case, the skid plate under the arm upper limit adjustment screw was loose
I removed the decayed double sided tape and epoxied the piece back into place:
I also exchanged the tracking sensor light source (black box):
After removal the tracking sensor aperture was revealed:
I installed a LED based replacement and I also replaced the sheet metal screw that holds the aperture to the arm base with a stainless M2x12 screw with a proper nut for better long term stability:
While the carriage is up, it is the perfect moment to restore the corroded contact terminals of the solenoid actuated switches. This shows the switches in their original condition. The terminals are oxidized and also bent out of shape, probably from earlier attempts to restore functionality by increasing the spring force (this never works, just makes things ugly): 
I removed the small boards with the switches:
I also extracted the corroded terminals from the board that carries the carriage position switches:
This shows the corroded terminals after I extracted them:
Then it was time to spend a couple hours in the garage for electroplating all the terminals, first with Ni and then with 24 carat gold. This is the result:
The next step was to re-implant them in their boards:
This shows the switches on the bottom side of the carriage PCB. I also replaced the solenoid current limiting resistor that is mounted next to the switch terminals (it is often blackened since it is a bit under dimensioned):
While all the above parts were removed I used the open range to get to the solenoid transistor and replaced it with a new TIP41:
I already encountered a few burnt out ones during my restoration projects, and therefore I made this replacement standard (as well as the associated transistors on the main PCB that drive this TIP41 - see below).
After bolting all the carriage related bits back together, it was time for a renewal of the main capacitor and AC motor section. This shows it in its original condition:

I extracted the capacitors and the motor. First I took the motor apart to immerse the housing, which contains the bearings in motor oil under vacuum:

While the bearings enjoyed the oil I replaced the old leaky capacitors with modern ones, held in place with a 3D printed fixture:

Then I rebuilt the main PCB. This shows the component side in its original condition:
I replaced all the electrolytic capacitors, the RPM relay and the RPM trimmers, as well as the solenoid transistors that feed the main TIP41, and the sensor arm transistor and its bias resistor:
This shows the new solenoid transistors magnified
and the sensor arm circuit. The blue box is a 25 turn trimmer that allows precisely adjusting the bias of the sensor transistor for reliable record detection:
While the PCB was up, I also replaced the push pull stage of the AC platter motor. These transistors also have a history of failing:
The next step was rebuilding the command center of the Beogram, the keypad cluster. I extracted it and opened it into service position. Of course, the strobe mirror came off while I handled it:
Again, 'beautifully' oxidized switch terminals. they were almost completely blackened:
I extracted them and did another session in the garage with my electroplating setup:
Then I soldered the terminals back into place:
Nice and shiny!
While the PCBs were up, I also replaced the original light bulbs that illuminate the position scale and the RPM trimmers from the back. This shows the bulbs:
and the LED replacements:
Then I epoxied the strobe mirrror back into place:
My next focus was on the arms. First came the sensor arm. I usually replace the bulbs with a flex-PCB based LED circuit that mimics a bulb in terms of current draw. This is necessary since the Beogram has circuitry that recognizes a burnt out bulb that it can disable the arm lowering mechanism to protect the stylus. This shows the original bulb and the replacement:
and the LED installed:
This shows it in action. The LED is a 'warm white' type that has enough red photons to illuminate the B&O logo similar to an incandescent light bulb:
Since the donor Beogram did not come with a tonearm, I had to transfer the arm from the corroded unit. While I had it out, I extracted the cartridge adapter. I usually replace it with a 3D printed replica, since the original plastic is usually brittle and it is easy to break the tab off when mounting or removing cartridges, which is not very Beolovely:
This shows the replacement already soldered to the signal wires:
This shows the new tab installed:
I completed my work on the signal path by installing a switch on the output terminal that allows connecting signal and system grounds in case there is a hum issue (often happens with RCA based receivers). This shows the original terminal
and with switch installed. The switch is shown in 'grounds connected' position: 
I also installed a new DIN5 plug with gold plated contact pins:
the next step was repairing the plinth that came with the corroded unit (the donor 4000 had a chipped corner). Someone previously glued the metal brackets back to the plinth, but unfortunately at an off angle and too low:
This required removing the bracket. This is best done by baking the plinth wrapped in aluminum foil for an hour at the 'warm' setting (~80-90C) in a kitchen range: 
After the baking process the metal brackets can be removed with a spatula or similar. Then I glued the bracket back into the correct position. This can be done with contact cement and two Beogram aluminum plates that have the exact right thickness to position the bracket at the right height. I use sheet of baking paper to prevent the glue from sticking to the side, while fusing the bracket to the front part of the plinth:
This shows the bracket back in the right spot:
Then I bolted the plinth back to the frame with new guidance washers. The original ones were cracked on both units:
This shows the new ones, one printed in black for the front center position so it cannot be seen through the gap between aluminum plate and plinth.
This shows one of them installed:
And now it was finally time for some measurements and adjustments. First I adjusted the motor voltage:
And then I verified that the sensor signal was solid. This shows the oscilloscope signal as measured at the collector of TR14:
The amplitude is better than 2V, which is solid. Each of the dips represents one of the black ribs on the platter passing underneath the sensor arm.
The next measurement was the RPM stability. Usually the AC motors do not have any issues, but I like to check anyway. This shows the BeoloverRPM device in action:

The BeloverRPM can log the RPM in 10s intervals for extended periods of time. This is a great way to detect intermittent RPM issues. This is the curve I measured after 24 hrs:

This is as good as it gets with the Beogram 4000!
After these measurements I adjusted the floating sub chassis and the platter height and parallelism. then it was time to adjust the tracking weight:
Another important adjustment is the arm lowering limit:
It should be set in a way that the needle can go slightly below the level at which a record sits, but not low enough to damage the stylus should there ever be a malfunction of the record detection system. Then I adjusted the tracking feedback. The trimmer on the LED light source comes in handy for fine tuning of the system:
After these adjustments I cleaned all the aluminum surfaces and polished the plinth a bit and then it was finally time for a first test spin! I selected Oscar Peterson's "Motions and Emotions" album from 1969, which I recently acquired. Of course this vintage album was ultrasonically cleaned with a CleanerVinyl Pro System, which restored it to its original glory.
This album is another awesome classic release from the German MPS label (MPS 15251 ST). It is latin influenced and usually gets me into a very relaxed and happy mood while working on Beograms! 
What a beautiful sight! It is always a very satisfying moment when a Beogram 4000 is back in business!
I will now play this unit for a couple weeks, and if it runs stable it will be time to send it to Sweden!