Beogram 4004 Type 5526 - California Project - Floating Chassis and Circuit Board Restoration

The previous post for this Beogram 4004 restoration finished up the initial disassembly and cleaning of Beogram.

In this post I finished up the mechanical restoration tasks and the restoration of the circuit boards to the point I was able to test some basic functions of the Beogram 4004.

Power circuitry, tangential arm control and the platter motor are now working.

Here is a photo of the initial start play test.

To get to this point I had to complete the tangential arm assembly restoration.

A lot of that was taken care of in the previous post for this restoration.

Continuing from there I disassembled and cleaned the tonearm lowering damper, then lubricated it with some PMX-200 silicone liquid and replaced the gasket of the piston.

While the floating chassis was still out of the cabinet by itself, I like to do as many of the mechanical adjustments and electrical tasks that are possible in that state.

The following photo shows the replacement of the platter detection sensor lamp with the DKaudiolover.com LED lamp assembly.  The photo also shows the adjustment of the arm to arm distance (7.7 mm) and the new locking nut on the tonearm counter-weight.

I was also able to check and adjust the Beogram 4004 vertical tracking angle (VTA).
That is the angle that the stylus contacts the record groove.
With the Beogram 4004 turntables, that angle is defined in the service manual as the adjustment of the top surface of the sensor arm to the top surface of the platter.
That distance is to be set at 23 mm.

On the Beogram 4004 turntables, the platter to arm height for the VTA is adjustable because the platter height can be moved.

The Beogram 4004 platter assembly sits on a platter bearing.
The platter bearing has threads and a locking nut so the platter bearing can raise or lower the Beogram 4004 platter.  Thus altering the VTA.

Also of note, there are three adjustment screws on the platter bearing assembly that affect the tilt of the platter.

Ideally none of these require adjusting but they can be re-adjusted if necessary.

In the case of this Beogram 4004 the 23 mm platter to arm distance was good.

Next, I adjusted the arm lowering limit and the tracking weight calibration.
I adjusted the tracking weight at the zero position with the counter-weight screw.
Then checked and set the tracking weight for 1.2 grams.

For the electrical restoration of PCB 1 (main board) and PCB 8 (output board) I replaced the electrolytic and tantalum capacitors with new capacitors.  On capacitors with values of 4.7uF and less, I switched to non-polarized WIMA MKS capacitor types.  On values greater that 4.7uF I used new electrolytic capacitors.

Here is PCB 1 after replacing the capacitors.

PCB 1 contains the motor control, sensor detection and control logic (done with analog circuitry).

For the platter sensor reliability we always change the fixed resistor 1R26 to a trimmer resistor so the platter sensing signal can be dialed in better during calibration.  We also check the 1TR3 gain to see if it is at a high enough value (typically greater than 500).  In this case, the gain was below 400 so I installed a new transistor for 1TR3.  It has a gain of around 700.

I mounted the new trimmer for resistor 1R26 on the trace side of the board.
That is just temporary so I can easily set the value before moving it permanently to the component side of PCB 1.

I also changed out the two speed adjustment trimmers (one for 33.33 RPM and one for 45 RPM) to multi-turn type trimmers as well as replacing the speed select relay to a DKaudiolover.com relay assembly.

I changed the two Darlington pair, power transistors, IC1 and IC4, to new transistors (TIP102 and TIP107 respectively).
IC1 requires an added filter capacitor to lower the ripple current on the 21 VDC supply.
If there is too much ripple on this 21 VDC supply voltage, the control logic of the Beogram 4004 can behave unreliably.  I will check what the 21 VDC voltage looks like on the oscilloscope later.

Last, I replaced the capacitors, audio muting relay (another DKaudiolover.com relay) and added a grounding switch to the output board, PCB 8.

Because phono audio signals can sometimes produce a hum in the signal due to different types of phono amplifiers the Beogram is connected to, the grounding switch allows a quick option to connect or disconnect the Beogram's signal ground from chassis ground on the phone cable connection.

At this point the restored Beogram 4004 parts were ready to be assembled together to check the restoration progress.

It was time to put the floating chassis back into the cabinet.

Here again, is the cleaned up cabinet base.  It is all clean but the outline of stain from whatever the spilled liquid was in the Beogram is still visible.

The three mounting and chassis locks for the Beogram have to be installed first.

The Beogram floating chassis contains the platter and the tonearm assembly so those components move together if there is any vibration or bump against the cabinet while playing a record.

To prevent the chassis from floating during transport, Bang & Olufsen engineers created a locking mechanism that tightens down and prevents the chassis from moving.

The way it works is the Beogram chassis is suspended on three leaf springs so the chassis is isolated from the cabinet base.

The lock for the chassis is a screw with two sections.  The upper section and lower section are threaded in opposite directions so when the screw turns, the locking nuts close together (for locking) or open up (for unlocking).

This picture shows the two sections of the transport lock screw and the two different locking nuts.
The top and bottom nuts are threaded differently so they must be installed to their respective ends.

The washers for the chassis locking mechanism are convex on one side.  I have noted in the photos their orientation.

Here are the stages of installing the Beogram 4004 floating chassis beginning (upper left photo)
with the underside of the floating chassis lock assembly in place and ready for the Beogram floating chassis to be installed.

That is followed by the installation of the floating chassis and then the topside of the lock assembly.

Also part of the cabinet reassembly was the 4000uF reservoir capacitor for the AC power for the Beogram.

For that component I used the new DKaudiolover.com Beogram 4002/4 capacitor assembly.
Very nice.

I then soldered the arm lowering solenoid and DKaudiolover.com tracking sensor lamp assembly to their respective solder pads.

The last restored components I installed were the tangential arm spindle, pulley for driving the tonearm assembly.

That brings me back to the first photo in this post showing the first start play test of this Beogram 4004.

I am very pleased with the restoration so far.

The next step is to calibrate and move the platter detection circuit 1TR26 trimmer to its permanent position on the component side of PCB1 and calibrate the record tracking sensor.
I will also measure voltages and signals with the oscilloscope.