Beogram 4000: Adjusting the Gap at the Carriage Servo Pulley Bearing

When I tried out the carriage mechanism of the Beogram 4000 that I am restoring right now after repairing and adjusting the tracking mechanism, I found that the threaded rod that advances the carriage had a lot of longitudinal play. Here is a picture of the setup:

The play manifested itself in an unwanted 'clacking' every time the carriage changed its direction. This turned out to be an easy fix. All I had to do was to remove the servo pulley and remove the damaged locking ring and the worn out plastic washer:

This is the pulley. It is held in place by a setscrew whose threads are in the small brass plate that is inserted into the slot above the shaft receptacle. It should be noted that it usually sits very tightly on the shaft and that it took some careful 'convincing' with a flat head screwdriver to pry it off the shaft.

I replaced the locking ring and the washer by a thick 2-56 nylon washer which fit perfectly on the shaft. The locking ring is not needed since the pulley is locked in place with the set screw:

I also lubricated the shaft in its bearing and the nylon washer with a bit of silicone grease.

Then I put the pulley back on:

Now it has only a fraction of a mm play and the carriage moves very precisely and without strange noises. I added a bit of silicone grease to the threaded section of the shaft and that quieted the mechanism further.

Beogram 4002/4004: New Belts and RPM Adjustment

A few months ago I received new Beogram 4002/4004 belts from 'Dillen' (Beoworld.org). Now was finally the time to install them into the Beogram 4002 that I am currently restoring. It seemed to need a new drive belt, since I had to considerably increase the speed trimmer to achieve 33 RPM. The Servo Belt also showed cracks. It seems it may have still been original. It had the prescribed triangular crossection, matching the groove in the pulley that drives the carriage spindle.

As in the case of the Beocord 5000, I thought it useful to take some measurements of the old and new belts for future reference (like when diving archeologists bring up this turntable in the year 2358 from the ocean floor where Florida used to be before global warming...;-):

Belt	Old ID	New ID	Old Crossection	New Crossection
Drive Belt (flat belt)	17” length when pressed flat	17” length when pressed flat	Width: 5 mm /0.2” Thickn.: 0.5 mm 0.020”	Width: 5 mm /0.2” Thickn.: 0.85mm/ 0.033”
Servo Belt	31 mm /1.2”	31.5 mm /1.23”	Triangular Height: 1.5 mm /0.059” Base: 2 mm /0.078”	Square 0.75 mm/0.030”

I really like the much tauter feel of the new, thicker, flat belt...and indeed, after I put the new drive belt in, the speed was considerably higher, indicating slippage with the old belt. Now the speed trimmer was much closer to the '0' mark for a precise 33 RPM, i.e. the slippage was much less. I decided to make this perfect, and adjusted the primary speed trimmers on the circuit board (R14 for 45 RMP and R15 for 33 RPM) for a correct RPM setting when the user accessible trimmers are set to '0'. I did this with a small all-metal watch maker screwdriver for 33 RPM while holding down the '33' key (as one does for sweeping the platter before starting to play a record). There I noticed a remarkable behavior: The speed of the platter would immediately increase, seemingly beyond 45 RPM, whenever I stuck the screwdriver into the slot of the trimmer. After extracting it the speed would normalize...it appears that my body's resistance path to GND via the key pad affected the time constant formed with C3, which then throws off the difference amplifier in IC3, resulting in a change of speed of the motor.

Lesson learned: Finally get rid of the all-metal screwdrivers...they got me in trouble before when I fried one of the outputs of my Beomaster 6000 4-Channel by a screwdriver-caused short when adjusting the quiescent current...;-).

Beogram 4002: Conversion of the Tracking Sensor Light Bulb to an LED and Adjustment of the Sensor

I started working on a Beogram 4002 that I obtained locally from Craigslist about a year ago. I thought it would be a nice practice run before I give the cherished Beogram 6000 4-Channel a go that I recently found.

I plugged it in after opening it up in the hope to see the location of the smoke should any develop...;-). No smoke, but no tracking either. Pretty much nothing worked, no tone arm lowering etc...also some traces of amateur repairs.

I determined that the light bulb in the tracking sensor was out, and I decided to replace it with an LED since the bulb cannot easily be replaced since it is glued into the sensor housing, i.e. cannot be separated. I also am a fan of LEDs since they have a much better long-term stability, especially should the unit be shipped at some point.

I ended up using a 3D printed part to hold the LED and the current limiting resistor. The part essentially replaced the light bulb fixture on the sensor housing. Here is a picture of the end result:

I made a video that explains the whole procedure including the adjustment of the tracking mechanism for the right tone arm angle relative to the record grooves. The video also discusses how the tracking mechanism works. Enjoy!:

On to the dead sensor arm!