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Wednesday, July 4, 2018

Beogram 4002 (5513): Service Manual Adjustments & Preparing for Use

I left off in the last post with the keypad cleaning, deoxidizing and the RPM panel. The keypad response is immediate but the cueing up and down button is too sensitive now. I will have to revisit that switch. There was a delay in the RPM panel re-installation because the molded, acrylic mounting fixtures were cracked. That is very common with these Beogram 4002 turntables. Often they are completely broken. Luckily the mounting fixtures on this Beogram just had one crack in each one and were still intact. I pressed the mount fixture together as I applied acrylic repair fluid in the crack. It dries very quick. That repair by itself is not strong enough if the RPM panel is removed and reinstalled very many times. To supplement the acrylic glue I cut two strips of Dura-Lar and put one strip on either side of the mounting block. I use Araldite AB rapid epoxy. It is clear.






















The patched mounting blocks attach to the aluminum frame by set screws underneath.























One thing to note is the mounting screw for the keypad and RPM indicator panel is right underneath the RPM panel so whenever there is a need to get to the keypad buttons or the phono output board, the RPM panel has to be removed. That is why reinforcing the mounting blocks is important.

Here is the panel back in place.























Next I want to make sure the voltages and signals of the Beogram detectors are good.
First up is the 4D1 LED and its 4IC1 optical sensor that detect where the tangential arm is. Markings in the acrylic slide alter the LED light to the 4IC1 sensor and Beogram circuitry reacts to those changes in the signal.

The service manual check for this sensor is to measure the collector of the 4IC1 sensor when a transparent section of the slide is between the LED and the sensor. The voltage level at the 4IC1 collector should be around 5V.  If it is not, the 1R88 trimmer (on the main board) is adjusted to set the voltage needed.

These pictures show the adjustment.





























































This is an important adjustment because the run-out groove detection at the end of playing a record is dependent on this voltage level being set correctly.

Next though is the fixed arm sensor that detects if a record is present (and if it is okay to lower the tonearm). The fixed arm has a light source that shines down on the platter. The reflected light is read by an optical sensor. If there is a record on the platter the reflected light will be at a low level and won't vary. That steady, weak signal is interpreted by the Beogram arm lowering logic as okay to set down the stylus. If there is no record the black ribs on the platter will affect the reflected light so that it returns as pulses to the sensor. The pulses cause the arm lowering logic to not allow the stylus to be lowered.

Obviously it is important for this signal to be working and is at a proper level.































This sensor is providing pulses that are above the required 6 volt level. No danger of accidently lowering on an empty platter.

I need to check the end groove, run-out stop circuit but first I will perform some more mechanical checks and adjustments to get the Beogram ready for record play.

I caught a break on the platter to tonearm height adjustment. The distance was already at the required 23mm.






















The arm lowering limit needs to be set so the tip of the stylus is about 1mm from the top of the first platter rib. This is a safety feature that prevents the cartridge from damage should the record detection circuit fail.























Another remaining mechanical adjustment is to check the tonearm for length and parallelism.
I run a string tangential to the Beogram arm assembly then check that the stylus travel path follows the string.






















The check that the tonearm is 7.7mm distance from the fixed arm and that they are parallel were checks I made earlier in the restoration when I took apart the tonearm assembly (for the oil cleanup).

We are getting close to record play now. At this point the tracking sensor sensitivity needs to be fine tuned, the run-off stop needs checking and the platter speeds need to be dialed in.

The tracking sensor adjustment went fairly smooth. It still took quite a few iterations to get the sensitivity set where the tangential arm motor moves after each revolution (2±1 revolutions initially, 1 revolution after that).

The run-off stop check measured good on the oscilloscope.




I was able to easily adjust 1R14 and 1R15 trimmers (on the main board) to set the 33⅓ RPM and 45 RPM speeds using the Beolover RPM tool.



Whew! A lot of adjustments and checks on these Beograms. I still love them though. What a great turntable design.

Now for a break for some food and fireworks (not with the Beogram :-).  Then I will do a quick adjustment on the keypad cueing button. After that I should be lying down and listening to some records.

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