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.

Beogram 4002 Type 5513: Finishing Up The Tangential Arm Position Sensor & The Fixed Arm Platter Sensor

It was great to see Beolover has the platter motor restored for this Beogram 4002 project. While it is being packed up for shipment back to me, I adjusted the voltages for two of the new sensor lamps in the Beogram.

The new LED replacement light source for the tangential arm position sensor was a little weak for me to leave it installed. It appears to work fine but with the 1R88, 25K ohms trimmer set for the highest LED output, the maximum voltage I could get on the 1TR17 base was 0.695 VDC. I would prefer for the 4D1 light source to have enough juice to max out the sensor then use 1R88 to dim the LED down where the base voltage of 1TR17 is 0.7 VDC.

That meant ordering a brighter LED. Here is the circuit as shown in the service manual. The 1R88 trimmer controls the current to the lamp source, 4D1.  That increases or reduces the LED light output to the sensor, 4IC1. The sensor controls the base of 1TR17.

With a new amber LED installed (Cree C503B-AAN-CY0B0251) for 4D1, there is plenty of light on the sensor now.

In operation the black bars on the tangential arm transport scale affect the light source on the sensor causing a change in voltage at 1TR17.  The output of 1TR17 drives analog control logic for the operation of the Beogram. It is quite interesting how the control system works on these analog 400x turntables compared to the 800x series which use a microcomputer for their control.

Here is a picture of the tangential arm position scale.

Here everything is put together.

The new LED does max out the sensor where I get a voltage of around 0.75 VDC at the base of 1TR17.  Using the 1R88 trimmer to adjust 1TR18 so the LED has less current, I got the desired 0.7 V at 1TR17's base.

Next up is the voltage adjustment on 1TR3 for the fixed arm sensor - the detector arm circuit. Two reference posts by Beolover show the relevant circuit and the installation of a 2M ohm trimmer in place of the 1R26, 1M ohm resistor (between the base and collector of 1TR3).

Here is the 1R26 resistor on the main board before switching it out with the 2M ohm trimmer.

Following Beolover's same procedure I temporarily installed the 2M ohm trimmer on the trace side of the board to make it an easier task of adjusting the trimmer setting while operating the Beogram.

Measuring the collector of 1TR3 to ground I adjusted the trimmer until I got to 4 VDC.

After the adjustment I de-soldered the trimmer and re-installed it on the component side of the board so it will be out of the path of the platter.

As a final confidence check of the detector arm circuit I connected a probe wire to the 1TR3 collector so I can view the resulting signal when the detector arm is over the platter.

I get a healthy 5.66 Vp-p minimum voltage when the arm is stationary over the platter and 6.16 Vp-p when the arm is moving across the empty platter.

These two sensor circuits are ready to go. The next tasks will be to run through some of the arm and platter mechanical service manual checks. After those I will adjust the tangential arm tracking of a vinyl record.