Beogram 4004 Type 5526: Finalizing the Fixed Arm Record Detection

In the last post comments I mentioned learning something new every time I work on these Beogram turntables. This is certainly one of those cases. I installed the Beolover LED module in the fixed arm sensor assembly that I had swapped with a spare Beogram 4004 turntable. That sensor measured exactly as I wanted using the original incandescent lamp so this new LED light source should work just as well. To my surprise the LED light source produced a weaker sensor signal than the incandescent lamp. The signal was better than with the original sensor assembly but it still wasn't at the level I need it to be in order to continue with this restoration.

The peak-to-peak voltage target is 6 V and the above signal is still short.

Even though the LED light source looks brighter than the incandescent bulb there are other factors involved like the angle of the light on the spot that the sensor is focused on and the wavelength of the light. I discussed this with Beolover and determined that I needed to adjust the LED position on the Beolover light source.

Here is the Beolover LED module. The blue arrow shows the direction I am moving the surface mounted LED. How convenient that Beolover designed the solder pads for mounting the LED so that its position can be adjusted.

That was all it needed. The new position of the light source provides the necessary light stimulus to the sensor where it can now produce a healthy 6 V peak-to-peak record detection signal.

I am now confident in this circuit so I can continue on to the Beogram 4004's platter height adjustments.

Beogram 4004 Type 5526: Record Detector Testing and Adjustments

This Beogram turntable has a new LED lamp source for the record detector circuit that is in the fixed arm. Part of that installation involved switching out a fixed 1MOhm resistor (1R26) with a 2MOhm trimmer resistor to allow better tuning of that detector circuit voltage. With the turntable parts re-installed I started the procedure of making an initial setting of the new 1R26 trimmer so the collector of 1TR3 is at 4 VDC when the fixed arm lamp is on.

I made that adjustment then moved the 1R26 trimmer resistor to the component side of the board where it belongs (so it is out of the way of the platter).

A quick test of the Beogram Start, without a record on the platter, passed with the sensor circuit correctly recognizing there was no record on the platter.

To complete this test however it is always a good idea to actually see the signal on an oscilloscope. I connected a probe wire to the 1TR3 collector and then to my scope. When I ran the fixed arm and tonearm over the platter I paused it so I could measure what the detector circuit is seeing.

I was surprised and disappointed to see that the peak to peak voltage of the signal was not from 0 V to around 6 V as expected. It only drops to about 2 V and goes up to 4.72 V.

So while the Beogram appears to detect an empty platter as it should, the signal is not ideal and could lead to problems later on. A little bit less peak to peak voltage and this circuit will not detect an empty platter.

I wanted to know if the problem was in the new Beolover light source assembly or with the fixed arm light sensor. The easy thing to try first was to use a powerful LED flashlight to enhance the light on the platter. When I did that I could make the 1TR3 collector signal go to its full range. But look how much light it took for the sensor to register that. Something must be wrong with the sensor.

As a second part of the test I pulled out the fixed arm lamp and sensor assembly a ways so I could move the Beolover LED module and watch the effect on the 1TR3 collector voltage.

Moving the LED light source could not make the signal peak to peak range any better. It could make it worse though and confirmed my fear that a little less voltage would cause the platter detection to fail.

Next step then is to swap out the fixed arm sensor. I decided to swap out the entire assembly first with one of my spares (from another Beogram 4004 turntable).

Here is the removal of the current sensor assembly that has the Beolover LED light source.

Here is the replacement sensor assembly. It still has an original incandescent light source.

Just like that, the 1TR3 collector signal went right to what is expected. The peak to peak voltage is from 0 V to about 6 V.

That voltage is with a dimmer light source as the Beolover LED light source is brighter than the incandescent lamp. The original fixed arm platter sensor is the problem and will have to be replaced.

Beogram 4004 Type 5526: Fixed Arm Work

This Beogram 4004 restoration is going to include changing out the fixed arm record sensor lamp with a Beolover LED replacement part. Since this operation is easiest with the fixed arm unbolted it is also an opportunity to clean and lubricate the pivot point of the tonearm lift/drop lever.

The fixed arm sensor lamp assembly is usually securely mounted in the end of the arm. It takes a firm grip and careful pressure to work the black plastic assembly out of the arm. That is why it is best done with the fixed arm unbolted.

Here is the Beolover replacement sensor lamp (LED). It is a special, flexible printed circuit board with surface mount components to replicate the original sensor lamp. The nice thing about it is there is no guess work in positioning the lamp for the lens. The Beolover part is shaped so it only fits one way and that way perfectly positions the new LED lamp over the lens.

After mounting the new sensor lamp and fishing the lead wires through it is a simple clamp, solder and trim operation to complete the installation.