We like to (insist really) that the 33⅓ RPM and 45 RPM indicator lamps be replaced with Beolover LED replacement assemblies. The reason for that is those lamps are part of the platter speed control circuit and our speed stability testing shows these LED replacement modules provide a measurable improvement to the platter motor performance stability.
Here is the original RPM indicator module removed and the installation of the replacement Beolover RPM indicator assemblies.
All of the electrical pieces are ready so it is time to reassemble things back in the Beogram cabinet.
One of my earlier posts showed that the suspension locking bushings on this Beogram were deteriorated to the point of crumbling away. The next pictures show the installation of the Beolover 3D printed replacement bushings.
I am not fully reinstalling the circuit boards yet. Just connected up so I can test power and basic functionality of the electrical work.
It is very common for these Beogram 4002/4004 units to work electrically after the work on the boards. Since this Beogram 4004 is in such pristine condition I fully expected that to be the case here.
To my surprise there were a couple of problems. The On automatic scanning for a record did not work. Neither did the Off automatic return or did cueing up and down. The manual scanning worked and the platter motor worked.
Looking at the sensor lamps in the Beogram I could see that the fixed arm lamp was working along with the tracking sensor lamp. The new RPM indicator lamps worked but the carriage position lamp did not. I figured the burned out carriage position lamp was the problem.
I have had to replace these before so I grabbed my usual amber LED replacement part.
Here is the failed, original carriage position lamp.
...and here is the replacement.
The new lamp works as expected and I adjusted the voltage on the anode side per the service manual to +5VDC when a clear part of the position slide scale is between the lamp and sensor.
The signal at the anode side of the LED looks correct when the tonearm carriage moves across the sensor.
Unfortunately the auto scanning and cueing functions still fail. That means there is some control function problem.
These Beogram 4002/4004 turntables from the seventies still look modern today. The way they operate will cause most unfamiliar observers to think this turntable is controlled by some computer chip of some kind. They would be wrong of course. The controller in this turntable is an analog control system. Optical sensors and microswitches feed the analog control circuits where transistor switches manage the logic. Pretty cool.
Looking at the circuit diagram for the start, stop and scanning I looked for where the fault could be.
The transistor switches I suspected are highlighted in red. 1TR12 and 1TR13 are supposed to latch for their respective forward (ON) and reverse (OFF) scanning. I desoldered and checked them with my transistor tester and they appeared to be good. I went ahead and tested 1TR11, 1TR14 and 1TR15. They also measured good. Next I tried 1TR8...also apparently good.
After a lot more probing and measuring I was at a loss to what the problem was. I finally decided that it had to be something involving those transistors or their connection. While the service manual says the transistors there are all BC237B transistors the board actually used BC547B transistors. I decided to go ahead and replace all six transistors...and the Beogram control logic started working correctly.
Interesting that the faulty transistor(s) measure good out of circuit but I have seen that before. It is something to keep in mind when diagnosing a problem.
Now I have a working Beogram 4004. I can move on to doing the service manual checks then play some records.
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