Beogram 4004 (5526): Repair of Record Detection Circuit and a New Reservoir Capacitor

I started working on the Beogram 4004 that I recently received. I decided to look into the 'does not lower arm at LP set-down point' issue first.

***I recommend downloading the Beogram 4002/4 service manual with the circuit diagram for reading this post.***

Usually, when a 400x Beogram does not stop at the LP point after pressing START (with a record on the platter), there are two standard causes:

1) It does not detect the record properly, or

2) the carriage position sensor does not work.

#2 is easily eliminated by watching if the deck switches to 45 RPM once the arm sweeps past the 7" singles setdown point. If it does, the position sensor is most likely up to the task.

The 4004 did the 45 RPM switch, i.e. it was time to look into the record detection circuit. The record detection circuit essentially needs to pass three tests:

1) Does the light bulb in the sensor arm work, and is it the correct bulb with the right current draw?

2) Does the platter induce a proper ~5.5-6V amplitude signal at the collector of TR3?

3) Does the circuit put out the proper signal at the collector of TR6 (~15.5V when there is a record; ~0V when there is none) when a record is on the platter?

Since the light bulb was working (lit B&O logo), I measured the voltage at the Collector of TR5 that drives the bulb and it showed 8.5V (spec is 9V, but that is o.k.). This voltage is used by the circuit to put out a 'no record present' signal when the bulb is dead (the voltage would be close to 21V in that case, which would permanently turn on TR6, i.e. its collector would be 0V - saying 'do not lower the arm'. This is a safety feature, since without it a dead bulb would give the sensor the same impression as the presence of a record, and the arm would be lowered on the platter.

With regard to #2, it is a good idea to check the DC voltage at the collector of TR3 first and measure if it is close to the prescribed 4V. This voltage is necessary to ensure that the sensor signal has a 6V amplitude when the ribs of the platter pull by beneath it. I measured 2.1, which was very low. This meant I needed to replace the biasing resistor R26 with a 2M trimmer for adjusting the bias of TR3 (in essence quench some of the CE current to achieve a higher C voltage). This is a very common issue in the 400x, and implanting the trimmer usually allows adjusting the bias to get 4V at the collector. I put in the trimmer and tried to adjust the collector voltage:

Unfortunately, even by completely maxing out the 2M resistance on the trimmer, I was only able to get to 3.4V. This meant that TR3 had an unusually large Hfe. In absence of an even larger trimmer (and the rule of thumb not to use large resistances/very small currents in such circuits if one can avoid it) I decided to put a new BC547B transistor in. This fixed it:

I measured the Hfe values of both transistors, and the original 548C had about 750, while the Hfe of the new one was only 430, which allowed the 2M trimmer to do its job.

While it was a good idea to fix the TR3 voltage, this was unlikely to have fixed the issue, since a too small sensor signal causes the opposite effect: It always detects a record whether there is one or not, i.e. the arm gets lowered onto the platter if there is an issue with the sensor signal.

The next step was to check on the output of the detection circuit, i.e. I needed to measure the voltage at the collector of TR6 when the carriage reached the setdown point. I did, and the voltage remained close to 0V, indicating that the circuit was not doing its job. First I thought TR6 was bad, but my transistor tester gave it a passing score. This meant something else was pulling the voltage down to zero.

I started the Beogram without TR6 in place. This essentially should tell the circuit "record present" since this causes the same situation like a turned off TR6. I measured the voltage again at the vacant collector terminal of TR6...and it was close to 0 volt again! So clearly there was a short somewhere. The most direct connection to 0V is probably via a shorted out C19 that would pull the collector pad down to ground via R41. I measure the capacitance of C19 in situ with my BK Precision 879B LCR meter, and it yielded 4.7 Ohms! This caused me a bit of head scratching, since there is essentially no other good path for this short circuit to happen in this circuit. I finally measured the resistance across C19 with my multimeter, and I got a stable reading of 132 Ohm! That should not happen with a working capacitor. So I took the capacitor out and replaced it with a new one. And the circuit worked again.

I thought this capacitance measurement was an interesting lesson. I measured the cap with my multimeter's capacitance setting, and it showed 0F. So it seems that the frequency based measurement method has hits weaknesses, when it comes to measuring borderline dead capacitors. The 879B did show an ESR value of 11 Ohm on this capacitor, which is very high. This should have tripped me off right away, but in the fog of battle...anyway, an enjoyable evening with a Beogram 4004 is coming to an end!...;-).

But wait, this Beogram had one more issue to rectify: The reservoir capacitor. Whoever tried to fix it before hot-wired a new reservoir cap of the cheapest kind into the unit:

I replaced it with a Beolover capacitor assembly that fits precisely under the mounting strap of the original one:

Beolovely again!