Beogram 4004 (5526): Tracking Sensor LED, Non-Working Carriage Switches, Final Adjustments and Test Drive with Eddie Henderson

The restoration of the Beogram 4004 (5526) that is on my bench is coming to an end. This post describes the final tasks of the project. After replacing the sensor arm bulb with an LED, I tested the carriage functionality and it behaved like the end switch and the turn off switch both did not work. It turned out that the wires that connect the switches to the main PCB had broken off from their solder points (white and grey wires):

I reconnected them (the grey wire needed to be extended a bit since it was pretty short after taking some insulation off for re-soldering):

After this repair everything seemed to work in terms of carriage motion.

The owner decided to also let me replace the bulb in the tracking sensor compartment with a LED assembly. I recommended this since this is also an essential light source, whose failure would disable the Beogram.

This shows the original tracking sensor light source in place:

I removed the bulb housing, which reveals the aperture that activates the sensor in the lower half of the assembly:

This shows the original bulb and the LED replacement in comparison:

The LED is in the same location like the filament of the bulb. This shows the LED assembly in place:

The blue trimmer allows fine tuning of the tracking sensor response, which makes it easier to adjust the system to spec. I also replaced the standard often rusty sheet metal screw that secures the aperture assembly in place with a M2 stainless socket head screw and a matching nut. This makes the initial coarse adjustment of the aperture position much easier.

Now it was time to adjust the platter height and the floating chassis horizontality. Then I moved on to the tonearm adjustments. An important item is adjusting the arm lowering limit. It needs to be set in a way that the needle 'misses' the ribs on the platter at the setdown points as an additional safeguard against needle damage should the record detection mechanism ever fail and the arm gets lowered onto the naked platter. This shows the adjustment result:

Then I moved on to the tracking weight. The first step is usually replacing the flimsy retaining clip on the counterweight screw with a M3 nut. This allows tightening the counter weight position down, which preserves the calibration over time and during shipping. This shows the original setup:

And with M3 nut:

Then it was time to calibrate the measured weight with the adjustment scale. I usually do it at 1.2g, since that is the weight at which most B&O cartridges track best. The weight scale on the adjustment wheel is notoriously imprecise, i.e. it is always best to confirm the weight with a digital gauge:

After this I adjusted the tracking feedback:

And then I tried to play a record...and all I got was a very loud hum from the speakers! Unbeolovely, entirely!

Hum usually means a broken connection in the grounding system between amplifier and turntable. It turned out that the shield connection in the DIN plug was disconnected:

Since the original DIN plugs cannot be opened up without destroying them, I needed to install a new plug:

The gold coated pins are a nice improvement in the signal path over the original corroded pins. This quenched the hum and the deck started sounding properly.

So it was time to listen to a nice record and enjoy this lovely 4004! I picked a recent addition to my collection, "Comin' Through" by Eddie Henderson. Of course this vinyl was cleaned with a CleanerVinyl Easy6 before I played it! Henderson recorded this album in 1977, and there are a few great funky tunes on it. My favorite is "Movin' On" on the first side. 

Beolovely! I will play this Beogram 4004 for a few more days and then it will be time for the return trip to its owner!

Beogram 4004 (5526): Replacement of Sensor Arm Bulb with a LED

After restoring the record detection circuit and the PCBs it was decided that the sensor arm light bulb should also be replaced with a LED to ensure operability of the deck down the road. These old light bulb like to fail, especially during shipping, i.e. it is a good idea to replace the bulbs with LEDs when restoring a Beogram. This shows the original light bulb in the sensor compartment together with the replacement flexPCB-based LED assembly:

I removed the bulb and inserted the LED board, which folds snugly into the compartment:

This shows the LED in action:

The LED is a warm-white type, i.e. there are enough red photons to ensure a properly colored B&O logo.

Whenever the light source in the sensor arm is replaced, the sensor response needs to be checked. The signal strength is highly dependent on the location of the light source in the compartment. The signal at the collector of the signal amplifying transistor TR3 needs to be close to 6V or better. This shows the measurement I made after the installation:

This Beogram record detection circuit is now operating better than spec (7.9V amplitude), i.e. we can confidently press the START button. 

Beogram 4004 (5526): Restoration of DC Platter Motor

After rebuilding the PCBs of the Beogram 4004 (5526) that is currently on my bench it was time to give the squealing DC platter motor a bit of attention. This shows the motor after extracting it from the enclosure:

I took it apart to get to the two Oilite brass bearings:

They are the two small donuts on the black pad up front. I immersed them in motor oil and pulled a vacuum:

Immediately, vigorous bubbling started from both of them. This indicated that the vacuum drew out the air inside the Oilite pores, making room for oil to diffuse back into the bearings. After ~48 hrs the bubbling stopped and I extracted the bearings:

Then it was time to reassemble the motor. For the upper bearing I used my 3D printed tool that allows to flatten the tabs on the retaining ring with sufficient force to get the bearing back into its original location:

This shows the two bearings back in place:

I put the reassembled motor back into the 4004 and installed my BeoloverRPM device for the performance of a 24 hrs RPM stability test:

This is the curve I measured the next day:

This is as good as it gets. This motor is back in business!

Beogram 4004 (5526): Restoration of the PCBs

In my last post about the Beogram 4004 (5526) that I am restoring right now, a bad capacitor turned out to be the reason why the arm would not drop at the LP setdown point. So we decided that the main board should be completely restored to prevent further capacitor issues. When one goes the others are probably not far behind. I replaced all electrolytic capacitors, as well as the RPM trimmers and relay:

This shows the 'RPM section' before

and after:

I always mount the RPM trimmers that the screws peek through the original adjustment openings so one can adjust the trimmer while the platter is turning:

While I had this board out, I also rebuilt the output board:

This shows the output relay section before

and after:

The red switch allows connecting signal and system grounds. A convenient feature in case the Beogram is used with a RCA adapter and there is some hum. Usually connecting the grounds cures that.

This concludes the restoration of the electronics...on to the DC motor, which made some nice loud shrieking noises when I tested it.

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!

Beogram 4004 (5526): A New Arrival from El Paso - First Look

A Beogram 4004 (5526) arrived from El Paso in Texas for some TLC. It arrived in a custom Beolover made shipping box that had been dispatched to the customer prior to shipping, simplifying the packaging process. The unit arrived in good condition. HD foam and double boxing has never failed the Beolover so far. Check out our 4002/4 page for a video how to package a Beogram properly.

I took the Beogram out of the box and had a first look: The aluminum panels are in pretty good condition (an important starting point for a successful restoration)

Unfortunately the platter has a scratch in the center:

but otherwise it is in good condition.

The hood has the usual scratches from careless storage:

Luckily there is not much damage on the inside, i.e. this hood can be polished back to a nice look.

I took the aluminum panels out and had a look below deck:

The unit seems mostly original except a very unprofessionally installed new reservoir capacitor:

Whoever did this did not even bother to tape it in place, almost ensuring a short circuit down the road  if the unit had been moved while plugged in. Not very Beolovely!

After making sure the capacitor leads would not touch anything, I plugged the Beogram in and pressed start. The carriage started moving, but did not stop at the LP setdown point, indicating an issue with the position sensor. At the same time the platter motor came alive with a shriek, suggesting dry bearings. Most Beograms with DC motor have this issue at this point in time.

In summary, this unit is a good starting point for a restoration, and I am looking forward to playing some nice vinyls on it when it is back in prime condition!