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 Type 5526: Last minute mod on the Beogram 4004 Turntable #1 from Dallas

The Beogram 4004 turntable number one (of two) from Texas was ready to return home to its owner when a special modification was requested. The owner has a modern integrated Bang & Olufsen system from the late nineties (Beocenter 2500) that is now the intended receiver for this Beogram. That presents a problem because the modern systems do not come with a built-in RIAA preamplifier which is required to prepare raw phono cartridge signals for amplification. The resurgence in the popularity of vinyl records has meant a whole bunch of amplifier systems out there without support for phonographs.

There are quite a few third party solutions of course. Those involve a small RIAA preamplifier box between the turntable and the modern amplifier. That works quite nicely but there is a better way when it comes to a Beogram 4002 or 4004 turntable.

Beolover has designed and built a custom Beogram 4002/4004 RIAA amplifier board that connects right into the Beogram (on the existing output board).

The beauty of this design is two-fold.

1. The modification is internal to the Beogram so it doesn't require an external box and additional cables. The original phono cable is used as-is.

2. The modification retains the original connections so the Beogram can still be used with amplifiers that have their own RIAA preamplifier.

The installation of the Beolover RIAA preamp is quite easy.  Here is the sequence.

In most cases the installation is just to remove the phono output muting relay and solder in the custom RIAA preamp board. For this case I also had to remove the grounding option switch I had installed earlier in the project.

The grounding option is not necessary when using the RIAA preamp connectors. If the owner wants to change the receiving amplifier for this Beogram later, to one that has an RIAA preamp, then the grounding option is still nice to have.  The switch I had installed before would be in the way so it has been replaced with a simple two-pin jumper between signal ground and system ground.

Here is the Beogram 4004 output board with the new RIAA preamp mod re-installed.
This picture shows the board connected up for use as the original Beogram....without any RIAA preamplifier.  I want to test that the original functionality works first.  Then I will switch it to use the Beolover RIAA preamp connectors and test the Beogram in that configuration.

If any other Beogram 4002/4004 owners would like to try this preamplifier for their Beogram just contact Beolover at beolover@gmail.com.

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!

Beomaster 8000 From Canada: Packing for shipment home

The day finally arrived to ship the Beomaster 8000 from Canada back home. All tests have passed and loose end tasks completed.

This receiver is quite heavy. The packed shipping weight can be from 57 pounds to 65 pounds depending on packing materials. I was able to get this shipment prepared at 57 pounds in a 30 inch x 20.5 inch x 12 inch box.

I glue the bottom flaps of the cardboard box together with a common wood glue.
After that I lined the bottom and sides with 1 inch thick, 2.2 pound rated, polyethylene foam. Those pieces are attached using 3M Super 77 spray adhesive. Polyethylene foam has some give to it but is sturdy enough to resist puncture. A good material for the first line of defense.

The old type styrofoam and styrofoam peanuts are the absolute worst packing material ever devised. They should never be used!
They generate a lot of static electricity and small pieces get into everything...especially the audio component you are shipping.

Bubble wrap is not very good either. It is okay for small, light things but heavy shipments can puncture the bubble wrap leaving no protection at all.

For this Beomaster 8000 my first step in wrapping it up for placement in the shipping box is to place it inside a clear plastic bag. That will protect against any liquid and keep packing materials from rubbing directly against the Beomaster.

Next I use 1½ inch thick eggcrate foam to wrap the Beomaster providing another layer of padded protection. This foam has more cushion than the polyethylene outer foam. I do one wrap lengthwise and one wrap crosswise.

The eggcrate foam wraps are taped and everything is placed into the shipping box. I use extra blocks of the polyethylene foam to fill in any gaps.

The last rectangular piece of polyethylene foam is the lid followed by taping shut the cardboard box.

This Beomaster 8000 receiver is now on its journey back home and I hope to hear happy results from the owner after it arrives.

Beomaster 1900 Type 2904: Preparing for recapping tasks

The Beomaster 1900 project continues with preparing the circuit boards for recapping. I don't like carefully navigating between wires and components to replace capacitors. My technique is to disassemble the Beomaster so I can easily get to the components I need to work with. On these vintage receivers I always reflow the solder joints on connectors so that is another reason to have clear access to everything on the boards.

The previous Beomaster 1900 project post left off with the receiver opened up for inspection. As I got ready to remove the tone control and FM tuning board I inspected the three slider controls a little closer. Several of the slide contacts are loose indicating this unit will need Martin's slider control repair kit.

The Bang & Olufsen engineers designed this compact assembly so the boards can be accessed pretty easy. The tone control and FM tuning control board unscrews, tilts up and lifts out (after disconnecting three connecters).

The lamp housings just unscrew and come out after releasing their plastic spring tabs.

That leaves the main board to get access to. The main board has screws that have to be removed plus the FM antenna connection panel and the FM tuner box. When those are all unbolted the main board can be lifted up from the right side towards the transformer on the left.

I tilt the Beomaster on its side with the transformer end down. That gives good access to everything on the main board.

Interestingly the middle area of the main board has holes and markings for circuitry that is not there.
That is because the Beomaster 1900 and Beomaster 2400 share the same main board but only the Beomaster 2400 has remote control capability.

Now it is time to start replacing the capacitors on this receiver.

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!