Beogram 8000: Repair of Platter Brake Circuit

After replacing the electrolytic capacitors of the Beogram 8000 that I am rebuilding right now and fixing the scraping subplatter issue the unit was again functional and ran smoothly at 33.33 RPM. However, I noticed that after pressing stop and the return of the arms to the home position the platter continued to spin for a long time until it finally came to a rest. The 8000 has a low friction bearing and there is no belt due to the linear platter drive. So there is very little friction overall once power is cut to the motor stator coils. That is why the designers of the Beogram gave it a motor brake that reverses the motor phases to essentially run it in the opposite direction for bringing it to a fast stop. That makes the humming noise when the platter comes to a fast stop.

This shows the relevant portion of the circuit diagram as shown in the Technical Product Information manual:

The motor stator coils are OL1. This are the two big coils that 'hug' the sub platter (which is the rotor of the motor). These coils are phase shifted by the motor cap 4C1. This is basically the same setup like in an AC motor 4002. The motor is driven by the Drive System, which is essentially a push-pull stage that follows the grid frequency taken from a dedicated transformer coil (4T1A) and which regulates the current through the stator coils that is produced by transformer winding 4T1B (the winding shown above the stator coils in the above schematic).
The brake circuit is is essentially formed by TR 31, TR32 and TR33. Once pin 37 goes low, TR31 is turned on (and the drive system turned off via TR27 and TR28). In the process TR32 turns on which then via D39 pulls down the base of TR33. TR33 turns on and the positive half wave of the 60Hz signal from the 4T1B winding is fed into the opposite end of the left coil of OL1 via D40. This reverses the motor phase relative to the normal signal.

The 'reverse drive' aspect of the brake system can be seen by a simple test: Connect the base of TR31 to ground with the unit in standby and the platter starts spinning backward!

It turned out that the brake malfunction in this unit was caused by a broken trace. The trace that connects the collector of TR31 to R110 was cut through, probably the consequence of a previous 'repair' attempt. This broke the chain of command between TR31 and TR32, so when the microcontoller said "brake!" TR32 did nothing, and TR33 remained off, which prevented the reverse phase signal to be applied to the stator coil. Since power to the coils was still cut via TR28, the platter simply spinned until mechanical friction finally stopped it...

Below is a photo of my fix: I soldered a small piece of 'magnet wire' between the relevant solder spots: Magnet wire is good for such tasks since it is coated with a special polyurethane coating (so one can wind a magnet without making short circuits between the windings) that burns off when touched with the soldering tip. Very convenient for making connections with short pieces of wire where it would be difficult to take the insulation of mechanically with a stripper tool.

After this repair the unit 'fired on all 8' again (ah the good old days when most real cars had a V8!)

On to mechanical adjustments and fixing the cosmetic issues of this unit!

Beomaster 8000: Step One - Rebuilding the Output Amplifiers

While we wait for Nick's awesome specially made pulley for the Beogram 4002 (5501) that I just finished up, it is time to get started on the Beomaster 8000 that will also go on to the UK once restored. It recently arrived and I gave it an external inspection, which suggested this 8000 is an excellent starting point for a full restoration.

My first step in any Beomaster 8000 project is to rebuild the output amplifiers. They are the most crucial part of the restoration of the 'power' part of the unit. A failure of their often corroded quiescent current trimmers usually neatly kills all output transistors with a bit of smoke emission on that channel before the main fuse protects the transformer (and the breaker of the house grid on which the 8000 resides while this burnout happens). These old trimmers often go open circuit during transport, so my approach is to not even turn the unit on when I receive it, but I go straight to the amplifiers and rebuild and test them to make sure that the above does not happen.

Here are a few impressions from this effort on this unit:

This shows the right channel as it came:

We see from the two clunky (white) emitter resistors that the above mentioned disaster already must have happened at some point. When the transistors burn out, the emitter resistors usually brown like a chicken in the oven due to the immediate heat emission. But usually this does not affect their performance down the road since they are wire wound types. An ugly replacement like seen here is definitely not an improvement in the Beolover's eyes! Also, whoever did this did not learn 'The Lesson'...he did not replace the trimmer! I always put in 25 turn precision encapsulated units to prevent this from ever happening (again). Multi-turn trimmers drift only very little and so the quiescent current adjustment is very stable over time. This is not the case for standard single turn trimmers, and that is one of the reasons that the 8000 often presents with one or two hot heat sinks, if it runs at all.

This shows the rebuilt board with new electrolytic 105C type capacitors and prettier resistors and the 25-turn trimmers:

Beautiful! I did the same to the left channel, which did not have new emitter resistors, but (slightly browned) original resistors. And then it was time to run these babies from external power supplies to test for any silicon failures, cracked traces and the like. This shows my hookup:

And here with the multimeter connected to the test points at the emitter resistors after powering the board up:

When installing a new trimmer make sure that the resistance is close to zero. This may be counterintuitive, but in this setup it turns off the output transistors preventing significant current flow between the +/- power rails. This shows my bench supplies at this point:

The left two are the - and + rails, which draw 60-70 mA while the quiescent current trimmer is close to zero Ohms. The right supply provides the 15V control voltage that is used by the Beomaster to enable the output by controlling the constant current source used for biasing the output transistors.

Adjusting the quiescent current means to ramp up the resistance in the potentiometer to set the working point of the output transistors that there is a 18mV voltage across the two emitter resistors:

At that point the power rails draw 0.1 (+) and 0.11(-) A:

I did the same for the right channel:

All good now in the output amplifier department! On to rebuilding the power supply board before giving this unit a first spin!

Beogram 4002 (5501): Final Touches - AC Motor Calibration, Adjusting Platter and Sub-Chassis and Tracking Force

The restoration of the Beogram 4002 (5501) that I am currently working on is coming to an end. I gave the aluminum panels and the platter a deep clean, and then did the sub-chassis and platter height adjustments followed by an adjustment of the tonearm lowering limit and the tracking weight.

There are two videos on my YouTube channel that show how to adjust the platter bearing and the subchassis (in a Beogram 4000, but the process is very similar in the 4002). There is also a video on my youTube channel that shows how to adjust the lowering limits. This is a very important procedure since the control system of the 4002 cannot guard against photosensor failure in the sensor arm. If the sensor fails the arm will be lowered whether there is a record or not, possibly endangering a very expensive cartridge. The correct adjustment of the arm lowering limit can prevent stylus failure if that happens. And finally, there is also a video about the tracking force calibration process.

After all that was done, I adjusted the AC motor waveforms. This is done by connecting an oscilloscope to the connection point between the motor capacitors and ground:

Then the oscillator trimmer is adjusted until clean waveforms of maximum amplitude are achieved. The end result looked like this for 33 RPM:

and for 45 RPM:

Then I adjusted the RPM precisely with my BeoloverRPM device. Personally, I still use the original design. The updated design is available to other enthusiasts. The BeoloverRPM is very convenient for the occasional RPM adjustment (yes, the belts do become elongated over time causing the RPM to slowly drop). Here are two pictures showing my adjustments for this Beogram with its new belt:

Very nice! And then it was time to set the unit up next to my Beogram 6000 4-Channel:

I celebrated this restoration with a record that I just bought: Gabor Szabo's "The Sorcerer":

This record is rapidly becoming one of my all time favorites. An amazing live concert recording that has a remarkably raw power that sucks you into Szabo's rhythm. 

Beogram 4002 (5501): Replacing the Original DIN5 Plug with a Modern All-Metal Gold Plated Unit

No restoration of a 4002 is complete without replacing the usually oxidized original DIN5 connector. Those precious low voltage signals deserve the best contacts possible. Gold does not oxidize, and therefore it is able to make low-resistance and long lasting contact. This shows the original plug that was on the Beogram 4002 (5501) that I am currently restoring:

I cut the plug off and prepared the cable for installation of the new gold plated all-metal plug:

Then it was time to solder the plug to the cable:

And this shows the assembled new plug in all its glory:

Beautiful! On to the final touches to get this Beogram ready for primetime!

Beogram 8000: Repair of a Scraping Sub-Platter with a 3D Printed Insert for the Main Bearing

The next item to look into with the Beogram 8000 that I am restoring right now was that the sub-platter scraped along on the chassis. This shows the interior of the 8000 with the sub platter still in place:

Somehow the platter was situated a few 1/10ths of a mm lower than usual, which made it touch the sub-chassis base plate in a few spots. This prevented the deck from maintaining a constant speed (it never showed 33.33 on the display, just 33, which is a signal that the RPM is off). Also it made an intolerable mechanical noise. Far from Beolovely! 

This seemed strange to me (there is no way to adjust platter height like one could in a Beogram 4002) and so I poked around a bit on the internet. I came across an uttering by one of the greats in this business, Dillen of Beoworld: "Typical symptom of a broken main bearing nylon insert. It happens often if the Beogram was transported with the heavy platter mounted or just put down too hard." (my customer confirmed that the ebay seller did not take out the main platter when he/she shipped it to him...).

He went on to say "The whole weight of the platter, hub and sub-platter rests on the very sharp pointy tip of the hub spindle. Only fix is to replace the nylon insert."

This shows the spindle after lifting out the sub-platter:

In the pic the tacho sensor is already out of the way (it swings away by moving the brass lever clockwise). In that position one can pull out the spindle:

The picture shows the pointy end of the spindle that rests on the plastic disc that Dillen mentions in his post.

The plastic disc can be pushed up using a small hex wrench or similar (there is a small hole on the bottom of the bearing that can be accessed from underneath the enclosure. This shows the white plastic (probably nylon) insert in the main bearing:

And after pushing it up:

First I tried to simply flipping it around, but there is no indentation on the back of the plastic part for holding the spindle in the center of the bearing. So my idea was to simply shimming the insert up a bit to lift the platter back to its normal height. The insert still seemed in decent condition, only the indentation was a tad too deep after the incident, causing the platter to be too low. 

I designed a small disk that I printed with my 3D printer:

After a few tryouts with different thicknesses it turned out that 0.7 mm was a perfect thickness to raise the platter just enough to not scrape anymore (test with the main platter and a record on top if you do that at home...).

The disc is easy to insert when the insert is up sideways:

Once the printed disc was next to the insert, it is straight forward to push the parts back to horizontal and down into the bearing.

Once I installed the spindle and the platter again, everything was fine. No more scraping, and the platter turned freely.

Beogram 8000: A New Arrival and Restoration of the Control System

A Beogram 8000 in need of some TLC recently arrived. It had been purchased on ebay with two MMC20 CL cartridges...very nice!. Unfortunately, it did not work anymore upon arrival at its new owners location. So it traveled on to my place and here we are:

The unit has the usual fallen off aluminum panels. Luckily someone catched them before they hit the floor and they are unscathed. Overall this unit is in pretty good condition and all seems original (if there just weren't the tough layer of Gorilla glue on the hinge of the smaller aluminum panel):

I opened it up and found a slightly dusty interior, but nothing otherwise unusual:

Then I had a look under the sub-platter where I found a metal tacho disk:

This is great news since the original plastic disks tend to delaminate, which causes severe RPM instability.

So far so good. When I tried to run it the platter would not spin, and the carriage showed some reluctance to move. Nothing unusual at this point in time for a Beogram 8000 in original condition. When the platter has trouble moving, it usually has to do with bad power block connections or a dead motor phase capacitor.

I decided to go ahead and rebuild the electronics. This involves replacing all electrolytic capacitors with modern Japanese major brand 105C types and reflow all the board to wire headers solder points. They are often cracked and this causes intermittent operation of the deck. Here are a few impressions. This shows the main boards taken out:

Most of the capacitors are straight forward to replace. The one that is usually a bit of a pain is the processor power supply decoupling capacitor in the EMI can. I took the can off the board and opened it up:

The processor was stuck to the can lid and popped out of its socket...In those days these chips were quite expensive, and so they used sockets instead of soldering them in directly. For exchanging the capacitor a removed processor is perfect. So I left it stuck to the lid for now. The picture below shows the original 47uF capacitor in question. It makes the GND connection (left) on top and not on the bottom solder point, which are not connected by a through plated via like one would expect on modern boards. This can be confusing since failure to solder it to the top contact pad will cause erratic processor behavior. I once spent an entertaining evening with an 'exploration' of this issue...see here for a description of that Beolover adventure...;-).

Anyway, I exchanged it:

and then I removed the processor from the can lid and inserted it back into the socket and replaced the lid and mounted the can back to the board. This shows the recapped board with the removed original components placed next to it:

After that I reflowed the solder points of the headers. This is best done by adding a bit of solder to each point. The boards were soldered with relatively sparse solder application, which may be one of the reasons that so many Beogram 8000s have bad joints. Indeed, I found several cracked points, mostly on the main power block header. This shows the pins where the motor phase capacitor is connected:

Both have a telltale ring around the soldered pin. This probably explains why the platter behaved erratically. I resoldered everything and then it was time to put the board back in. The next step was the replacement of the motor phase capacitor that is located in the power block:

I usually replace this big can with two modern 47uF bipolar units back to back. This turns them into a single 23.5uF unipolar capacitor, which works perfectly. Since modern capacitors are much smaller I recently designed a 3D printed insert that holds the two caps neatly in place:

After reinstalling the power block I rebuilt the power supply board that is next to the sub-platter:

It has only two reservoir capacitors that need exchanging:

This concluded the restoration of the PCBs, and it was time to do the carriage servo control voltages adjustment to the prescribed 620mV (err on the smaller side if you must, this adjustment can be a bit sensitive):

And then it was time for a test! And as expected the Beogram fired up normally with the platter moving smoothly and the carriage looking for a record. All good in control system land! On to the mechanical parts of this lovely deck!

Beogram 4002 (5501): Sensor Arm Light Bulb Replacement - Final Version

This is a follow up to my recent post about the replacement of incandescent sensor arm bulbs in Beogram 4002 turntables with a LED setup. Subsequent experiments revealed that the best position for the LED PCB is when it is pushed down by about 2.5 mm into the bulb cavity of the sensor compartment. This position yields the highest sensor response as measured at the collector of 1TR8 (5501; pre SSN 257556 notation), and I think this makes sense since the light bulbs have the filament always a bit lower than the upper rim due to the glass bulb.

I like reproducible solutions, and so I designed a small 3D printed wedge that can be epoxied on the outside of the flexPCB and that presses the LED down by the correct amount when the bulb compartment is closed:

This shows the LED powered up at 5.5V:

The board has a ballast resistor that adjusts the current draw to the prescribed ~60mA. This is necessary that the 'bulb malfunction detection circuit' is not triggered on the main PCB of the Beogram.

This shows the board installed and lit up:

This shows the TR8 signal:

Note how nicely the curves bottom out at 0V.

This shows the setup 'in action':

Note the nice red-orange glow of the B&O logo. This is a result of using a 2600K color temperature high intensity white light LED that has a considerable amount of red light in its emission spectrum which enables a 95% color rendering index (CRI). Also the light spot is nice and tight indicating that the light source is in the focal point of the exit collimator.

This pretty much concludes the development of the LED implant for the Beogram 400x sensor arm. Henceforth Beolover restorations can (if so desired by my customers) yield Beograms devoid of incandescent bulbs.

Beomaster 8000: Rattling Main Relays and a Broken 2MHz Oscillator Crystal

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.
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Oh well, nothing lasts for ever!...The fully restored (2011) Beomaster 8000 that I am using in our living room recently broke with a rather strange fault. I woke up at 4 am in the morning to the beeping of an uninterruptible power supply (UPS) in distress. First I thought we had a power outage, but it turned out that the UPS turned itself off due to a malfunction in the connected Beomaster 8000 (I run all my B&O equipment on UPSs to reduce the risk of damage from grid voltage spikes and the like). I restarted the UPS and immediately the main transformer power relays of the 8000 started going on/off in rapid sequence. And then the UPS caved in again, turned itself off and started beeping again. This immediately pointed to a severe fault in the 8000 since a UPS only shuts down if the current drawn exceeds the rated 10 or 16 amps. This suggested to not try running the Beomaster again before having a look inside.

I shut everything down and went back to bed. The next morning I swapped the 8000 out with one of the other 8000s I have around the house in less important locations. A few days later I opened the malfunctioning unit up and ran it. First everything was normal but after a while it started to rattle again. I switched it off immediately and started wondering what might cause such behavior. First I thought there is a problem in one of the outputs triggering the protection circuit. The reason was that I initially detected 15V at the output of the protection circuit (collector of 6TR15) during relay rattling phases. The 15V pull up the base of 6TR11 via 6D12, which then causes the relays to go off, cutting power to the main transformer. This is usually caused by malfunctioning output transistors or overheating of the outputs. However, in this case the 15V were rather a consequence of an entirely different root cause.

After some playing around with the unit and enjoying a few more UPS shutdowns and relay rattling events, I finally figured out that the crystal oscillator of 9IC4 on the main processor board had some issues. I figured this out due to two phenomena:

1) After one of the rattling episodes I was not able to turn the Beomaster back on. The standby LED was lit, but no more reaction to the keypad. Panic ensued since I thought I had accidentally fried the processor or some other disastrous event occurred. So I did a processor self-test by pressing the Monitor key first and then additionally the on/off bar on the keypad. This initiates a self-test sequence. And I finally saw my first error code on a 8000: TE8!

The manual yields a rather cryptic cause for this error:

"Error TE8:    Defect IC:9IC4(RAM)      Or short pin to chassis: 10-11"

Initially I thought I had really fried the slave processor that is responsible for the relays...but then ,after my initial panic subsided and normal brain functions kicked back in, I thought that my maxim "silicon usually dies last" should apply here too, since even a complete main power supply failure and dramatic short circuit etc...would have a hard time killing anything on the processor board. There is simply no direct connection. Furthermore, when I tried to turn the unit on again after an hour or so, it worked normally again indicating a healthy state of affairs on the processor end of things (silicon either is alive or dead...rarely there is an intermittent state in my experience). Also, once it worked again, the error code went away and I got this from the self-test:

A happy Test Passed (TP).

2) The second indication toward the oscillator issue came from an oscilloscope measurement I made after the error code episode:

The probe was hooked up to the base of 6TR11, which is connected to Pin 16 of 9IC4 via some resistors. At this point the unit was working and there was no rattling from the relays at all. But you see some type of digital random oscillation of the signal that normally should be just near-0V when the unit is on. The digital character of the signal already suggested to me that there might be a processor issue...I watched this for some time and then it changed to a more aggressive behavior:

A much stronger deviation from 0V and still pretty random and close to switching 6TR11! And then it went completely crazy and the relay clicking started again as 6TR11 did its job. I did not save the measurement at that point since I was busy turning off the unit as fast as I could...;-). This finally put me on the right track, however, and I started looking into 9IC4. The same random signals were also directly visible at Pin 16 of 9IC4, just on a 0-5V scale since at Pin 16 we are directly at the source of the signal and there are no resistors over which the voltage drops. This told me that indeed the processor was going 'crazy' occasionally. This combined with the error information above that alternatively to a processor malfunction also Pins 10-11 could be 'grounded' finally made me see the light since the oscillator crystal is connected to Pins 10 and 11!

I wiggled the crystal a bit and indeed, I was able to cause the Pin 16 signal to change from completely quiet 0V to the above shown random signals and to 'crazy' causing relay rattling. So I guess what happened was that the clock of the processor started to have random hiccups causing timing issues while processing its firmware, which then caused the normally constant output at Pin 16 to become random triggering the relay circuitry.

I ordered a few 2MHz crystals from Newark (21M6819) along with matching 18pF resonator capacitors (46P6436) - the original crystals run on 12 pF capacitors, i.e. they need to be exchanged along with the crystal to get a proper oscillator signal. After a few days I received the parts and put them in. This shows the original crystal with the two capacitors (brown, right below):

This shows the board with the parts removed:

This is the new smaller crystal in comparison to the original one:

Like most components modern ones are considerably smaller than the original ones...and this finally shows the new units implanted (I also exchanged the 9IC3 components assuming both the original crystals were from the same batch...;-):

The exchange is straight forward, but the right capacitors are difficult to remove since one of their legs is fed through a hollow via, which is a bit of a pain to unsolder due to the very small space between lead and via insert. One has to hold the solder tip to the via to liquefy the solder and then pull the cap out...three hands would be great for this process...;-)

One more thing: It is crucial to cut the leads on the solder side of the board very short to prevent short circuits to the EMI can that encloses the processors:

I had them too long initially, and I got some really spectacular readings on the displays when I turned the unit on. Essentially, it became completely unresponsive showing some random zeros on all of the displays. Another near-heart attack...;-). Cutting the leads solved that problem...live and learn...;-) 

I am running the unit now for a couple days back in the living room and it seems the issue has gone away. Also wiggling the crystal did not cause the issue anymore. So I am assuming we are back in business with this lovely Beomaster 8000!

Remarkably, a few days after this happened to my 8000 I received an email from an Australian customer whose 8000 I restored a couple years ago telling me that his Beomaster developed the exact same symptoms (I am sending him a couple crystals and the capacitors plus some instructions...this is Beolove!).

So I am thinking that exchanging the crystals should become a standard part of any Beomaster 8000 restoration...two at the same time is a rather strange coincidence, which points to a systematic issue with these crystals. Probably the leads are delaminating from the crystal material after 30-35 years...

Time to get back to my other restoration projects!