Beogram 8000: Adjusting Horizontal Parallelism and Glueing the 3D Printed Plastic Tabs to the Carriage

The Beogram 8000 seems to like its new 3D printed plastic tabs that I implemented a few days ago. So I decided to epoxy them in place for long term stability. To be able to clamp them I needed to remove the tonearm carriage from the rail that I could turn it up to apply the clamps:

After applying epoxy to the remaining 'stumps' I clamped the new tabs in place:

After 5 min the epoxy had settled, and I took the opportunity to also adjust the horizontal tonearm parallelism. The adjustment screw can only be accessed from underneath the carriage, so this was the moment:

After adjustment the tonearm was at the same height as the sensor arm:

Lovely!

Beogram 8000: Repair of Broken Off Plastic Tabs on Tonearm Carriage with a 3D Printed Part (II)

Allright! After fixing the broken off tab on the rear of the tonearm carriage of the Beogram 8000 that I am restoring right now, I came up with a similar fix for the broken off tab on the front. This one was a bit easier to access:

I decided to design a 'cap' that would fit on the remaining stump and hold the carriage down on the steep rod on which it travels:

This part was more challenging to print since on one side it had to be very thin to not alter the 'home' position of the carriage, and due to the oval orifice for the rod. Such structures are difficult to print due to th unavoidable gravitational forces that make it difficult to bridge voids with the printer. That is why the part looks a bit 'wobbly'. These pictures show the part installed. Here without steel rod:

And with steel rod in place:

The rod is easily taken out from the plastic receptacles that clamp it in place on either side, i.e. the part can be installed by removing and then inserting the rod into the part, and then clicking the rod back into its receptacles.

I ran the mechanism a few times and it seems to work very nicely...the part does not obstruct anything in the motion path of the carriage. So potentially this is fixed. Like with the tab in the back I had to iterate the part a bit until a good fit was obtained:

I will epoxy the part into place once I am certain it performs well. Without clue it would probably not stay in place during transport. 

Beogram 8000: Repair of Broken Off Plastic Tabs on Tonearm Carriage with a 3D Printed Part (I)

I finally got back to the Beogram 8000 that recently arrived from Japan. After fixing the spindle rotary encoder issue, I had to put this project on hold to complete a Beomaster 8000. One of the remaining major restoration tasks was the repair of the broken off plastic tabs that hold the arm carriage to the metal rods on which it rides. They must have broken off during a rough transport experience or a similar stressful moment. Anyway, there was not much choice other than replacing the carriage with one from a donor 8000 or going creative on this issue with a 3D printer. I chose the latter since I usually cannot accept Beograms being parted out.

This picture shows the broken off tab in the back of the carriage:

and the photo below shows the other broken off tab in the front of the carriage. The white stuff is evidence that someone tried to glue the broken off part back into place, but it must have come off again:

The repair of this one will be the topic of my next post. Here is a picture of the second tab in the back that was still in immaculate condition:

It was immediately clear what was missing from the broken off tab: The part that swings underneath the rail rod on which the carriage travels. This keeps the carriage on track even if gravitational forces are briefly suspended during transport etc...

Gluing the broken off parts is futile in such cases. There is no plastic glue that can be strong enough to fix this type of issue. There is just too much torque on the part, while the contact surfaces are really skimpy and no match for the acting forces. So I set out to design a creative solution based on a 3D printed part. This is what I came up with:

The small tab clamps into the gap below the tone arm counter weight and holds the part in place. The bent section of the part goes underneath the rail holding the carriage in place. Here are two pictures of the part installed:

The part clips into place with a solid click and pretty much holds itself in place without the need of glue etc...I will nonetheless glue it once I am confident that it does its job well, to make sure that the Beogram survives its return journey to Japan. It is a pretty interesting process to design such a custom part since it needs to fit an already existing part precisely. This required a number of trial and error iterations until a satisfactory fit and a happy 'click' were achieved. Here is a picture of all the iterations that I printed before I had the one that is shown installed above:

If you look real close, every one is different. A lot of effort, but now the carriage of this Beogram is again held in place safely! At least in the back, the repair of the broken tab in the front will be the topic of my next post.

Beogram 8000: Not Responding to Keyboard Input - Not Working Rotary Encoder Feedback

A Beogram 8000 hailed from Tokyo in search for some TLC. It was initially purchased from ebay and supposed to be in excellent condition. Once unboxed, however, it turned out that the unit did not work properly. There is already an extensive thread on Beoworld.org about the issues this unit displayed. In a nutshell, when pressing start the arm would run to the end of its range and then be stuck.

Yesterday, I opened the unit up and had a look. I immediately suspected a fault with the rotary encoder that is fitted to the spindle that drives the carriage with the arms. In difference to the earlier 400x models, the 800x employ a more modern positioning concept that is based on detecting the angular motion of the spindle and calculating the carriage position from that. This is a concept that is used in most modern control systems involving motors. The encoder unit is very similar to the ones used in the Beomaster 8000 for the volume and FM frequency wheels.

In the Beogram 8000 the evaluation of the encoder signals is done by the microcontroller, which has two inputs that detect the intensity fluctuations on two sensors. The fluctuations are generated via an aperture wheel that sits between the detectors and the IR emitter. Depending on the phase of the fluctuations on the two detectors relative to each other the system can detect the direction of the motion. The number of intensity oscillations tell the traveled distance. This way, the microcontroller always knows where the needle is. This 'feedback' is used to determine where to set down the arm, when it is time to lift and return home etc...

Therefore, when there is no feedback, the controller does not know anymore where the arm is. The firmware seems to be programmed with simple if...then conditionals that cause action depending on the position. If the position does not change in the mind of the controller (in the case of a non-working encoder system) the arm simply continues to travel until it hits the mechanical stop at the end.

All this suggested to me that I needed to investigate the encoder system to get to the bottom of the issue at hand. Below is the relevant section of the circuit diagram. Since I had similar problems already a couple times with Beomaster 8000s that I restored, I first checked on the functionality of the light detectors and the IR diode (OPE1). In the Beomasters the diodes had failed.

With the multimeter I determined 1.2V at the anode of the diode and this told me that it most likely was o.k. (they usually die by going OC). Then I measured the resistance on the photoresistors. They both showed about 1k when shining a strong LED light into the assembly and about 20-40k when just room light trickled into the setup. This seemed o.k., too. 

So I hooked up my oscilloscope and measured at the P2/6 and P2/4 whether I got pulses when manually turning the spindle. Of course the deck needs to be in PLAY mode or another on state to have things powered up. I had it in PLAY with the carriage belt removed, so the motor simply ran but the spindle did not move. This measurement yielded pulses for both encoder channels. The next step was checking the outputs of the opamps. And there I found a first clue to the problem: IC2 had no pulses at the output, while IC1 gave me nice 5V square pulses.

So I had a look at the PCB, and there it was:

The input of IC2 was short circuited to GND. In the above photo that is the IC pin that is associated with R5. It connects to the adjacent GND pad on the left via a gigantic solder ball. In fact if you look at the entire area it is apparent that someone with very little practice in electronics messed around and did some 'expert work'. Another great example of a 'perfect condition' or 'fully restored' ebay unit....(I hope this guy will smolder for a long time in Vintage Hifi Purgatory when his time comes).

Grounding the IC2 input of course results in a zero output signal since then there is nothing to amplify, and that explained my measurement. I fired up my Hakko desoldering gun and removed the solder from the pads and then resoldered them:

And this did the trick. The unit is working again. I put on my least favorite record (Sam Rivers) that I use for this type of testing, and pressed PLAY. The arm moved to the lead-in groove and lowered. Cueing worked and STOP. Very good!