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Monday, October 10, 2016

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!

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