Beogram 4000: Platter Motor Stopped Working

Last year I restored a Beogram 4000 from a customer in the UK. Unfortunately, he recently contact me since the platter motor stopped working. 

After receiving the unit I put it on the bench and started looking into the issue. first I checked the power rails and the reed relays. Everything tested fine. The next step was verifying the motor signal. I measured at the motor capacitor, and there indeed was no signal.

So I did the classic 'follow the signal' approach. I measured the oscillator signal at the collector of TR8, which checked out o.k.: 

The oscillation is not a perfect sine wave, since I had to replace the light bulb in the Wien oscillator with diodes when I restored it last year.  But so far so good...now I measured at the base of TR9, which should essentially show the same signal, just shifted to zero V since it is coupled via C6 to loose the DC component. But the signal was gone:

So it had to vanish somewhere between TR8 and TR9. I measured all the connections between TR8 and TR9, and it turned out that the signal vanished after the motor trimmer VR3. I replaced VR3 with a modern 5k 25 turn trimmer, but no cigar! The signal was still gone after the trimmer. At that point it dawned on me that there must be a short to GND somewhere. A closer inspection of the board yielded this:

R24 and R20 were touching! Something had bent the two resistors together, which effectively connected the base of TR9 to GND. I bent the resistors apart, and voila: The motor signal returned. This is the 33 RPM trace

and here is the 45 RPM.

So the trimmer had been replaced unnecessarily. This shows the new unit peeking through the access hole in the PCB for adjusting the motor voltage:

I put the platter on and a cartridge to see if I could play a record successfully. But the arm did not lower on the record. I realized that the sensor arm was dark, i.e. no power to the LED light source I had installed last year. I measured the voltage at the LED and it turned out that it was 0V. So I looked for interruptions in the connections between the PCB and the LED. And quickly I found the culprit: The yellow wire that connects 6V to the carriage had broken off its terminal on the PCB:

I soldered it back on, and now the deck is playing beautifully again! I will play a few more records, and then it should be time to travel back to the UK.

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!

Beogram 4004: START Key Not Working

A customer from New Zealand recently ordered a set of Beogram 400x transport bushings from the Beolover Shapeways Store for his 4004 (5525) model. He sent me a picture after he installed them:

Pretty! I like them in white!

A while later he contacted the Beolover Customer Service Department ;-) about an issue with the START button on his deck. When pressed nothing would happen, while he was able to start the deck by first pressing the 33 button and then using the "<" button to drive the tone arm towards the start groove of the vinyl. Furthermore, the START button worked once the arm was on the vinyl. This indicated that the switch itself was o.k.

The fact that the 33 button worked told me that the circuit mostly worked, and that the fault must have its root cause in the SO-switch override circuit that is activated by the START button. Here is the relevant section of my annotated version of the circuit diagram:

This is how it works: When the carriage is in its 'home' position the SO (switch off) switch pulls the base of IC1 down which disables the 21V power supply, turning the deck off. When start is pressed, 30V are applied to the base of IC1 via R5/D9/R3/D6 (follow the red dashed line from the << ON (=START) switch). The 22V Zener diode D7 makes sure that the voltage at the base is 22V. This turns the deck on. D5 prevents a short circuit to GND via the SO switch, while this process is unfolding. The <<ON switch also turns on the carriage motor (via the red dashed line to he left at the branch point below the <<ON switch) and then the carriage starts moving. After it clears the SO switch the <<ON override is no longer needed. C2 makes sure that the override is in place long enough until the SO switch is open.

The 33 button provides another way to override the SO switch, but it does not turn on the carriage motor. Only the platter starts turning. That is why my customer had to press the < button to move the carriage while pressing the 33 button to drive it far enough until the SO switch was open. After that the deck would work normally.

The above considerations immediately suggested to look for a problem in the R5/D9/R3/D6 path. And indeed, there was a dead solder joint on D6, which prevented the override voltage from making it to the base of IC1. A bit of soldering, and the START button started working properly.

Wonderful! Another Beogram back in business! This is Beolove...;-)!

Beomaster 8000 Microcomputer PCB Damage

Back to the Beomaster 8000. Today I looked at the missing display segments. First, I thought it is the usual dead LEDs on the boards. The display looked like this:

So I removed the three main displays. Luckily, I decided to plug them into the display 'harness' I made with a breadboard for the last display repairs. Powering them up yielded this:

This is the first Beomaster 8000 display I see that fully works. This told me the problem is elsewhere. I soldered the displays back in and looked at the circuit diagram. The particular LED segments that were out suggested a problem in the drain path to the SN74247 7-segment driver responsible for these segments (IC 2). I checked the signals on plug P-75 and indeed the pins responsible for the c and d segments were at 5V and did not show any strobe signal. This suggested a contact problem. I removed the display board and cleaned the plugs and resoldered the p-75 plug and socket. To no avail. The segments remained dead.

I followed the current path towards the SN74247 and found that the plug pins did not connect to the respective current limiting resistors R21 and R22. This meant that the PCB traces between P-75 and these resistors were broken somewhere. 

A closer look revealed that there had been the same problem at some point in the past with pin 3 as indicated by the white jumper soldered between capacitor and resistor:

I did the same for pins 7 and 8:

This brought the segments back:

The question now is whether to refurbish the displays anyway or leave them as they are.