Beogram 4002 (5503): New Monolithic Beolover Carriage Position Sensor for AC-Motor Beogram 4002 (Types 550x)

A little more than a year ago I designed a replacement part for the carriage position sensor PCB in DC platter motor Beogram 4002 and 4004 (Types 551x and 552x). This board takes the guesswork out when the carriage position sensor does not work properly. The original design is fairly sensitive to the alignment between the IR diode and the photoresistor, as well as the alignment of the ruler relative ot the sensor, and it can be tedious to get everything properly adjusted.

The Beolover Carriage Position PCB for Beogram 4002 and 4004 (Types 551x/552x) is based on a modern monolithic IR photo-interrupter and circuitry that generates a clean and precise digital output signal for driving 1TR17 on the main PCB.

Sadly, this solution only works in the later DC motor Beograms. The earlier Beogram 4002 with AC platter motor has a much more complicated hard-wired carriage position sensor PCB that is not very easily replaced. But when I recently restored an AC motor Type 5503 Beogram 4002, and it gave me grief with detecting the runout groove reliably, I thought it would be nice to have a solution for these models, too!

Since replacing the entire board is complicated and unnecessary, I designed a replacement for just the sensor part of the board. I designed a board that simply piggybacks onto the main PCB, replacing only the original sensor bulb and photoresistor. This board adapts the same reliable circuit that I used on the DC motor PCB. 

This shows the final version of the Beolover Carriage Position Sensor for Beogram 4002 (Types 550x):

Let's see how it is installed:

This shows the original setup. The photoresistor is in the black housing in front of the 'plexiglass ruler' bolted to the carriage assembly:

After removing the two screws that hold the ruler assembly in place, it can be removed. This reveals the black bulb housing under the ruler.

Here is a view from a different angle (I already had the orange capacitor and white solenoid resistor replaced when I took this photo):

After removal of the bulb cover, the bulb is visible:

The first step for the installation of the new Beolover Carriage Position Sensor for Beogram 4002 (Types 550x) is removing both the photoresistor and the bulb and cleaning the five solder pads indicated below of all solder:

This is best done with a desolder gun since the pads need to be clean and flat so that the Beolover board can be placed onto the original board surface. The board needs to be aligned as shown in the picture below:

This shows it aligned and soldered in place:

When aligned properly, the three round connection vias on the Beolover board are aligned with the respective solder pads where the board connects to power and the base of 1TR17. All that needs to be done at this point is to put a bit of solder inside the vias.

It is a good idea to solder one via first and then adjust the board precisely, while making sure it is fully flat on the original board surface. If it is not flat or misaligned, it may be difficult or impossible to adjust the plastic ruler for proper function of the position sensor.

This shows the plexiglass ruler assembly bolted back into place. 

Adjust the ruler that runs at a constant ~1mm distance from the front-facing part of the sensor during the entire travel of the carriage.

Flipping the switch in front of the sensor activates the on-board LED:

The LED makes it easy to test the proper functioning of the sensor. It should light up whenever a black bar passes between the legs of the sensor.

After installation of the sensor, the Beogram reliably recognized the runout grooves of all records I played. Beautifully!

Beogram 8002: Carriage Runs into Mechanical Limit During Carriage Return

A Beogram 8002 that I am currently working on exhibited a strange issue after I had it fully restored and mechanically adjusted (i.e. the arms were perfectly orthogonal to the carriage rods and the end switch was set to a position where the arm would drop properly at the LP setdown point): Whenever I activated STOP or the arm auto-returned after playing a record, the carriage would bump into its mechanical limit and tilt upwards a bit. Not very beolovely! This meant the carriage motor somehow did not shut down soon enough after the end switch activated. 

This was odd, since the end switch was adjusted for a perfect arm drop onto an LP run in groove, i.e. this issue could not be fixed by changing the end switch position a bit inward so the carriage would travel less far towards its home location. On another 8002 that I have currently on the bench everything worked nicely, i.e. the carriage traveled not as far after triggering the switch and the carriage did not bang into the rod support.

After a bit of head scratching I set up my portable DS-213 oscilloscope to do some measurements on these two Beograms. I measured one of the rotary encoders (blue) vs. the carriage motor voltage, hoping to learn something about the microcontroller timing of the carriage motor operation.

This figure compares the measurements for the problematic unit (top) and the normal one (bottom):

The problematic unit has a 100ms delay between switch activation and shut down of the carriage motor. I should point out here that the stop of the encoder signal coincides with the switch activation since the switch not only interfaces with pin 32 on the uController, but also directly turns off the power to the IR diode in the encoder.

A good question here is 'why did they set it up like this and not just use the encoder steps to determine an exact end of travel for the carriage?' Oh, the mysteries of vintage B&O!..;-). I would certainly not have designed it like this. Maybe a constraint was that not enough I/O pins were available and they had to combine a number of control items on a single pin.

Anyway, in contrast to this measurement the 'normal' 8002 only shows a ~70ms delay. In other words, the problematic 8002 runs the carriage motor about 50% longer than the good one after the end switch is triggered. This suggests that the two microcontrollers have different programming since the motor shutdown is directly controlled by pin 26 (">>") of the controller. In other words there was nothing to fix. Everything was mechanically and electronically in good shape, just the controller did some slightly different controlling!

My big question was how to alleviate this issue without the ability to change the firmware on the chip. In the end I saw only one way to deal with this: Slow down the carriage speed a bit during return to the home position. Luckily, there is a pretty straight forward way to do this!

This shows the relevant portion of the circuit diagram:

The shown circuit snippet is the half of the motor H-bridge that takes care of rightward (>) motion of the carriage. The opamp IC2 drives the TR3/4 push-pull stage that provides the motor current to OM1. The opamp itself is controlled by comparing the voltage at the + input with the voltage at the - input. The + input is connected to pin 26 of the uController via D7 and to the >> photoresistor in the black box on the keypad PCB via D8.

The voltage coming from the >> photoresistor ranges from 0.62V (if the calibration screws are adjusted properly) to about 3.2V depending on how hard the >> button is pressed.

This told me that the voltage control range tops out at about 3.2V, corresponding to full speed of the carriage. When carriage return is activated by pin 26, however, it applies the full logic voltage of about 4.8V or so, reduced to about 4.2V by D7. This means the the opamp is driven into saturation and puts out its maximum voltage.

My goal was to reduce the carriage speed enough so the carriage would stop slightly before from the mechanical end of travel during the 100ms after the end switch is triggered. This meant I had to reduce the voltage after D7 below 3.2V. I added a 1MOhm trimmer between D7 and the 1MOhm resistor R34, thereby forming a voltage divider that would allow me adjusting the voltage at the + input down to ~50% with the trimmer. For this I removed D7 and put it on the solder side of the board together with the trimmer for better access. Here an impression of this temporary experimental setup:

As expected the voltage yielding a sufficient speed reduction to prevent crashing the carriage was below 3.2V:

This voltage corresponded to ~840kOhm trimmer resistance (trimmer needs to be disconnected for an accurate measurement). With this resistance in place the carriage stopped about ~1mm before the mechanical end of travel, which in turn corresponded to a 9 mm distance between carriage and encoder housing:

All this was measured with the switch set to a position where the arm would drop perfectly into the center of the run in groove allowing time for the output relay to open before the needle would meet the music:

After this successful experiment, I replaced D7 with a small replacement circuit composed of a 820kOhm 0603 SMD resistor and the original diode on a Dremeled 1x6 piece stretch of prototyping board (it seems despite the 'analog' appearance of these 1980s boards they actually put the solder points on a 0.1" matrix!..;-):

Here an impression of the little board replacing the originally lonely D7 on the component side:

After this I tested the unit again and everything still worked! A promising sign!...;-). All good again in Beogram 8002 land! I should point out here that the 'apparent speed' of the carriage return only changed little, i.e. the casual observer will most likely not notice that the return speed was reduced a bit.

On to writing the main post about the restoration of this unit!

Beogram 4002 (Type 5513): Replacement of Carriage Position Sensor Photocell

Having worked on quite a few Beogram 400x in the last 10+ years I started thinking 'I saw it all'. But if there is one thing one can rely on with these beautiful designs: They will always come up with new challenges. This time it was a failing photocell in the carriage position sensor. Usually, this sensor fails due to a dead light bulb (or IR LED in later models). Or due to a broken photocell housing, causing the photocell to bend away from the 'plexiglass ruler', which can cause reliability issues.

So when I restored this unit, and it behaved inconsistently when trying to find the LP setdown point after pressing start, I naturally assumed it was the IR LED, which this later 5513 Type featured.

I replaced the LED with an orange LED and adjusted the brightness to get the prescribed 5V at the photo sensor cathode. Then I ran the unit again, and it found the LP setdown point! Case closed, I thought and I played a few more records.

Then out of a sudden it would not find the setdown point again and the cell voltage was off by a couple volts. I thought maybe the brightness adjustment trimmer had an issue due to oxidation, and so I replaced it with a modern 25-turn 25 kOhm encapsulated unit. Then I adjusted to get 5V and tried again. This time it immediately failed and so I started thinking, 'what is left to replace'. The photocell came to mind, but I never had to replace it so far, and I was able to adjust the 5V, so it should be o.k., I thought...that led me to replace TR17, which translates the photocell response to standardized ~20V pulses that can be interpreted by the control system of the deck. I put in a new 547B and tried again. To no avail.

At this point I finally believed it must be the photocell. I disconnected the carriage PCB and connected the cell to the multimeter set to resistance. Then I shone a strong LED flashlight on the sensor, and indeed the sensor responded intermittently. The resistance dropped, and then it went open contact, and then the process repeated itself. Whatever the reason, this was not o.k.

Since this diode-based photocell simply acts as a photoresistor in this circuit, I decided to try replacing it with a standard photoresistor. But first I had to re-design my photocell housing to be able holding such a resistor securely in place. This shows the redesigned set-up:

I basically gave the insert some additional features that matched the resistor shape. This shows the resistor inserted:

And while pushing the insert into the main housing:

This shows the new sensor in place opposite the orange LED that I installed earlier:

A shot from the back:

I had to try out a few different photoresistors before I was able to make this work properly. It turns out that the widely available GL5539 type works reliably under various ambient light scenarios. Note, that it may be necessary to adjust the LED intensity with R88 in order to get a good contrast on the photoresistor that TR17 is reliably switched off and on depending on whether a black stripe is in front of the sensor or not. This is best done with a voltmeter connected to the collector of TR17.

Beogram 4002 (5503): Restoration of the Carriage Position Sensor - Development of a New Beolover Part

The Beogram 4002 (5503) that I am currently restoring had a damaged carriage position sensor photo resistor housing, which had broken off mounting tabs:

I removed the photo resistor housing:

This shows it from the back:

After pulling out the bottom part, the photo resistor can be extracted:

I designed a replacement housing that I can print on a 3D printer. It consists of two parts. This shows them together with the extracted photo resistor:

This shows how to integrate the photo sensor with the assembly:

First straighten the leads of the photo sensor and put it on the insert part like this:

Then slide the main housing over the insert with the photo resistor:

This shows the assembled sensor from the bottom:

Now the assembly can be inserted into the original mounting cutouts:

Once the sensor has been clicked into the PCB, the leads can be soldered to the pads:

I decided to also replace the position sensor light bulb with an LED. This shows the original lightbulb lit up in its housing:

Removal of the housing reveals the light bulb soldered to its two pads:

The right pad is the 24V rail, while the left pad connects to GND via 1R91. It is easy to replace the bulb with a LED: Basically any LED will do. I selected an amber LED with a 2.1cd light output at 20mA that I had in my stash. Since it runs from 24V in this setup, a resistor needs to be put in series to limit the current. I selected a 3.3kOhm resistor that limits the current to about 7mA. This current lights the LED up in a low key way, similar to the output of the light bulb:

This shows the entire restored carriage PCB (I also replaced the 47u electrolytic capacitor and the 8.2 Ohm solenoid resistor):

After I installed the carriage again I tested the sensor output by measuring the collector voltage at TR23, which should give clean 24V signals every time a black segment of the 'plexiglass ruler' gets pulled between the LED and the photo resistor. This shows the signal that the 45 RPM pattern caused, while the carriage swept the platter:

This looks like it should, i.e. we can conclude that the restoration of the carriage position sensor was successful.

Beogram 4000: Restoring the Carriage Position and Solenoid Driven Switches

After rebuilding the keypad cluster of this Beogram 4000 it was time to see to the remaining mechanical switches that control the Beogram's performance. There are two clusters, one beneath the carriage where the switches determine the position of the tonearm, and the other next to the solenoid. The solenoid activated switches are responsible for the activating the tracking mechanism, opening the outputs when the needle is in the groove and to regulate the solenoid power down once the arm is lowered.

This shows the carriage position switches. For getting to them it is best to drive the carriage inwards and remove the red position indicator assembly. Then the board is fully accessible:

After unsoldering the two leads that power the carriage motor and removal of two screws the board can be pulled up and turned around:

Some oxidation is visible on the back of the terminal tabs. For some reason the carriage position switch terminals are usually less oxidized than the other switches under the keypad and next to the solenoid. I removed the terminals

and coated them with gold:

Then it was time to solder them back in and clean the plastic plungers that activate the switches from 40 year old hardened grease. Then I turned my attention to the switches next to the solenoid. The single switch assembly that controls the solenoid power can be directly accessed after removing the two screws that hold it in place:

I extracted the terminal and then removed the assembly that holds the output and tracking activation switches:

This assembly needs to be taken apart while not ripping the thin wires off that are connecting the switches to the control system:

Once this was done I extracted the three terminals. This shows all four terminals as extracted:

'Beautifully' oxidized, I'd say! I brushed the oxide off with 2000 grit paper and a fiber glass brush and then coated them with gold:

That looked much better! I put everything back together and gave the system a first test...Good news: so far everything seems to work...the carriage is now setting down the arm at the 12 inch point and the solenoid actuated properly. A significant milestone has been reached in this restoration!