Michigan Beogram 8002 Restoration: Sensor Measurements

The Beogram 8002 from Michigan is performing very good with its turntable functional tests.

I connected some test leads to the Beogram's position sensors, speed sensor, record detection sensor and the forward servo drive signal (IC2 Pin 7).  Reference Beolover's post on the servo motor check.

Here are my test points on the Beogram 8002 PCB 1 board.

First, the position sensors (1 & 2) along with the IC2 Pin 7 output to the servo motor.

This picture shows the signals when the Beogram 8002 is doing a forward scan over the platter.

The image on the left is of the Fast Forward Scan and the image on the right is the Slow Forward Scan.

I placed a test record on the platter and measured the sensors again while playing a record.

The servo IC2 Pin 7 signal varies as the servo motor advances the tonearm assembly to track the record groove.  It does this once every revolution of the platter.

The tonearm controls worked perfectly and the Beogram 8002 record play was great.

Next I swapped out the test lead on the Position 2 and IC2 Pin 7 test points with the Speed Sensor and Record Detection test points.

The Record Detection on the Beogram 8002 is interesting compared to the Beogram 8000.
The Beogram 8000 platter is very similar to the Beogram 4002/4004 platters so the platter detection in the fixed arm just measures a series of pulses by detecting the platter ribs.

With the Beogram 8002 B&O designed a special laser engraved platter with two different sections of black markings as well as a flat gray section.

Here are the Beogram 8000 and Beogram 8002 platters side by side.

The Beogram 8002 platter design results in a different record detection signal than the Beogram 8000.

Here are photos of the Record Detection sensor measurements and the Platter Speed sensor measurements.  I kept the Position 1 sensor as a reference.

This first set of images is with the platter speed at 33.33 RPM.
I actually set the speed to 33.33 RPM to show the Record Detection sensor at the black marking level and at the gray section level.  The photo on the left shows the Record Detection sensor over the black markings.

Here is he platter speed set to 45 RPM but I only captured the signals over the gray section of the platter.

I am satisfied that this Beogram 8002 is functioning properly now.

I will play a couple of records with the components still removed from the cabinet just to test the sound and make sure the audio mute relay is working.

Then I will do the cabinet repairs and reassemble the Beogram 8002 to complete the project.

Beogram 4002 Type 5513: Adjusting/Calibrating the Detector Arm Circuit

The Beogram 4002 turntable is almost finished. Most of the adjustments and calibrations are done.
I did say I would need to return and perform the adjustment of the record detection circuit as it measures a little low. I use Beolover's nice Blog Post about the Beogram 4002 record detection circuit.

While I am not changing the detector circuit lamp (in the fixed arm) at this time, I can adjust the circuit so it meets the specification in the service manual.

There are two components on the main board that are used to adjust the signal produced by the record detection circuit: 1R26 (a 1MΩ resistor) and 1TR3 (an NPN BC548C transistor).

This Beogram record detection signal measured a little low at  the 1TR3 collector.  Beolover's video prescribes a 1TR3 transistor with an hFE (gain) of at least 500.  Let's see what this one measures.

Very good. I will keep this transistor as the 1TR3 component.

I haven't changed the detection arm lamp yet. I want to see if the current lamp will adjust into range.

Another component in determining the detection circuit signal is 1R26.  It is a fixed resistor of 1MΩ.  I will change that resistor to a multi-turn, 2MΩ trimmer and adjust it to dial in the voltages for the detection circuit. Again, following Beolover's guide to adjusting this signal, I first place the new trimmer resistor on the trace side of the Beocord.

I put an oscilloscope probe on the collector of 1TR3 and rechecked the record detection signal when no record is present on the Beogram platter (as the tangential arm moves across it).

The measured signal was just as before...It works but measures lower than what the service manual calls for.

I adjusted the trimmer for 1R26 and remeasured the 1TR3 collector.  It took several adjustments and measurements but I finally got it set to what the service manual calls for.

The adjusted value for 1R26 was 1.667MΩ

Happy with that result I moved the 1R26 (new trimmer device) to the component side where it will live permanently.

Another adjustment completed. I may still revisit this adjustment if we decide to change out the detector arm lamp.

Beogram 4002 Type 5513: Going Through the Service Manual Checks and Adjustments

This weekend I started in on some of the Beogram 4002 service manual checks and adjustments.

There are both electrical things to do and mechanical.

Electrically I want to make sure photo sensors are generating good signals when they are activated by their light sources. The Beogram 4002 is an analog control system and it depends on correct voltages to work correctly.

Mechanically I will make sure the Beogram fixed arm and tonearm are aligned. I will need to set the platter height to the fixed arm (and tonearm) then set the lowering limit of the tonearm. The tonearm tracking force will have to be adjusted as well.

Finally there is kind of a duo electrical and mechanical adjustment, the tangential tracking sensor.
I will adjust that tracking per the service manual so the Beogram is properly advancing the tonearm (with phono cartridge) as it plays a record.

Those are a lot of tasks for sure and many of the mechanical adjustments affect each other. This process can sometimes be easy and other times take many repetitions.

My first adjustment was on the tangential drive spindle that moves the tangential arm assembly when playing a record. The tangential drive motor rotates the spindle which connects to a nylon nut on the tangential arm assembly (sled). That pushes the assembly forward and back.

The tangential drive spindle can be adjusted for vertical height from the floating chassis and for horizontal distance between the spindle and front tie-rod.

The next three pictures show checking the the vertical adjustment at different points on the spindle.

The horizontal position is similar. I used the calipers to check that the distance between the front tie-rod and spindle were the same at both ends of the spindle.

With the spindle driving the tangential arm assembly correctly I can check the arm position sensor to see if the voltage level of the sensor is correct.

Here is a photo of the arm position sensor and lamp again. The tangential arm assembly has an acrylic lens that passes between the lamp and sensor as the arm assembly moves across the platter.
Markings on the lens affect the sensor voltage which is used by the analog control system to determine what to do.

The arm position sensor is part of this circuit.

The service manual adjustment for this sensor is to move the Beogram tonearm to a position where the sensor lamp is hitting the sensor through a clear section of the position lens.

At that position the collector of the sensor (4IC1) should measure 5 VDC. If it does not, adjust trimmer 1R88 to get the 5 VDC. The result of that adjustment should also result in 0.7 VDC at the base of transistor 1TR17.

Since the original position sensor lamp (4D1) was burned out I had to replace it. The replacement LED is not the same as the original one so I expect to have to adjust 1R88.

Sure enough, the initial voltage at the sensor collector (4IC1) is only 1.5 VDC.

Using the trimmer on the main board (1R88) I was able to adjust the voltage for the 4IC1 collector.

With that adjustment out of the way I decided to take a preview look at the signal being produced by the fixed arm sensor.  Beolover has a great video about the arm sensor. How it works and what the proper sensor signals looks like.

On this Beogram 4002 project I connected an oscilloscope probe to the 1TR3 collector to measure the record detection signal.

The signal with no record on the platter looks like this.

The Beogram 4002 record detection circuit is working with that signal but we know that the signal level is lower than what the service manual calls for.  The peak should be closer to 6V and the dips in the signal should reach 0V.

That means I will return to this circuit and do the adjustment Beolover shows in the video.

The next part of the service manual adjustments I made were for the tonearm.
As I started playing with the Beogram tonearm movement (i.e. lowering, raising, pivoting) I noticed that when the tonearm lowered it would pull towards the fixed arm side. The tonearm did not lower straight down and raise straight up.

That pull usually means some parts that are supposed to move freely are rubbing together back at the tonearm pivot base. In addition, I noticed that the space between the fixed arm and tonearm was narrower than the 7.7mm gap specified in the service manual. So there were a couple of things out of alignment.

There are a number of adjustment screws to get the tonearm and fixed arm aligned. A problem is that they can affect each other so it often takes several iterations to get everything aligned to where they are per the service manual.

To address the distance between the two arms it took adjusting the fixed arm position and lowering/raising arm (the metal connecting arm between the tonearm and fixed arm).

Those adjustments addressed the parallelism between the two arms and put the gap at the prescribed 7.7mm.

During the adjustment of the lowering/raising arm I also adjusted the position of the tonearm counterweight. I found that it was set too close to the back of the tonearm and the side nearest the fixed arm was hitting the back of the fixed arm...causing the tonearm to lean towards the fixed arm when lowered. Adjusting that counterweight assembly fixed the arm lowering problem.

I need to calibrate the tracking force, arm lowering limit and stylus position but first I checked the height of the platter to the fixed arm (and tonearm).  The service manual calls for the distance from the metal platter surface to the top side of the fixed arm to be 23mm.

I have a measurement tool I keep set at 23 mm for that check. If the height is not correct the platter bearing must be lowered or raised as required. This Beogram only took a couple of iterations to get it set correctly.

Now for the tonearm lowering limit. The Beogram should be adjusted so the tonearm lowers at a maximum position where the phono cartridge stylus sits about 0.5mm above the lower rib section of the platter.

For the phono cartridge position on the platter and another parallelism check I use a piece of thread tied to the center spindle and run it to the right side of the Beogram. Then I check that the stylus sets down on the thread at the edge of the platter and at the spot where the end of a record will be.

The Beogram tonearm is now properly set up where I can complete the tracking force adjustment.
The tonearm counterweight is already pretty close from earlier when I adjusted it to allow proper pivoting of the tonearm.

Now I will measure the tracking force and set the tracking force knob to 1 gram.

All of the tonearm adjustments so far are predecessors for the Beogram 4002 record tracking adjustment.

For the tracking adjustment the tonearm will be set down at the center of a record album. The platter belt will be off so the turntable is not turning (by the motor).

The adjustment procedure is to manually rotate the platter (and record) so that the tangential arm motor (carriage motor) advances the arm assembly within one to two rotations of the record.  After that the motor should advance after every rotation.

Here are the adjustment components for the record tracking.

I typically set the eccentric screw for the aperture adjustment to the center position first. Then I adjust the diaphragm position so that the tangential arm is close to triggering the tracking sensor when is lowered. That way I know I am very close to where I need to set the aperture adjustment screw as well as knowing I have plenty of play in the screw to adjust with. Of course I also have the Beolover lamp intensity trimmer for any fine tuning later.

The Beogram is ready to connect up the platter motor again and check the two speeds (33¹⁄₃ RPM and 45 RPM). For the speed adjustments I will use the Beolover RPM measurement tool. This is the same tool he uses to test the platter motors after a rebuild of the motors. For people that work on or own a lot of Beogram tangential turntables it is a very nice test tool to have.

I am really pleased with this Beogram 4002 restoration so far.  I still have to go back and address a few things.  The record detection circuit needs calibrating so the detection circuit gets a full signal.
The two meters for the selected platter speed need to be replaced to the Beolover LED lamp modules. That will provide one more piece of stability in the motor...because the lamp is actually part of the speed control circuit. Once those lamps are changed I will recheck and readjust (if necessary) the platter speed trimmers.

Beogram 8000: A Fun Evening With the Record Detection Circuit (the Conclusion)

This is a follow up to this post. After I made the post about the failed transistors in the record detection circuit of a Beogram 8000, I received this forwarded input from the owner of the 8000 who has a friend who understands circuits:

“I think he [the Beolover] replaced every transistor he came across until he got to tr16.  Then he measured it, it was OK, so he didn’t replace it.  I think he should replace TR16.  The reason is TR16 operates with 6 ua of base current, maybe his curve tracer is a different current.  Clearly he has the right waveform going into TR16, and noise is still coming out.  With the beta he measured this should not be happening, so something is wrong with the measurement.  This thing where all the transistors wear out is disturbing.  Of course this circuit calls for very high beta in every stage, but it shows us how transient our technology is.  A transistor is only a transistor because of the sandwich of 3 different types of impurities, which we know move around. After 40 years, it seems they move around a lot.  Clearly these parts worked at one time, but they all changed enough to stop working.  It’s rare to see something this old with good enough documentation and people willing to keep it going.”

Point taken. I also wondered why this circuit was still not working properly even with TR16 measuring o.k., and I have no idea what exactly my little transistor tester does in terms of applied voltages etc...when it does its magic.

Only one way to find out: I restored R72 to 1M and put R70 back into the circuit, and then replaced TR16 with a 2N2222 that yielded an hfE of 225 on my transistor tester. It still did not detect the absence of a record. So I rummaged through my parts collection and finally found a new BC547B that yielded an hfE of 430, and put it into the circuit, and that fixed it:

The yellow trace is the measurement point between R72 and R71, and the green trace is HH (collector of TR16). This qualifies as a good logic LOW, and the micro controller apparently thought so, too. It swept the arms across the platter in search of smaller records, and then returned home and turned off, as it should!

So the conclusion is, yes, the circuit can work as designed (indeed, a satisfying result!...;-)

One question remains: Why does TR16 need such a high hfE? Sure, the base current is pretty low due to the 1M resistor. In this particular case, the minimum voltage in the sawtooth at R71/72 is about 6V, i.e. the minimum base current into TR16 should be about 5.3uA. The current across CE should be limited to 5V/33kOhm=151uA if the transistor is fully on. That is only about a factor 28, i.e. a hfE of 225 should be plenty to pull the collector of TR16 down with 5.3uA.

This got me motivated to look a bit deeper into measuring hfE values. It turns out they are strongly collector current dependent. This figure is taken from the BC547B datasheet provided by ONsemi:

So we see that at the low 151uA current that TR16 sees, the current gain is only about 60% of the maximum value at about 20mA. The 2N2222 has a similar curve (see here), and that (together with the strong variation of hfE values in general between individual transistors of the same series) may explain why the 2N2222 was not able to perform properly in this circuit. 

Beogram 8000: A Fun Evening With the Record Detection Circuit

*************************This post has a follow-up. Please, see here*****************************

I recently started working on a Beogram 8000 from California. As the usual, as a first step, I replaced the electrolytic capacitors and the motor AC cap in the power block, then re-flowed the solder points of the headers, after which it was time to give the unit a first try. I pressed start and the carriage started moving, and without a record on the platter the unit activated the tonearm lift at the LP setdown point. Not what it was supposed to do. I concluded there was an issue with the record detection circuit. 

This is the relevant part of the Beogram 8000 circuit diagram:

I should point out that the Beogram 8000 featured here did not have the trimmer R64 at the sensor input, i.e. is one of the earlier models.

The first spot to check is the collector of TR14, which amplifies the signal of the sensor 5PH1 as the platter ribs pass through under the sensor arm. I drove the arm manually further onto the platter to get a stable signal, and then put the oscilloscope probe to the collector. This is the signal I measured:

The dips correspond to the ribs passing under the sensor and blocking the light reflection. Unfortunately, there is no prescribed wave form in the manual, but judging from the very similar circuit in the 4002/4 models this signal was way too low.

the next step was to measure the sensor signal directly at C15, which yielded about the waveform shown in the manual:

Therefore, I suspected that TR14 had a too low gain (hfE), which is often encountered in the Beogram 4002/4 models. I extracted and measured it:

It only showed with a gain of 94, which is well below the minimum gain of 200 specified for the BC547B. I replaced it with a 2N2222 that demonstrated a gain of 215, and measured again:

Much better. The dips go to zero and the amplitude is almost 10V. Unfortunately, there is no spec in the manual for this measurement, but judging from the 4002s the above qualifies as a decent signal. I hoped at this point this would fix the issue. But no cigar. The Beogram again dropped the arm at the LP setdown point in absence of a record. 

If the sensor signal is good, then the next measurement spot is at the junction between R71 and R72. I got the yellow curve in the screen below:

The triangles peak at about 7V and dip down to about 3V, in contrast to the specified 10-to-7V drop shown in the manual. This means there is a too small voltage at the base of TR16, which may mean trouble pulling the 'Disc Detector' signal (HH) properly to a logic LOW state below 0.2V.

I measured HH and that is the green curve in the graph above. At this point I though, bingo, there must also be an issue with TR15 that does the tugging at the base of TR16. I extracted it for measurement:

It came up with an hfE of 158, which is also a bit low. I replaced it with a new BC557B with an hfE of 275, above the minimum 200 spec. So far so good. I measured again at R71/72 and now the curve was according to specification:

Unfortunately, record detection still did not work, and the green curve was a bit better, but the spikes, while a bit weaker, were still present. Ok, I thought, TR16 must also be out of spec. I measured:

...and got a happy hfE of 434 (it is normal that the hfE varies a lot, and all is good as long as it is above its minimum spec, at least for most circuits.

To make sure that the chain of command was intact between TR16 and the micro controller input I grounded the collector of TR16, which should permanently disable record detection, and it should always behave as if there were no record. I pressed Play, and indeed, the arms ran across the platter in search of a smaller record, and then turned around and went back home. So that was working.

At this point I was a bit at a loss. For some reason, the pull at the base of TR16 was not strong enough, and it did not fully turn on. A mystery!

I decided to play a bit with the circuit and I replaced R72 with a 100k resistor, increasing the current by a factor 10 into the base of TR16 (or so I thought). This did not fix the issue. It turns out that the higher current was depleting C19 too quickly, reducing its voltage too fast between the passing of the platter ribs, again causing spikes in HH. This meant I needed to increase the current into C19 also, which was easily done by removing R70, which partially drains the collector of TR15 away from C19. After eliminating R70 from the circuit, it started working properly with HH going fully to zero when the ribs were visible to the sensor:

Luckily, HH also quickly restored itself to 5V when covering the sensor (simulating a record), i.e. the record detection mechanism of this Beogram 8000 is doing what it is supposed to be doing.

At this point, I am not fully satisfied with this result since I do not fully understand what is going on. I guess another 8000 where this circuit works 'as is' will need to show me the light some other day. The B&O excitement just never ends!...;-)