Beomaster 1900 Type 2904: Reassembly and Testing

Continuing from the last post, I performed two of the Beomaster 1900 Service Manual adjustments.
The No-load Current Test (12mVdc across the output amplifier emitter resistors).

...and the P1 and P5 Tuning Voltage adjustments.

The Beomaster was now ready for reassembly and some audio testing.

The first step was to re-install the plastic masks for the Bass, Treble and Balance controls.

After that, the button panel is installed.

The button panel installation is a little tricky. There are 10 touch control buttons.  Each button has a metal spring connecting the panel button to a corresponding metal post soldered onto the main board.
When installing the button panel you have to make sure all of the button springs mate with their metal circuit board posts.  

My technique is to get most of the spring contacts to mate...8 or 9 out of 10 is good.

For the spring contacts that do not successfully mate, they can be corrected using a prong tool from underneath frame.

Here is an example of one of the springs not mating.

The picture on the right shows the corrected spring contact mated with the post.

I screwed down the button panel and powered the Beomaster 1900 on to check if, indeed, all ten buttons worked.

They do.

Now for the installation of the program panel and test play some music through the Tape input.

I used both speaker outputs of the Beomaster 1900 during this initial audio testing.
A pair of Beovox MC120.2 speakers on Speaker 1 and a pair of Beovox S-55 speakers on Speaker 2.

They sounded very good so I finished up the re-installation, installing the bottom cabinet plate.

I hooked up an FM antenna and did some listening tests on the tuner.  P1 through P5 all tuned successfully and also sound very good.

While a successful listening test carries a lot of weight in judging the success of a restoration, I like to do a basic harmonic distortion test at full power and a 1000 Hz sine wave input (on the Tape input).
I also like performing a 20 Hz to 20KHz frequency response test at full power (again, using the Tape input as the source).

For these audio measurements I use a QuantAsylum QA401 Audio Analyzer.  There is a newer QA403 available now but the QA401 is still very good and I continue to use it.

I always mention that my audio tests cannot be directly correlated with what is printed in the service manual (or B&O published specs) as the equipment in the mid-seventies was quite a bit different.
Also, many of the specs are a little vague regarding how, specifically, they were performed.

For that reason, I try to do my tests the same way each time using my audio analyzer, then compare my results to previous results I measured on other restoration projects.

My speaker loads for the audio tests are fixed resistors that provide an 8.08 ohm speaker load for each channel (Left and Right).

8.08 ohms because each 8 ohm load resistor assembly is made up of one 4 ohm load in series with a 0.08 ohm, sense load and two 2 ohm loads.

The reason for that is because the QA401 audio analyzer has a limit of 28 Vp-p (or 19.8 Vrms).
19.8 Vrms = 26 dBV which is the form of the input value QuantAsylum uses in stating the amount of input.

At full power of 20W output across 8 ohms, the Beomaster 1900 maximum output my QA401 should see (across the dummy load resistor assembly) is 22 dBV.  So I can place my QA401 differential measurement probes across the full 8.1 ohm dummy load safely.

If I really want to be at a safe measurement difference I could put my QA401 probes across one of the other sections of the dummy load (i.e. across the 4 ohm resistor, the 2 ohm resistor or the 0.08 ohm resistor).  However, the best results should come from using the highest output level I can safely use.

Here are my distortion measurements at maximum rated output power (or as close as I could get) across the 8.1 ohm dummy load. (The Tape input source was a 1KHz sine wave at -12dBV).  

Left THD: 0.038%

Right THD: 0.062%

The volume control for this Beomaster 1900 version is only adjustable in sixteen steps.  Later serial number units (serial number 1826011 and on) increased the volume selection steps to 128.  The output power is the same power but the volume adjustment is much finer.

Because of the coarser, sixteen step volume control, I played around with the volume control value and my source sine wave generator amplitude to achieve an output that was close to the maximum rated power for this amplifier. 

I am pleased with the harmonic distortion measurement results.  Although, as you can see, the Left channel is producing 20% more power than the Right channel.

I measured the audio test signal through the Beomaster 1900 pre-amplifier to its voltage amplifier stage in the output amplifier.

At the input to the output amplifier (Test Points TP201 and TP302 for the Left and Right channels respectively), the Right channel is about 10% lower than the Left channel.
When the signal is at the collector of transistors TR212 (Left channel) and TR312 (Right channel), the difference in the channels is 12%.

I will do some more investigating on the difference in output level between the Left and Right channels.
It appears that most of the difference occurs in the output amplifier.

Meanwhile, I continued on to a frequency response measurement at maximum power. 
I ended up with a little more volume for the measurement and the Left channel output was 5 Watts over its rated value.
The measurements are within the expected value for this amplifier but again, the Left channel output is higher than the Right channel (by 23%).

Beogram 8000 Type 5613: Wisconsin project is ready for record play

I finished the service manual checks and adjustments on this Beogram 8000 turntable.
The power supply voltages look good. The forward/reverse scanning LDR adjustments are set.
Tracking force is calibrated as well as the record tracking sensitivity.

A quick workshop test play of a record shows that the Beogram is ready for some record play listening tests so I will install the components back into the Beogram cabinet and enjoy some records.

Here are the supply voltage checks at the filter capacitors and at the output of the regulators.
From left to right are +15 VDC, -15 VDC and +5 VDC.

In this photo I show using my little three pin test connector for adjusting the forward and reverse scanning sensors (LDR devices).  I like to set the idle voltages for these around 650 mVdc.

Next was calibrating the tonearm tracking force at the 1.0 gram mark on the adjustment scale.
I adjusted the counterweight at the back of the tonearm until the 1.0 gram mark on the adjustment scale measured 1 gram on the scale.

The final adjustment is for the record tracking sensitivity.  This is the adjustment where the P4 connector is disconnected from PCB 1 so the platter doesn't turn.  

The procedure calls for manually turning the platter with a test record.  The stylus is dropped onto a track in the middle of the record and the tracking sensor sensitivity is adjusted so the tangential arm servo begins advancing the tonearm assembly between 1 or 2 revolutions of the platter.  After that the servo motor should advance the arm every revolution of the record.

After a few iterations of the adjustment screw the sensitivity was set.

Before putting the Beogram 8000 all back together in its cabinet I wanted to listen to a couple of records on it while it is in the workshop.  I connected it to my Workshop Beomaster 8000 and gave it a listen.  Using the Beomaster 8000 also tests the remote control functionality between the Beogram 8000 and the Beomaster 8000.

The MMC-20CL cartridge played beautifully on the Beogram.  My Beomaster 8000 is connected to a pair of Beovox S75 speakers and did their part in the record play enjoyment :-)

Note: I keep my Workshop Beomaster 8000 opened up all of the time. It is fully functional where I can use it to listen to music while I work but it is always available to perform board testing with and that is its main purpose in the shop.

Beogram 4004 Type 5526: Ready for record play

In this post I wrap up the service manual adjustments and reassemble the Beogram 4004 so I can begin playing records.

The first service manual adjustment is to set the platter height so the surface of the platter is 23mm from the top of the fixed arm.

I set my calipers to 23mm then adjusted the platter bearing screw so the platter measured 23mm from the top of the fixed arm.

Next, I adjusted the tonearm lowered position limit so the tip of the stylus is about 1mm from the top of the first platter rib.

Then I checked the stylus tracking path by stretching a string from the center of the platter to the edge of the Beogram cabinet.  As I placed the stylus at various points along the string I checked that the distance of the stylus to the string stayed consistent.

Following that adjustment procedure I checked the tracking sensor sensitivity.

With a test record on the platter, I lowered the Beogram 4004 tonearm onto one of the middle tracks.
The platter was not connected to the motor (the belt was removed) so nothing was moving.  

I manually rotated the platter and observed when the servo begins to move the spindle (to track the record)
.
The Beogram 4004 servo should start rotating the spindle pulley within one or two platter revolutions after the tonearm sets down.  After that the servo should move the spindle on every revolution.
I set the tracking sensitivity to rotate the spindle after one revolution.

All of the adjustments so far were without the platter belt installed.  Any moving of the platter was done manually.

Now it was time to install the platter belt and begin checking the platter speeds (33.3 and 45 RPM).
For the platter speed adjustment I used the two trimmers that were installed on the reworked PCB 1 board to adjust the 33.3 and 45 RPM turntable speeds. The speed adjustment knobs on the speed indicator panel were set to the 0 position.

I use the Beolover RPM Tool to check the platter speed as I adjust the speed trimmers. The following two pictures show the RPM tool and adjustment.

The last adjustments are to put the Beogram 4004 deck panels back together and check that everything aligns.  

The platter needs to be centered in the deck opening and the top of the platter should be even with the top of the deck plate.

Those adjustments can take quite a while to get correct and you often have to go through several iterations of the adjustments.  That was the case here although it wasn't really to bad.

Here are the pictures of the Beogram after all of the adjustments were complete.

This Beogram 4004 is ready for playing records.
I will test this turntable in a listening room for a few days to make sure everything works properly before returning the Beogram to its owner.

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.