Michigan Beogram 8002 Restoration: Scanning LDR Assembly

Here is a quick update on the Beogram 8002 restoration project from Michigan.

I left off with the Forward and Reverse scanning control not able to adjust to the values specified in the service manual.

To fix that I knew I would have to replace the two LDR devices and/or the light source.

It turned out that the two LDR devices measured pretty close to each other when testing them after I removed them.  The problem appeared to be with the light source.

I removed the original incandescent lamp and installed an 5mm LED with a 3300 ohm current limiting resistor.

I also tested a couple of new LDR replacements but none of them were as good as the two original devices.

The new LED light source worked great and I was able to adjust the forward and reverse scanning voltages to very close to the target 620mV for the "off" state of the buttons.

I tested the basic Beogram 8002 Play, Stop, Forward, Reverse operations with this updated control panel and it performed just as I expect it to.

With the Beogram 8002 components opened up like this I will solder on some test wires to measure the position sensor, record detect sensor and the platter speed sensor.

Looking at those on an oscilloscope will show how healthy those signals are.

The results of those tests will be in the next post.

Beomaster 1900 Type 2904: Solving the Voltage Difference Between the Channels

I ended the previous post with a problem regarding the output of the Beomaster 1900 amplifier.
Specifically, the Left channel output signal was about 20 percent stronger than the Right channel.

At first I thought the problem might be with the output amplifier transistors...either the Left channel or the Right channel.  However, I also had to keep in mind that the amplifier sounded good and while there were measurable differences, the distortion and frequency response measurements were very similar to other Beomaster 1900 units I have seen.

Still, I felt that a 20 percent difference in output between the channels was a little much.
So I did some audio signal tracing and measurements of the test signal through the various stages of the Beomaster 1900.

At the point of the input signal selection, right before going into the volume control, both the Left and Right channels are identical.

After the volume control, the difference begins...but it isn't always 20 percent.

Studying the Beomaster 1900 volume control again, I realized that the light source controlled, LDR volume control assembly had to be the most likely culprit.

The Beomaster 1900 (and 2400) volume control is an interesting design and is worth looking at here.

Owners love the sleek design of the Beomaster 1900 and in the mid-seventies, the touch control buttons were quite a modern marvel.  Source selection and Standby mode selection were simple on/off state changes while the up/down volume control was a more complicated matter.

The volume control has two parts to the control.

There is the control from a human finger holding the volume button up (or down).  Then there is the actual attenuation control of the audio signal.

First, the detection of a human user wanting to change the volume level.

This circuit and description show how the Beomaster 1900 takes commands from the volume control up/down buttons and turns them into different voltage levels.

The selected voltage level is used by the actual volume control of the Left and Right audio signals.
The way that works is the selected volume level voltage controls the amount of light a small lamp emits on a set of four LDR (Light Dependent Resistor) devices.

The LDR volume control assembly containing the light source (lamp) and the four LDR devices looks like this.

Disassembled, the Volume Control LDR Assembly looks like this.

As you can see, the volume control assembly comes apart and the discrete components can be replaced.
Martin Olsen (Beoparts.com) supplies a rebuild kit for these that consists of the lamp and four LDR devices.

The Beomaster 1900 Service Manual describes how the light source and LDR volume control works.

Here is the Beomaster 1900 schematic section that shows the Left and Right audio signals passing through the volume control and to the output amplifier.
The 8002293 device shown in red is the LDR volume control assembly.

The schematic above shows four key measurement points I used in looking at the audio signal.
TP200 and TP300, before the volume control device.
TP201 and TP301, at the entry to the output amplifier.

An important thing that stood out from the Service Manual description of the volume control device was that full volume occurs with a minimum amount of voltage on the lamp (typically 1 VDC) while the lowest volume level is typically with about 5 VDC applied to the lamp.  

That is opposite of what I would have thought before diving in to the inner workings of the volume control.
What that means though, is that the high end power (maximum output) of the Beomaster 1900 that I want to measure the performance of is at the lowest light source level.

Combining that information with what I observed in the Beomaster 1900 performance...
The Left and Right channel outputs at the speaker load were much closer to each other at output levels below 5 Watts.  That is when the light source is emitting more light.  From 5 Watts and up (to the maximum 20 Watts), the Left channel output got further away from the Right channel. That is when the light source is emitting very little light.

That looked to me like a volume control calibration problem.

I ended up playing around with three different volume control assemblies (two shown here).

Eventually I settled on the one I could adjust the best.

I have to mention that rebuilding this volume control device is more involved than simply de-soldering the old components and soldering in the new ones from the kit.

The calibration technique I used was to connect the lamp terminals of the volume control device to a bench DC power supply.

From there, I applied voltages from 1 VDC to 5 VDC and compared the resistance of each LDR device.  I wanted the L&R sets (R200/R300) to be close to each other and (R201/R301) to be as close as possible to each other.

To adjust the LDR devices to accomplish that requires moving the LDR devices a bit towards (or away) from the light source.  Even tilting them a bit changes their value.

The adjustment procedure means applying a voltage to the lamp and measuring the LDR resistance with the metal cover on the volume control assembly.  To adjust an LDR means having to remove the metal cover, readjust the LDR, then replace the cover to measure the resistance again.

The procedure is quite tedious and I ended up decided on a source voltage of about 1.3 VDC as the target level I wanted to adjust the LDR devices to. 

That corresponds to a pretty high volume level and wasn't too difficult to measure with a DMM.

When I was finally happy with my result, I reinstalled the volume control device and retested the Beomaster 1900 amplifier.

The THD measurement immediately showed an improvement.
 

The Left and Right channel outputs at maximum rated audio power into 8 ohms are now within almost 6 percent instead of over 20 percent.

The THD measurement for both channels at that maximum output level is in the 0.03% range.
That is a very good value for this amplifier model.

The Frequency Response measurement for the two channels also shows the signal amplitudes being closer than before the rework on the volume control LDR device.
The Frequency Response measurement is within the limits I have previously seen on the Beomaster 1900 units although the Right channel does appear to have a better response trace below 1KHz than the Left channel.

I will move this Beomaster 1900 into a listening room for better real world listening tests (than out in the workshop).  Unless I don't like something I hear during those tests, I will consider the bench testing complete.

Cosmetically there is still the issue of peeling veneer from the plastic side panels.
I need to see if I can save those while the Beomaster 1900 is in the listening room.

Beogram 8002 From North Texas: Testing the restoration work so far

This Beogram 8002 is ready to do some testing of the restored components before starting in on the service manual adjustments.

The recapping of the circuit board, transformer and chassis capacitors was completed in the previous post. 
There aren't any capacitors in the button control panel but there are two LDR devices plus a lamp that will need to be adjusted during the service manual checks. On rare occasions those LDR devices and the lamp require replacement but I usually leave them alone. However, I do like to install a small test connector that is handy to use in testing the functionality of those two LDR devices.

Here is the Beogram 8002 button control panel removed and looking at the underside.
The photo shows the two adjustment screws for the LDR devices. Unfortunately there are not easily accessible test points for measuring the LDR voltages while doing the adjustment.

There is a small set screw and plastic tabs that keep the circuit board in place on the panel. You have to be careful in removing the board as I have come across quite a few panels where the plastic tabs are broken off...likely by someone that didn't know what they were doing.

Turning the circuit board over I noticed that the chassis ground connection was loose.

I cleaned off the copper area and resoldered the wire.

The LDR adjustment calls for measuring the two LDR devices (reverse and forward) to ground so I will add a small 3-pin connector with those signals.

The test connector is a female connector so there are no exposed pins to short out when the test connector is not being used. When I make the LDR service manual adjustment I will insert a 3-pin male plug for my testing.

I also plan on measuring the various Beogram 8002 sensors with an oscilloscope so I soldered on some temporary test wires for those signals on the back of the main PCB.

That takes care of what I will need for electrical measurements while testing the Beogram.
Now I just need to reassemble the tonearm assembly in the floating chassis.

I had removed the tonearm assembly for cleaning and lubricating the Beogram.  
I cleaned off all of the old lubricants and am ready to apply new lubricants to the Beogram 8002 spindle and tangential arm assembly rails.

You will notice that the rear rail is longer than the front rail and has a rubber sleeve (with a metal end cap) on each end. This type of rear rail is only on Beogram 8002 models that were built for markets where the AC line is 60 Hz. Bang & Olufsen engineers determined that the 60 Hz models were susceptible to some vibration that needed damping.  This is a USA model Beogram 8002 so it has the rear rail built with the damper. Because these turntables are over 30 years old I always clean and put some rubber conditioner on the ends.

To lubricate these parts I use some Tri-Flow synthetic grease on the mounting points of the spindle followed by a mixture of Rocol MTS 2000 and ESSO NUTO HP32 (1:1 ratio) on the spindle itself. On the front and rear rails that the tonearm assembly slides on I use some Tri-Flow dry lube. I have also used white D.C. M-kote paste on the rails as I use on the Beogram 400x turntables.

While the tonearm assembly is removed from the spindle and rails there is one service manual check/adjustment to go ahead and make.

The horizontal parallelism adjustment screw is on the bottom of the tangential arm assembly. So it is not accessible when the arm assembly is installed.  

This adjustment is to line up the top surfaces of the tonearm and the fixed arm so they are even with each other. Here is a photo of the adjustment screw.

Another adjustment screw that is only accessible from underneath the tonearm is the screw that holds the tracking force slider control in place. Sometimes this screw can become loose and the tracking force slider slides too easily. Often drifting away from the setting an owner sets it to. 

While I have easy access to the screw I made sure it is securely in place and the slider does not move too easily (there should be a little bit of friction).

Reinstalling the rails, spindle, spindle nut and tonearm assembly requires a bit of maneuvering of the parts all at once but it isn't too difficult. The rear rail must be fit through the tonearm assembly base and through the bracket that attaches to the spindle nut. Obviously the orientation of the parts are critical as well.

Everything can now, gently be tilted over to fit properly on the floating chassis.

The tangential arm servo motor belt and the position, rotary sensor assembly can now be reinstalled.

I can now start connecting floating chassis components together to see how the restored parts work.

A couple of quick checks are needed though.
Check that the center hub of the Beogram 8002 (with the tachodisk for the platter speed detection) spins easily and does not contact any part of the sensor assembly.  This one is good.

The top platter is not necessary for testing the basic Beogram 8002 functionality but the black, sub-platter is. The metal sub-platter passes between the curved slot of the two tangential drive motor components and is actually part of the motor. Without the sub-platter in place nothing will turn and the Beogram 8002 will try to drive the motor anyway ... and could blow a fuse.

Finally...here we go with the first power on test of the restored parts.
Everything that needs to be connected is connected.

You can see that the Standby dot is illuminated and ready to go.

Pressing play I get an operating platter, the tonearm assembly moves and the Beogram 8002 control logic is able to lock in the platter speeds.

As I expect to observe (and hear) the Beogram 8002 platter and tangential arm assembly operate extremely quiet and very smoothly.

That is it for this post. 
Next time I will measure the power supply voltages and view some sensor signals on the oscilloscope.
I will also finish up the service manual adjustments for the tonearm tracking force, record tracking and the forward/reverse LDR scanning voltages.

A Pair of Beomaster 1900 Receivers for Restoration: Unit #2 Performance Testing and a Volume Control LDR Problem

Today I finished some of the Beomaster 1900 service manual adjustments and began looking at its performance with THD (total harmonic distortion) and Frequency Response.

First a couple of service manual adjustments to make.
The Beomaster 1900 service manual calls for adjusting the tuning voltage for FM 1 and FM 5.
Specifically, the FM 1 tuning potentiometer (4R3) is to be set on the 88MHz dial position. 
With FM 1 selected as the Beomaster 1900 music source the 4R2 trimmer is adjusted so test point 4TP1 measures 4.5 VDC. 

Similarily, the FM 5 tuning potentiometer (4R7) is set to 88MHz.  With FM 5 as the selected music source trimmer 4R8 is adjusted to get 4.6 VDC ... also at test point 4TP1.

Here are those measurement and adjustment points.

The last service manual adjustment I made was to adjust the output level trimmer 2R136.
For this adjustment an input signal of 200mVrms, 1KHz (sine wave) is applied to the Beomaster 1900 Tape input. With the Beomaster 1900 volume preset on the middle setting the Beomaster is turned on by selecting the Tape source button. At that volume level the output of the Beomaster (as measured at the speaker outputs) should be at 100mV.  Trimmer 2R136 is used to adjust that level.

I set the test up by configuring a 200mVrms, 1KHz test signal out of the QuantAsylum QA401 Audio Analyzer.  I verified the signal with an oscilloscope then applied it to the Beomaster 1900 tape inputs.
I set the volume preset to the middle button and selected the Tape source button to turn the Beomaster 1900 on.  Then I measured the voltage at speaker loads.

Here is the test setup and the measurement results.

The left and right channels differ in output by 20mVrms. That isn't horrible but I expect them to be closer.

The Balance control was correctly in the center. The no-load current adjustments are almost identical.
I guessed that the problem must be in the Beomaster 1900 volume control LDR device.
That is a light controlled resistor device that uses a lamp whose intensity changes via electronic control. The amount of light then changes the resistance values of a set of photo resistors to control the Beomaster 1900 volume levels.

Just to investigate a little further I went ahead and checked the Beomaster 1900 THD (total harmonic distortion) at 30Watts (with 8 ohm dummy speaker loads). 

The THD values are good but the left channel is at about 20 Watts while the right channel is at about 30 Watts.

I needed to investigate the volume control LDR device.

Here are some photos of it.

Here is a photo of the volume control LDR device disassembled.

A single lamp in the center of the device emits light to four LDR devices.

When there is no light the resistance level of the LDR is at its highest and corresponds to full volume of the Beomaster 1900 amplifier.

As the lamp begins to emit light the LDR resistance decreases and attenuates the audio signal.

The photo shows four LDR devices around the lamp in the volume control device.
Two LDR devices are for the left channel and two are for the right channel.
One set (L & R) are for voltage levels in the tone control circuit and the other set (L & R) are for the voltage level in the output amplifier.

To test this device out I connected the two wires for the lamp to a DC power supply and set the voltage to 5 VDC.  Then I measured the resistance of each of the four resistors to see how close in value they were to each other. 

I discovered that the four LDR devices in the volume control of this Beomaster 1900 unit did not have a consistent resistance value for my test voltage.

I measured several LDR devices with my test setup and selected four LDR devices that were very close to each other in value. 

I reinstalled those into the volume control casing and then back into the Beomaster 1900.
Then I repeated the Tape input level adjustment.

This time the difference in the left and right channel is about 7mVrms.  That is much better than the previous 20mVrms difference.

What about the left and right channels at maximum power output?

The Beomaster 1900 spec sheet says the maximum power output of the Beomaster 1900 amplifier is rated at 2 x 20 W/8 ohms. The maximum total harmonic distortion is listed as less than 0.2%.
Due to the nature of the touch sensitive volume control it is difficult to dial in a precise volume level for this check.  I ended up with measurements at output levels of 15W, 28W and 32W.

All three measured well below the target 0.2% THD value.

Here is the THD measurement at 28 Watts across my dummy 8.1 Ohm speaker loads.

Those are nice distortion percentages and at a higher output level than the specification sheet.
Also notice that the left and right channel output levels are much closer to each other than before I worked on the volume control LDR device.  I am satisfied with these results.

The last test is a frequency response test. I set the voltage level to output 26W of output from the Beomaster 1900 amplifier across the 8.1Ohm dummy speaker loads. 
Here are the measurements from my QA401 Audio Analyzer.

That measurement is right in line with what I have been seeing on the Beomaster 1900 and 2400 amplifiers. The frequency response is comfortably between ±1.5dBV from 20Hz to 20KHz.  This Beomaster 1900 is actually performing at ±1dBV through that frequency range. Plus that is above the expected maximum output level. 

I will continue on with the reassembly of the Beomaster cabinet parts then put the unit into listening testing.

While I am doing that I will begin a couple of fun Beogram turntable restorations and a change of belts on a Beogram CD50 unit.