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.
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.
