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%).