Beomaster 1900 Type 2904: Testing and wrapping up the project

This Beomaster 1900 receiver is finally ready to perform the job it was intended for.

I don't have as many options for connecting up my audio analyzer probes inside the Beomaster 1900 like I did on the Beomaster 8000 but I am able to perform the main tests I wanted to do which are a total harmonic distortion (THD) test at maximum output into my dummy 8Ω speaker load using a 1KHz sine wave source signal and a frequency response test.

Here is my newest testing apparatus, two new dummy speaker loads.

My previous dummy speaker loads were made with a single 8Ω resistor for each channel. The new test loads have a 4Ω resistor in series with two 2Ω resistors and a 0.08Ω sensing resistor.  This gives me more connection points for measurement probes to use in combination with the audio analzyer attenuators...so I can setup the measured signal at a level most suited for the analyzer.

I still expect to primarily connect up the analyzer probes differentially across the 0.08Ω sensing resistor as shown in the picture.

The DMM probes are across the full 8Ω load so I can monitor what the amplifier output level is actually at. This is especially useful on the Beomaster 1900 where you really don't get a numeric value for the volume. I find it difficult to visually tell if I have the same volume level when using the touch controls unless I can see the result on a DMM.

The connection to the QuantAsylum QA401 audio analyzer from the 0.08Ω sensing resistor is via the two differential BNC connectors.

The first tests I ran were for the total harmonic distortion (THD).  The Beomaster 1900 - Type 2904 service manual states the maximum THD should measure less than 0.2%.  I take that to mean at full power output (20W into an 8Ω speaker load).

As mentioned in other Beomaster amplifier tests here on the Beolover Blog, we can't duplicate the test setup exactly as Bang & Olufsen did when the Beomaster 1900 was manufactured. However, we should come close and we collect these new test results to use as a reference for other restoration projects of the same Beomaster models....so keep in mind that this testing is an ongoing learning experience.

Here are the Beomaster 1900 left and right channel THD measurements. The QA401 audio analyzer generated the 1KHz sine wave test signal to the Beomaster 1900 Tape input. I adjusted the volume on the Beomaster as best I could to just under 12.7Vrms (which is 20W across 8Ω).  The Beomaster 1900 is a little tricky to dial in a specific volume level value (like 12.7Vrms).  My measurements ended up being more in the range of 11.7Vrms to 12Vrms. 

The right channel measured 0.13% THD which is comfortably below the spec value of 0.2%. 
I was happy with that but then surprised when the left channel measured 0.034%. 

Both channels are obviously in spec but not having measured this Beomaster 1900 before I have no reference data whether this is typical.

To be safe I decided I would run through and recheck the no-load current settings of the output amplifiers. Perhaps that adjustment had changed.

This meant removing the Beomaster 1900 bottom cover and the tone control/FM tuning cover in order to access the measurement and adjustment points. The probe connections across the two emitter resistors for the no-load current adjustment are too difficult to reach on the component side of the board so I connect from underneath.

The adjustment trimmers are also a little hard to reach.

I ended up spending a couple of hours on the readjustments.  Initially I checked them with the amplifier cold. Both channels measured up close to 14mV across the emitter resistors (R256 and R356). Eventually they settled to the low 11mV range. I adjusted them both back to 12mV then reconnected the dummy speaker loads to drive the outputs for a while up to 20W across the 8Ω loads again. After that I lowered the volume level to zero and removed the speaker loads again to check the no-load current measurements. Since the amplifier was warmed up it took a while for the voltages to settle back down. Both channels dropped to about 11.5mV.  I adjusted them back to 12mV one more time and repeated the test. 

This time the no-load adjustment check measured close to 12mV on each channel's emitter resistor.

After all of that I remeasured the THD for 1KHz at 20W and...got the same results as before.
Right channel THD is 0.13% and the left channel THD is 0.03%.  I did try measuring the THD for both channels at a lower volume (10W).  The right channel THD dropped to 0.09% but the left channel remained at 0.03%. 

It would seem like a lot of effort for no gain but I feel better knowing that the output amplifier is adjusted as good as I can get it.

In addition, while the two channels are not identical in their THD test, both channels do meet the Beomaster 1900 spec value. I am satisfied with the THD result now.

The last performance measurement I wanted to do is a frequency response test. The Beomaster 1900 spec for frequency response on the Tape input is 20Hz to 20KHz ±1.5dB.

The frequency response measurement is another test that is an ongoing development for me with my current test equipment set up. For this first time Beomaster 1900 frequency response check I decided to try the QA401 new frequency response plotting feature. Like the THD tests I used the QA401 differential input probes across the 0.08Ω sensing resistor in my dummy speaker load.

The test automatically sweeps test frequencies from a selected range and plots the amplitude of the output signal. I selected a sweep from 15Hz to 40KHz.

Here are my results.

The frequency plot for both channels is within the ±1.5dB specified by the Beomaster 1900 spec from 20Hz to 20KHz so I am happy with that. I marked the 1KHz test signal amplitude as the zero reference to check the ±1.5dB amplitude to. I tried to lock the volume level to the same position for both channel frequency response test but I ended up with the left channel running at a slightly higher level.

Now for a final play test and this Beomaster 1900 should be ready to return home.
I decided to connect up this receiver to my spare Beosystem 8000 where the Beomaster 8000 project recently vacated from. I was interested to see how a Beomaster 1900 would do driving a pair of Beovox M100.2 speakers.

Not a problem for the "little" Beomaster 1900. 20W of maximum power is plenty for a pair of Beovox M100.2 speakers in a small listening room (12 foot x 12 foot).

The final performance evaluation is now my own ears enjoying music controlled through this great little amplifier from the seventies.