Today, I completed some service manual adjustments on the Beomaster 1900 and started evaluating its performance by measuring THD and Frequency Response.

The first adjustments involved setting the tuning voltage for FM 1 and FM 5 according to the Beomaster 1900 service manual. The procedure involves setting the FM 1 tuning potentiometer (4R3) to the 88MHz dial position and adjusting trimmer 4R2 to achieve a reading of 4.5 VDC at test point 4TP1 with FM 1 selected as the audio source.

Similarly, the FM 5 tuning potentiometer (4R7) is set to 88MHz. Then, trimmer 4R8 is adjusted to obtain a reading of 4.6 VDC at test point 4TP1 with FM 5 chosen as the audio source.

Images of these measurement and adjustment points are provided below.

The final service manual adjustment involved setting the output level using trimmer 2R136. This required feeding a 200mVrms, 1KHz sine wave signal into the Beomaster 1900’s Tape input. With the volume preset at the midpoint and the Beomaster 1900 turned on via the Tape source button, the output at the speaker terminals should measure 100mV. Trimmer 2R136 is adjusted to reach this level.

I set up the test using a QuantAsylum QA401 Audio Analyzer to generate the 200mVrms, 1KHz test signal. After verifying the signal with an oscilloscope, I applied it to the Beomaster 1900 tape inputs. I set the volume preset to the middle position and selected the Tape source to power on the Beomaster 1900. Finally, I measured the voltage at the speaker outputs.

The test setup and measurement results are shown below.

There was a 20mVrms output discrepancy between the left and right channels, which, while not ideal, was acceptable.

The balance control was confirmed to be centered, and the no-load current adjustments were nearly identical. I suspected the issue stemmed from the Beomaster 1900’s volume control LDR (Light Dependent Resistor).

This LDR is controlled by the varying intensity of a lamp. The lamp’s light level alters the resistance of the photoresistors, thereby controlling the Beomaster 1900’s volume.

To delve further, I measured the Beomaster 1900’s THD at 30 Watts using 8-ohm dummy speaker loads.

Although the THD values were good, the left channel was outputting approximately 20 Watts, while the right channel was outputting around 30 Watts.

This discrepancy indicated a need to examine the volume control LDR.

Photos of the LDR are shown below.

The disassembled volume control LDR is depicted below. A centrally located lamp illuminates four LDRs.

With no light, the LDRs have maximum resistance, corresponding to full volume from the Beomaster 1900 amplifier. As the lamp brightens, the LDR resistance decreases, attenuating the audio signal.

The photo reveals four LDRs surrounding the lamp within the volume control unit.

The left channel utilizes two LDRs, and the right channel employs the remaining two. One set (left and right) manages voltage levels within the tone control circuit, while the other (left and right) handles voltage levels in the output amplifier.

To test the LDR, I connected its lamp terminals to a DC power supply set to 5 VDC. Subsequently, I measured the resistance of all four resistors to ascertain their value consistency.

My findings indicated that the four LDRs within this Beomaster 1900 unit’s volume control did not exhibit uniform resistance values at the test voltage.

I proceeded to measure numerous LDRs using this setup and selected four that displayed the closest resistance values to one another.

After installing these LDRs into the volume control housing and reinstalling the assembly back into the Beomaster 1900, I repeated the Tape input level adjustment.

The difference between the left and right channels was reduced to approximately 7mVrms, a significant improvement from the prior 20mVrms discrepancy.

Next, I examined the left and right channel outputs at maximum power.

The Beomaster 1900 specifications state a maximum amplifier output of 2 x 20 W/8 ohms and a total harmonic distortion of less than 0.2%. The touch-sensitive volume control’s nature made it challenging to achieve a precise volume setting for this test. Ultimately, I took measurements at output levels of 15W, 28W, and 32W.

All three measurements fell well below the 0.2% THD target.

The THD measurement at 28 Watts across the 8.1 Ohm dummy speaker loads is shown below.

These distortion percentages were excellent, especially at an output exceeding the specifications. Notably, the left and right channel output levels were now much closer after addressing the volume control LDR. These results were satisfactory.

Lastly, I conducted a frequency response test. I set the output to 26W across the 8.1-ohm dummy speaker loads.

The QA401 Audio Analyzer measurements are depicted below.

These findings align with my observations of the Beomaster 1900 and 2400 amplifiers. The frequency response remained within ±1.5dBV from 20Hz to 20KHz. Notably, this Beomaster 1900 achieved ±1dBV across the entire frequency spectrum, surpassing its anticipated maximum output.

The next steps involve reassembling the Beomaster’s cabinet and conducting listening tests.

Concurrently, I will commence the restoration of a couple of Beogram turntables and replace the belts on a Beogram CD50.