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Monday, January 12, 2015

Beocord 5000 (4715/4716): Calibrating the Peak Program Meters (VU Meters)

I put the Beocord 5000 (4715/4716) back together. It seems to work now. I made a recording using a signal my waveform generator, and it seems the playback is undistorted across the frequency range and at a reasonable level. So I started to do some measurements to check if the deck is within spec. Before getting into measurements like frequency response etc...I thought it would make sense to start with calibrating the "Peak Program Meters" (PPM) as the meters are proudly called in the service manual. Seems fast electronic audio level meters were something new in consumer units back then in 1978...I studied the service manual to get some advice for their calibration. The manual suggests to directly feed 300 Hz from the waveform generator into the input of the Dolby B ICs, and then do the adjustment of the PPMs.

I chose a slightly different path and simply fed a 300Hz 25 mV amplitude (not pp) signal into the inputs of the Beocord, switched the deck into 'record' (paused), and then used the recording level meters to adjust each of the channels to get the prescribed 740 mV RMS at the outputs of the Dolby B ICs (pin 7). This can be measured with a multimeter switched to AC. This will give the RMS voltage of the AC signal. Once the 740 mV RMS are at the output, the trimmers for the PPMs can be adjusted for 0 dB (first red light).
Here is a pic of the part of the Operation Control PCB (#2) where the trimmers are located:

The two larger trimmers are for the 0 dB adjustment. Instructed by the service manual, I adjusted them until the 0 dB lamps just lit up. The next step was to do the -25 dB adjustment. This calibrates the slope of the meter. This adjustment requires to lower the output signal at the Dolby B ICs to -20 dB relative to the 0 dB 740mV RMS signal. -20 dB corresponds to a factor 10 of the amplitude of the signal, i.e. I adjusted the record level sliders to get 74 mV amplitudes at the Dolby B outputs. The service manual prescribes to adjust the -25 dB trimmers in a way that the -25 dB lamps just light up. Not sure why they recommend to get the -25 dB lamps to light up, and not the -20 dB ones, considering that the signal level was lowered by 20 dB...anyway, I decided to stick with the service manual, and did the adjustment. Then I did the 0 dB adjustment one more time (as suggested by the manual). After this adjustment I can now trust the meters, which sets the foundation for a successful frequency response measurement.

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