PM5126 sine/square-wave oscillator
![PM5126-oblique.jpg](PM5126-oblique.jpg)
The PM5126 is a transistorised LF generator covering a range of 10 Hz to 1 MHz. It delivers an output voltage of 0-12 V on two outputs: a 600 Ω and a low-Z output. The output signal is a low-distortion sine was or a square wave. An output meter indicates the output voltage. The oscillator also has a synchronisation input. Though I didn't find any production codes on the components, I suspect it dates from the early seventies. The documentation I found is from 1972. My specimen has a test & maintenance sticker that refers to a date in 1978.
This AF generator consists of a number of circuit boards. They plug into edge connectors that are interconnected by wires. The power transformer and both power transistors of the power supply are mounted on the back panel
The PM5126 seen on the back, power transistor on back panel.
Simplified diagram of the PM5126 oscillator.
That concludes the oscillator board, The other circuit boards contain:
Measurements of the signals on the oscillator board.
Fortunately, somewhere in a corner of the Net there was a manual. First I checked the bias of the transistors in the oscillator but could find nothing. I exchanged the electrolytic (not in the diagram) that serves as a coupling capacitor and keeps the DC component of the output amplifier from the NTC resistor. That didn't make any difference. I started to fear that maybe the NTC was bad, which would be quite an awkward situation as the chances of finding this very special component were close to zero. Finally it dawned to me that the frequency range switch might be corroded. I sprayed some contact cleaner on a Q-tip and cleaned the switch. Black goo. I dried the switch using some more Q-tips and carefully applied some contact lubricant. After this, the oscillator worked fine! The meter still went to the right hand corner just after power-on and changing frequency, but that was just the overshoot of the control circuit around the NTC resistor. The oscillator didn't halt any more.
Now my PM5126 was ok and I had a decent, reliable LF generator.
Copyright © 2012 by Onno's E-page published 2012-09-29