PM5126 sine/square-wave oscillator
The PM5126 is a transistorised LF generator
covering a range of 10 Hz to 1 MHz.
A really useful instrument for the electronics workbench.
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
I assumed that it would be a Wien bridge oscillator like
many other AF generators.
When I set out to fix this generator and read the schematics, I was
surprised, by the circuitry.
Philips had used a different type of RC oscillator.
No doubt, at the time they have patented this circuit to gain an advantage
over manufacturers of “ordinary”
Wien bridge oscillators.
This is not an oddball circuit, though. It works well.
The oscillator consists of three amplifier sections each consisting
of three transistors in a Darlington arrangement.
Two stages are used for tuning.
A phase shifting RC member is connected between
the emitter and collector of the output transistor of these stages.
The output signal of the third stage is taken from the
collector, so this stage is inverting and has an amplification
of more than 1.
At the oscillating frequency, the two phase shifting amplifier stages
give a phase shift of 90 degrees each.
With the 180 degrees of the inverting stage, this adds up to
360 degrees.
The loop gain is regulated by an NTC resistor in the collector load
of the output stage of the third amplifier.
This is yet another difference from the classical
Wien bridge oscillator, where usually a small incandescent lamp is
used as a PTC resistor in the feedback loop of the oscillator.
All amplifier stages are DC coupled.
A zener diode (not in the diagram) is used to bias the
stages such that the output
transistor has enough headroom.
That concludes the oscillator board, The other circuit boards contain:
- The squarer and meter circuits,
- The output amplifier,
- The power supply (except for the power transistors).
The frequency range switch and attenuator switch are mounted to
the front and under the circuit boards.
I bought this generator through an Internet auction site, together with a
PM5100 valved LF generator
and a PM2454 LF voltmeter.
At first sight my PM5126 seemed to work fine, but after
some time I noticed it was a bit unstable.
This became slightly worse over time.
Every time I changed the frequency setting, the output meter
swung wildly and the oscillator often stopped working for tens
of seconds.
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.