Philips PM 5132 : try to repair

[jeannot]

Hi,

I was given a Philips PM 5132 (function generator) as it was not working anymore. I would like to repair it.

First thing I did was connecting the output to my scope. The trace is flat, quite noisy and its behavior does not change with any button press or pot turns. I tried all the different wave shapes, frequencies and amplitude but it did not change the output. See picture for the trace on scope.


By chance I found the service manual online. See here https://elektrotanya.com/philips_pm5132_sm.pdf/download.html. The schematics are on the pages 40 and 41.
I thus opened up the device and measured the power rails voltages. The supply section is working as it produces the +/- 22VDC.

Next step was to check the main oscillator.  I measured the "alpha" point (between resistors 702 and 703) with my scope. The oscillator seems to be mostly working as there is an oscillation. However, I don't think the oscillation should look like this. If we take a look at the block diagram on page 36, the main oscillator section should output a triangular signal or a square one. Moreover, the oscillation is not perfectly periodic as we can see on the picture.

Do you know what could cause this? Am I wrong on the expected output of this main oscillator ?

An interessant thing is that this main oscillator behavior is different based on the state of the "duty cycle" button.


Last, the TTL output seems to be working according as it produces the same oscillation in the range 0->5V (on the picture, yellow is main oscillator and blue is TTL output). So i think the problem hides between the "controlled current source" and "main oscillator".

Thanks for you help!

[shakalnokturn]

I haven't looked at service manual yet.
Is the +/- 22V the only supply? If not check the others too.
One of my main concerns considering age would be switch contacts.
Does this use Philips blue axial electrolytics? They're ofen way past their time...

[jeannot]

yep there is only one supply.

but without actuating any switch, the main oscillator should produce a stable signal. I will take a look at the switch contacts.

there are indeed blue cylindrical electrolytics, I will check them.