Philips PM 5105 LF Generator Alignment

I have a Philips PM 5105 Low Frequencies Generator and I need some tips for the alignment.

I have the service manual but the alignment potentiometers and trimmers (610, 516, 513, 605 and 621-523) are not sufficient to make the equipment work as it should.

As an example, the 400 Hz scale is reading 316 Hz on my oscilloscopes (Hantek and Siglent).

1 KHz is reading 2,4 KHz on the oscilloscopes.

The tension on the diode 452 is -16,7 V as expected.

Maybe the equipment is consuming more than the specified: It should be 4,5 W but I did measure 7,1 W on a non reliable meter. The same device is measuring 80mA  for the PM 5105.

I did try the equipment (the first time that I did turn it on) on a dim bulb tester and variac and it was OK.

I did try to adjust it on the trimmers according with the manual to no avail.

Are the electrolytics the culprits? Are the transistors?

Thanks,

Alvaro Georg

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Those blue Philips electrolytics can be suspect after so many years. In particular, the older ones with grey or beige rubber around the positive terminal have a tendency to dry out, while the newer ones with black rubber usually survive aging better but instead they have a tendency to develop shorts.

Transistor faults are very unlikely in this case.

An oscilloscope, in particular an analog scope, is not very useful for precision measurements of frequency unless you can use it for zero-beat or lissajous comparison with a known and stable frequency (like the mains frequency, the horizontal frequency of a tv transmission or the 19 kHz pilot tone of a FM stereo transmission). The DTMF transmitter of a touch-tone telephone can also, with some ingeniuity, be used as a reasonably accurate frequency reference.

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Many thanks for your answer Mr. Forsman, it will be very useful.

Yes, I've decided to change all the electrolytics.

I am using a Siglent SDS 1102 CML+ Digital Storage Oscilloscope and a Hantek 2D72 Handheld Oscilloscope, both are digital ones. For my purposes with the oscillator it will be OK without an accurate precision. But, maybe in the future, I will know how to make a proper precise alignment as you suggested.

Alvaro,

Rio de Janeiro

Brazil

 

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Dear, Alvaro,

if the Sine Wave looks good in all Ranges, and only the Mismatch of Frequency is the Issue, than please check, if the Difference is the same in different Ranges, i.E. if You set at 400hz and get 320, try next Range, where you expect 4khz. If You get apx. 3.2khz, then there is most likely a Problem with the Frequency Pot.

Either the Knob with the Scale is mounted in a wrong Position, or the Pot is worn out ( due to often turning the Pot to different Frequencies over the Years the Trace within the Pot may change the Value while getting thinner.

Please check, if the Pot is 50kOhms as expected. If not, then this may also be the Issue.

Best Regards,

Henning

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Many thanks to your input Henning, I will check that.

Alvaro.

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Just a follow-up.

I did change all 7 electrolytics and all were of less capacitance than the specs (one was open, but maybe I did apply too much hot with the solder iron on the leads when I was taking it out). It was the capacitor that was more close to the two big resistors on the power supply.

All the resistors were within the specs except one that I did change.

I think that the real problem was the frequency pot.  They  were supposed to be 2 of 50K in tandem. They were not the original ones and in place there were 2 of 15K each. Mr Henning Oelkers got it right.

I guess that even if the pots are working well, since they are out of specs there will be no way to get the oscillator working properly.

The oscillator is working but with the same incongruities as before.

Other than that, only if there is some problem with the transistors or the film capacitors...

I will try to anex a picture of the schematics but the schematics are already on the page of the model.

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