Philips GM2882 problems

[LA9CHA Thorvald]

I have a problem with my generator as the output signal level changes quite much from o,1 ---10Mhz to 10-30 and 30-60Mhz. It becomes much lower, and on the 30-60 band it does not oscillate. I have replaced the EF50 valves, but no improvement. The frequency does follow the scale, but amplitude decreases. Looking for hints and tips for solving the problems!

[trh01uk]

I have never heard of the Philips PM2882, and Google provides no "hits" at all! If its using EF50 valves, then it probably dates from the 1940s or 1950s - and it's probably a rare vintage bit of kit in its own right!

If its that old, it will probably have many faults, some showing up as a failure to oscillate, and change in output level. It will benefit from a general restoration as follows:

1. Replace all leaky capacitors.

2. Replace all out of tolerance resistors.

3. Test all the valves and replace any that are out of spec.

4. Clean all the switches and internal connectors, particularly rotary switches with an "open" construction where dust, grease and dirt can easily enter.

5. Switch on, and check the power supply rail voltages are correct.

That recipe is quite a lot of work, but you may well fix the faults you are describing, along with a lot of other "faults" that haven't yet shown up as problems.

If you would like more detailed advice, please post the schematic, or if possible the whole manual for the unit.


Richard

[LA9CHA Thorvald]

Hello Richard and thanks for answer. Sorry for misprint, gear shall be GM2882. and the critical components must be in the coil turret itself. Here are no e-lyt, mostly quality resistors. EF50 have been tested on my AVO tester and show ok. Perhaps a replacement of the components related to the main oscillator wil improve the function!

[trh01uk]

I see from a quick scan of a manual I picked up - see attached file - that there is an EF50 oscillator using a turret coil system, followed by another EF50 as amplifier and modulator.

You describe the fault as a variable output level on certain bands - particularly higher ones - and it doesn't oscillate at all on the top band (30 - 60MHz). Given this is happening on a number of bands, I would think individual components on specific bands in the turret as less likely - they would all have to be showing a similar fault at the same time - possible, but an unlikely coincidence I suggest.

So I would begin by looking for something that is common to all the bands, and gets worse as frequency increases. What about the turret contacts? These are common to all bands. Maybe they have plating that has worn?


Richard

[Jon_G4MDC]

There could be an alc action happening in the output valve since no oscillator will have constant output over such a wide range. This is where the problem is likely to be. The trouble is my Dutch is weak!

It's described here. Would any Dutch speaker like to translate please? Translate programs don't seem to want to attempt it. Probably becasue it's pdf.
Needs to go through "Dutch OCR" first I expect.

Later edit - my phone managed it as a screenshot. Not ALC as such then.

"In order to be able to meet the most favorable conditions for a constant output with a minimum number of harmonics over a range, a correctly dimensioned coil combination has been designed for each frequency range. These are then switched in their entirety by means of switching roller A1. The H.F. amplifier tube L2 has a resistor as an element in the anode line. In order to obtain a frequency-independent gain, this anode resistor is selected long. Via C15, hot H.F. signal is fed to the attenuator (R1 to R11), from where the signal is applied by the shielded cable to the potentiometer R31 / R32. The artificial antennas, consisting of 025 / R33 and C26 / R34, are connected to this. the whole frequency coupling"

[trh01uk]

Looking at the output amplifier, the only sign I can see of "AGC action" is the negative feedback from the anode load, which is split between R19 (2.2k) and R20 (2.2k), with feedback via C14 to the cathode resistor R18 (39R). I agree these would be components to check.

However, if the OP is correct that there is no oscillation at all on the 30-60MHz band, its hard to see how these components could produce that result. That points to problems in the oscillator stage, and most likely around the turret connections I would guess.


Richard

[Jac]

Originally Posted by Jon_G4MDC

There could be an alc action happening in the output valve since no oscillator will have constant output over such a wide range. This is where the problem is likely to be. The trouble is my Dutch is weak!

It's described here. Would any Dutch speaker like to translate please? Translate programs don't seem to want to attempt it. Probably becasue it's pdf.

Translation of your pdf:

In order to get a constant output with minimal harmonic content, a correctly dimensioned coil-combination has been designed for each frequency range.
These are switched with switch section A1.
The HF amplifier valve L2 has a resistor in the anode feed as a coupling element.
In order to get a frequency independent amplification, a low value anode resistor has been chosen.
Via C15 the HF signal is fed to the attenuator (R1 - R11), from which the signal goes through the screened cable to the potentiometer (divider) R31/R32. Connected to this are the artificial aerials, made up by C25/R33 and C26/R34.

Hope this helps. I didn't look at the circuit diagram, it is just a translation of the text.

Jac

[LA9CHA Thorvald]

Many thanks for answers from all of you, great forum with lots of experience and knowledge! I now have more "fun" to investigate!

[Jon_G4MDC]

Thanks Jac. That is much better to read than the translate program output.
Chosing a "long" anode resistor was something that left me totally puzzled!

Good luck to OP finding what is the matter with it.