Q-Max Grid Dip Oscillator (GDO) Question

[Humber888]

I've just refurbished my geriatric Q-Max GDO-2 grid dip oscillator with new carbon composition resistors and a new smoothing electrolytic and am very pleased with the way it is now performing. It is acceptably accurate when loosely coupled near a frequency counter and the meter deflection is now up to near full scale on the lower ranges. However, I had never used the headphone socket before and, having tried it now, all I can hear is a very faint hum. I don't have the manual for the GDO so wondered if anyone knows what type of 'phones it wants (low impedance, high impedance, etc.) and what I should expect to hear.

Many thanks.

Mike

A beat freqeuncy near an oscillator coil (oscillating of course). Or the radio if coupled to an aerial.

[Skywave]

Most GDOs can be re-configured to act as an absorption wavemeter. I think that you'll find that the purpose of the headphone socket it to hear the modulation on an amplitude-modulated signal when the GDO is being used in that mode.
So, if you have a sig. gen. handy that will do A.M. . . . . .

Al. / Oct. 27, '13

[Humber888]

Thanks to both respondents. The GDO will act as an absorption wavemeter if you switch off the HT. I checked it with a signal generator wound up to 10V p-p output and a loose coupling around the GDO's coil of unshielded wire connected to the inner BNC output pin. I realize that ideally I should make up a proper coupling coil for the sig gen, but this was good enough to get a small deflection on the GDO's meter. Also, you could hear the 1kHz modulation from the sig gen on headphones . More sensitive headphones would be better than the low impedance stereo pair I tried, but the results were good enougn to prove the point.

Thanks again.

Mike

[G6Tanuki]

Another use for the headphones output on a GDO is to calibrate the GDO itself - you need a crystal oscillator whose output you can loosely couple to the GDO's coil - then you tune the GDO across the crystal's frequency and you should at some point hear a "whoop" in the headphones.

Compare the crystal's frequency with the frequency now shown on the GDO's dial and you'll have a feel for how accurate the GDO's calibration is.

[neutronic]

Originally Posted by Humber888

I don't have the manual for the GDO so wondered if anyone knows what type of 'phones it wants (low impedance, high impedance, etc.) and what I should expect to hear.

In those old days carbon headphones. High-ohmic ,something like 1K.

And you will hear nothing!
Neither my Philips GM3121,two Grundigs 701/709 nor two Millens 90651/96052
have ever produced any sound other than hum.
The plug is only useful for injecting an audio-tone

Jard N.

[HamishBoxer]

A lot of GDO,s were supplied with an ear piece like a deaf aid,not actually headphones.

[neutronic]

Yes,the newer(often transistor) ones from US and Japan like my Kyoritsu K-126A.
But the real old ones were ment for old fashion headphones as can be seen by those banana-plugs or big sized jack plugs in the GDO.

Nice example : Millen 90751 Audio plug-in unit.

Jard N.

[Humber888]

It appears that my Q-Max is doing better than Jard's GDOs. At least I get some (quiet) sounds from my headphones. (How many GDOs do you need Jard??) Anyway, the refurbishment is complete and I can put it to bed until needed. The original rationale for digging out the GDO was to help set up a ferrite rod aerial to resonate at 198kHz as an aerial for my Halcyon off-air frequency standard. Unfortunately the GDO only goes down to 1.5MHz so is of no use! Plan B I think.

Cheers.

Mike

[Herald1360]

I should think plan B would be just to connect a germanium diode in series with a uA meter across the ferrite rod coil then just tune for max deflection if you know it's a long wave coil already.