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Heathkit GD-1190

BFO Guy

New Member
Joined
May 18, 2020
Messages
23
Hey folks, I’m just looking at the GD-1190 schematic (available online/google it) and I’d like someone to help me understand what type of detector this is. It seems a little strange. It appears to be a mono coil and the only mono coils I readily understand are BFO’s. But this does not look like a BFO. I believe I read somewhere that it was a VLF but not sure. Certainly doesn’t look like a PI. It appears to have a second oscillator for Audio Tone but I don’t fully understand the way it’s working along with the coil oscillator to generate output. The coil oscillator is driving (actually pinging not oscillating) the coil at 58khz (which oddly is the tank circuits tuned frequency but this is not a TR machine nor a PI I don’t think) and the audio tone is around 200 hz.
Also noticed that the signal going into the amp from the coil is only .6 volts peak to peak. That seems like not much drive on the search coil but maybe those voltages on the shematic are just for some test mode.
And even the discrimination control circuit near the coil has me a little puzzled.
I must admit to being a little confused by this circuit. Maybe it’s late and my brain isn’t working but i’d Like to discuss this with someone a bit.

Thanks.
 
Thanks. I have never heard of “off-resonance” per that thread at geotech. Googling it doesn’t help much except it seems to have something to do with achieving maximum power and maximum efficiency or rather a compromise of both in driving a tank circuit. I did notice a slight calculated difference in the tank tuned frequency and the oscillator frequency but did not suspect it was an intentional difference. Rather just difference caused by the use of standard value components. Something to look into deeper I think but I’m not actually convinced that this is really a thing here. If the tank was perfect it’s resonant freq would be 58.8 kHz while the oscillator freq is 57.2 kHz. Not that much difference, easily within the bandwidth sweet spot of an LC tank at this frequency.

Thoughts?
 
I also looked at the schematic and didn't really understand how it works.
But having the oscillator 'off' the resonant frequency of the LC Tank would cause a change in the amplitude when metal comes near the coil.

Maybe you can register on Geotech and ask. There are many people on there that have been playing with detector circuits for a long time.
 
I am actually experimenting with ringing a tank very far off resonance and I am seeing an amplitude change due to metal all across the ring time except for the first couple of cycles where the energy is inserted. So even at the tanks own chosen resonant frequency the amplitude is affected when no additional energy is applied. In the case of a pure oscillator circuit I’m not sure but I don’t remember amplitude being affected. At least not this much. This Heathkit design is neither of these scenarios. It is a very near off resonance scenario and I have not tested that.it looks like the applied energy signal is attenuated quite a bit. And there is an adjustable capacitor there. I’d have to build this up to see what it’s doing. It’s either very simple or it’s some sort of radio magic that I’m just not seeing. Heathkit was known for there radio kits after all.
 
It finally clicked. After your comments about amplitude changing when metal is present due to the off-resonance driver. I now understand what this is doing. It’s a PWM thing. Once tuned, as metal is placed near the coil the tank frequency shifts into the sweet spot and the amplitude increases. This increases the up time above the reference voltage at the input to IC 1 which increases the duty cycle at the output of IC 1. This increased duty cycle 58 kHz signal is mixed in with the low frequency audio tone from IC 2 C causing a disturbance in the speaker output.

It’s pretty clever but I don’t know if this is any more sensitive than other designs. There are so many ways to “skin the cat!”. Metal detectors all seem to share the same depth limitation due to coil field penetration being simply physics. My old late 60’s Garrett BFO can pick up common coins at nearly the same depth as my new Garrett Ace 300. Most of the advancement has just been in the discrimination capability and data presentation. Microprocessors give us a little edge on depth due to signal processing being better than our eyes and ears but it seems marginal. Modern components with lower losses adds depth too I guess but again marginal. I used to think that simply adding more power would increase the depth but you hit diminishing returns very quickly. Which is why most detectors run on pretty low voltage.
Anyway, let me know what you think of my analysis of the circuit. Hopefully I’m not totally wrong. Lol.
 
Your analysis looks correct (I looked at schematic again).

Yes, in general all detectors are limited by the physics of the EM coil field and targets.
Increasing TX power does not help much. Sensitivity is mainly limited by the RX side.

Newer detectors are more stable, smaller control box, and better disc.
The Newest detectors using multiple frequencies handle changing ground mineralization better which can add a little more depth and does better on disc.
 
Looking for Heathkit GD-1190 assembly manual and calibration info

I am trying to repair a Heathkit GD-1190. I am looking for the assembly manual and calibration info. Does anyone have one they can copy or lend me?

Roy Ashkenaz K2RMA
 
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