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 Post subject: Thord Grid dip meter GDM-3
PostPosted: Apr Tue 07, 2020 8:55 pm 
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I have acquired a Thord grid dip meter GDM-3. It was made in Japan. Looking for an instruction manual and schematic. Thanks


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Tue 07, 2020 9:05 pm 
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Location: Sanford Fla 32771 (USA)
This might help, it has a picture of your meter. Stated it the same as Midland GDO.
https://www.qsl.net/n4xy/GDO_Midland_Th ... eries.html


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Tue 07, 2020 9:15 pm 
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I believe the brand name is Thoro.

http://www.n4xy.com/GDO_Midland_Thoro-Test_Series.html

I have a similar/possibly the same meter i asked about here looking for information,

viewtopic.php?f=8&t=214285

I didn't get any responses, so I hope someone can help out here :D

-Steve

EDIT: Sorry I didn't see Paul's comment before posting mine :oops:

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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Tue 07, 2020 9:23 pm 
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Thanks for the clues! after a little searching, I found the schematic at http://blogimages.seniorennet.be/on4btw ... /27946.pdf


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Tue 07, 2020 10:14 pm 
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wow, thank you!
I'll save that for my own :D

-Steve

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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 4:06 am 
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It would be nice to know what's going on with the coils! No info in the linked manual. Different ones must use different connections on the 7 pin socket.

Perhaps someone will trace them out.

And, how does the meter driver transistor get some base bias? The diode looks to be turned the wrong way. Surely they didn't just use a leaky transistor???

Ted


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 6:22 am 
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It would appear the transistor would get its bias from rectifying the signal from the oscillator, but I agree, the diode appears backwards to generate a negative going bias for the PNP transistor.

I could dig mine out and see if I could check it, and/or try to figure out the coils.
That is unless Eric gets it done first. :D

I'm not sure how quick I'll get to mine.

here is the only interior pic I have of it.

-Steve


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Last edited by azenithnut on Apr Wed 08, 2020 4:57 pm, edited 1 time in total.
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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 8:19 am 
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OK, since I could not sleep due to sever thunderstorms rolling through, I dug out the GDO and looked things over.

I'm not 100% sure on the coils, but this was the best I could come up with.
A, B, and C are mostly the same configuration with different resistor values.

It does appear the diode is shown backwards. The banded end goes to ground.

-Steve

EDIT: Interesting the manual does not show the coil ranges.

A 0.5 to 1.6 MC AM Broadcast
B 1.6 to 5 MC
C 5 to 15 MC
D 15 to 40 MC
E 40 to 90 MC
F 90 to 150 MC Barely makes it past 2m


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 5:49 pm 
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Thank you Steve for the coil diagrams. Those should help me troubleshoot my next problem. I found the A coil would not oscillate. The other coils worked. I then found by replacing the 9 volt battery with a bench supply and raising the voltage to 11 volts, the A coil started to oscillate and the meter functioned.

Steve, Does your A coil work with the 9 volt battery?


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 6:10 pm 
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Yes, I tried it with a pocket transistor radio I have handy, and I can hear the oscillator all across the band.

I tried dipping the antenna coil and it does work, but the dips are very shallow. Not the easiest to see.

I would confirm the 7 pin socket is making good connection on all contacts.
I had to scrape them a little to get good continuity readings on the coils.

-Steve

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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 8:14 pm 
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That particular GDO dates from the late 1960's and was made by a Japanese OEM that sold them to anyone they could and labeled them with the brand name of the buyer's USA company. The company where I worked ordered several thousand and they were brand labeled "Quement." We sold them with ads in the back pages of CQ and 73 Magazine when we were not advertising our SWR bridges and QSL card holders.

Originally there was one of those tiny folded paper sheets included with a schematic and brief instructions, I can't recall a manual as such. By the mid-1970's the Yen was unfavorable against the Dollar and we stopped buying anything like this. I am not sure if it was the same OEM but we also sold single and dual meter SWR bridges, which others were also importing, and they had other names such as Calrad, Swan, Midland, etc..

I have a complete or nearly complete set of coils for one of these GDO's around here somewhere if anyone gets one of these without them. It just may take a telephone psychic to remind me where they are...

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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Wed 08, 2020 8:49 pm 
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Ted
You are correct. The diode is shown backwards in the schematic. I had a few minutes so I confirmed the cathode is going to ground in the actual unit.
Eric


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Thu 09, 2020 5:51 am 
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Location: Lexington, KY USA
Thanks for all the information!

the A coil would not oscillate. The other coils worked. I then found by replacing the 9 volt battery with a bench supply and raising the voltage to 11 volts, the A coil started to oscillate and the meter functioned.

Be sure the pot is good and that you get the full battery voltage going to the oscillator when it is turned up all the way.

A quick fix would be to reduce the value of the resistor in the coil. This resistor controls the gain.

You might look for a resistor that's drifted too far.

A hotter transistor might make it go. Even a silicon one would probably work. Either or both of the oscillator transistors.

Ted


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Fri 10, 2020 10:28 pm 
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Here was our version:


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 Post subject: Re: Thord Grid dip meter GDM-3
PostPosted: Apr Sun 12, 2020 10:17 pm 
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Interesting that yours (Quement) and the Thoro-Test both used model number GDM-3 which probably was the OEM manufacturers model.

Midland used their own model number.

Thanks for sharing

-Steve

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