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Sanjib.A
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CEO
Re: More on the Dip Meter...
Sanjib.A   11/30/2013 12:15:33 PM
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Cool video!! Are these instruments still in production? If I can find I would try to grab one and would like to experiment for debugging EMI/EMC issues.

ANON1249426187345
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Rookie
"Display"? That's what we call a meter, sonny..
ANON1249426187345   11/29/2013 1:28:24 PM
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Now you're making me feel old.  What you call a "display" is a "meter", which is short for "galvanometer", but I don't think anyone called them that after the dawn of radio.  Permanent magnet and a coil of very fine wire.  Current through the wire makes a magnetic field which moves the needle. 

The GDO's meter has only a single scale, but meters used in VOMs and VTVMs often had several scales for various ranges.  You had to be sure you were looking that the correct one.  But we were used to that from negotiating the multiple scales of our slide rules...  Some meters (such as my 1970-vintage Olson VOM) had a strip of mirror on the surface with the scales, so that you could line up the needle with its image and avoid parallax errors in your reading.

Regarding the "gigantic" capacitor - that one's actually pretty small (as implied by the "mighty midget" name) - only 15 uF and 150 volts.  Capacitors in the TVs I took apart in my youth often had electrolytics with voltage ratings up to 450 or more.  Often the "cans" were 1.5 to 2 inches in diamaeter and 3 or 4 inches long and contained three or four capacitors.

Note the capacitance is given "MF", using "M" for "micro" rather than the Greek letter mu.  Small capacitors were specified in "mmf", or "micro microfards", rather than todays picofarads.  Instead of nanofarads, you had values like "0.001 MF".  Luckily, there was plenty of room on the parts to print this sort of thing.

zeeglen
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Re: My cousin had one!
zeeglen   11/29/2013 10:44:03 AM
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>Caleb " I think I actually paid $20!"


Love these old Heathkit teardowns!  One minor correction, the bottom slide switch is actually the diode/osc mode select; the power switch is on the sensitivity control.  In diode mode the B+ to the tube plate (anode) is turned off, then the unit could be used as a wavemeter or AM demodulator into the headphones due to rectification between the grid and cathode.


I bought a used successor the GD-1B from a fellow ham while in high school.  Think I only paid $15.  Still have it, and will email you a photo of the plug-in coil set.


grg9999
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Re: My cousin had one!
grg9999   11/28/2013 12:05:02 PM
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I had one of these years back.   Very useful gadget.  It's basically an RF oscillator with a grid current meter.  So you can use it as a signal generator to test out radios and the like.  And since it has a grid-current meter, you can put it next to a L/C circuit and you'll get a "Dip" in the grid current at resonance, so you can verify the working and frequency and approximate "Q" of a tuned  circuit.  very useful, as old radios had a lot of tuned circuits.  If you dial back the knob until it just stops oscillating, it becomes a tuned RF voltmeter, very useful in debugging and tuning up live oscillator and transmitter stages.    All of the frequency and voltage and stability aspects were very approximate and not going to put Boonton or HP out of business, but good enough for many purposes.    Heath made a smaller, newer one years later using transistors and tunnel diodes, but they were not as good as this old tube-based one.   You just could not get as good a "dip" with those low-impedance transistors and tunel diodes.

 

 

Susan Rambo
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Blogger
Re: Grid dip meter
Susan Rambo   11/28/2013 11:41:58 AM
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Very nice, @Jonathan Allen. Would you paste some images of it in the comments so we can see your Eico 710? Thanks!

Michael.Thomas_#5
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Rookie
Re: Heathkits!
Michael.Thomas_#5   11/28/2013 4:54:26 AM
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Early PCBs made with paper and phenolic or glue broke down under high voltage. No arcing or leakage over 1" of air.

Some military designs used a half-way system of epoxy boards with solder wrap posts. Made flying components a bit more controlled.

Cremo0
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Frequency range
Cremo0   11/28/2013 3:53:50 AM
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The frequency range covers 2MHz to 250MHZ

Bert22306
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CEO
Re: Heathkits!
Bert22306   11/27/2013 11:41:22 PM
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I too noticed all those "flying" components. That's the way most tube circuits we designed, I'd say until the 1960s perhaps. That's when you started seeing PCBs, even in some tubed electronics (miniature tubes mostly).

I bought a Knight Kit VOM way back Freshman year in college, and am still using it to this day. No tubes. Just passive components hanginmg out in mid air, as it were.

David Ashton
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Re: My cousin had one!
David Ashton   11/27/2013 9:57:07 PM
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Pity you did not get the coils Caleb, I daresay it would work still.....

I have a friend who is a "ham" and he wanted me to help him make a GDO.  I found a nice circuit with a FET that does from the 100's of KHz up to the mid 100's of MHz.     If it works, maybe I should get into the kit business.....

David Ashton
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Re: Heathkits!
David Ashton   11/27/2013 9:49:28 PM
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@robotgrrl....    "Noticed the 'mighty' capacitor in your GD-1A. .... Why were capacitors so big back then?"

They have decreased a bit in size, especially high voltage electrolytics....think it;s something to do with the etching of the foil plates - and thinner foil.

But smaller is not always better - as I once found out:

http://www.eetimes.com/author.asp?section_id=36&doc_id=1284545

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