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 Post subject: GenRad Impedance Bridge hook up
PostPosted: Jul Mon 15, 2013 8:04 pm 
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Can the audio output of a signal generator drive the external generator function of this General Radio impedance bridge (model 650-a) and enable measurement of coil inductance? Also does the input frequency affect results?
I have verified all resistors and changed out caps c1 and c2. The bridge measures resistors on DC with good accuracy. When using the above set up for capacitor measurement all seems ok but I am not interested in using this instrument except for inductance and can’t seem to achieve null.
The internal hummer on this bridge is not working so with switch set to ext. on/ internal off, I hooked up a pair of amplified computer speakers to the external detector posts rather than earphones.
I know some of you are saying…..why put yourself through this with this old instrument but in defense, it’s just interesting and I want to understand how it worked back in the day. Also it makes for a good workout moving this thing around the bench. :(


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Mon 15, 2013 8:43 pm 
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Location: Long Island NY
The GR 650A is still a perfectly good and viable bridge. While it may eat up more space and not be as simple or convenient as a modern "digital wonder," in the hands of an experienced user, it can reveal more about components than a meter does.

You can easily drive it with an audio oscillator; in fact, GR made an AC oscillator/power supply (model 650-P1) that fits into the battery compartment of the 650A. It provides DC and has a one-tube, 1-kHz audio oscillator to power the bridge. It also contains an amplifier which may be used to extend the sensitivity of the headphones much the same as you are doing with your amplified speaker.

As for running the bridge on other frequencies, yes you can, but accuracy begins to suffer at higher frequencies due to the capacitances and inductances of the components and wiring in the unit. The manual gives the accuracy de-rating to apply for frequencies beyond 1-kHz.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Mon 15, 2013 11:00 pm 
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Though not popular these days, and can be had at give-away prices, the 650 is still a very capable bridge. What frequency do you want to measure at? What's the estimated value of the inductor? Also, I can't remember if the generator or detector needs to be floating or if they can have a common ground. The detector of choice back in the day was a pair of high impedance headphones, not the low impedance ones you find today. I still have a 650 with the AC module.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Mon 15, 2013 11:46 pm 
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I have one and a parts one upstairs. If you used a scope as detector, you could
use a battery oscillator, like the plastic Heathkit model.

The hummer needs a battery.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Tue 16, 2013 12:28 am 
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Thanks for the replies.
As far as the frequency that I will be measuring at.....based on Chris's info I am going to haul out a Eico audio generator as it will provide better signal that a signal generator but it will only go to 20hz.
I have a bunch of various small coils to measure and they are marked with previous owners readings. Mostly 20-150uh.
I did hook up 4 C cell batteries for the DC supply but the hummer did not respond. I partially disassembled and verified continuity coming off transformer but no dice.
Anyway I will continue to experiment with getting null on inductors. I did read in the manual that they may need to be a distance away from the tester in certain circumstances but don't think that is the issue.
As always.....thanks for the feedback.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Tue 16, 2013 12:48 am 
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As I remember, 10kHz is not much of a stretch, but beyond that you'll need some correction factors. A few microhenries may be difficult to resolve at such a low frequency, and the detector output may drop off.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Tue 16, 2013 12:03 pm 
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Thanks Alan. Guess I will have to look for different method for oscillator. Has anyone substituted a Piezo tone alarm for the buzzer in this bridge? Might work.
Guess I will try an pick up the P1 add on oscillator that GenRad made for this bridge.
Appreciate the info.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Tue 16, 2013 9:01 pm 
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There might be sealed reed oscillators around. Piezo might work if you can get the
voltage off the sounder and it is sin wave.

This 8 kHz one runs on 5 volts and was salvaged of an old circuit board.

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frequency standard 8 kHz.jpg
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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Wed 17, 2013 1:16 am 
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OK, it's been a while since I used my 650, so I dragged it off the shelf. First, I don't think the 650 contains a transformer so you can't just hook a non-isolated sig-gen to it. Even if all you have is a filament transformer, use that between the sig-gen and the bridge. An actual bridge transformer would be the bomb but those are very hard to find these days. Maybe some audio coupling tranny would be the next best thing. IMO, the best solution is to fix the hummer.

Next, you'll be looking at very small signals at null, sub millivolt. A scope is just adequate but a GR null detector is better because it has more gain and filters out the noise. An HP AC voltmeter is also pretty good. Ground to the G terminal of the pair. IMO, the bridge does fine at 1 kHz and little is gained by going higher. 20 kHz is the max; above that errors increase rapidly.

Now, here's the bad news. For small RF inductors of about 100 uH and low Q at audio frequencies, the bridge exhibits the dreaded sliding balance. That means you can get a pretty good balance at an infinite number of places across the dial. I think the manual suggests how to deal with this, but I didn't have much luck. The null, however, was plenty deep with the isolation transformer between the bridge and generator. Without it no measurements could be made at all. I think if one were using a more sensitive detector it might be possible to tell one sliding null from another, but not on my 5 mV/div scope.

I need to go back and play some more but one key is to have a marked inductor that you have some confidence in. When you can measure that accurately, then you can have confidence in other measurements.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Wed 17, 2013 2:09 am 
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The Heathkit vacuum tube bridge, which was the one they offered following the hummer
bridge has a bridge transformer wound with a large space between windings. A junker
IB 2 might be a good source.

Anything in the low pF or uH range really needs a Q meter.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Wed 17, 2013 11:24 am 
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Steve / Conrad....thanks for the additional info. I did play around with a 1khz external signal yesterday while monitoring proper input voltage per the manual. But as you said Conrad, had sliding nulls and not deep enough to be confident in readings.
So this additional info might help.
The internal genrad hummer looks pretty simple but there does not appear to be any info on repairing this device.
Will be going to Estes auction Saturday and maybe I can find a junker heathkit for that bridge transformer.
The other thing that is confusing me is the relationship of the DQ dial vs CLR dial reading. The manual states to only use
CLR reading multiplied by CLR switch setting, disregarding the DQ setting. Am I understanding this correctly?
Scott

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Wed 17, 2013 7:41 pm 
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DQ is for measuring dissipation of a capacitor or Q of a coil. The dial needs to be set to get a good null, but you don't need to know what value it's reading.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Wed 17, 2013 8:15 pm 
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Adding, for RF coils measured at audio frequencies the Q will be very low, like 0.1 or so, even though they will have much higher Q at the frequency they were designed to operate at. Thus the Dairy Queen knob will usually be set near zero.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Thu 18, 2013 12:06 am 
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Have you tried the hummer in your bridge with a power supply heavier than 4 D-cells ? It was intended to operate with 4 #6 cells, which can supply considerably more current. The hummer is a simple buzzer device with an adjustable contact screw.

--Chuck


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Thu 18, 2013 12:15 am 
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Had some time to play a bit more. Sometimes when using the old stuff it pays to do it just the way they did it back then. I fixed my reluctant hummer- there's a screw that locks the spring loaded contact with the knurled ring. It just needed to find its own position and the hummer fired right up. These things used 4 #6 dry cells as said above, so use that (ha!) or a 6 V cell, or any other power supply. Finally, use some headphones, not a meter. Mine aren't very sensitive but even those worked OK.

You can't walk the DQ and value pots to get to the minimum. Set the DQ pot to something low, say 0.2 and then find the best null with the big value pot. You'll hear a bit of tone at the null. Move the DQ pot slightly one way or the other. Find the null with the value pot again. If the tone at null is louder, go the other way with the DQ pot. It's very sensitive, but you should find a DQ setting where you get a dead silent null as you sweep through it with the value pot. It was pretty accurate with the 90 uH low Q inductor I tried.

I have a lot of stuff in my lab, including other bridges and null detectors. No doubt one could extract more from the 650 by adding other sources and detectors, but using it the old fashioned way, just as they intended, gives a very respectable result.


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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Thu 18, 2013 11:35 am 
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After reading the last two entries I increased the voltage slightly and also turned the knurled knob of the hummer while adjusting the adjustment screw on top of the mounting block. It is very touchy but after some trial and error I have a pretty decent hum. Things are looking up. Will be experimenting with null today.
Probably a dumb question but does the meter null indicator come in to play at all when measuring inductors or is it strictly the use of the hummer null?
Thanks for the help.
Conrad, thanks a lot for the detailed explanation on the DQ and CLR dials. Very helpful info.

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 Post subject: Re: GenRad Impedance Bridge hook up
PostPosted: Jul Thu 18, 2013 12:13 pm 
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The meter is only for DC measurements. GR would have you use a set of high-impedance headphones as the null detector for the hummer. You need an amplifier to use modern headphones. Your computer speakers are probably adequate for that.

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