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Author Topic: testing transformers  (Read 4942 times)
G3RZP
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Posts: 4325




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« Reply #15 on: October 07, 2012, 09:17:45 AM »

>A ham should have a VOM and a dummy antenna at the very least.<

Yes.

How much other test gear depends on what you are doing, but an SWR /power meter and something for monitoring your signal quality is useful, too - I use a WW2 BC221.

The DVM has some problems. Firstly, I haven't met one that didn't go somewhat, if not completely, mad with RF around.  Secondly, because of the high input impedance, capacitive effects can suggest a leakage of AC where there isn't one. Thirdly, they all seem to put some noise out on the input terminals, although it's not usually a problem. While adjusting something for a peak or a null, you need one with a bargraph display - or an analogue meter.

There is a problem that because it gives a 4 or however many digit readout, people believe all the digits...Plus when was it last calibrated?
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OLDSWAB
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« Reply #16 on: October 08, 2012, 05:01:58 PM »

The last time I had a problem in a scope it was the transformer. However do you have the print for your scope? If so remove the high voltage tubes and see what voltage you have going to the high voltage tube according to the schemetic with a meter able to go that high. If you blow a fuse you need not go any futher bad transformer. Next insert the high voltage tubes and attach a meter to the output of that circuit. Now as mentioned earlier put a light bulb in series with the transformer on one side of the power line. Say a 40 watt bulb. Check the high voltage output. If it is working the voltage will be lower than on the schemetic because of the light bulb. I have had high voltage caps go bad and heat up internally and short out. I got a schemetic from     www.vintage-radio.info/heathkit,    and looks like the output of the 1V2 is around 1100 volts. Be careful with the meter you have not to burn it out. That is another reason to use a reduced input voltage for testing. The two capacitors on that output look to be a .1 or a .2 mfd at 1600 and 1200 volts respectively.They can go bad.   Good hunting and be careful. KC9TRV
« Last Edit: October 08, 2012, 05:28:00 PM by OLDSWAB » Logged
K8AXW
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« Reply #17 on: October 08, 2012, 08:55:54 PM »

Peter:  You make very valid points on the DVM vs. the analog meter.  Another one is the ability to catch a glimpse of a cap charging with an analog meter which you can't with the DVM. 

Almost everyone thinks "digital" these days and fail to appreciate the unique capabilities of the analog meters.  I use the DVM most of the time but with the understanding of the limitations or shortcomings it presents.

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N0SOY
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« Reply #18 on: October 10, 2012, 06:57:28 PM »

I think I blew up one of my DVM.  I am not sure but it is going simply Bat S.  I am going to dig out one of my old VTVM  Simply because they are less sensitive.  The HV caps are suspected as well as the transformer.  I do have a Cap Bridge so it is easy enough to test those (most likely going to recap the scope anyway.  )

Thanks for your ideas (I like the light bulb idea)

Any other ideas are always welcome

Best Regards

David
N0soy
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KA4POL
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« Reply #19 on: October 10, 2012, 10:17:35 PM »

I think I blew up one of my DVM.  I am not sure but it is going simply Bat S. 
That might be the battery being low.
Does the fuse still blow with the secondary side disconnected?
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N0SOY
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« Reply #20 on: October 11, 2012, 03:57:44 PM »

Battery is new. 
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KA4POL
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« Reply #21 on: October 11, 2012, 09:58:20 PM »

Is there a fuse in the DVM that could have blown?
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N0SOY
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« Reply #22 on: October 11, 2012, 10:01:37 PM »

I ran some more tests and I think the problem is the caps on the secondary side.  The Primary side showed  resistance and was stable using 2 meters.  When the suggested tubes were removed the fuse did not blow.  I suppect that it is two big paper filter caps and a particular hard to get multi segment electrolytic.  I am just going to recap the whole thing.

Thanks for the help

David

Btw I figured out how a Fubared my meter.  When I tested the amperage across the blown fuse I had the + line going to the wrong input.  Fortunately I go my $ out of it long ago, but I liked that meter.  
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KA4POL
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« Reply #23 on: October 12, 2012, 02:06:10 AM »

So you're lucky after all that it is not the transformer.
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G3RZP
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« Reply #24 on: October 12, 2012, 02:25:40 AM »

The DVM may be repairable. Mine had the protective spark gap arc over at 650 volts when it is supposed to be 750 and took out a 100 ohm 1/8 watt resistor. Replacing that restored operation. I did take the opportunity to check the calibration, and it was still spot on. Worth having a look.
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