How to you measure the voltage

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atomic.dave
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How to you measure the voltage

Post by atomic.dave »

I want to figure where to measure the voltage that the HV is putting out with a meter. Do you put postitive from the meter on TP2, and the negative on the negative of where it connects to the tube? or what?
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mightyohm
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Re: How to you measure the voltage

Post by mightyohm »

You will need a 1 GigaOhm resistor as described here:
viewtopic.php?f=15&t=487

Once you have the resistor in series with your DMM, you connect the resistor to TP2 and the DMM ground to the "-" battery terminal or pin 3 on the PULSE connector.
wbp
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Re: How to you measure the voltage

Post by wbp »

I purchased the suggested 1G resistor from Mouser. I've swapped R5 & R6 per another thread. Using a Fluke 117 (10M input) The LOWEST value I can get at TP2 is 5.00 Vdc, with the pot all the way to the left. With the pot all the way up I measure around 9.5 Vdc. Seems like something isn't working correctly in the HV circuit.

Any suggestions?

Thanks,
William
wbp
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Re: How to you measure the voltage

Post by wbp »

Changed R8 from 15 to 22 ohms. Now I can have the correct voltage range - 300 to 600.

William
wbp
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Re: How to you measure the voltage

Post by wbp »

I adjusted the pot for 400V (measured 4.00 V with the 1G resistor using a Fluke 289, 10M input). When I measure TP2 directly, I get 185 volts. This might be useful for those who have a similar meter but no 1G resistor.

William
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mightyohm
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Re: How to you measure the voltage

Post by mightyohm »

wbp wrote:Changed R8 from 15 to 22 ohms. Now I can have the correct voltage range - 300 to 600.

William
Hmm, that is quite strange. Are you using fresh alkaline batteries?
wbp
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Re: How to you measure the voltage

Post by wbp »

Yes, fresh alkaline. Tried 2 different sets to be sure. Wouldn't low battery make the voltage lower? Original 15 ohm r8 measured right at 14.9.
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Re: How to you measure the voltage

Post by mightyohm »

The HV supply increases voltage as the battery voltage drops. That's why it's a good idea to set the voltage to the lower end of the tube operating voltage (350-400V) with fresh batteries installed.

It sounds like you fixed the problem, but if you want to poke around more:

Check that all of the resistors in the circuit match the bill of materials (except for the swapped R6/R5).

Make sure Q1 and Q2 aren't swapped. Same for D1/D2.
Check to make sure L1 says "103" on top.
Check that C3 is marked 102 (and isn't swapped with C2).
Make sure your DMM is actually 10M impedance in the voltage range you're using (often the lowest voltage range will be different, for example).

It's normal for there to be some variation in the HV output from kit to kit, that's why there is an adjustment. But this is the first time I've heard of the need to replace R8. I'll look into this more.
wbp
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Re: How to you measure the voltage

Post by wbp »

I just went over it again. Checked Q1, Q2, D1, D2, L1, C3 is the smallest of the caps. Except for R5/R6 it looks just like the image on Flickr.

I have two Fluke meters, a 117 and a 289. Specs for both give 10M Ohm input impedance for DC V. I will try putting the 289 in manual range and see if that makes any difference. I also have a Fluke HV probe but it is 75M ohm, which is probably not really high enough to not pull the circuit down.

Wondering what effect swapping R5/R6 has on this?

Wonderful kit, by the way, really enjoying it. Had an old smoke detector, removed the pill from that to use for testing...

William
wbp
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Re: How to you measure the voltage

Post by wbp »

Interesting. Put both meters into manual range. On the lowest range, I get 4.0 V. On any of the higher ranges, I get 3.75 V. So there IS a change in input impedance in the lowest range, but if I do the math right, it's a 6% increase, not a decrease. Interesting that both meters exhibit the same behavior. No way of knowing which range(s) are actually 10M at this point.

Still, even with this, with the original value of 15 ohms for R8, the lowest I could get was either 5.0 V or 4.7 V - even allowing for the difference in the way the meter reads that's still too high.

I have submitted a request for clarification on this to Fluke.

William
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