Adjustment HV for SBM20

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Adjustment HV for SBM20

Postby etoktar » Tue Nov 01, 2011 5:45 pm

Hello, I bought the kit and assembly it without problems. When I did the measure about HV to a SBM20, the min is 180V e max 330V, usign a Fluke 87 III model. I follow setp-bysetp and there is no error in assembly using the parts e visually is the same in the Geiger home page. I checked again and I cant see any worng.

When I turn the power switch on, I can hear a the piezo speaker and the led blink... I have a SBM20 and SI-3BG, but there is no difference in voltage changing the tubes. Removing the Geiger tube doesn't change the voltage on TP2. When it turn on, I got some beeps. I inspected the solder joints, and they all look okay.

If I let VR1 in middle position, the voltage is about 230V only. Could you help me and give some tips about the voltage on transistors to get the correct 400VDC HV DC? How is a new kit, it supposed to work fine.

Thanks in advance.

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Re: Adjustment HV for SBM20

Postby mightyohm » Wed Nov 02, 2011 8:58 pm

Removing the tube should not affect the voltage on TP2.

The impedance of your Fluke 87V is spec'd at 10 MegOhms. This is low enough to load down the output of the HVPS and significantly affect your measurement.

I use a 1 GigaOhm resistor in series with my DMM and calculate the actual voltage based on the ratio of impedance (approx 100:1 for the Fluke 87V).

There's a nice example of this technique here:

Mouser sells 1G resistors for a couple dollars, this is the one I use: ... 1%2f2-F-1G

Basically you put the 1 Gig resistor in series with the + lead of your DMM, and multiply the corresponding reading by 100. Make sure you're in the 60V or 600V range, the input impedance of the 87V isn't exactly 10 Meg in the 6V range.

Hope this helps.