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Old 2nd January 2012, 12:22 AM   #1
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Default VTVM troubleshooting help

I acquired a Precise 909 VTVM and i am in the process of refurbishing it. I have not been able to find a schematic but i am following the concept from other similar units, like Eico and Knight. I changed all the paper caps and the electrolytic in the power supply.

I have two issues.

ac:

I can do the calibration adjustments of ac balance (6al5) and the ac cal using a 1 k signal. The problem is that the frequency response is not flat, it progressively drops with higher frequencies. I am interested in the audio range. The replacement caps around the 6al5 rectifier are .01uf.

Ohms:

I replaced the D battery with a common D alkaline. With the battery installed i cant adjust the ohms pot to full scale on the meter. I think all the metal in the battery must be inducing something in the circuit. If i take the battery off and connect an external 1.5v supply (to the same battery leads) it works fine.

Any ideas? resolving the frequency reponse problem is my primary target .

Alfredo
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Old 3rd January 2012, 02:56 PM   #2
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Old 3rd January 2012, 07:31 PM   #3
Jebem is offline Jebem  Portugal
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Vacuum Tube Voltmeter 909 Equipment Precise Electronics

Is this one?

Anyway, this link has got the PDF service guide...
http://www.afrudolph.de/precise/pr909us.pdf

Last edited by Jebem; 3rd January 2012 at 07:36 PM. Reason: found the schematics
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Old 3rd January 2012, 08:58 PM   #4
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Yes, awesome !

Thank you so much. Hopefully I can do a proper calibration

Alfredo
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Old 7th January 2012, 10:31 PM   #5
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I compared the circuit today and found a difference in the value of R9. Mine was 1.3M while the schematic calls for 820K. I changed it and there is an improvement in the frequency response. It is still not flat. I guess this VTVM was not designed for a flat response of the audio frequencies. I am getting -1.5db at 20Khz. My Simpson 260 is flat past 20K. Luckily I only spent $10, it may still have its uses for High Voltages.

I did notice that if I bypass R9 that I get a flat response past 20K, but by bypassing it I limit the AC usage to less that 100 volts as anything higher will damage the 6al5 rectifier.

Jebem, thanks again.

Alfredo
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Old 7th January 2012, 10:35 PM   #6
Jebem is offline Jebem  Portugal
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Well done, Alfredo. Old equipment seems to have one or another bad component, in particular resistors and capacitors, so it pays the time we spend checking for it.
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