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

Bump

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

Yes, awesome !

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

Alfredo

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

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.