VTVM troubleshooting help

avincenty · 2nd January 2012, 12:22 AM

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

avincenty · 3rd January 2012, 02:56 PM

Bump

Jebem · 07:36 PM

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

avincenty · 3rd January 2012, 08:58 PM

Yes, awesome !

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

Alfredo

avincenty · 7th January 2012, 10:31 PM

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

Jebem · 7th January 2012, 10:35 PM

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.

2nd January 2012, 12:22 AM	#1
avincenty diyAudio Member Join Date: Jan 2008 Location: Southern California	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

3rd January 2012, 07:31 PM	#3
Jebem diyAudio Member Join Date: Dec 2011 Location: Lisbon	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

7th January 2012, 10:35 PM	#6
Jebem diyAudio Member Join Date: Dec 2011 Location: Lisbon	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. __________________ If You need something done properly, do it yourself (and ask help from DIYaudio... ) One of my SE 211 tube amps: http://sites.google.com/site/sunajeb...ificador-211se

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
VTVM substitute?	Firepowerforfreedom	Tubes / Valves	22	22nd March 2011 04:48 AM
diy Heathkit VTVM probe...	sklimek	Parts	11	2nd December 2010 11:11 PM
Battery Question vs VTVM	Alaska Bill	Equipment & Tools	5	27th November 2010 04:32 AM
HP 410B VTVM (refurbished and recalibrated)	n_maher	Swap Meet	3	23rd September 2008 05:23 PM
Is a VTVM useful?	d1camero	Tubes / Valves	10	31st January 2008 05:12 AM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

3rd January 2012, 02:56 PM	#2
avincenty diyAudio Member Join Date: Jan 2008 Location: Southern California	Bump

3rd January 2012, 08:58 PM	#4
avincenty diyAudio Member Join Date: Jan 2008 Location: Southern California	Yes, awesome ! Thank you so much. Hopefully I can do a proper calibration Alfredo

7th January 2012, 10:31 PM	#5
avincenty diyAudio Member Join Date: Jan 2008 Location: Southern California	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

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read