AVO MARK IV backing off high sensitivity - UK Vintage Radio Repair and Restoration Discussion Forum

bud allen · 24th Jan 2019, 9:49 pm

Now rebuilt and calibrated and all is measuring to spec and functions up to the backing off to read the mA/V or gm, which is very jumpy, wavers a bit up and down slowly.

In returning to the last position to fine adjusting the backing off, 4 of 5 times it has drifted up or down from zero, by more than a division. Advancing to read mA/V again, it does the same thing again. I do not detect loss of continuity. New tubes are bit less prone to this effect.

I have cleaned the course pot, from the rear, with electronic cleaner, rotated the wiper, and applied a bit of Progold to the wiper. Does not seem to have made much difference. The selector has also been cleaned, no Progold applied. New diode installed.

I am about to clean the fine pot, I just don't think it is the problem, but will do it anyway. Could the wiper be worn flatish so that it touches the adjacent windings just by vibration?

Has anyone suffered this situation? Is it just part of the beast? Normal?

I am wondering if the two pots need replacing, however the AVO does not look all that used, less the octal has been replaced.

I am monitoring the line mains for jumpy AC, and not seeing much more than a few 10th of a volts change.

Best greetings
bud

bud allen · 25th Jan 2019, 9:41 pm

After setting up a variety of meters to monitor incoming voltages, and the AVO anode voltage and current using Fluke 83 mark III, and Mark 5, plus two other meters, The problem is in the AVO, all the voltages and current are steady. I thought it might be our dirty electricity which I have monitored before as varying upward to 3 and 4 percent from time to time.

The problem is the backing-off pots. As soon a the knob is pulled forward and paperboard slipped behind, the readings are 95% to 100% stable. So it seems the spring tension on the wiper has relaxed and barely making contact with the winding.

I am going to remove the knobs, open the C clip that holds the shaft to the pot and slip it out the back. On this one the wiper simply rotates on contact ring. If the arc of the wiper is still good, and no valley has been carved into it, then I shall try to give some tension to it and replace it.

Hope this helps anyone who may have this issus on an AVO.

Dekatron · 26th Jan 2019, 6:47 pm

Thanks for sharing what you found out!

I usually replace the backing off potentiometers whenever I can, the original type of potentiometers are available from auction sites as well as from surplus sellers. To make the process easier for me, I keep the original shaft and move the slider assembly from the new shaft to the old shaft whenever possible, sometimes by soldering the new slider assembly onto the old.

I've also drilled holes in the new shafts with the help of a steel tube with a pre-drilled hole in the correct position using it as a guide to get the hole in the correct position in the center of the shaft.

/Martin

24th Jan 2019, 9:49 pm	#1
bud allen Triode Join Date: Nov 2015 Location: Key West, Florida, USA. Posts: 36	AVO MARK IV backing off high sensitivity Now rebuilt and calibrated and all is measuring to spec and functions up to the backing off to read the mA/V or gm, which is very jumpy, wavers a bit up and down slowly. In returning to the last position to fine adjusting the backing off, 4 of 5 times it has drifted up or down from zero, by more than a division. Advancing to read mA/V again, it does the same thing again. I do not detect loss of continuity. New tubes are bit less prone to this effect. I have cleaned the course pot, from the rear, with electronic cleaner, rotated the wiper, and applied a bit of Progold to the wiper. Does not seem to have made much difference. The selector has also been cleaned, no Progold applied. New diode installed. I am about to clean the fine pot, I just don't think it is the problem, but will do it anyway. Could the wiper be worn flatish so that it touches the adjacent windings just by vibration? Has anyone suffered this situation? Is it just part of the beast? Normal? I am wondering if the two pots need replacing, however the AVO does not look all that used, less the octal has been replaced. I am monitoring the line mains for jumpy AC, and not seeing much more than a few 10th of a volts change. Best greetings bud

25th Jan 2019, 9:41 pm	#2
bud allen Triode Join Date: Nov 2015 Location: Key West, Florida, USA. Posts: 36	Re: AVO MARK IV backing off high sensitivity After setting up a variety of meters to monitor incoming voltages, and the AVO anode voltage and current using Fluke 83 mark III, and Mark 5, plus two other meters, The problem is in the AVO, all the voltages and current are steady. I thought it might be our dirty electricity which I have monitored before as varying upward to 3 and 4 percent from time to time. The problem is the backing-off pots. As soon a the knob is pulled forward and paperboard slipped behind, the readings are 95% to 100% stable. So it seems the spring tension on the wiper has relaxed and barely making contact with the winding. I am going to remove the knobs, open the C clip that holds the shaft to the pot and slip it out the back. On this one the wiper simply rotates on contact ring. If the arc of the wiper is still good, and no valley has been carved into it, then I shall try to give some tension to it and replace it. Hope this helps anyone who may have this issus on an AVO.

26th Jan 2019, 6:47 pm	#3
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO MARK IV backing off high sensitivity Thanks for sharing what you found out! I usually replace the backing off potentiometers whenever I can, the original type of potentiometers are available from auction sites as well as from surplus sellers. To make the process easier for me, I keep the original shaft and move the slider assembly from the new shaft to the old shaft whenever possible, sometimes by soldering the new slider assembly onto the old. I've also drilled holes in the new shafts with the help of a steel tube with a pre-drilled hole in the correct position using it as a guide to get the hole in the correct position in the center of the shaft. /Martin __________________ Martin, Sweden