AVO valve tester meter servicing - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

HBWOODY · 25th May 2020, 6:39 pm

Martin I should have mentioned that the original MK3 meter (no swamp resistor)is dead (coil is shorted). The meter referred to in my more recent posts is a meter from a CT160 which is in need of a neg grid pot. I was determined to get the meter back to specification to enable correct calibration of the Mk3.
The original 10k shunt measured 43K so is about to be replaced with a new 10K resistor.
I now have a new problem to sort out. When I switch on the MK3 the needle on the meter does not rise to the SET position in fact it does not rise at all. I checked the heater, anode volts which are good but when moving on to set the neg grid volts I do not get the expected volts so I tomorrow I will have to check out that part of the circuit.

Dekatron · 25th May 2020, 6:48 pm

Thanks, that explains a lot. Had me confused for a while there!

Station X · 26th May 2020, 7:40 am

Quote:

Originally Posted by HBWOODY

Martin I should have mentioned that the original MK3 meter (no swamp resistor)is dead (coil is shorted). The meter referred to in my more recent posts is a meter from a CT160 which is in need of a neg grid pot. I was determined to get the meter back to specification to enable correct calibration of the Mk3.

To be clear, this thread is about an ex CT160 meter, with a swamp resistor which is to be used in a VCM MK3??

Quote:

Originally Posted by HBWOODY

The original 10k shunt measured 43k so is about to be replaced with a new 10K resistor.

That's an awfully big change in value for a 10k resistor. I would suggest testing the meter disconnected from the tester with a new 10k resistor wired directly across the series combination of the movement (2269R) swamp resistor (675R) and correction resistor (306R) a total of 3250R, to make sure it has an FSD of 39.75uA. That way the meter can be given a clean bill of health and you'll know that any tester faults aren't down to the meter.

Quote:

I now have a new problem to sort out. When I switch on the MK3 the needle on the meter does not rise to the SET position in fact it does not rise at all. I checked the heater, anode volts which are good but when moving on to set the neg grid volts I do not get the expected volts so I tomorrow I will have to check out that part of the circuit.

Please discuss this in another thread, it's confusing the issue.

HBWOODY · 26th May 2020, 9:58 am

Graham have done what you suggested and get FSD at 40uA, is that close enough? I will start a new thread about the fault which has turned up.

Station X · 26th May 2020, 10:01 am

Yes. 40uA is good enough to proceed with testing.

25th May 2020, 6:39 pm	#41
HBWOODY Pentode Join Date: Mar 2020 Location: Bodmin, Cornwall, UK. Posts: 212	Re: AVO valve tester meter servicing Martin I should have mentioned that the original MK3 meter (no swamp resistor)is dead (coil is shorted). The meter referred to in my more recent posts is a meter from a CT160 which is in need of a neg grid pot. I was determined to get the meter back to specification to enable correct calibration of the Mk3. The original 10k shunt measured 43K so is about to be replaced with a new 10K resistor. I now have a new problem to sort out. When I switch on the MK3 the needle on the meter does not rise to the SET position in fact it does not rise at all. I checked the heater, anode volts which are good but when moving on to set the neg grid volts I do not get the expected volts so I tomorrow I will have to check out that part of the circuit.

25th May 2020, 6:48 pm	#42
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO valve tester meter servicing Thanks, that explains a lot. Had me confused for a while there! __________________ Martin, Sweden

26th May 2020, 9:58 am	#44
HBWOODY Pentode Join Date: Mar 2020 Location: Bodmin, Cornwall, UK. Posts: 212	Re: AVO valve tester meter servicing Graham have done what you suggested and get FSD at 40uA, is that close enough? I will start a new thread about the fault which has turned up.

26th May 2020, 10:01 am	#45
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: AVO valve tester meter servicing Yes. 40uA is good enough to proceed with testing. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.