6A8G.
Society Members
Hard working kiwi soundmixer.
Posts: 69
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Post by 6A8G. on Nov 7, 2021 12:07:36 GMT 12
Bit of a luxury item this one... the AV-2 is an AC-only meter. It's very sensitive too, the ranges starting at 01.v, 0.3v, .1v, .3v, 1v, 3v, 10v, 30v, 100v & 300v RMS. I know the average voltmeter will cover most of this but it was surprisingly cheap... so I got it!
Restoration went well enough - I stuck to the Heathkit layout & replaced like with like. At the big switch on the meter needle was jittery. Checking the output of the voltage amplifier shows it's motorboating. I'm completely at a loss to explain this. When I disconnect the capacitor (2 uF) which feeds the meter diodes the motorboating stops.
Does anyone have any brilliant ideas as to what's causing this please? Yours, confused, John
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Post by elorens on Nov 11, 2021 17:58:36 GMT 12
Hi John, motorboating is usually caused by low-frequency feedback along an inadequately decoupled HT supply line. Not a guaranteed solution, but one thing you could try is to replace the two 20 uF caps with new ones. Good luck!
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6A8G.
Society Members
Hard working kiwi soundmixer.
Posts: 69
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Post by 6A8G. on Nov 21, 2021 10:54:27 GMT 12
I did just that - no difference:(
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
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Post by peter on Nov 23, 2021 2:52:26 GMT 12
Righto - the thought I have is that the rectified mains is leaking slightly through either the 2uf cap just before the meter or the 1uf on the other side of the meter (on grid 6 of the 6AU6). So my first thought would be to replace those - the O.002uf feeding to the 6AT6 grid is probably ceramic or plastic so OK but if it's wax paper it could be leaky - if you have a scope you could look for ripple on the rectified mains at the 10k resistor by the mail filter caps (and check it's not gone low). Next I'd check the 10meg resistor at the 6AT6 to see it it's gone high or low then the other resistors around that valve - the 47k 470k and the 220k.
I must admit looking at the power supply it's fairly crude with half wave rectification followed by a pi smoothing filter which is going to have a bit of ripple on it regardless, but the caps by the meter should sort/stop that. If those steps don't cure the problem, then it's the 6AT6 valve playing up or the meter diodes.
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