On Sunday, 19 August 2018 16:17:51 UTC+1, Cursitor Doom wrote:
Gentlemen,

One of the drawbacks of attempting to fix vintage stuff is the expected
voltage readings given in the service manuals of the day. The manuals
usually state that the readings given were measured with analogue VMs of
a certain ohms-per-volt rating - most commonly IME 20k. Consequently if
you measure with a modern DVM with stupendously high Zin you're screwed
and will get unrealistically high values. That's never worried me as I
keep a vintage AVO for just such circs. All the British service manuals
seem to reference 20k OpV AVOs. However, I'm currently TS on a mid 70s Tek
scope the manual for which states the readings given are valid for a
meter with a Zin of between 100k and 200k (specifically a Triplett 630NS
see link).

Anyone come up with a solution to the problem of making voltage readings
on high impedance parts of a circuit with a meter of a different Zin to
that used by the people who wrote the service manual?

Never heard of an analogue meter with such a high Zin, but here it is:

https://tinyurl.com/ycjz9l4o

100-200k is 5-10v scale on a 20k/V meter. Or use a digital & add your R.
High R meters give a more realistic reading than old analogues on high R circuitry.


NT