Marconi VTVM repair

[mansaxel]

Hi,

(this is my first post here, so bear with me, ladies and gentlemen....)

I've recently gotten my fingers on a Marconi ex-MOD VTVM, the TF1041b. I've found the manual, and started to undo the shipping damage. (No, contrary to popular belief, a sturdy plastic bag and some bubble wrap across the front is not appropriate packaging to ship a VTVM.  )

 The manual is located.

I'm trying to verify it and restore working condition. My aim is to have a working instrument that helps me in the workshop. Yes, I have modern meters. And 2 scopes. And more than one cup of T.E.A. Mechanical meters are NICE and ergonomic. Even if they aren't as precise as the Flukes and hp's, I like them and they help me. So, working it is, and it's more important than "vintage" or "pristine". A full re-cap is planned.

I have three questions, so far:

* The component list (proudly, I might add) lists a bunch of semiconductors, of which some are selenium rectifiers.   Those will be replaced with 1N4007 and a resistor; if need arises. Some other places in the schematic, in the meter protection circuit (see attached image) have a "S.T.C Unistor Type MQ8/1" which is drawn as a diode. The STC databook I've found lists not that part number, but "Q6/1" which is a miniature selenium ("SenTerCel") rectifier. What to replace with, if need arises?

* I'm trying to go through the tests in the manual and am applying a 2V 1KHz sine wave to the AC probe. Regardless of voltage setting on the sensitivity rotary switch, applying the voltage pegs the meter. What should I look for?

No more questions. I had one more from the start, but found enough old datasheets to establish the thermistor values I might need. Doing your homework helps.

/Måns.

[mansaxel]

So, I once more prove to myself that I have lots to learn.  I had my scope connected in parallel to the probe of the VTVM. And a strange 10KHz source.  Once I replaced the 10KHz source with the oscillator from my Tek distorsion analyser (which also needs repair, but not the oscillator, it is the analyser part that is weird) and only looked at the waveform with my Scopemeter (which is floating) the VTVM is accurate to within 0,4% to both ScopeMeter and hp 3438a (and they agree to within 0,2%). The clue here is "floating", I think.

Did a brief test of DC volts and we're good there too. 13,6V is 13.6V.

Tried to volt-nut myself and connect every meter I've got in parallel, but once I got to the Avo 8 it loaded the source causing a voltage drop of ~ 0,1V and did not cope very well with 10KHz. Came to my senses. Obvious conclusion: I need more high-impedance broad bandwidth meters. And a second Avo 8 to check the first.

Tomorrow: Assembly and resistance tests.