AVO VCM MK1 calibration

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AVO VCM MK1 calibration

Postby Muzzwell » Fri Dec 08, 2017 8:48 pm

Hi, I wondered if someone could help. I am trying to calibrate a MK1 VCM using the instructions in the MK2 manual. These were written in Dutch and English and produced by Hein Ros who has since sadly passed away. I am a Brit living in Holland and happen to speak Dutch. In his Dutch version of the calibrating instructions he refers to the use of the 5 hole connector on the top of the unit. These are labelled on the machine as: G1 - S - A1 - A2 - D1. Frustratingly in his instructions he keeps on referring to 'C'. For example: Attach testmeter (Avometer 8) to the contacts A1 & C on the 5 pole connector. This section is to check the mains voltage. He later keeps referring to contact point C. Does anyone know what he means by 'C' ?

Also in the instructions to the MK2 unit there are extra adjustable resistors it seems that were not on the MK1 model. I have done my best to calibrate using both the Dutch and English versions. Of course there are many variables. Not least of which being that the machine seems to need to warm up properly first. I have some reference tubes EL84 that were tested on another machine. Best I have to go on. Would anyone happen to know of a MK1 calibration article, I have searched including through the EMER manuals.

Any help much appreciated. I am not expecting this machine to be a precision instrument.

Kind regards,

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Re: AVO VCM MK1 calibration

Postby Jac » Fri Jan 19, 2018 8:36 am

That is sad to hear about Hein Ros, I didn't know that. He made a lot of information available.

When he refers to "C", I expect he means the Cathode, and that is just the chassis ground of the tester.

For calibration of AVO, people always try not to prepare the tester and themself well, and just try to fiddle around with the calibration screws. That is not how it works.

There is the calibration procedure, yes. But before that comes a full check-up procedure. And full means not partially. Take the Mk4 manual as a reference, for how to generally check your tester first, if you don't have that information for the Mk1. So ALL transformer windings, etc, the whole mess, and leave nothing away. Extreme care needs to be taken here, since these machines are 70 years old, and it was of course never anticipated they would be used for so long.

Then in the check up procedure comes the chapter: "Check the panel meter". For this, take out the meter. And check it very precise. Check linearity, very precise, also in different positions, as the meter is upright under a angle. Often they have a deformed coils by overload. That will give unlinearity. Check stick-slip also. Don't try to "fix" the meter quickly when it's your first meter, you rather damage it instead. Let somebody do this, who knows how it's done.

Then make SURE, 100% SURE, the mains calibration works as it should. This makes no sense with a faultly meter inside.

Next, you need some calibated tubes. As a minimum a calibated set of 6SN7 and 7SL7 (Can be bought via the jacmusic website)

Only after all of that, comes calibration.

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