Heathkit AV-3U calibration

[Smith3535]

I am trying to calibrate my Heathkit AV-3U, all works fine it's just I can't get the 10R wirewound pot to move the pointer while on the 300 volt range and connected to the mains, as per instructions in the set up guide. What I've done so far is to have the pot out, opened it up cleaned put back together and reads perfect ohms on the DMM. Checked all voltages on all valves and everything is only a couple of volts out apart from one, that is pin 2 on the 12AT7 (SA) valve. That should be 38volt but is only 13 volts. I'm not sure if that is causing the calibration not to work, but I'm at a loss to find why its so low.
Any help will be very much welcomed

This is the schematic of the Heathkit AV-3U VTVM

[Skywave]

If I read the cct. diag. correctly, from pin 2 to pin 3 of valve SA there is a resistor of 22 MΩ: change it. The 0.0022 µF at pin 2 needs checking for leakage too. And while you are in at the circuitry, check values of the 100 kΩ to pin 6 & the 220 kΩ to pin 1. High ohmic value resistors have a habit of going high resistance.

Al.

[ronbryan]

Have you checked that the range switch wiper is making a good connection to the switch fixed contact on the 300V range? If the connection is open, the grid pin 7 of the first 12AT7 (SA) will float, causing low anode volts and low volts on SA pin 2. You could easily confirm the connection with the instrument switched off and an ohms measurement between SA pin 7 and ground, where it should read around 31 ohms.

Ron

[Smith3535]

I have checked the pin 7 to ground and it reads 31.8R. I should have mentioned about the 22M resistor on pin 2 it is in fact a 2.2M on the unit and on the full manual I downloaded but not sure which is right now it measures 2.5M so is a little higher. The 100k and 220k were slightly higher so changed them anyway but still so response from the 10R calibration w/pot
This is the full manual i downloaded which shows the 22M as a 2.2M....Oh! can't upload it the file is too big. Eeerrrmm ? OK I've scanned the schematic from the manual and uploaded that it's a bit blurry but the 2.2M is visible.

[PJL]

I think we need all voltages on the SA valve to work out what's wrong.

The manual is misleading as it shows pin2=38V and pin3=52V or a grid bias of -14V which would completely cut the 12AT7 valve off. The reason for this will be the loading of the meter.

What type of meter are you using?

PS: The 25uF decoupling capacitor on the 680 ohms will also be critical for calibration to work.

[ronbryan]

On the AV-3U circuits that I have, the grid resistor in question is 2.2Meg.

You should be aware of the loading effect of a meter when measuring voltages in a high impedance circuit, such as the triode with the 2.2M grid leak. You can make allowance for this, if you are aware of the meter resistance however.

Although there is no reason to associate a faulty crystal diode bridge with a specific range, such as 300V, have you checked the diodes for forward and reverse resistance out of circuit?

Ron

[Smith3535]

I did change the 25uF and the 680R with no difference on the calibration setting, but as far as what multimeter I am using, it is a cheapo DT810B from a market stall. I like it only because it's small so I tried my other meter a 26 year old Precision Gold DMM and on pin 2 it reads 43.2V instead of 14V on the cheapo meter that's amazing there is so much difference in the meter readings, but the calibration preset is still not moving the needle, well only about a 1mm at the very most, not really sure how much it should move.

The meter reading is while on the 300V range. While connected to the mains is showing 2.5 on the 3V scale and the manual says turn the 10R preset so to get 2.3 (230V) but that is still not happening.

[Smith3535]

I haven't checked the diodes yet I'll do that next,

[PJL]

We also need you to measure the voltages on the valves to help with diagnosis. Both SA and SB. Use the old meter for now as the input impedance is probably 10M. Pins 1, 2, 3, 6, 7, and 8.

PS: The calibration works by altering the degree of negative feedback.

[Smith3535]

These are the valve readings I've just taken on the Old 10MR DMM

[PJL]

The voltages look OK. Measure your mains voltage using the DMM then try calibrating to the same voltage.

[Skywave]

Can you just confirm for us that the fault is only present when the 300-v. range is selected? (And, therefore, that the fault is not present on any other range?) If those two checks are still valid, that result should narrow down the possibilities by quite a lot.

Al.

[PJL]

I would expect the meter deflection to be proportional to the value of the resistance from the wiper of the calibration control to ground. What is the resistance at either end of rotation? Does the deflection vary as much as the resistance changes? From your description it sounds as if this is not the case?

Is the problem that the meter is unable to deflect beyond a certain point on the dial?

[Smith3535]

That's just it i can't calibrate it, the DMM on the mains is about 240v and the vtvm is showing about 251v.
I don't suppose changing the 10R w/pot would help? I have opened the pot up and cleaned it and it reads perfectly on the ohmmeter.

[Station X]

Have you checked the diodes in the bridge?

[Smith3535]

The meter needle deflects perfectly and is unrestricted. The calibration pot only moves the needle about 1-1.5mm seems the waste of time to me and that's on V and mv ranges,

No I haven't checked the diodes yet. There's a large cap in the middle of the bridge across the meter as well, I presume that's some kind of smoothing?

[PJL]

Can you check the resistance from the wiper of the calibration pot to ground at both ends of the pot swing. It should go from 30-40 ohms.

The 2uF electrolytic capacitor between the meter and the 1K resistor is a possibility as leakage would result in DC current through the meter which would be unaffected by calibration.

The meter capacitance is also worth replacing although the problem you describe is too much deflection.

[Skywave]

Originally Posted by Smith3535

The calibration pot only moves the needle about 1-1.5mm.: seems the waste of time to me and that's on V and mV ranges.

Ah! So the fault is not restricted to the 300 v. range only - which was implied in your OP. O.K, we now have a clearer picture of the fault conditions, even if the scope for that fault is now a lot wider.

Al.

[Smith3535]

PJL you are a treasure it was the 2uf capacitor i changed that and now have about a 40v swing on the calibration pot. I think someone else mentioned that earlier on but its when PJL said the calibration is set by altering the degree of negative feedback then that could only be via the return on the right hand side.

The cap was a waxed Dubilier type 410 when i tested it on the ESR it was showing as good if less than 200uf and on the cap meter read about 3.5uf,

Brilliant! thanks everyone

[Skywave]

Good to hear that it's eventually fixed: persistence paid off.

And the moral of that story is when faced with a bit of vintage gear which malfunctions, start by checking capacitors whose values are, say, from 25 µF and up - such as cathode by-pass caps. and . . . err . . . PSU smoothing caps. But of course, we all know that, don't we? Ho hum.

Al.