AVO VCM163 - operator issues

[drewaudio]

Hi,

My Avo 163 was working fine until i unknowingly put a 6SN7 with a cathode short to the filament. The tester overload light lit up so i immediately turned if off.

Now I cannot get test results off a'. I note that a" is ok?

What any I missing here?

Please help!

Andrew

[Radio Wrangler]

The same tester circuitry is in use for both sections of a multiple section valve, with only switching for the anodes, cathodes and grids being different for the two sections.

So that suggests a damaged wire somewhere, or a damaged switch. If the failure wasn't a coincidence (and coincidences do happen sometimes) then i'd go looking for something in the cathode circuit which got burned out by heater current.

These testers were designed for just that, testing unknown valves. So they were designed around performing a series of tests in a planned sequence. Early tests weeded out problem valves where those problems were ones which could cause damage to the tester in later tests. So the sequence should always be started from the beginning, and all the test steps performed. Jumping in later on is risky. The risk is low, but at the hyped prices of those testers, it's a risk best avoided.

David

[drewaudio]

Thank you for the reply.

I'm still lost on this one! A' does not work on ma or Gm. But A" and A"' etc all work fine....



I'll explore further based on your suggestion.

Cheers

Andrew

[Dekatron]

Do have a look in the Sticky thread here : https://www.vintage-radio.net/forum/...ad.php?t=53598 for a correct schematic in post #7. Then also check R35, see post #6, on the mA/V switch while you have a good look at the Electrode Selector switch itself, you can see that the Electrode Selector switch sometimes also has problems with the contacts and the wiper in post #14. It is possible to replace the Electrode Selector switch, it is the only switch that is easy to replace in the whole tester!

I suggest that you use a DMM to check continuity from the valve socket down through the Circuit Selector switch to pushbutton switch S7 : mA/V - g2 and then down to switch S8 : mA/V (Cal., 0-6, 0-20, 0-60) checking each position.

You can also try to rotate the roller selector which selects the a' electrode a few turns to see if you get contact again, also check if you get contact in any other position with that switch.

Also check the cathode path which should pass through the roller selector and also the Circuit Selector switch.

Don't forget to check the links on the top of the valve panel as they sometimes get burned if they don't have good contact.

/Martin

[drewaudio]

Thanks Martin,

That's a some great insights! I'll let you know have I get on....


Kind Regards
Andrew

[Radio Wrangler]

During 'Test' the cathode goes to ground.

So does heater (-)

If you have fitted a valve with the other heater pin (Heater +) shorted to cathode, and selected 'Test' you have created a short circuit on the output of the heater transformer via the heater voltage selector switches, the electrode selector switch (cathode wafer and heater+ wafer) the roller selector switches, the valve socket and the valve itself. You'll have to work your way round the whole path looking for something that has burned out, or as Martin says, it may be a link.

Because one valve section works and not the other and because a heater short seems to be the cause, then I wonder about the connection from the valve holder cathode pins to the roller switch, and then to the point where the cathodes are commoned?

For this reason, you should always do the heater-cathode insulation test before moving on.

These testers can be damaged by unfortunate settings of the controls and some of the vulnerability to damage due to testing faulty valves is handled by only doing all the tests and in the planned order.

Avo didn't do much to add protection from wrong knob positions and they didn't really make the importance of the sequence of testing clear enough.

David

[Dekatron]

Can you see if both heaters glow?

Check if you get heater voltage on both triode halves. If one side doesn't glow you know which side to check for heater voltage, then it is probably that roller selector switch which needs to be checked, sometimes a few turns help sometimes not.

/Martin

[drewaudio]

Good news....all is working again!!

The valve that caused my trouble was a 6SN7.

I checked this morning and the valve heater circuit works. I think the issue was in the selector rollers as moving them around and back to the 6SN7 settings fixed everything (thanks to Martin for that suggestion above).

I really appreciate the helpful ideas to fix the tester...I really thought I had done some major damage.

I've learnt a lesson in operations and won't make the same mistake again.

Thanks everyone!

Kind Regards
Andrew

[Radio Wrangler]

Just remember that you CAN do major damage with wrong settings on controls, or if you jump in part way through with a damaged valve.

David

[Dekatron]

I'm happy to hear that it works again!

As David says, do the tests in the order of the switch to minimise faults cropping up in the valve under test.

Also let the valve sit for some time on the CHECK(H) step before advancing to the TEST position, that heats up the heater and some failures will be caught then and other failures at the test position will sometimes be caught faster in the test position meaning less stress on the tester. You can also let the valve sit in the c/h INS test for some time as other errors might be caught then too.

/Martin

[drewaudio]

Thanks for everyone's help!

PS I am missing one of the knobs, does anyone know where I could get a replacement??

Cheers
Andrew