Ohm meter will not zero - Page 1
Forum section: electronics / projects

author: clay1905
author_rank: Regular Contributor
timestamp: January 28, 2020, 01:56:04 am
content: Hello folks,Of late I have been working towards the completion of a restoration project, an early V.T.V.M.Part of the reason behind this restoration is to get a better understanding of the workings of radio valves.OK, the meter seems to be pretty good on all functions except for Ohms.In this function the diode bridge is out of balance. Obtaining a movement zero with the probes shorted isn't possible. However, zeroing is quite possible in all other functions.From what I presently think I know, this imbalance boils down to the voltage difference on the grids of V3. In particular, the negative voltage on G1. At about 7.6 volts negative relative to its cathode, that section should be near cutoff.In any case, almost all of the components test very well. Marked resistors are all well within 10%. The capacitor is mica and isn't leaking. The wire wound resistors are the only ones I can't be certain of, but the two of 21K test very well.So the question is, what to do? There are a few places where I could make change, but I'm wary of them all. The meter must have worked at some time in the past with these components. Now it doesn't and I'm not too sure why.Increasing resistor value for R10 may get the drop required, but I"m concerned about pushing up the voltages on the grids of V2 as a consequence. It might make no difference, but I'm still a bit too green to make that call. Shorting R's 9 to the chassis gets a good response from the movement, but taking them out of circuit seems the wrong thing to do.I would like a little guidance here on determining the best course of action to get the bridge back into balance on the Ohms range. I don't want to be doctoring things, but rather finish a proper restoration where everything is as it was originally intended. And if possible, I'd like to be a little the wiser too.Clay.

author: trevatxtal
author_rank: Contributor
timestamp: January 28, 2020, 06:59:42 am
content: Sorry if this is a silly question!Have you checked the 3 volt battery ?If it is down you will never zero the scale!

author: clay1905
author_rank: Regular Contributor
timestamp: January 29, 2020, 01:41:01 am
content: Hi trevatxtal,Yes, the battery has charge. Worth revisiting to be sure though. Thanks for the prompting.In the mean time I've hit upon a snag. I disassembled the movement and chassis from the front panel in preparation for a repaint. No components were harmed in this process, or so I would have thought. Something has developed a big weird short.Now, there is no subtlety about the instrument. In two functions, DC+ and Ohms the needle swings hard downscale, and in the other two functions is a hard upscale swing.Hmm, seems I've provoked something that is simple, and consequently, damnably hard to diagnose.I'll be back soon (I hope).

author: Prehistoricman
author_rank: Regular Contributor
timestamp: January 29, 2020, 10:57:22 am
content: If you've not made a short, then you may have cold solder joints that got disturbed.

author: trevatxtal
author_rank: Contributor
timestamp: January 29, 2020, 04:42:13 pm
content: hello again I noteQUOTE the battery has charge END QuoteThat was not what I said if it is a rechargeable battery it will not be 3 Volt I suspect you may have the wrong batteries. if so it will never zero.Trevor

author: clay1905
author_rank: Regular Contributor
timestamp: January 31, 2020, 01:00:58 am
content: Yep,I have disconnected the link between the cathodes of the first stage and the grids for the second stage. This eliminates all influence on the movement of batteries, resistor trees or just about anything else. All I have now is one twin triode with grounded grids, circulating current between itself and the power supply; and a movement registering any balancing current flowing between the triodes' cathodes.As things stand, with the function selector in any voltage position, the movement behaves pretty much as normal. The Ohms setting is producing a strong downscale swing that can't be brought up to zero using the zeroing control.The only other thing left in circuit now is the function selector switch. This has to be where the fault lies. Visual inspection doesn't reveal any obvious problems though. It's getting to the point where I'll be trying a good substitute.Just now I'm not feeling all that great. Soon as able, I'll try the experiment to see what happens.Below is a modified schematic, showing where the disconnection took place.

author: clay1905
author_rank: Regular Contributor
timestamp: January 31, 2020, 01:06:11 am
content: It's not a V.O.M. This meter is a V.T.V.M. I'm not talking about setting infinite resistance using the Ohms Set control, I'm talking about achieving movement zero using the zero set control (Not the mechanical zero button either).With these meters, it's normal for a partly flat battery to make achieving a zero easier. But, as indicated, this battery has charge.As you'll read below, the problem is resolving down to one rotor within the function selector.

author: clay1905
author_rank: Regular Contributor
timestamp: February 02, 2020, 03:34:56 am
content: After a lot of hair pulling and beard stroking, I found that there's a bit of a trap here. The spindles on the variable resistors are not insulated from their wipers. Thus, if the insulating washer and tube assemblies are put together out of order, there's a short to ground that messes things up a bit.Things are better, but not good yet.The Ohms function looks to be much better. I can get the needle to read zero with probes shorted, and infinite with probes open.A.C. Volts produces a downscale needle swing. I can't bring the needle up to zero with the zero adjust control.D.C. volts, any polarity, I can get a nice symmetrical zero, meaning I can set the zero control in either polarity, and switching to the other polarity produces no needle movement. However, connecting in a 6 volt battery in the 10 volt range is like saying boo to the dead. No reaction at all.Ah. A peek down between the fronts of the selector switches and the front panel, and I see they both have two insulating washers each, but there's no evidence of the tube that goes between. I'll see what I might have that I could make substitutes out of. Wonder if this will make a difference.

author: clay1905
author_rank: Regular Contributor
timestamp: February 02, 2020, 03:51:50 am
content: Rats teeth!It wasn't that easy. The holes for mounting the switches are smaller than those for the potentiometers, and there's no room for insulating tubes.

author: clay1905
author_rank: Regular Contributor
timestamp: February 02, 2020, 05:27:07 am
content: A dab of solder to a cold joint on the junction of the lead in from the D.C. volts socket has cured the lack of interest the movement showed previously.The Ohms function reads as precisely as I think I could expect.When A.C. Volts is selected, the needle stands at mid-scale. I don't think it was intended to be a centre reading meter as there is no centre zero point.I'm not too sure how to proceed from here. I have figured out that this meter uses an active A.C. probe and I'm not clear about how having it connected affects the circuit. It didn't come with the meter.I have an idea the meter should read zero with the A.C. probe disconnected as there would be nothing to throw things off and cause the bridge to become unbalanced.If I have to keep this up for too much longer, I think I might become unbalanced and throw myself off a bridge.

author: clay1905
author_rank: Regular Contributor
timestamp: February 02, 2020, 05:50:05 am
content: I think this might be the last bit. There is a mica capacitor associated with the function selector. Exactly what it's function was I hadn't fully grasped. And it was soldered into the wrong place on the switch in any case.Another read of the schematic made it plain that this capacitor is for A.C. coupling and should form the ground path for an A.C. voltage under test.Rearranging the capacitor to conform with the idea of it being the ground path has the meter zeroing on all functions.Now it appears that all the functions are working as they ought.OK! Now it's coat of paint time.