Heathkit IM-5228 always reads zero ohms

[jamesadoty]

Hi all:

I'm new here.

I have a Heathkit IM-5228 VTVM that I'm restoring. I replaced the electrolytic capacitor and the paper capacitor. I also replaced the battery.

I brought the meter up on my dim bulb circuit plugged into my variac which in turn is plugged into an isolation transformer.

I was able to calibrate DC and AC volts and I can properly zero the meter.

In resistance mode I can set the meter movement to infinity ohms, and when I touch my AC/Ohms lead to my ground alligator clip the meter goes to zero ohms.

At this point I was feeling good, everything is working as it should. Or so I thought.

I went to test a known good and tested a 1K ohm resistor and no matter what range I set the range knob to it reads zero ohms.

I double checked the battery, and power from the battery going into the meter.

I calculated the series resistance of the resistors on the C and the B pads of the range switch and they add up pretty close, though admittedly I didn't check each resistor individually.

I found a resistor lead near one of the vacuum tube sockets that was shorted to another resistor lead which it shouldn't have been touching and corrected that, but still no go, I'm not getting any impedance readings at all, it just shows zero ohms.

Sometime in the meter's past someone switched out the vacuum tubes for a pair of Fetrons from Heathkit. I ordered replacement vacuum tubes to try, I own a Hickok 6000A vacuum tube tester I recently restored and it works, so I can test the vacuum tubes.

I'm not saying that either or both of the Fetrons are bad, nor am I saying they're the cause of the issue, but honestly the inside of these meters are pretty simple and I'm pretty much out of ideas.

I took two years of electronic engineering classes starting in 1980 but I haven't used those skills a whole lot since then so I'm really stuck and would appreciate some input from those of you in the know.

Thanks.

 

[aliarifat794]

Inspect all wire connections around the resistance measurement circuitry for possible shorts or miswiring.

 

[jamesadoty]

Inspect all wire connections around the resistance measurement circuitry for possible shorts or miswiring.

I'll try to get a look at it tonight.

 

[Les Jones]

Looking at the schematic I suspect it could be contact problems on the function switch wafer A rear. When the function switch is set to resistance there should be continuity between the input and the wiper on the range switch wafer C rear. This wafer on the range switch connects the input to the 1.5 volt battery via a resistor selected by that wafer on the range switch.
Here is a link to the manual. https://www.manualslib.com/manual/1531169/Heathkit-Im-5228.html
Les.

 

[jamesadoty]

Looking at the schematic I suspect it could be contact problems on the function switch wafer A rear. When the function switch is set to resistance there should be continuity between the input and the wiper on the range switch wafer C rear. This wafer on the range switch connects the input to the 1.5 volt battery via a resistor selected by that wafer on the range switch.
Here is a link to the manual. https://www.manualslib.com/manual/1531169/Heathkit-Im-5228.html
Les.

I've been pouring through the schematic for the better part of an hour and am leaning toward an issue with range switch wafer c, I hadn't gotten as far as checking wafer a yet.

I'm at work at the moment so I can't check the vtvm yet, but I plan to go back and revisit this section. There's got to be a reason the range resistors on wafer c are all being "bypassed"

 

[Les Jones]

I quick test would be to to test the voltage between the input socket pin and ground with a DVM. You should get a reading of 1.5 volts when set to any of the lower resistance ranges. On the highest resistance setting you would only read about 0.75 volts due to the loading effect of the 10 meg ohm input resistance of the DVM.

Les.

 

[jamesadoty]

I quick test would be to to test the voltage between the input socket pin and ground with a DVM. You should get a reading of 1.5 volts when set to any of the lower resistance ranges. On the highest resistance setting you would only read about 0.75 volts due to the loading effect of the 10 meg ohm input resistance of the DVM.

Les.

I'll give that a try. Thanks.

 

[jamesadoty]

I quick test would be to to test the voltage between the input socket pin and ground with a DVM. You should get a reading of 1.5 volts when set to any of the lower resistance ranges. On the highest resistance setting you would only read about 0.75 volts due to the loading effect of the 10 meg ohm input resistance of the DVM.

Les.

That check gave the results you were looking for.

I'm still looking around.

 

[jamesadoty]

Looking at the schematic I suspect it could be contact problems on the function switch wafer A rear. When the function switch is set to resistance there should be continuity between the input and the wiper on the range switch wafer C rear. This wafer on the range switch connects the input to the 1.5 volt battery via a resistor selected by that wafer on the range switch.
Here is a link to the manual. https://www.manualslib.com/manual/1531169/Heathkit-Im-5228.html
Les.

That check came out normal too. More head scratching.

 

[jamesadoty]

I checked the ohms range resisters in circuit and each seem to be within specs, or at least close enough.

Since AC, DC+, and DC- ranges and readings seem to be correct, I'm confused and don't know where to go from here.

Unless I missed one, I checked all the other resistors in circuit and they all seemed to be in spec too, or at least close enough.

 

[Les Jones]

As you say in post #9 that you are seeing a voltage at the input terminal from the variable current source then the problem must be the path from the input terminal to the voltage measuring part of the circuit. I think that path is via function switch wafer A rear but I cant see from the picture of the contacts how taht can happen. I think thereshould be a path between between connections 2,3,9 on that wafer when the switch is set to the resistance position. I can't see how that can happen by looking at the picture of that wafer.

Les.

 

[jamesadoty]

As you say in post #9 that you are seeing a voltage at the input terminal from the variable current source then the problem must be the path from the input terminal to the voltage measuring part of the circuit. I think that path is via function switch wafer A rear but I cant see from the picture of the contacts how taht can happen. I think thereshould be a path between between connections 2,3,9 on that wafer when the switch is set to the resistance position. I can't see how that can happen by looking at the picture of that wafer.

Les.

I'll test for continuity between those three pins and work backwards.