Heathkit IM-5228 always reads zero ohms

J

jamesadoty

New Member
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.
 
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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.
 
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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"
 
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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.
 
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I'll give that a try. Thanks.
 
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That check gave the results you were looking for.

I'm still looking around.
 
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That check came out normal too. More head scratching.
 
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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.
 
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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.
 
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I'll test for continuity between those three pins and work backwards.
 
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I did a lot more testing and just don't see where the problem lies. I rechecked all the range resistors in the ohms circuit, though come to think of it, I didn't recheck the additional range resistors for AC/DC voltages.

I went through and rechecked all of the other resistors and they're pretty normal looking.

I rechecked connectivity from the probe connector to the function switch.

Here's how you can tell my skills with trouble shooting electronics is poor. Could the fetron that replaced the 12AU7 be faulty?
 
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As it works on volts, then the fetron can't be the problem either.

There's not much it can be?, all it does on ohms is feed 1.5V DC through different value resistors to the input socket, and the differential amplifier (double triode) amplifies that voltage and feeds the meter.

So it's essentially just a potential divider, with the bottom half the resistance you're measuring.

As you can set the FSD using the pot, then the meter part is working fine, but presumably the resistor(s) from battery to input socket are high resistance somewhere, which means you're permanently on a high resistance range.

Have you tried reading a high value resistor? (in the mohms), and does that then not go to zero?.

Simple way to check, put an ammeter across the input socket, and set the VTVM to ohms and x1 range - the external meter should read 150mA, then try the x10 range, it should read 15mA, and so on as you go up the ranges. I suspect any current ranges will be extremely low?, which means it's working on a very high range.

Assuming it is very low current, try the external meter directly on the switch wafers - leave the VTVM set to ohms x1, and measure the current to chassis with the external meter. Pin 3 of range switch wafer C is the ohms output current, so should read 150mA - if that's OK, then try on the Function switch, pins 2 and 3 of wafer A. Pin 2 goes directly to wafer C, and should read identically to the previous test, and pin 3 goes to the input socket, and should read exactly the same as well as the wafer joins them together.

If the test for 150mA on pin 3 of wafer C fails, then try on pin 11, as that's the 150mA input from R30.

So measure those currents, and get back to us.

Really, it can only be the 1.5V battery, R30 (9.1 ohms) or either of the two switch wafers - the tests will prove which.
 
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I tried two things, both very telling, or would be if I knew where to go from here.

I dug up a 330K ohm resistor, set the range switch to 100k and tested the resistor. It measured 200k ohms. I then tested the resistor with my Heathkit IM-13 and it read about 310k ohms. So the ohm meter, at least in the 100k range is reading 100k ohms low.

I then put my Fluke in mA mode and set the range switch to x1 and if there were any mA going through my Fluke, it couldn't "see" it.

I verified that I have 1.5 volts across the AC/Ohms probe lead and ground. I also connected my 330K ohm resistor to the leads and read 1.2 volts across the resistor.
 
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I noticed after reading resistance that it takes an unusual amount of time for the meter to return to full deflection. It doesn't do that when reading AC or DC voltages.
 
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So go and check on the wafers, as I suggested above - if R30 is easily reachable, you might try the current from R30 (output) down to chassis.
 
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