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.

 

[Les Jones]

With the power to the meter switched and the function switch set to resistnce do the following resistance tests The first test with one meter lead connected to the centre pin of the input socket . Set the range switch to the lowest resistance range and put the other meter lead on the junction R30and R29 Note the resistance reading. set the range switch to the next range up and move the meter lead to the junction of R29 and R28 note the reading. Repeat this until your meter lead is on the right hand end of R24.
Next test. With one meter lead stil on the input socket measure the resistance to the left hand end of R 12 and note the reading.
Connect one meter lead to ground. connect the other meter lead to the junction of R30 and R29 note the reading. with that meter lead still connected to the junction of 30 and R29 connect a 10 ohm resistor between this point and ground.
Noe the reading.
Report the results.

Les.

 

[jamesadoty]

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.

Will do.

 

[jamesadoty]

Because there's FAR too little current on the resistance ranges.

Agreed. I wonder if the battery terminals are jenky. As well as checking with R30 I think I'll pull the battery and connect a power supply to ground and R30 to see what happens.

 

[jamesadoty]

Hi all:

Good news. First, I found the problem. Second, I'm a total idiot.

I didn't have time Friday or Saturday to work on the VTVM but I did have time today.

Last night I was in bed and I couldn't stop thinking about the weird issue of the meter movement moving back to INF after touching my ohms/AC lead to the ground lead and was wondering if what ever was causing the problem with the ohms function was also causing the weird meter issue.

This morning I took the battery out of the meter, set my very crappy Chinese bench top power supply to 1.5 volts and connected to ground and the input to R30, and powered up the meter.

I made sure the meter zeroed properly, set the range switch to X100, connected my 1k ohm resistor and the meter read exactly 1k ohms. I then set the range switch to 100k and connected my 330k ohm resistor and got 320kohms.

I disconnected the power supply and verified my C cell battery and it was fine.

I took the spring end of the battery holder apart and found that the metal was very tarnished. As luck would have it, two weeks ago I ordered an after market battery terminal repair kit for the meter so I replaced the spring, bolt, nut, lock washer, and spacer with the new ones. I don't like the replacement spring, it doesn't surround the battery body and is a total pain in the rear to keep from folding over while installing the battery but it otherwise works pretty well.

Once I had the battery installed I tried my resistor tests again and got the same results, 1k ohm and 320k ohms, so the problem was the very tarnished negative battery spring.

Lesson learned on thoroughly examining battery terminals. I couldn't see how tarnished the battery spring was while it was in place.