Newbie electronics question

[MrBond]

I found an old Heathkit in the attic a month or so ago and decided to try my hand at building it. I'd never soldered anything more than some speaker wires, so I figured it'd be a good project to learn on.

I get it all assembled and do the preliminary wiring check before I plug it in. It says to connect my multimeter to the power cord and to a particular resistor. I'm supposed to get a reading of 2.2M ohms when I do that, but the meter reads off-scale most of the time, occasionally flashing 1.7-1.9 MOhms at me.

It looks like I've got everything soldered correctly with no bridges, so I'm stumped. Why wouldn't I be getting a constant reading on the meter? Can electronic components go bad? This kit is probably close to 30 years old.

 

[0]

Originally posted by: MrBond
I found an old Heathkit in the attic a month or so ago and decided to try my hand at building it. I'd never soldered anything more than some speaker wires, so I figured it'd be a good project to learn on.

I get it all assembled and do the preliminary wiring check before I plug it in. It says to connect my multimeter to the power cord and to a particular resistor. I'm supposed to get a reading of 2.2M ohms when I do that, but the meter reads off-scale most of the time, occasionally flashing 1.7-1.9 MOhms at me.

It looks like I've got everything soldered correctly with no bridges, so I'm stumped. Why wouldn't I be getting a constant reading on the meter? Can electronic components go bad? This kit is probably close to 30 years old.

I'm not sure what you are actually checking. Could it be beyond the capability of the multimeter? If its a DMM, that's probably not likely. Its hard to understand what exactly it is that you are measuring. Can you be more specific? Give some component values?

 

[MrBond]

It's a DMM from Harbor Freight, so it's not exactly high quality.

Here's what the step says:

Connect the negative lead of the ohmmeter to the indicated lead of the 1.0 micro farad Mylar capacitor. Set the Ohmmeter to it's highest range. With the positive ohmmeter lead, measure first one line cord plug, then the other. In both cases, the meter should read 2.2 m(ohm) or higher

In stead of getting a value on the multimeter that stays there, it just flashes a value between 1700-1900, then it goes away.

Like I said, I'm an electronics newbie.

 

[nakedfrog]

Yes, components can go bad. It's entirely possible that capacitor isn't in such great shape.

 

[CraKaJaX]

Make sure the DMM is set for DC.

 

[MrBond]

I'm fairly sure I have the DMM set correctly. I read the manual .

I'm starting to wonder if my meter just can't read it for whatever reason. I don't have another meter here with a higher range, but I may have one at work I can borrow.

The whole "bad component" thing could be an issue too. I'll try and track down a better meter, but failing that I may just plug it into the wall and see if she sparks

 

[CraKaJaX]

Originally posted by: MrBond
I'm fairly sure I have the DMM set correctly. I read the manual .

I'm starting to wonder if my meter just can't read it for whatever reason. I don't have another meter here with a higher range, but I may have one at work I can borrow.

The whole "bad component" thing could be an issue too. I'll try and track down a better meter, but failing that I may just plug it into the wall and see if she sparks

Do it.

Sparks, smoke, fire... all of the above means you did it correctly.

 

[RossGr]

This is one place where a good old fashioned analog meter movement is superior to a digital. It can be difficult to tell the difference between an open and a short on a digital, so first be sure you are set to read resistance, then short your leads together and observe the display, that is the intication of a short.

Next, separate the leads and observe the open condition.

Now you can do your measurement, is it showing some thing like the short display or the open display. If it is showing open...


time for a smoke test.

 

[Linflas]

Just out of curiosity what kit did you build? When I was a kid I built a GR-64 and it still works.

 

[futuristicmonkey]

Just to add, whenever checking the resistance in a high range (ie over 1Mohms) many multimeters tend to be *very* sensitive to any movement of the test leads( there are dedicated units built specifically for this). Also, the resistance between my hands (tested with a Fluke 179, definitely not a hunk of junk) tends to be anywhere from 250 to 750k ohms. This can also influence your reading. My advice would be to secure your leads to the measurement points with alligator clips, just to make sure its not a trivial problem.

Edit: And considering the age of the kit, there could be some oxidation/patina on the measurement points that may be causing your sporadic readings. Try re-heating the solder for a few seconds.

 

[MrBond]

Originally posted by: Linflas
Just out of curiosity what kit did you build? When I was a kid I built a GR-64 and it still works.

It's a relative humidity meter. I don't really have a use for it, but it seemed like a waste to just let the kit sit unassembled in the attic, plus I needed a winter project I could work on in my apartment.

It's showing an open condition, the meter reads "1" on the extreme left of the display. If I touch the leads together, it reads zero.

If I touch a lead to each of the line cord prongs, it reads very near zero (0.2ish). That part of the wiring check it passed. I'm really tempted to just plug it in and see if it works. It's taking up about a quarter of my kitchen table right now so I want to finish it .