Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
Hi everyone hope this is the right place for this.
I was hoping someone could help me troubleshoot my Heathkit Im-28 VTVM. When I put the dial to DC+ the meter moves all the way over to the right then after a few seconds it goes back to the left which from what I understand is normal in either DC+ or DC- positions. Now, when it moves back to the left My zero adjust doesn't do anything, it wont move the meter at all. If I move the selector to DC- the meter then stays to the right and doesn't move back to the left. The troubleshooting page suggests to check capacitor c6 and resistors R35, 37 and 38 but they are good. I even changed C6 to 22uf 450v to be sure it was good. I did notice that with the variac set to 110volts coming in I have 123 volts ac coming out of the transformer and across c6, and if you measure DC at the + side of c6 I get 108 volts instead of 50 to 70 volts and on the - lead I get 42 volts which should be 60 to 85 volts. I have attached the schematic, I am hoping someone has an idea of what might be going on here. Thanks anyone for any time on this matter, I don't really want to buy another vtvm so I am hoping I can find the problem here. Warren
[Image: http://i1348.photobucket.com/albums/p721...84cbb8.jpg]
(This post was last modified: 07-12-2014, 11:18 PM by Warren.)
Posts: 15,299
Threads: 531
Joined: Oct 2011
City: Jackson, NJ
Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
I added it about a min after the post. Thx.. 
Posts: 7,213
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Hi Warren
How about the resistors around the zero adj pot?? I'd give them the evil eye and did you swap the 12AU7??
GL my friend
Terry
ps Is D1 a soulineium rectifier or silicon. Sorry I'm not a good speller but neither is my spell check!
(This post was last modified: 07-13-2014, 05:49 AM by Radioroslyn.)
Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
I have an older Heath and in the assymbley book it says if the meter does not zero or is erratic in either ac or dc to reverse 2 wires inside. But I am not at home (on vacation) and can't remember which wires it said to reverse. Maybe you can look to see if your manual says anything about it.
Posts: 2,128
Threads: 18
Joined: Oct 2008
City: Merrick, Long Island, NY
Agree, and I'm sure you changed out the battery and cleaned the connections, cleaned all the switches and pots, tuvbae sockets, and replaced all the capacitors and made sure probe is OK. Yup, resistors are next, but they did sometimes use odd values. 1% replacements are now cheap enough even if you have to series/parallel a couple to be close enough. You can also get a couple of spare tubes pretty cheaply if you don't have 'em sitting around. So long as the meter movement is OK, not much else could be wrong --- but it may have been assembled by a kid who is now a grandfather like me.
Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
I did check the 3 resistors around the zero adjust pot, they are good.. I haven't switched the 12au7 tube but it does test good on my tube tester. Do you think I should try another even though it tests good? The diode is a silicon diode.
My manual doesn't say anything about switching 2 wires so im not sure which ones they would be..
I changed c6 cap, the rest tested good.
Maybe I could just replace all the caps and resistors but that wouldn't really be troubleshooting it and I could probably get another one for what that might cost..
(This post was last modified: 07-13-2014, 03:40 PM by Warren.)
Posts: 599
Threads: 24
Joined: Jan 2014
City: Edmonton AB CA
I agree with Codefox1, clean those switches good with some Deoxit and a tooth brush or Q-tip or what ever you can get in there and install a fresh battery. I have a couple of these and if the tubes are good and the switches are clean every one I have ever seen works just fine. Its a funny thing, I have worked on a lot of Heathkit, Eico, and assorted other kits from years gone by and every one was build with excellent workmanship. Seems kit builders back in the day did good work.
Gregb
Posts: 2,118
Threads: 112
Joined: Jun 2010
City: Medford OR (OR what?)
I wish I could read your schematic - it is a bit - furry.
You should replace all of the caps anyway, even if you got another one.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
hmmm, strange I can read every word on the schematic. Im not sure why you cant.by the way, I thought the battery was only for the ohms reading part of the meter, does it make a difference for volts measurment also? I guess I will order caps for it and change them all with the battery and see what happens from there. Thanks for everyones input so far.
(This post was last modified: 07-13-2014, 08:22 PM by Warren.)
Posts: 7,213
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Yes Warren you are correct the battery is only for the ohms circuit. I should have mention as others have the switch contacts need to be very clean and tight. Any resistance at the contacts will cause problems. Have had problems with tubes that testers say are ok, not often.
Terry
Posts: 2,353
Threads: 92
Joined: May 2010
City: Clayton, NC
Check all three of the calibration pots. They are not the best quality and have been known to go bad. Test for total ohms and for adjustment.
Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
I will be trying what all of you suggest, if anyone thinks of something else let me know. I will keep you posted on what I find.
Posts: 2,353
Threads: 92
Joined: May 2010
City: Clayton, NC
Let me know if you find my car-keys in there.
Posts: 736
Threads: 26
Joined: Jun 2013
City: Hayward, California
I guess Im not the only one to lose their car keys... 
Users browsing this thread: 1 Guest(s)
|
