Another oddity Fairchild 7050. - Page 1
Forum section: electronics / repair

author: neo
author_rank: Super Contributor
timestamp: September 11, 2017, 02:18:24 am
content: I have yet another oddity, a multimeter that only turns on if you shake it.It works until jostled of moved but upon either it has to be shaken to make it work again, i checked the line cord, the fuses, the switch and there is voltage going to the transformer but for whatever reason it will not turn on regularly.When it won't turn on the only thing to do to make it turn on is to shake it.I seem to attract weird problems, either that or i don't know enough yet. Any suggestions for this one? Please and thank you.

author: Vtile
author_rank: Super Contributor
timestamp: September 11, 2017, 02:23:50 pm
content: Bad transformer solderjoint??

author: neo
author_rank: Super Contributor
timestamp: September 11, 2017, 09:35:36 pm
content: Quote from: Vtile on September 11, 2017, 02:23:50 pmI checked but i guess it wouldn't do any harm to resolder them. The transformer is very well secured to the chassis.

author: Armadillo
author_rank: Super Contributor
timestamp: September 11, 2017, 11:46:44 pm
content: Temporary Bridge the sliding ON/OFF switch?

author: neo
author_rank: Super Contributor
timestamp: September 15, 2017, 08:37:27 am
content: Well i have narrowed it down to the transformer, really quite interesting in that it only works if just so and it is not a solder joint.

author: neo
author_rank: Super Contributor
timestamp: September 16, 2017, 01:11:39 pm
content: Well i am genuinely stumped, there is continuity and on the bottom mains IS present, everything works. Switch works, the solder joints i know are fine because i have literally flooded them with solder at this and yet when i probe the leads on the transformer there is no voltage at ALL unless the meter is at the right angle which changes every time. The thing that is driving me batty is that continuity exists!HELP!! PLEASE!!

author: coromonadalix
author_rank: Super Contributor
timestamp: September 16, 2017, 01:22:58 pm
content: ''flooding" with solder doesnt mean youll repair something ... check carefully, if wires, touch them one by one and make a physical stress to see bad contacts, check solders with magnifying glasstry to find a schematics, you may probing the wrong way

author: neo
author_rank: Super Contributor
timestamp: September 16, 2017, 02:05:48 pm
content: Quote from: coromonadalix on September 16, 2017, 01:22:58 pmI have checked the wires, nothing that i can see it out of order, even if i connect it directly with a jumper it does not change anything.As for probing it the wrong way that would be rather hard given how simple it is, literally the mains comes in through a fuse to a switch and to the transformer. All of which i have checked three times, there is no problem except that the voltage somehow magically disappears despite there being continuity.

author: SMdude
author_rank: Frequent Contributor
timestamp: September 16, 2017, 03:32:03 pm
content: Perhaps there is a bad contact or broken wire in the transformer. When you test it with the multimeter, there is only a very small amount of current, however on mains it requires more current and the bad connection fails. Also, sometimes transformers have a thermal switch in them.Have you tried heating / cooling it?

author: neo
author_rank: Super Contributor
timestamp: September 16, 2017, 04:30:32 pm
content: Quote from: SMdude on September 16, 2017, 03:32:03 pmI have not however it is a very old one, 1973 type old. The leads into the PCB are just plain metal and i am probing across the two mains lead on the transformer and there is no voltage unless it is just right.

author: SMdude
author_rank: Frequent Contributor
timestamp: September 17, 2017, 12:20:15 am
content: Have you tried measuring the current on the primary side?This will confirm if the primary side of tranny is open cct.Are you sure that the secondary is not the problem?You might be having trouble measuring voltage on the transformer due to a varnish coating..?Measure the current, it never lies

author: neo
author_rank: Super Contributor
timestamp: September 17, 2017, 12:48:31 am
content: Quote from: SMdude on September 17, 2017, 12:20:15 amWhen working the voltage out is logical. No varnish on these wires.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 17, 2017, 12:56:18 am
content: This reminds me of a problem I had with an old Triplett frequency counter that was completely dead.It turned out that both of the primary windings on the transformer were open. The thin wire had broken right at the solder joint where the winding connected to the transformer's terminals. I had to peel off a couple of layers of insulation to get at them, but I was able to resolder both windings and get the counter working again.Ed

author: neo
author_rank: Super Contributor
timestamp: September 17, 2017, 01:09:23 am
content: Quote from: edpalmer42 on September 17, 2017, 12:56:18 amThat gives me an idea, thank you.

author: neo
author_rank: Super Contributor
timestamp: September 25, 2017, 01:45:05 pm
content: For once the problem with something is as funny as it is infuriating. I checked the cords continuity 6 times and the voltage another 6 and it always seemed to be something other than the cord.Finally in a fit of annoyance i removed the cord and applied power via alligator clips and my variac. It worked, so i cut the end off an IEC cable and soldered it on and using it all problems, except for the capacitor that broke off when i touched it, are solved. As much as i hated mutilating the IEC cable it was a perfect math to the original, only difference is that it is slightly longer which i don't mind.Luckily this meter only uses 4 common value capacitors, so common i already had one and could ebay the other 3.