How to test Cans.

[longhaireddwb]

I'm talking about the square little metal boxes that have the tuning capability. I think they are variable caps?

I've ran across a couple radios lately with bad ones and I just r and r them and find there bad but...

How do you really test them? DVOM? Cap tester? How to go about it?

Please help a guy to learn something new.:redface:

 

[mechanic]

Tuning capacitors in older equipment were of the air gap type, the plates are separated with air that is a dielectric or insulator. So there should be no continuity with an ohm meter between the plates. New styles have a dielectric made from mica sheets, made for a small size. The same test is used.
Of course there is a breakdown voltage for these types but it would be with a high current or frequency that "jumps" the gap. Known to happen in amps or antenna tuners. :blink:
Trimmer caps can be just about the same build as tuning caps and can be checked in the same way. Moisture is a common failure with tuning caps in a receiver.

73 mechanic

 

[AudioShockwav]

the square little metal boxes that have the tuning capability.

Are you talking about Variable inductors ??

73
Jeff

 

[Captain Kilowatt]

Are you talking about Variable inductors ??

73
Jeff

DING! DING! DING! They are either variable inductors (coils) or small transformers.

 

[mechanic]

If they are inductors or IF cans (transformers), they can be measured with a LCR meter. I have seen them as low as 40 dollars in combination with a DVM.
When you want to be a bit on the technical side, the LCR meter acts as a Wein Bridge and the inductor is the component being measured against a known resistor and capacitor. The inductor cannot be in circuit for obvious reasons. :mellow:
Failure does happen with these type inductors from over voltage to physical damage, even more so when the ferrite core is damaged.... "screw driver jockeys"

73 mechanic

 

[longhaireddwb]

There we go! Variable inductors. I'm new to all this so go easy on me. Thank you all for the reply's. I'll check into one of those LCR Meters. If that is what it takes.

 

[jazzsinger]

There we go! Variable inductors. I'm new to all this so go easy on me. Thank you all for the reply's. I'll check into one of those LCR Meters. If that is what it takes.

if they only have one coil on the schematic its a variable inductor,if it has two coils its a variable transformer.

 

[Rogerbird1]

THe simplest test would be to make sure the coil side(s) have continuity.. If they are burnt open, they are bad and wont adjust. a simple ohm or continuity test would prove that.

An LCR meter test would be the best.

Roger

 

[liquidh8]

Thread from the dead I know.

I have a radio I think may have some bad Cans, The cores are good. I'll be looking into one of these metes I guess.

But, where would one get replacements?

 

[KM3F]

The cans are interstage coupling transformers.
They are designed to tune a specific IF frequency.
Many are 455 khz or 262 khz but may be other frequiencies.
Some are center tapped. Sometimes the center tap is not used in a peticular circuit.
Unless you know what frequency they are they won't be usefull and the impedence the windings are supposed to match.
Testing would be difficult unless you have a test setup at the frequency they are supposed to tune at.
The original factory testing was the same way for a customer's spec.
Then the circuit has to match.
Very often at Hamfest some of these 'cans' show up, but unless they match the circuit you put them in they either will cause low gain or not work at all.
Used in a tube circuit, the plate voltage usually is in the 135 to 150 DC range for small receivers.
Good luck.

 

[unit_399]

A "quick and dirty" check would be as follows:

Check the primary side and the secondary side for continuity.

Check that the primary is not shorted to the secondary. There should be NO continuity between them.

Make certain there is no continuity between either winding and the metal can.

Remove the ferrite core and check it out.

Unless you have a meter that reads inductance, there's not much else you can do.
DIGI-KEY is a good source for most cans used in CBs.

- 399

 

[psycho]

Are you talking about Variable inductors ??

73
Jeff

That's what I was thinking.

 

[liquidh8]

Yes, the variable inductors. The the transformers. i will look through digikey and the like.

unit399, this is for the 8719 washington I have another thread about.

Thanks guys.

 

[PLUMMER]

Any examples of how these cans go bad , other than the golden screwdriver? I mean will more than 15 volts, or age , other component failure?

Also what are symptoms of a bad can?

Thanks, I would have never thought these go bad....

 

[liquidh8]

I don't know is all the ways, but I know over voltage can do it. That is why I ask. The power supply in this washington took a dump, and a few caps exploded. After rebuilding the power supply and replacing all the caps (electro), it is still low on power, am an ssb. Ky was thinking one of the inductors might be messed up. All the cores are good.