Knight Cap Tester Doesn't Like Electrolytic Caps

[Tubejunke]

Originally Posted by jr_tech

Are you indicating that the eye never closes or never opens? Im confused.

Have you measured the voltage applied to the capacitor check terminals during leakage tests at all set voltages and both modes (paper/mica and electrolytic) ?

jr

I'm sorry Jr, I see that I did misstate this. I said that there is no reaction of the eye tube when I should have said that it shuts when voltage is applied and it remains shut on any electrolytic. The voltages are there in both modes. Paper mica functions seem fine. A good cap gives a momentary closure of the eye tube and then a reopening.

Thanks as well to Bill who confirms that these units do test electrolytics contrary to the statement I quoted from the manual in my prior post. I thought it odd that it is written in the manual and then mine so far isn't testing them. And a complete recap is always a good thing, but I have found not always necessary. There are known problem child caps in these type units like the big 2 mfd non polarized (just to name one) which remains to be seen here. I just haven't had time to hook it up to my Heathkit IT-28 yet. Heck, there's not many caps in the whole darned thing. LOL! Not like I'm doing a 40s RCA TV set......

[jr_tech]

The 2mf cap is not in the circuit during leakage testing, so I don't think that it is the cause of of the closed eye problem.

Just a wag, but is the switch on the power factor pot working ok? might try some De-Ox on it. If the switch is closing ok are the 3 resistors connected to it ok and connected correctly? If the 1k was swapped with the 470k, the switch would have little effect on the current supplied to charge the cap in leakage mode.

I think that the statement in the manual is just a cya statment, since in some cases a large electrolytic could draw enough current to keep the eye closed (5 to 10 ma) and still function fine... really depends on the circuit.

jr

[Tubejunke]

Originally Posted by jr_tech

The 2mf cap is not in the circuit during leakage testing, so I don't think that it is the cause of of the closed eye problem.

Just a wag, but is the switch on the power factor pot working ok? might try some De-Ox on it. If the switch is closing ok are the 3 resistors connected to it ok and connected correctly? If the 1k was swapped with the 470k, the switch would have little effect on the current supplied to charge the cap in leakage mode.

I think that the statement in the manual is just a cya statment, since in some cases a large electrolytic could draw enough current to keep the eye closed (5 to 10 ma) and still function fine... really depends on the circuit.

jr

I will check the switch. I assumed it was working as when in bridge mode the pot will alter the eye opening. At first there was flutter as the pot was dirty, but I cleaned that early on. I will have to look at things like the two resistors potentially not being connected correctly. Anything could be connected wrong if the builder installed the wafer function switch upside down.

I really appreciate the input from both of you guys. I have several cap checkers, but I have never had a Knight and you don't see them much. That statement from the manual held me back from the logical troubleshooting that we are beginning to talk about. Now I know its capability and I should be able to sort this out.

As I (for whatever reason) enjoy messing with these old things, I want to develop a better understanding of the circuits and also current flow. Cadillac units like Sprague TelOhmikes' have a panel ammeter for a reason.

[jr_tech]

Originally Posted by Tubejunke

I want to develop a better understanding of the circuits and also current flow. Cadillac units like Sprague TelOhmikes' have a panel ammeter for a reason.

A good test for your meter, and an aid to understanding, is to experiment with a couple of resistors accross the "C" terminals and observe the closing angle of the eye. I just tried a 10 meg and a 10 k resistor on my C-3 Heathkit.
With the 10 k resistor, the eye stays closed at every leakage voltage test position when the pf switch is in paper and mica mode, and will open to varying degrees at 150 volts and below in electrolytic mode.
With the 10 meg resistor, the eye never closes at any voltage setting when the tester is in electrolytic mode, and will close completely at test voltages above 150 volts in paper and mica mode.
The C-3 is about 1000 tomes more sensitive to leakage in the paper and mica mode than it is in electrolytic mode. Your Knight tester is functionally the same as the C-3 and should behave in a similar manner, if the sensitivity is changed correctly by the pf switch.

jr

[Tubejunke]

Originally Posted by jr_tech

Just a wag, but is the switch on the power factor pot working ok? might try some De-Ox on it. If the switch is closing ok are the 3 resistors connected to it ok and connected correctly? If the 1k was swapped with the 470k, the switch would have little effect on the current supplied to charge the cap in leakage mode.
jr

Ok, the Knight is now working and this statement early on at least put me in the right area. It turns out that the builder wired the 220K and the 470K Ohm resistors so that they were parallel to one another when the switch closed when switch closure by my take on the schematic is supposed to short the two resistances. In simpler terms everything was wired 90 degrees off. And remember the S2 function selector was 180 deg off. My God, its a wonder the thing didn't go up in flames!. The guy did a decent job otherwise as far as termination and such, so it really took some looking to find his mistakes. Then there were my mistakes ie:

"I will check the switch. I assumed it was working as when in bridge mode the pot will alter the eye opening. At first there was flutter as the pot was dirty, but I cleaned that early on. I will have to look at things like the two resistors potentially not being connected correctly. Anything could be connected wrong if the builder installed the wafer function switch upside down. "

First, I only verified mechanical opening and closing of the switch. Second, the pot alters the eye in electrolytic mode (I meant that the S2 was in a bridge function at the time). Third, the potentiometer action really has nothing to do with the condition of the switch. The last part of my statement was as it turns out the answer.

So now I have a brand new/old Knight cap checker to add to my collection. Thanks as always to everyone for the pointers and comments.