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  #1  
Old 04-13-2016, 01:29 AM
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Tubejunke Tubejunke is offline
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Knight Cap Tester Doesn't Like Electrolytic Caps

This began with my posting for a manual for a Knight 83YT24 so that I could properly wire in the rotary wafer function switch that the kit builder had put in upside down. I got the manual and went through the mess of disconnecting all of the wires and components and flipping the switch. NOT really a fun job. but it did bring the unit back to life.

So I ended up with a near mint condition unit that could never have been used as it was put together. A slow power up proved that the unit actually had a fairly accurate bridge for cap and resistor values and the DC leakage test voltages were all reasonably close. The problem that I have is that wax paper, or basically non electrolytic caps work fine on the DC leakage tests. But when I connect any electrolytic to the unit there is no reaction at all on the shadow angle of the "magic eye" tube. I have read that electrolytics can be more difficult to test on these units; and I know that the charge time is longer, but I could hold the leakage switch 10 min and get no reaction. I can take any of my test caps to my Heathkit IT-28 and get quick results.

I'm just not sure what would be the likely cause of this. Low B+ perhaps?? The two 10mfd 500vdc caps in the power supply checked out ok (not perfect) in that they didn't quite make it to 500v on the IT-28 leakage test. Both got a little warm after some use. I only had one new one on hand, so I changed one of them. I guess the other cap is a likely culprit and I know it should go if I plan on using the thing much. I just don't know if that one cap would cause the problem mentioned. The power transformer runs nice and cool. Just can't test the lytics.

Am I on the right track here?
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Old 04-13-2016, 02:52 AM
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Did you switch the "power factor" switch from "paper and mica" to "electrolytic"?

jr
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Old 04-17-2016, 12:55 AM
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Yes, most definitely. I have all sorts of these old things and they all have the power factor selector which evidently is the same basic thing as modern e.s.r. in theory and it is always used with electrolytics. Actually, you may have lead me to the potential problem as I noticed that the power factor control makes no change in the "magic eye" shadow angle, so something is wrong there.

I don't have my schematic here, but I think that the HUGE 2mfd wax paper caps found in many different makes/models of this type tester is about guaranteed to be bad. I will have to look into that, or the power factor circuit in general as that is the one strictly electrolytic testing circuit. Thanks.
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Old 04-22-2016, 03:53 AM
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OK, this is getting weirder. I go through the manual for this thing and in the instructions for leakage testing is the following, "Electrolytic capacitors do not respond to leakage tests because they are manufactured using and electrolytic which conducts current to a limited degree. Therefore, a partial closing of the eye on LEAKAGE does not ALONE indicate a defective capacitor because an electrolytic is inherently leaky."

Now is this Knights way of saying that their almost carbon copy unit (to Heathkit C3 etc) simply isn't capable of doing an electolytic leakage test? It does bear a few circuit differences and both tubes are different than Heathkit (C3) which I use as a reference due to being very common and similar in design. If this quote from the manual is true than further bother with the issue is a waste of time and the unit will not be a keeper as the rated voltage leakage test capability is what makes these really handy today in vintage radio and TV service. The Wien bridge capacitance and resistance functions aren't accurate when compared to even cheap modern digital multi meters.

I hope someone here had had or seen one of this model Knight units in action and can tell me that they can in fact test the electrolytics.
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Old 04-22-2016, 11:49 AM
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Quote:
Originally Posted by Tubejunke View Post
The problem that I have is that wax paper, or basically non electrolytic caps work fine on the DC leakage tests. But when I connect any electrolytic to the unit there is no reaction at all on the shadow angle of the "magic eye" tube. I have read that electrolytics can be more difficult to test on these units; and I know that the charge time is longer, but I could hold the leakage switch 10 min and get no reaction. I can take any of my test caps to my Heathkit IT-28 and get quick results.
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
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Old 04-22-2016, 05:28 PM
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I have one and mine tests electrolytics just fine. I did a complete recap and it works great.
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Old 04-23-2016, 01:01 AM
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Quote:
Originally Posted by jr_tech View Post
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......
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Old 04-23-2016, 02:05 AM
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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
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Old 04-23-2016, 10:55 AM
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Quote:
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I have one and mine tests electrolytics just fine. I did a complete recap and it works great.
Actually recap is an understatement. The man that originally built it did a very good job in the 60's. It worked well into the 70's. What I did was reassemble it. I completely disassembled it and using a manual I completely reassembled it. I replaced every out of spec componant, and rechecked all the switch connections, and cleaned the controls. This was overkill, but I wanted to clean some of the chassis.
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Old 04-23-2016, 03:51 PM
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wow! Overkill is an understatement, but great work Bill. That's a for sure way of having a real dependable unit & squeaky clean to boot.
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Old 04-23-2016, 04:46 PM
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Quote:
Originally Posted by jr_tech View Post
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.
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Old 04-25-2016, 04:20 PM
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Quote:
Originally Posted by Tubejunke View Post
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
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Old 04-25-2016, 07:34 PM
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Ok, since these old units are steadily gaining popularity as anyone can see that has been watching the prices go up considerably in the past year or two, we may as well have some good reference posts for others that wind up getting their own although they will more than likely be a Heahkit or Sprague. So I'll throw out the latest on the Knight.

A very nice gentleman that has the website that is dedicated to vintage test equipment has given me some advice and asked a few questions. One question was that he wanted to know what current was across the cap test terminals when the unit was set for electrolytic testing and eye closed; voltage applied was 50V and my Variac powered the unit at 100V. I measured about 5mA and sent him my results. I haven't heard what he thinks yet, but I believe his page on this unit says 5 or so mA is the magic number.

So I'm still dead in the water with the original problem and thus far no solutions. I even replaced the .25mfd 400VDC cap right beside the leakage switch. My Heathkit IT-28 showed this cap to be leaking at pretty low voltages, however my C3 didn't show the cap as leaky until I was in hundreds of volts. Either way it leaks, but a new replacement cap made no difference. This could be another thread!

Just out of curiosity I decided to hook my ammeter in series with either open leg of an electrolytic cap and see what I get first on a known good C3 and then the Knight. The C3 proved to show and expected initial rise in current when voltage was applied via the leakage switch; followed by a steady decline as the capacitor charges, followed by almost no current flow when charge is complete. NOW, I go to the Knight and I get a completely different reaction. I get first a small amount of current and then a not so slow sharp rise in current. It will be interesting when I find out what this devil is doing, but that sure isn't what I want it to do
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Old 04-29-2016, 04:50 PM
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Originally Posted by jr_tech View Post
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.
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