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 Post subject: Manual for a Sencore LC-75
PostPosted: Jul Wed 10, 2013 12:18 am 
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Location: Monroe,NY
Hi,
I'm looking for a free download of the operation/ repair manual with schematic for a Sencore LC75 Inductance and Capacitance Tester. Every link I've checked is a dead end wanting credit card # and such. Does anyone know of a real place to download this?
Thanks

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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Wed 10, 2013 12:29 am 
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PM me your E-mail and I will send you a copy.


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Wed 10, 2013 2:17 am 
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Be forewarned if you don't have the proper, low capacity special test cable, you're not going to get proper results with the meter. A standard bnc to clip leads with rg-58 will not deliver accurate readings. The original cable is made from a 35" piece of weird 93 ohm cable RG-62 with the positive lead connected to a 2 1/2" piece of 50 ohm RG-174 to the positive mini grabber clip with the shield grounded to the rg-62 shield and the grabber end unterminated . The negative grabber clip is connected by a 3" piece of about #14 wire.


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Wed 10, 2013 2:47 am 
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Wow,
Who would have thought that it was some weird cable! I'll have to look at that carefully so I can try to duplicate it.
Thanks for telling me,

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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Mon 15, 2013 11:52 am 
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I was at a hamfest yesterday, and found a piece of RG-62U ( solid core ) cable. I know that I should get the flex core one, but just for experimenting, I had to try it. What I found was using a 4 ft RG-58 cable ( 50 ohm ) with grabbers, and pressing the capacitance button, it read about 120 pf with nothing attached to it. Trying to use the zeroing knob was past it's range ( I could only get down to about 100 pf ) I tried testing various value caps and concluded that indeed, it added about 120 pf to the results, making it impossible to check small value caps. next, I tried a piece of the RG-62U ( 93 ohm ) about 48 " long with no grabbers, just separated the ends and put alligator clips on it. Immediately when pressing the capacitance button I noticed the difference. It read about 14 pf and I could adjust it with the zeroing knob and get accurate small value testing results. Then I cut it shorter to about 40 " and tried again. This time when pressing the capacitance button, it was about 6 pf and easily adjusted.
My question is how did you come up with the length of 35" for the RG-62, did you measure an original? I think I may try shortening it a little more to 35-6 " and see how close I can come to exactly what I need, but thanks for the heads up! I would have thought that there was something wrong with this tester, had you not explained the lead issue to me.
Regards,

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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Mon 15, 2013 1:33 pm 
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OK, so there is no abiguity, the measurement is taken from the back of the BNC collar which is a crimp type to the very end of exposed center insulator and bit of center conductor that's soldered to the RG-174. I just took apart the center Y cover and measured again.

You haven't added the 2 1/2" RG-174 mini coax as I described to the mini grabber have you?
You're going from 93 ohms to 50 ohm coax at that point. That may be your problem.

The red and black grabber leads on mine are not the same length. The black grabber lead is just wire and 1/2" longer. This is a factory made cable and there has been no changes to what came out of the box when I bought it in the mid 90's. The P/N is 39G143. The grabbers are marked EZ Hook for the red and Pamona for the black. The RG-62BU cable is not very flexible and is manufactured by Essex p/n 21-771. The Y cover is made of a triangular shapped, folded plastic and kept together with a mini Ty Wrap and the Sencore P?N and "Low Capacity Test Leads" is printed on the side. The RG-174 and black wire just exit on the corners. This isn't one of those molded Y's you see on regular test leads presumably to accomodate the RG-174 soldered junction.

Google the part number and make sure you're sitting down.

Edit: The replacement Sencore p/n is now 39G219


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Wed 17, 2013 1:22 am 
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Ok,
I measured the rg-62a/u that I have to use and attached the proper length rg-174 and #14 wire for the black lead. Once again, thank you, Dawn for the exact measurements of the test lead, I believe that now I have a good lead. I followed the procedure on page 36, for basic calibration. My question is about how this thing works. I measured some caps, with my B&K 820 digital cap meter, and then with the Sencore LC75. Most are close except the larger electrolytics. When I read the manual, it appears that the LC75 uses an R/C time measurement to determine capacitance, while the B&K 820 uses an AC signal and a bridge circuit. This would account for this. Then I started measuring some inductors and chokes. I had a Hammond 155b new in the box which should measure at 6 mh which measured at 3.8 mh and would not pass the ringing test. Then measured a 100 uh new which measured at 93 uh, within the 10% on the label of the inductor. Then an 8.5 h choke which measured 9.6 h again, did not pass the ringing test, and lastly a 2.5 mh rf choke which measured at 2.7 mh. The 2.5 mh choke and the 100 uh choke measured near what I expect, and both passed the ringing test, but I'm questioning if this is working correctly. I don't have an inductance bridge to make any comparisons, but what do you think?
Thanks,

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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Wed 17, 2013 3:51 am 
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It's been a while since I used any of the inductance features. I rarely have the need to check inductors. However., first assumption that I'm making is that the '77 is supposed to be the same meter as the 53 with the exception of an odd method of calculating ESR that was added later. I have the '53 which looks identical otherwise and the information about the cable I recall applied to all the units.

One of my employers gave me mine without the cable. The only problem outside the cable was it needed to be callibrated again by the mid 90's. If your's is much the same, the ring test involves setting a trip point for a relay besides the range callibration with standards. I assume you set that correctly. Then there's the issue of ringing test having many caveats such as inductances ranges, iron cores and other coil contruction that affects damping and the counts. It's been a while and besides laminated cores or resistive forms, I don't recall the other circumstances that will afect the test but there were several regarding core materials.


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Fri 19, 2013 9:54 pm 
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Location: Grand Rapids MI
Hi There!

I can tell that you're trying to do this on the cheap, but I'm guessing that Sencore has the test lead set and that is probably the best way to go with this. A fudged cable set will not give you correct readings and the rest of the effort to make this thing work will not turn out right. Just a suggestion. Sencore is in business and they've been helpful on my LC-53.

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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Jul Sat 20, 2013 1:07 am 
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Location: Latham NY
The ringing test was mainly for yokes and flybacks, don't think it works for power transformers.


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Sep Fri 27, 2013 4:33 am 
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Joined: Sep Fri 27, 2013 4:20 am
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Sorry to come in late to the party, but after getting a Commercial Copy of the LC75 "Manual" I find it totally lacking anything really helpful toward repairing it!
So ,I too am asking the musical question "who's got a Schematic -or better- on this thing"?

I need a DNA like document, not a users guide.

Thanks for any possible help ,
Dave Fragale


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 Post subject: Re: Manual for a Sencore LC-75
PostPosted: Sep Sat 28, 2013 4:27 pm 
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Location: Ohio 45177
I guess I always assumed that any shielded test lead used with the Sencores would work as long as the test lead could be zeroed out. I have an LC 102 here. I took a SM cap that I once tested on a very precision bridge and selected it from a group of 100pF caps to actually come out to 100 on that bridge. I zeroed out the factory leads on my Sencore and tested that cap. 101.5 plus or minus a half pF due to movement, etc. Next a 4 foot RG 58 lead with a commercial bnc to small alligator clips adapter. Zeroed it out, and got 100 plus or minus a half. Next an unknown inductor toroid. 6.66 mH on the factory test lead, and 6.84 on the coax with lead adapter. The Sencore factory lead has a shield all the way to the clip on the end. I am fine with the factory lead but with constant use I see them fall apart. They can be repaired once or twice with care, but a replacement is expensive, as I recall. Makes me wonder how a X1 straight thru scope probe would work with that tester, as they are generally low cap cables, I thought.


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