I picked up a Sencore LC53 cap tester at the Baltimore hamfest last weekend. The seller said it worked perfectly, but his definition is different from mine.

In fact it won't zero when turned on - a number appears in the display but it appears to be random. The zero knob does nothing. The LEDs work. Pushing a test button seems to do nothing.

Anyone has any advice or a service manual? I found the op manual on the net, but not the service.

Just FYI - if you did not get the factory probe with it, I don't think you'll be able to fully get it to register to zero. If you intend to build a proper probe for this, I've learned that you'll need to use RG-62 coax (13pf/ft), which is a lower pf/foot capacitance than say RG-58 or RG-59. When powered up you should simply see "-000" on the display. The front panel zero adjustment won't make any changes to the display with nothing attached. There is a lead zero calibration pot on the rear of the unit, which will never completely zero the display unless you have a proper capacitance probe attached - this may be causing the random display you mention - try tweaking that control to see if your random digit issue clears up. If you simply attach a cap for testing, you should easily be able to see if the unit is working properly.

I've got the following manuals, of which I located on the Internet. If you can't locate these, PM me with your email address and I'll send 'em your way:

- Sencore LC53 "Z meter" Capacitor - Inductor Analyzer. Operation, Application, and Maintenance Manual
- Sencore LC53 "Z meter" Capacitor - Inductor Analyzer. Schematic and Parts list.
- Sencore LC53 Calibration procedures

Ah, yes. the "hamfest special". "Worked last time I plugged it in(15 years ago)(guy I got it from said it worked)" Nice to be able to plug stuff in at those affairs and see if it does actually work. I myself have been fairly lucky buying at those events. But I usually like to hunt down an AC outlet somewhere before the seller can pack up and leave. One piece of unguaranteed gear, a Gen. Microwave power meter, had problems but it only needed a filter cap replaced in the power supply. And a 465 scope with intermittent attenuators. Bought a junker to salvage parts from for that one. Maybe also a HP sig gen. that became unstable after awhile and I could not find the cause. That I disposed of. THat being said, those probes for the Sencores do break internally sometimes, usually at the back of the clip leads that I have seen. Perhaps if you have a factory lead, do a continuity or shorts check. Move the leads to check for intermittents.

Thanks everyone! I did find the schematic and parts list once I used those terms - I had been searching for "service manual".

The unit has no probes, but I don't think that's the problem. I do have a cable with two alligator clips and a BNC and realize it might not zero, but it doesn't get that far. On powerup, it display a number like 00353 and then just stays there. If I power down and back up, it'll display a different number. The Zero adjust has no effect at all.

I'll start out tonight by checking power supply voltages. I'm glad to get other suggestions from anyone who has them.

abraves-
If I recall, the input connector to this has a fuse in a holder. Depress the BNC, and rotate, to remove it? It's been a while, can't remember for sure. Make sure the fuse is good, and all metal contacts are really clean. This unit also has stacked pc boards. Again, clean all pins/sockets. It isn't easy to clean those connectors, but unless you have, you are shooting in the dark. There is no way around it. And 'cleaning' doesn't mean just a spray of contact cleaner. All contact surfaces need to have the gunk and corrosion of decades removed before you can even attempt to see if you really have a problem.

It is a real good device when it's working. Good luck!

John S.

I thought I'd bump this with the latest on my war with this LC53. I do have the schematics for it and that has certainly helped, but I'm no farther along - other than having checked lots of things. Summary: It powers up and displays but the zero controls do nothing and it will not test any cap or inductor. Each power up results in what appears to be a random number in the display. Here's what I've done:

1. The power supply works and the +7, -7, 20V and 5V are good.
2. The cap working voltages are selectable as they should be and appear on the test lead when you press "leakage" so the fuse/holder, etc are all good.
3. The display board is good
4. The analog board and digital board connections are all good - cleaned and tested.
5. There is a counter/oscillator on the digital board where the schematic shows pins with 1kHz, 10K, 100K and 1mHz. All of these check good and dead on.
6. Nothing appears broken, loose or otherwise visually suspect
7. An ESR check of the small electrolytics show they are OK.

So, that's about all I can do without a service manual that has a troubleshooting section. This thing is a rats nest of dozens of CMOS and TTL gates and switching transistors that all interconnect so I really can't develop an approach that let's me check things in a logical order.

I hate to give up on it, but unless I get lucky or some service lit exists (or both) it's probably beyond me to fix.

Here are some troubleshooting tips I received from Sencore Service several years ago when I was troubleshooting a problem on my LC53. Helped me find the problem with my unit. Good luck.

Incorrect cap values and no response when trying to zero leads:

Check for pulse entering analog board at pin 11 and leaving at pin 7
Pulse should change when Caps Value button is pressed
Check for shorted or open transistors TR20 and TR21 (2N5771) on analog board
If TR20 is shorted the pulse will be only partial level

Follow pulse through system to look for bad gates

Check IC12 pin 12 on digital board for 200hz square wave.

Inductor test problems:

Check for 20 volts DC at pin 20 of board interconnect
Check for open series pass transistor TR11 (2N4401) on analog (top) board

On mine, I had loose connections where the pins for the interconnecting plugs were soldered onto the PC boards; this gave me a missing digit on the display. Use a magnifying glass and check them all.

Tim, did Sencore ever publish the theory of operation of that beast? It's hard to figure out what they are doing, when you have to chase the circuit from one sheet to the other and back again. I'd be interested in any more troubleshooting tips, especially concerning the inductance function. Mine works pretty well, but I'd like to get the inductance ranges calibrated closer. Before I tweaked on it, some ranges wouldn't work at all.

Tim:

I do not know if anything was ever published. I got this info from calling their service department. They were very helpful.

A quick update on this repair in the hope that it may help some future reader - or me.

I tested all the items that Tim had suggested. The 200Hz signal is perfect, the transistors are good and all the other items in that list checked out fine.

Still, nothing has changed. The display powers up with three seemingly random numbers. The zero controls do nothing.

I recently decided to check out the BCD counter (IC7) which sends the signals to the display board to tell it which segments to light, thinking that this chip might cause an unchanging display. There are four BCD outputs - two have strong pulses at all times and two have no pulses ever. This doesn't seem right. In checking this chip, the clock is good, the Memory Reset is low (as it should be), the Disable pin is low (again this is correct) but the Latch Enable pin only has a small pulse of about 2Hz at about 3V while VCC is 7V. Tracking this latch enable back is difficult since it's connected to lots of things.

but I don't think this is the problem. For one thing, I can make all numbers come up on the LED if I turn the unit on and off enough times. Sooner or later, each number appears.

My next area to consider is the Zero adjustment itself. It appears that this feeds some op amps that function as a Digital Volt Meter. The calibration manual even has you calibrate the DVM first by applying .9V on one of the inputs to these op amps and then adjusting for 900 on the display. The output of the DVM portion is pin 1 of IC6 on the analog board. I read very little voltage at this point and adjusting the Zero does not change it at all.

Going back to the Lead Zero pot, I can read no voltage to speak of on any of its leads. I'd think this would need a voltage on one leg (the other is ground) so the center tap could adjust it.

So far I haven't found that voltage source as the blankety blank schematic is so convoluted. I'll give it another try though.

Good luck. I hope my LC75 never has problems.