Convenient excuses & expenditure justification

LCR meter kit from Jackson Harbor Press (thanks Bob)

LCR meter kit from Jackson Harbor Press (thanks Bob)

I’ve been wanting an LCR meter for a lot of years now but have refrained from buying one due to my skepticism of their accuracy. A voice in the back of my head is telling me that I could probably measure the value of a small-value capacitor a number of times and get a number of different results.

The variation would be due to factors like overall meter accuracy & resolution, lead capacitance, physical arrangment of the leads, body capacitance as the measurement is made, etc. But maybe I’m wrong, as often is the case…

In reading over the manual for the new Ft. Tuthill 80m kit, it appears that there are ten 0.1uF capacitors that should be matched. If not, the rig stands a small chance of breaking into self-oscillation.

From the assembly manual:

The kit will work best if the 0.1 uF capacitors of the filter section are matched. This kit has a total of 40 0.1 uF capacitors, and 10 of them are in the filter section. These are marked “104”. It is strongly recommended that all 40 capacitors are measured and the closest 10 caps in value are used as the filter capacitor parts.

The matching process involves getting a blank piece of paper, measuring each 0.1 uF capacitor, placing the part on the paper next to the recorded value. I suggest recording three digits if the measured value is over 0.1 uF (0.1xx uF) or two digits (0.09x) if the value is less than 0.1 uF.

Matching is not strictly necessary, but there is the outside chance that a filter section could go unstable and oscillate if 0.1 uF caps are used that happen to be far apart in value. Matching the 0.1 uF caps produces a very nice filter response with a relatively flat pass band.

LCR meters are cheap enough – I guess I’ll see if they’re of any real value in measuring component values we normally deal with in QRP construction projects. But the price of the kit just went from $50 to a bit over $100…… make that $65 thanks to Bob’s suggestion in the comments section – a decent enough price for a transceiver and a piece of test equipment.

.

.

  4 comments for “Convenient excuses & expenditure justification

  1. January 26, 2010 at 1:19 pm

    You and I have been on exactly the same journey as regards this particular project John. I have a meter somewhere that measures capacitance, but have a feeling that it only does small values in the picoforads.

    The lure of a direct conversion receiver with well designed and carefully thought out distributed filtering is just too great. I’ll be joining you in line to buy a capacitance meter pretty soon I think.

  2. Bob Reisenweber
    January 26, 2010 at 2:06 pm

    Hi John,

    I’ve had good luck with the Jackson Harbor Press L/C meter kit. Probably do a good job matching capacitors and for $15 bucks, or so, you can’t go wrong. However a nice professional unit would be nice also.
    73 de Bob W3BBO

  3. January 26, 2010 at 3:39 pm

    Dave – this whole idea of yours to get this kit is very bad – you’re going to Sunny Diego! Beaches, excellent weather, good food and chicas. You’re not gonna be breathing solder fumes amidst all that, are you?

    Actually I was starting to rethink this kit till I read Bob’s suggestion and I have now placed an order for the kit you mentioned, Bob. Here’s a link to it that also has the manual, schematic, etc:
    http://home.att.net/~jacksonharbor/lcmeter.htm

    I’ve always had good results with anything from JHP which includes various keyers and their VLF converter. Now I’ve got a kit to help me build a kit…

  4. January 27, 2010 at 1:26 pm

    I lived in the Los Angeles area for 21 years John, so sunshine and beaches don’t faze me enough to get in the way of my natural nerd tendencies. As for the chicas – well, thankfully, they are everywhere (bless ‘em!)

Leave a Reply

Your email address will not be published.