Altronics K2533 LC meter - troubleshooting - Page 1
Forum section: electronics / projects

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: January 18, 2013, 11:58:30 am
content: Just wondering if anyone's successfully troubleshooted the K2533 LC meter - as featured in SC May 2008 and reviewed athttp://tronixstuff.wordpress.com/tag/k2533/The reason for asking is I'm grappling with a just built kit.The unit turns on, showing 14.8 nF (should be a few pF). All this is tied to the local oscillator's frequency, which is discussed next in the manual.The unit succesfully generates F1 with a display reading of about 51000 (it needs to be between 42000 and 58000) with the jumper in LK2.However I think it stops oscillating when moved to LK1. The LCD reading is 000028 when it should be about 71% of 51000.* The first thing I did was try another 100uH inductor. This was because the old one was hard up against the metal of the switch and this might cause the oscillator to drop out. Same results - except frequency a little higher (53000) and nothing when on LK2.* Next I questioned the value of C2 (as suggested in the manual). Replaced it with a 1000pF disc ceramic. Again no change.* The relay was then suspect. So I wired the added ceramic across C1. Success! Output dropped down to 38000 which is about right.* So I then tested the relay. Oddly this was also OK.All this is consistent with an oscillator that oscillates reliably at the higher frequency (LK1) but not at the lower frequency with the extra capacitance + relay. But it will work OK with the extra parallel capacitance without the relay.I am tempted to change values a bit to get reliable oscillation on about the specified frequency. eg a 47uH inductor and 2.2 nF capacitors may be more reliable. Or conversely 220uH and 470pF.Any thoughts on a fix would be appreciated!Thanks,

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: January 27, 2013, 02:06:26 am
content: Thought I'd post a follow up after a few days of sitting on it.It now works!The problem was with the oscillator's reliability.I ended up putting another 100uH in series with the existing one.Instead of being parallel to the board they now stand up, with the connection between the two soldered off the board.Fortunately they don't stick up above the switch so the front panel can still be installed.The greater inductance has meant a lower output frequency. Instead of 50000/35000 it's now 35000/25000. But the oscillator now starts reliably on both jumper test settings.This does not seem to have affected the readings or accuracy. As the ratio between 1 and 2 capactitors in the circult seems to be more important than their actual values I ended up retaining the original supplied 1nF caps.While the design is fine, aspects of its execution (eg the PC board design and the metal-sided switch supplied) may have caused the above problems with the particular parts used. A variation in values solved the problem.RF chokes used as elements of a tuned circuit should be kept away from metal (at least 5- 10 mm?). The kit's use of a metal-sided switch hard-up against the side of the choke may have detuned it or made the oscillator hooked up to it a less reliable starter with certain parts values.Maybe the kit was developed with a plastic sided switch, or other chokes allowed more reliable oscillation on many other units built.All this illustrates the subtleties of translating a design into a workable and reproducible product. In this case allow a bit of space around tuned circuits.

author: johnboxall
author_rank: Unknown
timestamp: January 30, 2013, 05:46:39 am
content: Nice fix Peter. You should tell Altronics about it, kit feedback is crucial for success and the production of more kits.http://www.altronics.com.au/index.asp?area=cont