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 Post subject: LCR-4 MEGA328 power supply mod...
PostPosted: May Tue 19, 2020 3:29 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6467
Location: Indianapolis, IN
Hello All,

Had read about these awhile back when they sold for around $20, but didn't want to chance getting a bad one. A guy on FB recently recommended 'em again during discussion of a transistor radio problem, so looked on Ebay and found one in Ohio for around $7 with free-shipping and took a chance. It arrived with a loose LCD, but did seem to work very well for something so cheap. And other than the LCD issue (adhesive border in back-light frame no good), the build quality impressed me.

Noticed when assembling the acrylic case that the small ribbon cable on display would have to be bent tightly against back edge so that the rear panel could be installed. Didn't want to create any stress on the unit, so drilled and filed a rectangular "relief" hole so that the cable didn't have any additional tension.

9-Volt batteries aren't cheap and wanted to safely power this with a "wall-wart" so built a LM317 based regulator on a small piece of Vectorboard which I mounted inside the acrylic enclosure. Had a used coaxial power cable receptacle in parts cabinet which mated perfectly with plug on a 12V "wall-wart" in stash, so drilled necessary mounting holes for it very carefully using drill-press. Set up regulator to provide 9.2 Volts and it works great !

Along with the internal 9V regulator and LCD ribbon relief hole, added four small rubber feet using the enclosure screws by placing nut within.

Ready for action !

Imagine several here have these---what have been your experiences ? Has anyone added the connector so that firmware could be flashed ?

John


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 Post subject: Re: LCR-4 MEGA328 power supply mod...
PostPosted: May Wed 20, 2020 1:42 pm 
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Joined: Jul Wed 22, 2009 3:07 pm
Posts: 836
Hi John,

Just a quick note on that acrylic case you have for your tester.
By handling that case you could zap your tester!

It happened to me, I have built that small DSO-138 digital scope kit, and I also got an acrylic case for it.
Having built the scope it worked for about a week with the case installed. Then the TL084 input chip got damaged.
Having replaced the TL084 fixed the scope.
I am theorizing that it was electrostatic discharge because of the case.
Since then I installed protection diodes on the input.
https://jyetech.com/forum/viewtopic.php?f=18&t=1906

If you read the engineering manual for that Transistor Tester, it does not have any input protection in stock form.
The manual outlines ways of implementing input protection. From your pictures it looks like you do not have that.

I also have a similar tester, also got the case for it, but have not installed it in the case yet (no protection either).

Regards, Peter


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 Post subject: Re: LCR-4 MEGA328 power supply mod...
PostPosted: May Wed 20, 2020 3:47 pm 
Member

Joined: Oct Mon 26, 2009 10:02 pm
Posts: 6467
Location: Indianapolis, IN
Thanks for the warning, Peter !

The humidity is fairly high now in house (heat-pump failed), so ESD isn't much of an issue. But should get that done before fall & winter. Please let me know how you do it for your similar device...

John


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 Post subject: Re: LCR-4 MEGA328 power supply mod...
PostPosted: May Wed 20, 2020 4:38 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 836
Hi John,

Here is the manual I mentioned:
https://github.com/svn2github/transistortester/blob/master/Doku/tags/english/ttester_eng112k.pdf

I do not find the protection circuits in that manual, but I am sure I have seen it somewhere...
I will let you know if I find it.

The EEVblog forum has a long running thread (7 years and still going) on these testers:
https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/

EDIT:
Did find the protection, it is titled "Protection of the ATmega inputs", pg. 11, Chapter 2.2.1. in the manual.
I guess it was too obvious ;-)

Regards, Peter


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