MAX7219 Based 4 Digit Display Matrix

[Jerry]

Hi everyone,the pitfalls of buying dirt cheap China items from ebay,i use ebay a lot and have a generally good experience.There are a lot of fakes out there, but for experimental non production hobby parts,i dont see a problem,also bear in mind it makes sense to have a known full spec device (from Farnell/RS etc to swap out to eliminate out of spec faults causing headakes!!! Thats what i do.
ok, to the latest problem.about a couple of months ago i ordered 5x 4 digit matrix led displays which are based on the old but useful MAX7219 serial led display driver,the displays are cheap (under £4) so i didn't expect high quality.When they arrived 3 weeks later, i got to play with them,i must say there build quality was v good,with 8x8 matrix displays that were in turned pin socketets.So i thought i would make a bedside clock that was a bit different,animated effects,auto brightness,alarm,nothing to special, my eyes had difficulty seeing my watch from a distance,i needed a big display.About a month of programming,without using any libraries from the net!!! i had a working system and i must say i'm very pleased with the outcome.Remember this display has been reliable for several weeks,today i decided to add some more animation,powered on pic and programmer,display just had all leds on apart from digit 4 (first in serial line) which had 1 line of 8 leds that was v bright,i reprogrammed the pic and same happened again,did the usual and checked wiring,and whilst i had my hand on back of display it felt warm, so i took out the corresponding display and felt the top of the SMT 7219,it was hot(they usually run cold) as this is the first display in the serial line,it was obvious it was causing all the others in the chain to not get the expected serial commands etc.So basically its failed.These matrix displays i modify by replacing the fitted 10k Iset resistor with a 150k as i dont req. a very bright display(its pretty bright still),as this limits the led current to a very small amount, im surprised its failed.I will be in the workshop tomorrow to replace the 7219 and will report back with my findings.By the way all the 7219 chips have their part no rubbed off? interesting,having worked for a few electronic companies in the R&D depd,the only time that we scrubbed off the no's was to stop rivals from copying the design,as these displays say there 7219's in the ad,its a bit weird there scrubbed,might be to save law suits.
Will be back with my findings,its got to be the hot chip,but has it damaged the other 3,doubtful in my experience.
Ok so i unsoldered the offending 7219 and did a VI trace on the pins,all seemed ok (no shorts anyway) apart from pin 4,9 (both internally connected to GND) and 19 (V+),across these pins resistance was very low,around 10-15 Ohms,that would explane why the 1A psu didnt shut down! and the chip getting hot.Easily sorted it by soldering a known good one in its place from one of the other boards.My failure theory being,as this 7219 was first in the serial line,its inputs are connected to the 5 pin connector on the edge of the pcb,so could be static damaged,or tester error at the factory(presumably they do a quick test before shipping)?,remember static doesnt always knock out the silicon,but can weaken it,ive got a fully static safe workstation,so i doubt it was me,but who knows.Thats it for today,its fully working now,pleased it was a simple fix.Having worked in the electronic industry for 30+ years,ics that go short as opposed to just not working,is pretty common,i would guess input protection diodes failing,but without microscopic inspection,its anyones guess.
Happy New Year to you all.
J