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Electronics => Repair => Topic started by: Photon939 on July 05, 2016, 09:14:52 pm

Title: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: Photon939 on July 05, 2016, 09:14:52 pm
I picked up a rather sad and dirty looking BK Precision 2800 meter at a local hamfest for $2 on Saturday. After going to the store for some C cell batteries it seems to power up; the main IC seems to be ok but the two rightmost digits do not work.

From google images, mine is significantly dirtier
(http://www.davmar.org/images/TE/BK280.jpg)

Data into IC4 (75491) looks good, but there seems to only be a bit of sawtooth shaped ripple on the output pins. Bypassing the output pins with a 1K resistor to ground causes the respective LED display to glow so the displays are OK.

There is currently one guy on ebay in the US selling NOS Radioshack Archer branded 75491 chips, everything else is probably counterfeit junk from china. I would rather not pay nearly $10 for an IC if a modern chip is available with the same pinout.

I will post images and some waveform shots once I get home.
Thanks in advance
Title: Re: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: lowimpedance on July 06, 2016, 01:49:47 am
A quick look reveals nothing pin for pin compatable with the 75491.
The 'closest' internal functionaly speaking is the UL200x series,  BUT not pin for pin and also has common emitters (and 6 drivers not 4)
 www.diodes.com/_files/datasheets/ULN200xA.pdf (http://www.diodes.com/_files/datasheets/ULN200xA.pdf)
So if fixing the meter is something you must do (dont we all like a challenge!) then perhaps you could 'shoe horn' one of the UL types in , although I have not looked at the 2800 circuits to see if its feasible.
 Ultimately the $10 NOS part may be the easiest and cheapest option.
Good luck
Title: Re: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: Photon939 on July 08, 2016, 02:40:13 am
Thanks, I guess if I cant find any others I'll fork out the bucks for the NOS Archer part.

Here's a photo of the guts:
(https://dl.dropboxusercontent.com/u/1181169/20160702_161113th.jpg)
Title: Re: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: Kleinstein on July 08, 2016, 11:56:07 am
Functionally there is a chance to use just 4 FETs like 2N7000 to replace the function of the chip, if it is just switching to GND. At least one could try with 1 digit.
Title: Re: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: JFJ on July 08, 2016, 12:19:12 pm
Quote from: Photon939 on July 05, 2016, 09:14:52 pm
... everything else is probably counterfeit junk from china.

An eBay seller in the UK has a small quantity of SN75491N IC's listed that don't appear to be the unbranded version from China:
http://www.ebay.com/itm/SN75491N-/361618441090 (http://www.ebay.com/itm/SN75491N-/361618441090)
Title: Re: BK Precision 2800 LED DMM from 1976 - replacement for 75491?
Post by: FlyingHacker on July 10, 2016, 09:56:14 pm
Pretty slick meter. Gotta love LEDs.