Monthly Archives: May 2014

Canned air

Little bit of troubleshooting tonight.

Bought a can of air (Component dusting spray, some complex molecule) at Radio Shack tonight and began dousing various components on the board until I got a good response.
After about five seconds of having been on, this one melted away the frost..

Gave it a good solid spray and switched on again. BEE-DOOP

Nothing useful on the screen but a POST beep is good.

Cleaned it all up and dried it out.
20140529_200109 by renault9gta, on Flickr

All good and well.. ordered a replacement. Decided in the interim to grab the soldering iron.

SHAZAM

Yup, that’ll do. Tried powering the board up without it in place. BEE-DOOP.

So, what is it?

It’s a SCN2681, a dual UART. It sits right up against and interfaces with the 8088 below it and also the funny socketed HP chip also.

Seems to be fairly central to the entire architecture (well, that figures really) so hopefully a replacement will liberate some life from it and allow some trash, or better, sensible data onto the screen,

I’m just hoping that “BEE-DOOP” is a good beep code..

…at least it’s A beep code.

Back to the 700/44

Back onto this after nearly a year of sat under the table. My Wyse burned up another 330k 1W resistor, so that got fitted with a 2W version with good ventilation.. back up and operating my hi-fi playlists.

Decided to turn my attention back to the HP. Decided just to shotgun the electrolytic caps to take that variable out of the picture.

First power-up I got blieurp-BLIIIIP out of the speaker and it filled the screen with stripes. The next power-up it went BEE-DOOP and did nothing… then nothing but a raster thereafter.

Sounds to me heat-related elsewhere on the board- there’s signs that this design runs kinda toasty so I think next up is a can of duster spray on all the diodes and chips to see if I can get it to come to life.

It’s probably going to be a Zener gone soft and bleeding voltage off to make a rail too low. I see several occurrences of 2.7V on the board, which is a strange and un-natural voltage (but then again, so is +38V) so that may be the culprit. It seems to try to work for the first few goes until it gets warmed up whereby it dies.

Still no service manual