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HP 700/44 terminal

PhilipA

PhilipA

Experienced Member
Joined
Aug 4, 2013
Messages
476
Location
Larose, LA, USA
All,

Rescued an HP 700/44 terminal from work today. Going to be discarded so I figured what the hey and snagged it.

Got it home, tried the power on it to check for bangs and smoke. It liberated neither, but it didn't come on either. No raster, no beeps.

Pulled it apart, checked the voltages.
12 was about 7.6, 5 was dead in the millivolt range and 38 was a healthy 38 on the nose.

Lifted the board and tube out. Sometime in the past it's had a repair, the warranty on it was out in 2001. Probed around the power supply section of the board, two of the three 33D diodes tested funny and looked a bit toasty so I desoldered then and tested them out of circuit. They tested good so went back in.

I'm at a bit of a loss with it. Does anyone have the service manual for one of these? I've found the user manual but that's not much help right now.

I don't think I can power it up out of the case either because the HT is integral to the board. The cathode heater glows, and that's about it. Somehow I've managed to stop the latch mechanism on the power switch from working also, the power switch is now just momentary...

Thanks

Phil
 
No schematics, but found a few leaky caps. Pulled them off the board, the rails went to 4.7, 11, 38 and the odd one out, the -12 was at about +4.7... see a pattern emerging?

Going to trace the -12 and see if it's decoupled anywhere.

Looks like this has been a problem for a while because the -12 smoothing capacitor has bulged out. Yuk.

Phil
 
haha oops. I was using the wrong "ground", I mis-read the labeling, it was somewhat hard to read with the back of the case on.

Poked about today and found the +38 good, the +12 at about +11, the -12 at about -10 and the +5 at 0, which is kinda odd because it uses the +5 as reference, as far as I can see. Or maybe it's the +12, it's not that easy to trace the board because it's covered in a big sticker and only silk-screened on the component side.

Working by the logic that if there's somewhere reasonable voltages present (at least in the right ball-park) then the power supply is mostly doing alright and the +5 is being dragged down to ground somewhere else in the circuit.

This'll be fun to trace, only 80% of the board uses the +5 rail...


--Phil
 
Replaced a bunch of burned-up looking PH-33D diodes with an approximate match (1N4006) and got the voltages on the board aroundabout where they should be- (+5.03, +12.45, +38.70, -11.3) and was greeted by a raster and a couple of beeps

hi-lo... low-low.

Which I presume is an error code (gonna go read the manual) but since turning it off then on again the raster has now shrunk and it doesn't beep any more. More investigation required.



20130901_175439 by renault9gta, on Flickr

But, have a picture, because, well, pictures.

--Phil
 
Usefully, HP didn't think to include the beep codes in the user manual.

Ech. Gotta pull the back off again to find why it refuses to boot.


--Phil
 
Without a schematic you're really shooting in the dark. Looks like you've made some progress though.

That model terminal is quite rare. If it becomes too much of a PITA, I'd just get a 700/92 or 700/96. They're much easier to come by.
 
Technically I don't need these; they fell into my custody, and as such deserve better than just being pitched into the dumpster. I'll get it going, but typical HP, it's fairly complex and proprietary under the hood.
Two steps forward, one step back. I think this one's gonna be a labor of love rather than anything else..

Phil
 
Horrible photograph, as I was trying to balance the tube (it's held in by gravity and will alone) and not let the board fall out and not electrocute myself AND take a picture at the same time:


20130902_141237 by renault9gta, on Flickr

Did manage to liberate this from it earlier. I know it's the computer equivalent of humming whilst strumming your lips but it does at least show the character ROM is working and there's a certain amount of life left in it.
It did that twice then refused to do anything else again. Slowly getting somewhere.


--Phil
 
Someone kindly pointed me in the direction of the HP "Service" manual for a terminal in this family.

According to HP, the recommended course of action is to "replace the main PCB". Manufacturers have a lot to answer for when it comes to today's throw-away ideals... it's not just the consumer.


Troubleshooting continues.

--Phil
 
PhilipA said:
Someone kindly pointed me in the direction of the HP "Service" manual for a terminal in this family.

According to HP, the recommended course of action is to "replace the main PCB". Manufacturers have a lot to answer for when it comes to today's throw-away ideals... it's not just the consumer.


Troubleshooting continues.

--Phil
Click to expand...

Does the Service manual have a schematic?
 
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


--Phil
 
Little bit of troubleshooting tonight.
20140529_194858 by renault9gta, on Flickr

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

20140529_205619 by renault9gta, on Flickr

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

So, what is it?

20140529_205629 by renault9gta, on Flickr

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.


--Phil
 
Just wanted to say good work and great pictures! Documenting the process is something I should be better at...sometimes I just get too busy to take pictures while I'm working.

Kyle
 
I've found it can be useful, especially if there's long periods of doing nothing inbetween.. I tend to forget where I left off lately. My memory used to be a lot better!

--Phil
 
Well, that didn't work.

Voltages have all drifted off also. Need to look at the PSU, and I really hope the EEPROM hasn't been wiped blank.

--Phil
 
No, never could find the documentation for it.

It's still under my kitchen table. I might shotgun the caps and diodes on it one day to see if that brings it back to life.
 
Thread revival!

Pulled this off the shelf today (new house, it remained broken though) and stuck it on the bench.

I took a look-see and was disappointed to find I had redone all the e-caps on the board already. That didn't fix it.

The voltages were a bit off, I had a thought though. Possibly the battery being dead is a reason for no POST. I'll try see if changing it (welded CR2032) makes the slightest bit of difference.

Phil
 
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