EEVblog #1354 - Compaq Portable Repair - Part 2

Dave breaths life into the old Compaq Portable from 1984.

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[adriansdigitalbasement]

Awesome! Yeah the 18khz/50hz signal you were seeing was the MDA scan-rate. The internal monitor on these is multi-scan and can run at either 15.6khz/60hz or the MDA rate of 18khz/50hz. Incidentally even though that scan converter says it can handle "CGA" it really means the 15.6khz scan rate... it's expecting an analog level input and no tthe TTL 5V signal that is coming from that card.

[KinChungE]

16:43 The menu says:
1. Image
2. Position
3. Display
4. Language
.. Exit

[Petertronic]

That GBS scan converter board is typically used in old arcade machines when changing them from CRT to flat screen. Used them a few times, with mixed results. It's cheap, but the more expensive ones give a much cleaner output!

[Oldgamingfart]

Perhaps it needs to be in the case for all the grounds to tie-up? A 'chassis ground', so-to-speak..

[tndabone]

The internal monitor is a different scan rate than the external 15K. The dip switches on the motherboard pick which kind of video card, not what video modes come out of the card.
Hook to the 9pin for standard video signals and hit ctrl+alt+< for 15k.. Also, your keyboard is probably shot because of the foam going to crap over the years. (I rebuilt a compaq portable last year, including the power supply)

[frankowalker]

The composite issue looks like the same grounding problem I had a few months ago.
Chuck in an ATX PSU and hook up a wire from Composite GND to PSU GND.
Great vid, Dave. Looking forward to the next part.

[memadmax]

From what I can remember, negative voltages are for the tape and/or disk drives?

Been a long time, memory gets hazy for that time period lol
Great work, Nice vid.

[sneugler]

A completed repair on the EEVBlog? Impossible!

[stanburton]

I worked with the guy who designed this machine. Back in the day.

[davidg]

You are a brave man to work on a Compaq portable. I remember working on those Compaqs back when they were still used and they were a nightmare compared to any desktop of the same era, as you have noticed. My "repairs" consisted of buying a replacement board and plugging it in, I never heard of anyone doing board level repair on those.

[nazteeb]

My second work computer! Fantastic machine! :)

[GadgetUK]

I wonder if the composite video bouncing all over the place is related to the missing ground on its connector? Surely that should be grounded?

[mangoman]

Only two days after watching the first video, someone in my apartment building dumpstered a November 1983 unit! It was the craziest coincidence I've ever had.

I've disassembled it fully and tested the voltages on the power supply, but I'm apprehensive of putting it back together and plugging it in for fear of the tantalums going full nuclear. I've ordered a bunch of 10uF 25V replacements, but is there anything else I should watch out for?

The unit came with quite a few upgrades, notably a multifunction 384K RAM expansion card and a 40MB Quantum HardCard.

I'm eager to get this thing running, and I'll definitely be watching along!

[fpmacko]

Thanks for this, Mate. I still have my Compaq Portable purchased as a Christmas gift in 1982. It's totally dead but now Dave has given me the urge to crack it open to see what's what. (If I can find it.) Dave --- I have a photocopy of the factory Maintenance and Service Guide. Lemme know if you want a copy. I also have both factory diagnostic disks but they're 5-1/4 inch so no easy way to dupe them.

[emmettturner]

I've had one of those GBS-8200 CGA2VGA converters back I'm 2016 for use with arcade boards. Since then, the video game community has turned it into a VERY competent scaler using custom firmware with some small tweaks/mods and an ESP8266 board to control it all via WiFi. The project is on GITHUB as "GBS-Control." I find it particularly useful since it properly handles 240p and consumer RGB (0.7v peak to peak, 75-ohm impedence) when it is very difficult to do anything with 240p RGB in NTSC land. :)

[FarrellMcGovern]

Well, the MDA and CGA are two different cards. MDA is a higher resolution monochrome card (max 720×348), while CGA is a lower resolution colour card ( max 640×200). You would need a different monitor to display an MDA card's output. We used to call the MDA compatible monitors TTL monitors, the CGA compatible monitors were composite monitors (cheaper), or RGB (More expensive!).

Although, in theory, the MDA card could output graphics, it didn't so. The Hercules monochrome card, on the other hand, could output graphics at the 720×348 resolution. This made it very popular for doing CAD. After the Hercules card came out, it took over the MDA market...no one bought an MDA card...I think that the Hercules card was actually cheaper than IBM's MDA card. A very popular use for the Hercules card was for AutoCAD. In fact, it was that combination that made AutoCAD the defacto standard. Much in the same way today that we use Blender for benchmarking systems, people used AutoCAD on a Hercules card (or clone thereof) drawing the Firehose Nozzle as a good all-system benchmark.

[flymypg]

Nice launch and belly-flop, but I was expecting more explosions at the end.

[rfmerrill]

Some older DRAM chips need +12 and -5 to work.

[leroycasterline]

Ah, the Compaq Portable - that was one fine (if heavy) machine. Portable computing and a nice workout in one package.

ComputerLand (remember them?) sent me to the 2nd ever Compaq training class for techs down in Texas. Before I left ComputerLand I bought one with every conceivable option (and damn near everything was an option - like a serial port or even a clock card so you didn't have to type in the date and time on every boot). I think I had nearly $6K in that thing!

[TheEPROM]

Consider your self lucky:
My IBM 5150 which I restored was dead when I got it. I had to replace all the buffer chips, Most oif the RAM & even the ROM as that had suffered bit rot. The last hurdel was the RAM dipswitches. But I was so happy when I got it to boot from MS DOS for the first time. Oh & there were RIFFA caps in the PSU, some did go pop.