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Dead IBM 5151 MDA Monitor

3lectr1c

3lectr1c

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Lucked into a 5151 today for free! It’s a bit dirty but in good shape seemingly, cosmetics wise.
I have a completely unknown MDA card pulled from a rusted out PC 5170. The card is corrosion free and looks perfect. Original full length card circa 1984. I plugged it into my Compaq Deskpro 286, and it gives the normal post beep with it inserted. I get no power to the monitor though. It stays silent, no sounds. Monitor is plugged into the wall.
Also tried with the monitor plugged directly into my 5170’s power supply through the pass through, no change. Any ideas? Am I just doing this all wrong? Is it my MDA card?
IMG_5430.jpeg
 
Was the deskpro configured for a color monitor or mono? Maybe, if it were set for color, it’s not initializing the MDA card?
 
Are there DIP switches I’d need to change? I got the normal two quick beeps, I’d expect there to be a different beep code if there was an error with the video card. I’m not well versed in this stuff though honestly. My experience lies with 90s stuff and beyond, mostly.
 
I think the deskpro 286 is configured with a setup floppy instead of switches. The 8086 version has switches.
 
Ah, yes, I do have the setup floppy. I’d expect to at least get video out first though? Or not? Not sure.

Another thing - the cable off the monitor is missing a couple pins in the connector. I assume that’s normal, or is mine damaged?
 
I’m not sure about how the deskpro reacts with an MDA card. I’ve got one I could potentially use to replicate your setup, but I’ve recently had hernia surgery, and I’m pretty sure vintage Compaqs exceed how much I’m allowed to lift for a few weeks.

It is normal for there to only be a few pins, since there are no color signals.

Modem7 has put some helpful information here
 
That link is useful - thanks. I’m sure hoping it’s just a config issue with my MDA card.
Noobish question - can I have the Deskpro’s normal VGA card, and the MDA card attached at once, and run both monitors at once? If so, I could try hooking up both, booting from the setup disk, and then setting to MDA and then see what happens.

Is it normal behavior though for the monitor to act 100% dead without an MDA card? It is completely quiet when I have it plugged in and with the computer running. No high voltage, nothing.
 
You might be able to just use the vga card, run setup to set it for mono, and then power it off and swap cards. My only concern would be if you cannot get the mono to work then you might get stuck. Though most of my XT class and later systems will still initialize a VGA card, even if set for something else, and just complain about it at first, but I cannot assure you that your deskpro will behave the same.

That is correct the 5151 will be completely dead without a signal.
 
Hmm. I don’t think the Deskpro getting stuck on dead mono would be an issue as there’s no CMOS battery in it anyway, it would just reset if it lost power. I’ll give that a go tomorrow and report back. If that doesn’t work, I’ll crack the back off the 5151 and check for anything obviously wrong.
I will add though that the 5151 is oddly light for what I’d expect a CRT to weigh - is that normal, or is there a chance mine’s missing internal components? The tube’s definitely there at least of course.
 
IMG_5431.jpeg
There is no option in setup for changing the video type. I’d assume based on that that it’s auto detect?
 
Nothing at all. It just flashes and goes back to the menu.

Here’s the MDA card:
IMG_5432.jpeg
I just noticed a dip switch diagram though - I’m going to have a look, ones monitor related.
 
Have an oscilloscope ?
If so, you could monitor the MDA card's horizontal and vertical sync pins to see if:
* Any sync pulses at all, and
* Frequency and polarity match what is shown in the 'MDA' column of the 'Sync Pulses' section of [here].
 
Thanks for the info - I don’t have an oscilloscope unfortunately. Will keep your note in mind when I get the back cracked.
 
The 5151 has a vertical scan oscillator which runs even if the V sync pulse is not present.

However, the H scan and EHT generation for the CRT relies on the circuit in the 5151 receiving the horizontal drive pulse, an 18.4KHz pulse coming out of the video card. And ideally this pulse frequency is correct or it can damage the 5151. Without that pulse the CRT would be blacked out.

If you open up the 5151, if the power supply rail is ok, you should see an orange glow from the CRT heater. And you can check the main power supply with the meter.

You can then check the V sync and H drive signals with a logic probe.

If the H drive (HOR) pulse is there, and the H driver stage, output transistor, flyback transformer are ok, then there should be auxiliary voltages from the flyback transformer, for example the 55V rail, that you can check with the meter. If the H drive pulse is present, but the output stage not working, possibly among other causes, the horizontal output transistor has failed (it is the "at risk" part in this design).

Without a scope or logic probe, you can check if the V sync pulse is there (likely then the H pulse would be too) by using a small speaker with a series 2.2k resistor to listen for a buzz. This is called an "audio probe"

Unfortunately, unless you are very young and can hear up to 18.4KHz the audio probe method is not much use to check for the presence of the H drive signal and ideally the tool is the scope, because you can also check the frequency and the waveform.

If the H pulse is present and the H output stage/flyback etc is generating EHT, often it is possible to hear an electrostatic cracking sound at turn on and turn off too.
 
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