Power Supply problem (?)

General discussions related to the Altair 8800 Clone

Power Supply problem (?)

Postby cluttered » October 5th, 2016, 6:55 am

[I tried to email Mike directly but his Yahoo address bounces. So I'll use the forum instead!]

I bought an Altair Clones a few years ago but it has stopped working and I’m looking for advice. I'm not a hardware guy at all, but I can work a screwdriver without injuring myself too severely.

The symptom is that the system appears completely dead; I think the power supply is no longer working. The fan does not turn, and I’ve measured about 0.4V DC coming out of the power supply even though 240V AC is going in to it (I’m in Australia.)

I was hoping to find a fuse that I could easily replace but I can’t see one. The PSU is a Mean Well RS-15-12.

Am I missing something obvious, or should I just buy a replacement PSU online somewhere? Or might the fault be something else?
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Re: Power Supply problem (?)

Postby Synoptic » October 5th, 2016, 7:15 am

Try an AC adaptor of same output rating than your actual power supply and see if it fixes it.
I think the thing runs on 12v. It should be marked on the sticker of the power supply somewhere.
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Re: Power Supply problem (?)

Postby AltairClone » October 5th, 2016, 9:11 am

Congratulations! You're the first documented failure I've received of Altair Clone hardware :)

My email address hasn't changed - not sure why that's not working for you. Email is deramp5113 followed by ASCII 0x40 followed by the yahoo domain name.

Most any 12 volt A/C adapter with the proper barrel plug (center contact is positive) will plug into the back of the front panel circuit board and work. The unit draws very little power (the fan draws far more than the electronics).

You can replace the Meanwell supply also. It's probably available from distributors in Australia so that you don't have to pay so much for shipping. If you can't find one, let me know and I can get you a power supply.

Mike
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Re: Power Supply problem (?)

Postby AltairClone » October 5th, 2016, 9:16 am

Actually, you may want to do some additional trouble-shooting first.

With the computer unplugged from AC, unplug the DC connector from the back of the front panel board. Then plug your computer into AC power and measure the DC output of the power supply right on the power supply terminals. Is it now 12 volts with no load? If not, then yes, the power supply has gone bad. If you measure 12 volts with no load, the problem is probably elsewhere.

If you find 12 volts at the supply, I'd next unplug the fan from the back of the front panel board and then connect the DC plug from the power supply into the back of the front panel board. If the computer works and the DC voltage at the supply is still 12 volts, then the fan was the problem. Otherwise, there is some sort of power to ground short on the front panel board.

Let me know what you find.

Mike
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Re: Power Supply problem (?)

Postby cluttered » October 5th, 2016, 3:17 pm

Thanks for the suggestions, I'll try them this evening.

As for the email address, I just realised that out of habit I used the ".au" variant of the domain. Oops!
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Re: Power Supply problem (?)

Postby cluttered » October 6th, 2016, 6:07 am

Hi Mike,

I've done the tests as you suggested and it looks like its the PSU. But I'm much happier that it's the PSU that is unwell rather than the rest of the system.

It looks like I can buy a replacement PSU (Mean Well RS-15-12) locally without too much difficulty or expense so I'll give that a try.

Thanks.
cluttered
 
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Location: Australia

Re: Power Supply problem (?)

Postby cluttered » November 15th, 2016, 6:26 am

Good news: all fixed!

I bought a replacement PSU from eBay for not much money which eventually arrived from overseas.

Because we moved house two weeks ago the Altair Clone was in storage for a while along with my tools, and it wasn't until today that I've had time to actually work on it.

It was quite straight-forward. I just needed to take notes of which connections were used for which, and then remove the errant PSU and swap in the new one, taking care to get the connections right and also to verify correct continuity using my multimeter.

So far I've only done a quick test using the front-panel switches but things seem to be behaving correctly.

I've also recently bought a VT340, so I now finally have a proper serial terminal for it, as soon as I buy a serial cable! The VT340 isn't ideal, it's a bit on the modern side, I'd prefer something more in keeping with the Altair era, so I'll keep looking for a more appropriate serial terminal. Although modifying some of my CP/M programs to support colour could be fun.

Looking forward to a proper session with the system this weekend assuming I find a cable beforehand. I can't remember if I need a crossover so I'll get one of those as well just in case.
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Re: Power Supply problem (?)

Postby AltairClone » November 15th, 2016, 11:30 pm

You shouldn't need a crossover for the Altair to connect to a terminal - just straight through.

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