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Equalizer700
June 7th, 2009, 08:08 AM
Does anyone have any suggestions for how to safely power up an Altair 8800 that has not been turned on in 20+ years? It has been in a storage locker, appears to be in good condition, and isn't even dusty. One of the electrolytics was replaced 20+ years ago, the others are original. I am concerned about damaging the front panel electronics if the power supply is messed up...??? Is it safe to just power it up with no boards installed?

I seem to remember seeing a post somewhere about how to gently power one up, but I can't find it.

I have a Variac, if that makes any difference.

Any help is appreciated.

John

Erik
June 7th, 2009, 08:49 AM
Use the Variac to bring it up.

It's up to you if you want to do it once with no boards (to ensure that the PS isn't fried) and again with the boards to recondition the caps on the cards.

For most similar gear I take 4-6 hours to go from 10-120 in small increments. I've heard both that slower is better and that faster will work. . . YMMV. :)

RichCini
June 7th, 2009, 11:39 AM
Use the Variac to bring it up.

It's up to you if you want to do it once with no boards (to ensure that the PS isn't fried) and again with the boards to recondition the caps on the cards.

For most similar gear I take 4-6 hours to go from 10-120 in small increments. I've heard both that slower is better and that faster will work. . . YMMV. :)

I use this method as well on my Altair and my IMSAI when I first got them. I've also used a 40w or 60w bulb in series with the primary to identify a shorted transformer. I personally have never blown a cap, but it can happen.

Last time I did this, I increased the voltage about 5v every 10 minutes or so. On newer caps, I've done it faster (10v increments). YMMV.

You should also inspect the tantalum caps on the S100 cards and replace any that might be cracked. I've heard of them blowing, too.

Chuck(G)
June 7th, 2009, 02:18 PM
I'd be tempted to bite the bullet and replace all of the PSU electrolytics right off the bat. Compared to the value of a MITS 8800, it's not that much expense and easy to do. While the Variac method may avoid a shorted electrolytic, it's not going to diagnose the dried-out ones.

tezza
June 7th, 2009, 05:44 PM
Use the Variac to bring it up.


I have a question which will no doubt expose my great ignorance regarding these kind of things.

There has been a few posts lately about "computer rot" and computers failing when switched on after a long period in storgage. Would the use of something like a Voriac be of help here? Could you DAMAGE a computer (say an Osborne 1 for example) by gradually racking things up over a few hours?

Tez

Chuck(G)
June 7th, 2009, 06:28 PM
There has been a few posts lately about "computer rot" and computers failing when switched on after a long period in storgage. Would the use of something like a Voriac be of help here? Could you DAMAGE a computer (say an Osborne 1 for example) by gradually racking things up over a few hours?

This is a subject that you can find endless debate on in various web forums, with no agreement.

The theory runs like this: Wet electrolytic capacitors consist of a rolled-up pair of aluminum foil electrodes separated by a porous insulator and filled with an electrolyte. When power is first applied, a layer of oxide forms an insulating layer.

The idea is that the oxide layer degrades over time and that gradually applying a voltage helps to "reform" it.

This probably is somewhat true of antique capacitors, but in my experience, a modern bad capacitor that starts out bad doesn't get better, no matter how you apply voltage. Many old capacitors simply dry out and leave you with little more than an empty can.

So I'm not a Variac fan (I do have a few), but using a medium-to-high wattage incandescent lamp in series with the AC line the first time a unit is powered up isn't a bad idea for avoiding "magic smoke".

One more item to note is that the Variac method should never be used on a unit with a switching-mode power supply, such as a PC. The artificially low voltage can cause the power supply to be damaged due to excessive current flow.

nige the hippy
June 8th, 2009, 02:35 AM
I'm very much with the "bulb in series" idea, I used to use this a lot when fixing amplifiers etc. A dead short just turns the bulb on!

A couple of things I've noticed (I don't know how relevant) :-

RIFA filter capacitors on the mains inputs ALWAYS go bang (often with flames etc), I've taken to replacing them without trying to test them. The resin encapsulation cracks, and the internals seem to be hygroscopic, big mess!

Tantalum capacitors go short more often than is convenient. Sometimes they go bang and conveniently remove themselves from circuit. Other times they take something else out, I learned the hard way when a shorted tant took one of the fingers off my S100 backplane. I now have a small test backplane with polyswitch resettable fuses (and LEDs) on the power rails.

tezza
June 8th, 2009, 03:00 AM
Thanks for those comments Chuck(G)

In my brief experience with these things I concur with all those experiences Nige.

In my Obsorne 1 and BBC b the filter capacitors exploded in flames on first power up.

On one apple disk drive I had a shorted tantalum, on another I had one go pop with much drama.

Tez

arjoll
June 8th, 2009, 03:24 AM
The 'lamp limiter' is a great, and very simple, device. I've repaired some vintage valve electronics (radios and tape recorders) and its ideal - you get a current-limited supply very cheaply. One example is here (http://www.vintage-radio.com/projects/lamp-limiter.html) (UK site).

A variac may work fine with a linear power supply (mains -> transformer -> rectifier -> regulator) to test up to the regulator, but if you're running the thing on 1/2 power then you're giving the (say) 5V chips 2.5V - they're not designed for that, and I"m not sure if its a particularly good idea.

If your machine has a switch mode supply (many do) then I don't see how a variac would be that useful!

In terms of cap types - there are cases of sucessfully reforming electrolytics, but I don't see the point! If you're restoring a 1930's radio and want everything to look authentic then yes, there is a reason for doing so (you can also stuff modern ones into old cans), but I'd just replace any you're not sure of. Check the ripple, and check the current flowing through 'em, and replace if they're leaking (current) or doing nothing. If you're not sure, spend a dollar or two and replace them.

Tantalums - horrible things. Be careful, they explode violently. If there are any signs of distress replace them, but usually they just let go all of a sudden. Probably more likely if there is rubbish on the supply rail, so make sure your power is clean - back to checking the electrolytics....

Caps across the mains (like Tezza's Osbourne and BBC) - replace with X2 caps at an appropriate rating for your mains supply. They are designed to cope with transients on the mains without expiring violently! After having a supressor cap release its magic smoke (http://en.wikipedia.org/wiki/Magic_smoke) in a Sord M23 I'm replacing others as I find them.

Equalizer700
June 21st, 2009, 08:36 AM
Okay, I finally started powering up the Altair. I put a 60 watt bulb in series with the power cord, and hooked the whole thing up to a Variac. With the Altair boards all out and the power switch on, at about 20 or 25 volts on the Variac the 60 watt bulb begins to glow dimly. The Altair hasn't been powered on since 1981 or 82.

Is a 60 watt bulb a big enough wattage bulb for this?

The system has a Parasitic Engineering power supply upgrade installed. It replaced the original transformers and rectifier and added a very big (3ufd, 440VAC) capacitor. As I remember it, this supply was supposed to provide better regulation over fluctuating line voltages.

Assuming there is a short somewhere, what is the best way to troubleshoot it? I have a digital VOM and an oscilloscope. But it's been a LONG time since I had to troubleshoot a power supply. The schematic is available at http://96.0.36.155/parasitic.pdf.

Any advice is appreciated.

:D

John

nige the hippy
June 21st, 2009, 02:51 PM
Thought so....

quote from http://www.allaboutcircuits.com/vol_2/chpt_9/6.html ....

"No exposition on transformer regulation could be called complete without mention of an unusual device called a ferroresonant transformer. “Ferroresonance” is a phenomenon associated with the behavior of iron cores while operating near a point of magnetic saturation (where the core is so strongly magnetized that further increases in winding current results in little or no increase in magnetic flux).

While being somewhat difficult to describe without going deep into electromagnetic theory, the ferroresonant transformer is a power transformer engineered to operate in a condition of persistent core saturation. That is, its iron core is “stuffed full” of magnetic lines of flux for a large portion of the AC cycle so that variations in supply voltage (primary winding current) have little effect on the core's magnetic flux density, which means the secondary winding outputs a nearly constant voltage despite significant variations in supply (primary winding) voltage. Normally, core saturation in a transformer results in distortion of the sinewave shape, and the ferroresonant transformer is no exception. To combat this side effect, ferroresonant transformers have an auxiliary secondary winding paralleled with one or more capacitors, forming a resonant circuit tuned to the power supply frequency. This “tank circuit” serves as a filter to reject harmonics created by the core saturation, and provides the added benefit of storing energy in the form of AC oscillations, which is available for sustaining output winding voltage for brief periods of input voltage loss (milliseconds' worth of time, but certainly better than nothing).

In addition to blocking harmonics created by the saturated core, this resonant circuit also “filters out” harmonic frequencies generated by nonlinear (switching) loads in the secondary winding circuit and any harmonics present in the source voltage, providing “clean” power to the load.

Ferroresonant transformers offer several features useful in AC power conditioning: constant output voltage given substantial variations in input voltage, harmonic filtering between the power source and the load, and the ability to “ride through” brief losses in power by keeping a reserve of energy in its resonant tank circuit. These transformers are also highly tolerant of excessive loading and transient (momentary) voltage surges. They are so tolerant, in fact, that some may be briefly paralleled with unsynchronized AC power sources, allowing a load to be switched from one source of power to another in a “make-before-break” fashion with no interruption of power on the secondary side!

Unfortunately, these devices have equally noteworthy disadvantages: they waste a lot of energy (due to hysteresis losses in the saturated core), generating significant heat in the process, and are intolerant of frequency variations, which means they don't work very well when powered by small engine-driven generators having poor speed regulation. Voltages produced in the resonant winding/capacitor circuit tend to be very high, necessitating expensive capacitors and presenting the service technician with very dangerous working voltages. Some applications, though, may prioritize the ferroresonant transformer's advantages over its disadvantages. Semiconductor circuits exist to “condition” AC power as an alternative to ferroresonant devices, but none can compete with this transformer in terms of sheer simplicity."


I haven't a real clue about how ferroresonant transformers work at low input voltages, and whether that could explain the glowing lamp. More likely that it is some core loss & a bit of leaky capacitor drawing current. anyway you're almost certainly doing the right thing so far.
When it's cooked for a few hours at low voltage, I'd be inclined to test the "resistance" of the DC capacitors with your DMM, power off & wait for the caps to discharge first! Then look for the resistance reading increasing over a second or so up to infinity. Use the right polarity on the test leads. (I might be teaching my granny to suck eggs here!) and just check all is happy before cranking the voltage up a bit more.

incidentally, you're nowhere that uses 50Hz mains are you!

Chuck(G)
June 21st, 2009, 03:10 PM
You used to see Sola FR transformers in a lot of equipment; you could even get them as line isolation transformers. The reason they're not seen much today is because they do distort the AC waveform quite a bit and can create problems downstream.

If this were my system, I'd start by disconnecting the transformer from the rectifiers and then apply (with an incandescent in series) full line voltage to the transformer.

If the lamp lights to near-full brilliance, there are two possibilities: The 3 uF capacitor across the topmost winding is shorted, or (worst case) one of the windings is shorted.

If however, the lamp lights only dimly, that's good news.

Next, reconnect the +8v circuitry (green wires on the transformer). Apply current with your variac and series lamp. If you can get to near full voltage without the lamp glowing brightly, measure the voltage across to the +8 output--it should actually be more than +8 v, since 470 ohms isn't much of a load.

Repeat with the +/-16v supply (yellow leads).

That should at least get you to the point where you know what part of the power supply isn't working right.

Equalizer700
June 22nd, 2009, 12:36 PM
Chuck, Nige:
Thanks to both of you for great advice. I myself wondered if that capacitor across the transformer winding might be causing this. I'll check it out tonight, using both your suggestions.
John

Equalizer700
June 29th, 2009, 08:45 PM
I hooked the power supply up to a 0-120 vac variac with a 60 watt light bulb in series with the output, figuring that if the light lights there is a short somewhere. When I turned it on and got the variac up to about 25 vac, the lamp started glowing. I disconnected all the rectifiers from the power supply transformer and tried again with the same result. I disconnected the big 3 uf 330 vac capacitor that is across one of the power supply transformer secondaries and got no glow up to about 60 volts, where I stopped. I discharged the power supply 3 uf capacitor and measured across it with a vom. That showed no short in either direction (it looks open).

So I decided to start by replacing all the caps. They all shipped today, except the 440vac one, which will take 10 days. I'll be able to work on it this weekend and will report progress. Once that is done it should work, unless the transformer is bad :-)

Thanks for the help and advice!

Equalizer700
July 3rd, 2009, 09:42 PM
Would you believe the schematic I have is wrong? 3300uf capacitors are marked 330uf on the drawing, and a 25v one is marked 2.5v. I didn't notice it until I started to replace parts. So I have parts, but some are wrong. Replacements have been ordered and should be here about the same time as the 440vac cap. I was hoping to see some blinking front panel lights for the 4th, but I guess it will have to wait until next weekend :-(

Chuck(G)
July 4th, 2009, 08:25 PM
Would you believe the schematic I have is wrong? 3300uf capacitors are marked 330uf on the drawing, and a 25v one is marked 2.5v. I didn't notice it until I started to replace parts. So I have parts, but some are wrong. Replacements have been ordered and should be here about the same time as the 440vac cap. I was hoping to see some blinking front panel lights for the 4th, but I guess it will have to wait until next weekend :-(

Well, at least the 440VAC capacitor wasn't marked 44V!

Keep us posted.... :)