ASTEC AA13780 (1988 vintage)

[itwentboom]

I have problems with an ASTEC AA13780 SMPSU from 1988 in an old Mac II, basically the output voltage is a tad low causing the computer to fail, the first clue was the buzzing sound in the audio loudspeakers suggesting the caps are on their way out. So the plan is to replace the caps, easy thing to do, but I wonder if it is possible to adjust the output voltage a tad. There seems not to be a preset (variable resistor) so I assume the circuit uses a zener or some other reference.
I guess the answer is no, but anyone have a circuit diagram? Or know how the PSU gets a reference.

thanks

[PeteS in CA]

Re: ASTEC AA13780 (1988 vintage)

The chances of just about every lytic being nearly dead is high. Astec made (and makes, I assume) good stuff, but the thing is 25 years old. And therein lies the problem. Common low impedance series like Nichicon PW, Panasonic FC and UCC LXZ may be so much better - i.e. lower in impedance - than what was common 25 years ago that they could cause stability issues if used as output caps. That won't present a problem with the input lytics, and you may have a couple of "pleasant" options.

Starting with those input lytics ... you can do one of two things (or compromise and get some of the benefit of both choices). You won't have to worry about matching the ripple current capability. Modern parts will be better without causing problems. If you match the value, the new part will be shorter, possibly much shorter. This will improve airflow and cooling, which is a good thing. On the other hand, if you match the case size, the parts will be significantly higher capacity, resulting in better input voltage filtering (probably invisible to any user) and much better hold-up time during a line voltage drop-out or sag (this could be nice for rural people and places with thunderstorms). Or it might be possible to pick a part that is both smaller and higher in capacitance, just not the smallest or the highest. I think I'd suggest going that route, improve both the cooling and the hold-up time.

Now, the output caps ... a lot depends on what you have available to you. Nichicon seems to obsolete older series that are popular more slowly that UCC (NCC) or Panasonic. Nichicon's PM series is the RoHS version of the PL series, which was introduced some time around 1988 or 1989. So if you have PM series parts available to you, you can use those. Nichicon's PS series is the RoHS version of their PR series, which came out around that time as well. The next better series that would probably be usable without causing a stability problem would be Nichicon's PJ series or UCC/NCC's LXV series (basically equivalent to each other). Unless it's the "worst" available to you, I'd recommend not using Nichicon PM or UCC/NCC LXZ or lower impedance series. Their ESR could be so low as to cause stability issues. The other "issue" may be case size. If you match the value and had to use PJ or LXV there is some chance the case will be smaller. As long as the lead spacing matches (mainly an ease of installation issue), shorter and/or smaller outside diameter will simply mean better airflow and cooling.

[PeteS in CA]

Re: ASTEC AA13780 (1988 vintage)

As to voltage adjustment, pretty much any regulated P/S has to have a reference - a zener, a TL431 or other reference IC, or a reference inside a PWM IC. IIRC, Astec liked TL430/TL431 reference ICs, which is in a TO-92 package (looks like a small plastic transistor). Pots are more expensive than fixed resistors and less reliable (comparatively) over time. So Astec may have used manually trimmed resistors in the output sense circuit (2 or 3 in parallel, with one snipped out at test to adjust the voltage), or they may have had one of the values selected and installed at test. So look for what looks like a missing or snipped out resistor on the output side, and look for a reference IC (very possibly a TL431 or TL430).