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Replacing capacitors in Astec PSUs

SteveH

SteveH

Experienced Member
Joined
Apr 30, 2003
Messages
301
Location
Shropshire, ENGLAND
Hi,

Last year I re-capped the Astec PSUs in my TRS-80 Model III. As advised by you guys here, I replaced the two metalized paper Rifa class-x caps with modern metalized polypropylene equivalents. The thread was here.

Anyway, I’ve just acquired a non-working Digital Microsystems 816 terminal that happens to have an Astec PSU of very similar build to that in my TRS-80. However this PSU appears to have four metalized paper caps.

The attached images show the minor difference between the TRS-80 and DMS-816 PSUs.

1. TRS-80 III PSU showing 2x metalized caps
1st_psu_top.jpg

2. DMS-816 PSU showing 4x metalized caps.
2nd_psu_top.jpg

So my questions are:
1. Is the cap marked 0.22uF K an x-class cap or not? The markings definitely show a K and not an X.
2. Is it OK to test these PSUs without applying a load?

Thanks,
Steve

[/edit 1]
Although it doesn't answer my questions, I've just found the description and schematic for this Astec AC 8151 PSU on bitsavers. http://bitsavers.trailing-edge.com/pdf/astec/Astec_AC8151.pdf
The PSU in my DMS-816 is the AC 8151, it's just that the PSU model number isn't visible in the picture.
[edit 1]

[/edit 2]
I've now found a repair manual for the Astec AC 8151 PSU here http://primrosebank.net/computers/mtx/documents/Astec_AC8151_rm2.pdf (3Mb download).
This answers my second question regarding applying a test load, and it even shows the 0.22uF cap as being a MTL Poly Cap 0.25uF +- 20% 250V. But is that x-class or not?
Thanks.
[edit 2]
 
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No, I don't believe the other two caps are X-rated--in fact, one looks to be a plain old ceramic. Generally, you want X caps when they're placed directly across the AC line or from one leg of the AC line to ground or chassis.

The idea is that X caps, should they develop a short, quickly melt the thin metal film around the short circuit, so they do not fail shorted. Ceramic caps can and usually do fail shorted--so the result can be a fire or shock.

There's nothing wrong with using X-caps in all 4 positions, but it's not necessary.
 
The 0.22s aren't class X (judging by size) try to get a self-healing type (metallised film) also check the electrolytics on the output - 3 in a row, they look a bit bulgy.
apply a load. it's a way of testing the psu, rather than merely powering it up. a headlight bulb on the 5V outputs is usually ok. - always use the 5V output if you're going to load anything, because it's the one that the output voltage is usually measured on to provide feedback.
 
I usually use an old hard drive as a load for that sort of thing, since it uses both 5V and 12V.

Just don't use a drive you care about, in case there is something wrong with the PSU.
 
Thanks Chuck, nige, OT,

Looks like I was editing my question while you were all replying. As I've already got several x-class 0.1uF and 0.01uF, I'll use those and I'll get hold of a non x-class 0.22uF. I'll also apply a load as per my edited question. I'll try those changes first before looking to change the 3 x 1000uF electrolytics.

Thanks.
Steve
 
It's times like these that hoarding does have it's advantages (much to the annoyance of her indoors). I've got one such drive that spins up but isn't recognised. I'll just refer to it as my new load test device :D
 
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I have just got a non working 816 terminal cheap as it will not power up, with this thread I should get it working, if I am lucky all I will have to do is replace all the electrolytics in the psu with new hi temp ones, thanks for the info
 
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Hi Rory,
So you bought the other one they had? I couldn't get the PSU working in the end, despite replacing the filter caps, the electrolytic caps and some diodes that had also failed. So I put that PSU to one side and made up a connector to allow me to hook it up to a mini ATX PSU. It works and for the time being I've fit the ATX PSU, less case but with some shielding, inside the monitor.
Good luck with your PSU.
Steve
 
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