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Fixing a PS502 that blows the fuse in the TM500 power module

 

I've got this PS502 that I've been trying to fix literally for years. I got it as a set with a TM503 + DM501 and DC503 from the surplus outlet of my local university (when I was still close to a student there, some time between 1998 and 2003). I think I didn't power it on when I got it, but when I tried powering it up a few years ago the fuse in the TM503 immediately blew.

It didn't take much to figure that the PS503 was the culprit. I replaced the main filter caps, which were completely dry, and that did not fix the problem. From there I was (at the time) completely stumped, and then I got distracted by other projects. Since then, however, I have gotten much more versed in the ways of old test equipment. This weekend I went on a Tantalum hunting spree, and managed to fix half a dozen dead 7k plug-ins just by finding shorted tants (only one of which showed any outward sign of failure).

With the heady rush of success I decided to have a look at the PS502 again, and sure enough it looks like there is a shorted tantalum cap (C35, just below and behind the LM309 5V regulator).

Looking at the schematic for the PS502, however, I am confused: the positive and negative outputs are mirror images of each other, coming out of U20 (dual tracking regulator), except that there is this 4.7 uF cap between the negative output and sensing lines, while the positive output and sensing lines have no capacitor between them (it should be labeled C25, if it existed). Why would you have this cap in the negative rail but not in the positive rail?

Also, it's not clear to me how this shorted cap could be causing the main fuse in the TM503 to blow?

-- Jeff Dutky
 

PS502 or PS503? You've said both.
 

Oh, sorry: it is a PS502, dual tracking supply plus fixed 5 V supply. Fairly empty inside. Here¡¯s the TekWiki page

 

Have you checked CR10 for shorts?

On 2022-04-13 1:37 a.m., Jeff Dutky wrote:
I've got this PS502 that I've been trying to fix literally for years. I got it as a set with a TM503 + DM501 and DC503 from the surplus outlet of my local university (when I was still close to a student there, some time between 1998 and 2003). I think I didn't power it on when I got it, but when I tried powering it up a few years ago the fuse in the TM503 immediately blew.

It didn't take much to figure that the PS503 was the culprit. I replaced the main filter caps, which were completely dry, and that did not fix the problem. From there I was (at the time) completely stumped, and then I got distracted by other projects. Since then, however, I have gotten much more versed in the ways of old test equipment. This weekend I went on a Tantalum hunting spree, and managed to fix half a dozen dead 7k plug-ins just by finding shorted tants (only one of which showed any outward sign of failure).

With the heady rush of success I decided to have a look at the PS502 again, and sure enough it looks like there is a shorted tantalum cap (C35, just below and behind the LM309 5V regulator).

Looking at the schematic for the PS502, however, I am confused: the positive and negative outputs are mirror images of each other, coming out of U20 (dual tracking regulator), except that there is this 4.7 uF cap between the negative output and sensing lines, while the positive output and sensing lines have no capacitor between them (it should be labeled C25, if it existed). Why would you have this cap in the negative rail but not in the positive rail?

Also, it's not clear to me how this shorted cap could be causing the main fuse in the TM503 to blow?

-- Jeff Dutky



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Murchison,

I thought that I had checked the bridge rectifier (CR10) for shorts, but either I had not, or I had not made good contact when doing so, because now I see that two terminals are shorted.

The solder joints have way too much solder on them, and a bunch of flux residue, maybe that's why I didn't find the short before? It took a couple tries to get good contact with the multimeter leads, but maybe I just didn't know what I was doing a year or two ago.

The part was replaced by a previous owner because what I see on the board is a generic NTE167. The original Tek part (152-0488-00) is specced as 200 V @ 1A, surge of 50 A. It looks like I can use a KBP202G or KBP204G as a replacement.

-- Jeff Dutky

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