Astron VS-35M blowing its fuse

[K3GHH]

Since I didn't know how to test the 2502's, I replaced the pair. As I think about it, though, my replacements are 5010s, and the reason Astron used two 25-amp rectifiers was to wire them in parallel for the extra current capacity... so I bet I could have replaced the two 2502's with ONE of the 5010's.

But I'm obviously a novice at working with solid-state components, and just wanted to get my PS working quickly, so I wasn't inclined to experiment.

[K8AXW]

GHH:  You made the correct move.  Whenever you replace one of two identical components with a different model number you run the risk of failure.  In other words, by replacing the parallel diodes with the one high value diode, it's a good move to do the same with the other pair.

I feel as you do.... let's just get the darn thing fixed!  Good job!

[KB5ZSM]

If I understand you right, you still have the rectifier in circuit. Do you have an Ohm Meter? Why hasn't anyone suggested using an Ohm meter? Disconnect one of the A.C. leads from the transformer to the rectifier and then check the rectifier for shorts (I assume the red and black lead on the rectifier are still disconnected). The bridge rectifier is just four diodes in one package with four leads (+,-,AC,AC). No pair of leads may show a short circuit. Every pair of leads you test should show a diode action where the leads in one direction will show infinity and in the other direction a much lower Resistance (at least a ten-to-one ratio if not a lot more). Any shorts and you found your culprit.
I also like the idea of checking the MOV that was mentioned earlier. That was the disk looking thing on the primary side of the transformer that you mentioned also.
If the pass transistors were shorted, they would cause your output voltage to go way up causing the crowbar circuit to engage but since you disconnected that part of the circuit, you already eliminated that as a probable cause.
Good luck!

Win (KB5ZSM)
Tip Top TV

[K3GHH]

If I understand you right, you still have the rectifier in circuit. Do you have an Ohm Meter? Why hasn't anyone suggested using an Ohm meter?
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Good luck!

Win (KB5ZSM)
Tip Top TV

Win, the power supply has been repaired and is working fine. The original KBPC2502 bridge rectifiers are sitting on my desk; I replaced them with two KBPC5010's from a recent hamfest. K8AXW (and probably KD8DEY) explained how to test the originals with an ohmmeter, and I'll do that just for curiosity. But this little adventure is OVER!

--John K3GHH

Edit: Win, I should add that I very much appreciate your willingness to share your expertise (expertise I obviously do not have!) and the time and thought you devoted to your advice. The fact that the PS is now working doesn't change that in the slightest!

[N4EF]

I had the same trouble with my RS-35A blowing fuses after a lightning strike.  I found the technical hand-holding very helpful and the comments from experienced electronic techs were awesome. Thank you.

My Astron RS-35A is now working again: one of the 2N3055 pass transistors was bad. For the sake of clarity for those (like me) who haven't dissected their Astrons before, that the 4 pass transistors (on the back of the power supply, within the heat sink) can be replaced in 4 minutes without soldering as they unscrew and can be pried out. The new transistors plug in only one way. Easy.

Dave
n 4 e f

[K8AXW]

ZSM:  Win, you're obviously correct.  It would have been a simple matter of checking the diodes with an ohmmeter..... for you, me and many others faced with the same problem.  However, with GHHs lack of expertise in solid state electronics, I felt it was easier to suggest what components could be bad and just replace them.  Assuming that he did properly test the rectifiers and found one or all bad, he was going to have to find replacements anyhow.

As it was, it worked out.  No doubt, after experimenting with the defunct diode(s) he'll learn and know the next time. 

Your good advice will no doubt be noted and remembered by other readers of this forum.