Studiomaster EP&-N power supply question

[Randall]

I have a Studiomaster Trilogy 166 in house with one working and one dead external power supply type EP7-N. No schematics to be found, but it's a pretty straight forward supply. It has two sections, a 1A bridge rectifier, diodes and caps for the 48v supply, and another rectifier, diodes and caps for the +/- 15v regulators. The secondary of the PT has two windings. One supplies the 48v side through a fuse. The other winding supplies the +/- rails through two fuses, one on each AC leg of the bridge rectifier, and both of these fuses were blown.


Since it is nice to have working one to compare to, I checked everything I could think of and did side by side comparisons, but found nothing suspect. All the diodes, both rectifiers, filter caps, and regulators looked OK. So I put it on my bulb limiter and switched it on. Bulb barely lit up, the board lights came on, so I switched it off and plugged direct into the AC and it blew both fuses again (albiet they were 500mA, not the 1.6 amp it calls for). more checking, still nothing, except for one thing. I noticed a washer securing one of the regulators on the back of the heatsink was touching the ground buss, and I was getting 10 or so ohms from it to ground, so I loosened and corrected that, but I don't think it was the problem because it looks like everything is insulated. But to satisfy my OCD, I remedied it.

Then I put it back on the bulb limiter and have all my voltages on the connector, +/-15v and 48v. I don't understand this. I don't have the proper value fuses on hand, but from what I have described I half expect it to still be blowing those fuses since I haven't actually found anything wrong. So, my question is, how could this be? Given the inexpensive parts, I am almost set to order up a LM7815, LM7915, and a bridge rectifier, but I know Enzo won't care for that approach. Is it possible that one of these devices is working on the bulb, but shorting under actual load?

[Leo_Gnardo]

You're smart to use the bulb limiter BUT it may be those fuses - undersized - pop when presented with full charging current at switch on without the bulb limiter in place. I thought you had a variac - if you do now's the time to break it out & dial up the voltage over the course of a couple seconds. If the fuses hold, then commit to the specified fuses & you're probably good to go.

And why don't you have some + and - regulators in the parts drawers? Heck I order 10 at a time from Mouser or Newark or Digikey, it may take a couple years to use 'em all but I figure it's cheaper to stock 'em rather than pay UPS FEDEX or USPS to deliver one at a time. Shipping, crikeys it'll kill ya! (Sez the guy who just put in 2 orders from CE and Mojo, $20+ shipping each ) Like a good scout, be prepared.

[Jazz P Bass]

Say what?

The supplies call for 1.6 A fuses & you are curious why the 500 ma fuses are blowing?

[Randall]

No, I'm curious why the 1.6A fuses blew in the first place.

[Leo_Gnardo]

No, I'm curious why the 1.6A fuses blew in the first place.

There's the usual reasons, overcurrent from some fault. And there's old age. Typically there's a current peak every time the fused item is switched on, after a couple thousand times the fuse wire gets brittle from overheating & flexing with every power-up. If there's a fuse say on each of a pair of transformer secondary leads as in some console power supplies, one lets go, then the supply tries to charge up thru the other, that lasts a couple milliseconds and the second fuse opens up. Seen it happen with Yamaha console supplies the size of big power amps, 15 amp fuses and they always go in pairs. Guess who always had spare fuses?

[Enzo]

You found a washer shorting a regulator tab to ground, corrected it, the fuses no longer blow, and it is still a mystery?


Look at the data sheet for the 7915, and what function the tab serves.

[Randall]

Enzo as always I value your input, you have taught me much. I knew the tab is connected to the middle leg of the 7915, but it seemed to me with the isolation washer and mica that it wasn't probably the issue. Please correct me if I'm wrong. I saw no short between the tab and ground, only the washer touching the ground trace, which I am not sure was the problem.

[Jazz P Bass]

It may very well be that you will never know why the fuses came in blown.
(seeing that the unit powers up fine on a lamp limiter)

Not too sure what a metal washer floating around can do but it can't be good.

If you want to know what current is going through the fuse block, either hook up a meter to read current or insert a resistor, measure the voltage drop & calculate the current.

[Randall]

Today I put a 500mA fuse on one input of the BR in question , and a meter to measure current on the other. I brought it up on my variac under load, and it works. And it draws next to nothing, so the fuses must be rated for power on surge current. So perhaps it was the regulator mounting washer, maybe it was a little nut that was floating around loose in it, maybe it was a power surge ( I do live in the lightning capital of the USA), or as someone said, maybe I will never know why the fuses blew. But it seems to be working now. So as always thanks so very much for the insight. I do deeply appreciate this place.

Oh, and I did order some +/- 15v regulators to have on hand.