4PM filter board power supply issues

Discontinued products Shruthi
#1

Hi all,

Just got my 4PM filter board stuck together, but I’m only getting a +5V on the red-dotted chip pins, not -5V on the blue pins. Also the LT1054 chip is getting worryingly warm after a couple of minutes. I checked and IC12, the 7905, has no -ve voltage on its O pin. It does actually start at -5V immediately after switching on, but drops to 0 over a few seconds. I measured some pin voltages:

IC12, 7905:
Gnd: 0V
I: starts at -8.1V, drops to -2V
O: -V, drops to 0V

IC11, 7805:
I: 8.1V
Gnd: 0V
O: +5V

IC7 LM1054 (all measured steady-state, after +ve has settled at 0V):
1: 2.2V
2: 1.6V
3: 0V
4: -0.64V
5: -2V
6: 2.6V
7: 1.35V
8: 8.1V

There is no continuity between ground at +5/-5 rails.

The DC power supply is delivering a nice steady 8.95V and is middle-pin posivie, so it’s being powered correctly. I’ve checked all the soldering on the power supply components, and generally and (although not the tidiest) it all seems sound.

ANY help would be hugely welcome, as I’m pretty much a noob with electronics!

Cheers,
Pete

#2

P.s. Output pin of 7905 should say “-5v, drops to 0v”

#3

is anything getting really hot? Sounds like a short on the -5 rail

#4

Check the polarity of the capacitors in the power supply region (the electrolytics and the tantalium one).

#5

Ahhhhhhhhhhhhhhhhh, the 10u cap is polarised? Damn, it looked similar to all my ceramic caps so I just assumed it was unpolarised Do you think I’ve blown it, or can I just reverse it and try again?

#6

Should’ve seen the + on the pcb… schoolboy error.

#7

Try it. It’s desirable to have it but it was optional on many of the filter boards anyway.

#8

Wooohoo, I have a working power supply circuit. Good ol sturdy capacitor, even after some drunken re-soldering :slight_smile: