Re: Modifications to 577 Curve Tracer Main Power Supply

Ed Breya

Do you have enough access to make live measurements in this section while it's hooked up and powered? If so, you should be able to check some operating voltages and find the problem. Is the -30V zero, or some partial level?

If access during operation is very difficult, a good trick is to pick certain nodes, and tack probing wires onto them, long enough to be brought out into the open. You have to of course be very careful to not short any of these temporary test points.

There is no "lock-up" condition you alluded to - it should start up just fine, unless there's a failure. If VR772 has no current from the -30V output via R772, there should still be plenty (a few mA) to pull it down via R774, into a regulation range where it will produce some output, and then get VR772's operating current to the normal level. This arrangement may be to provide some degree of foldback current limiting, but it's more likely for improvement of line ripple rejection.

The usual suspects in old Tek gear of this vintage, are Ta caps. Only C781 and C772 appear to be candidates for close inspection. First determine what types they are, then act accordingly. If C772 is shorted (or partially shorted), for instance, then VR772's voltage would always be stuck near ground, and the -30V output would only reach a few volts. C781 would of course short the output, if failed short. The next suspect would be Q772 - if it's shorted or breaks down, the output will be cut off completely. And so on down the line.

You may have checked all these parts already, but not necessarily at operating conditions. That's why the best way to diagnose is with actual circuit measurements, directly or via temporary test points. I'd suggest the minimal set would be the bases of Q772 and Q786, which will tell a lot. If temporary TPs are needed, and it's difficult to remove and replace the PS board, then lots of wires will minimize the number of times this is needed. In this case, the BEC of Q772, the BC of Q786, and E of Q788 should be accessed. These should quite quickly and thoroughly show what's going on - it's actually a pretty simple circuit.

One last thing to consider is "mod failure" - if the various parts indicated with value changes or are tacked onto the circuit, look at them closely for proper installation and connections.

Good luck.

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