Dave,

Yes the sockets have caused intermittent connections.? The sockets are form Keystone Electronics P/N 4608 which are still available.? It appears that the brass contacts get annealed over time and the insertion force on one of the units I worked on had gone to ZERO!? After taking the screws out of the collector, the transistor would have fallen out if it were not for the sill pad insulator.?? With new sockets the transistors fit tight!

Yes the voltage across C6 has to be measured at the cap because it is floating relative to ground.?

?I am thinking of designing a small board that will crowbar the -12.6 output if it goes beyond -14V.? I wonder if anyone would be interested in such a module?

Mike

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Dave_G0WBX via groups.io
Sent: Friday, September 13, 2024 10:04 AM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 141T Power Supply Help Needed

Take care with taking voltage measurements around the 141T PSU -12.6V regulator.

It is not "ground referenced" at the input.? It is essentially a floating Positive regulator with it's Positive output tied to "Ground", resulting in C6 "floating" at some arbitrary voltage on it's Positive side, though it's Negative side "Should" be at (or very close to) the
-12.6 Output voltage.

To measure the raw DC voltage, you need to measure across C6, not from it's +ve terminal to Ground!

Re power transistor sockets.? Over the years I've always found them to be a major cause of all sorts of weirdness.? Best abandon them, as once the pin sockets have lost their spring temper, you'll never get a reliable contact between them and the power transistor Base and Emitter leads.? Just fit the device conventionally, taking care with any needed insulation between the device case, and the heatsink, including insulating washers for the fixings etc.

If the collector of Q4 does get shorted to "Ground", the -12.6V will end up at an uncontrolled -ve voltage more(or less) equal to the raw voltage across C6.? Not good for some of the down stream loads, such as perhaps, the CRT flood gun filaments!? Especially if there is no (or faulty) downstream crowbar protection, that should fire and "Blow" F4 if such a thing happens.

Apologies if I'm preaching to the converted, I'm not trying to teach anyone to suck eggs!? Just to raise awareness re the layout of that particular regulator circuit, as it is not entirely straightforward.

73.

Dave B.



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