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PG506 calibration unit question


Bill E
 

I just repaired a PG506, usual shorted cap in power supply. Getting to the caps required disassembling the beast (assembly cost must have been high on this).
Anyway, I found 2 mica washers on the floor, and sure enough they're in the parts list. But, I don't know where they go, can't see any obvious place. Anyone know?
Thanks, Bill


Craig Cramb
 

usual shorted cap in power supply.


What cap are you talking about. I’ve been trying to find an issue for low +,- 16 voltages for a while.

Craig


Jared Cabot
 

I had a quick look in my unit, but I couldn't see any obvious mica washers without dissassembling the unit. I did see two possible locations though.

On the left side, just behind the display is a metal can transistor bolted to the chassis in a little holder, there seems to be some sort of insulator there.

Also on the same side at the bottom near the front end is a TO-220 package screwed to the PCB with a sil-pad.

I can take as many photos as needed tonight after work if you like.


Jared.

On Wed, Nov 11, 2020 at 08:52 AM, Bill E wrote:


I just repaired a PG506, usual shorted cap in power supply. Getting to the
caps required disassembling the beast (assembly cost must have been high on
this).
Anyway, I found 2 mica washers on the floor, and sure enough they're in the
parts list. But, I don't know where they go, can't see any obvious place.
Anyone know?
Thanks, Bill


Dan G
 

On Tue, Nov 10, 2020 at 06:52 PM, Bill E wrote:

Anyway, I found 2 mica washers on the floor, and sure enough they're in the
parts list. But, I don't know where they go, can't see any obvious place.
Anyone know?
Based on my service manuals, there should be a mica washer placed between
the A1 board and the bottom rear frame tab. There is a trace that runs less than 1mm
away from the edge of the tab on my plug-in, so I imagine the washer is meant
to prevent accidental shorting.

That explains one mica washer. I am not sure where the second one would go.
I see only one mica washer listed in the mechanical parts list, both in the
early (pre-B040000) and the more recent manuals.


dan


Bill E
 

On the main board, right hand side looking from the front, upper rear corner near the inverter transformer. There are 4 caps, 2 4.7uf 25v, 2 47 uf 25v. For some reason, Tek put one 4.7 - 47 pair on the front of the board, the other pair on the back. They look like dipped tantalums, no printed markings, but color code stripes. They're pretty obvious, yellow tops. However, I have a relatively early unit (rev A main board), your mileage may vary. One pair is for the +16, one for the -16. I replaced them all with new electrolytics.
Whichever cap had a problem was totally killing my +16, and dragging all the other voltages way down.


Bill E
 

Excellent, thanks. I rechecked the parts list, one washer called for. Maybe it split with age. And, I just noticed the assembly diagrams, duh. Guess I never scrolled far enough down in the manual.


Bill E
 

Whew. After replacing the ps caps, voltages ok but various functions didn't. Function, that is. Put in the mica washer, all better. Yep, it was there for a reason, that reason obviously to correct a layout error on the board. But wait, the var amplitude wouldn't var. A tedious bit of schematic reading, test probing, the problem tuned out to be one contact in the switch associated with the var button, stuck closed. Even more tedious disassembly, managed to get that working. Now, finally, everything works. Even the calibration is pretty good. Isn't all this stuff fun?

Bill


Tom Lee
 

Great job, Bill! That beast is one of the more jam-packed PIs of the series. I know what a PITA it can be to work on, so way to hang in there!

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/11/2020 10:35, Bill E wrote:
Whew. After replacing the ps caps, voltages ok but various functions didn't. Function, that is. Put in the mica washer, all better. Yep, it was there for a reason, that reason obviously to correct a layout error on the board. But wait, the var amplitude wouldn't var. A tedious bit of schematic reading, test probing, the problem tuned out to be one contact in the switch associated with the var button, stuck closed. Even more tedious disassembly, managed to get that working. Now, finally, everything works. Even the calibration is pretty good. Isn't all this stuff fun?

Bill