PG506  Repair - 20v & 50v standard amplitude is low - Page 1
Forum section: electronics / testgear

author: N3SWR
author_rank: Newbie
timestamp: February 18, 2019, 05:13:58 pm
content: Looking for a little guidance by those that know this calibrator better than I do....The calibrator in question is of the higher serial number vintage - above B0500000 if I can recall - I don't have the calibrator in front of me at the moment so just going on memory here. It's more for letting you know wait schematic is being used here.I have four 2400 series scopes to calibrate after repair so I need to get this up and running. I went through the calibrator and did a recap of old electrolytic's and suspect tantalum's as well. I cleared up a few issues along the way doing that and replaced an open (overheated) transistor too . Apparently this unit was put aside for a "we will look at another day" excuse. I saved it from being tossed out some years ago and now I need it. It's in decent shape as well.After running through the calibration procedure - with pulse switch in the up position, all standard voltages came back to life except the 20v and 50v. They all indicate low. 20V is approx. 17.3v and the 50v is approx. 45v or so. The 100V is spot on as well as all the rest.I took a hard look at the -72V rail and that is fine - except for when the 20 and 50v voltages are selected via the dial. The rail drops to around -45V so, it's being loaded down I feel.That large rotary amplitude switch - not necessarily a nightmare but man it will send you across the schematics in a few directions that boggle my mind. For the 20V and 50V positions, I see switch 8 and 9 switched in accordingly and of course separate circuits.So before I start pulling legs up from components and chance doing damage as it is a bit cluttered in there, anyone have an idea where the problem may potentially be? At this point I'm not sure where to try and split the problem in half from a troubleshooters perspective just to look for the bad section and help me narrow this down.Been through what I could find on the forum but I think I honestly ran out of leads unless I missed something...Thoughts???Thanks to all either way as this site helped me big time in keeping some valuable gear from the dumpster

author: MaxFrister
author_rank: Regular Contributor
timestamp: February 19, 2019, 02:29:31 am
content: I repaired one of those awhile ago. Not a fun format to work on. I'm looking at a B040.. schematic so perhaps things have changed.Somewhat strange. I'm not seeing a lot that would break 20v and 50v but allow 100v to still function.1. Check R276-278 divider.2. Dirty contacts on that switch.Neither of these really explains dragging the power supply down.

author: N3SWR
author_rank: Newbie
timestamp: February 19, 2019, 01:07:49 pm
content: Exactly - that's why I'm scratching my head on this. I may attempt to remove the rotary switch but I really don't want to go there.I will have a look at that divider over the next day or so. I can't get any bench time for a bit so replies will be spread out a little. However, thanks for the effort!

author: siggi
author_rank: Regular Contributor
timestamp: February 21, 2019, 11:13:08 pm
content: Quote from: MaxFrister on February 19, 2019, 02:29:31 amYups - not much else to do there. If you measure the voltage over CR280A, (Vcr280a) then shunt it with a current meter, you should see a current of (100+Vcr280a)/(9950+6037+1212.8+2800), nominally 5.00mA. I'd guess the top resistor in the divider chain has drifted high, and you can calculate this from the voltages you've measured. I'd save you the trouble except I don't know how you measured these voltages, and note that this output is high impedance. Its accuracy depends on being shunted with a 1MOhm scope input.Quote from: MaxFrister on February 19, 2019, 02:29:31 amThe -72V supply isn't connected to anything unless the generator is in HIGH AMPL mode, there's a relay that cuts both the ground and the "hot". I'd suspect measurement error, as you can't measure that supply against the common ground while in STD AMPL mode.

author: L_Euler
author_rank: Regular Contributor
timestamp: May 17, 2019, 06:32:59 pm
content: Quote from: N3SWR on February 19, 2019, 01:07:49 pmRelative to bench time, how are you accessing the internals? Do you have an extender card? I just got a 506 that is a complete dud (the indicators come up but no output at all), and I need to dig into it to see what's going on. I pulled the TM503 case cover off and the sides of the PG506 while plugged into one of the end slots. That configuration gives me reasonable access, but an extender card would be optimum. Actually, an extender cable would be optimum and relieve me of having to keep alignment correct between the 506 and TM.