PG506 Repair - 20v & 50v standard amplitude is low


N3SWR
 

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???

I posted this on the eevblog which is a rather good site but I figured I would give it a try here just to see what happens since it's going to be
a short while before I get back to the bench and lets see if anyone else has seen this before...

Thanks to all either way as this site has also helped me big time in keeping some valuable gear from the dumpster ;)


Siggi
 

On Thu, Feb 21, 2019 at 9:39 AM Outerealm via Groups.Io <weis63=
ptd.net@groups.io> wrote:

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.

The 50/20V are simply divided down from the 100V rail, so there aren't
many components that could cause this. You should be able to figure this
out by simply measuring down the resistor divider string (R275-R278).
Note that the 50/20V output voltages rely on the being loaded by a 1M scope
input. If you're using a high-impedance DMM, I would have expected them to
read higher than nominal.


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.

The -72V rail is not in use, it's not even connected to a common ground
unless you're in the "High Amplitude" mode, so this is probably irrelevant
or possibly a measurement error.


bobh@joba.com
 

The 1A contacts and the 10B contact are also closed in those positions.  I don't have a PG506 with me now to check further.

Bob.

On 2/21/2019 7:39 AM, Outerealm via Groups.Io wrote:
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.


N3SWR
 

Thanks Siggi, I'll try and have a look at it once again after this weekend. I know it's probably something simple.


N3SWR
 

Solved....

After tearing into the PG506 for a while and re-verifying the divider networks to no avail, I discovered a bad BNC cable between the 3456A and GenCal resistor decade I was using. The darn thing had 38KΩ of resistance between the center conductor and shield - just enough to load down the network I was testing. That drove me crazy for days...

I figured I would post it here to close this out and not just leave it hanging in the breeze which irritates me, and others may learn from it - verify your test gear always ;)

Thanks to the group for the help as well ;)