HP3455A offset/drift problems in 100+1000V DC range

[Robert Obermayer]

Hi,

im currently troubleshooting a HP3455A for a annoying problem.
10V and smaller ranges all seem perfectly okay, as is ohms and AC
voltage, while 100 and 1000V DC dont.
Just after Power-Up the TEST mode gives me 7, 5 and 4 and readings in
the 100v range are way off (too small), increasing to almost the proper
value with every autocal click then drifting back quickly.With autocal
disabled, the reading is drifting down quickly as well.
After a short time, the error 7 disapperas and the behaviour gets a
little better but still drifting badly and way out of spec readings.
Trying to isolate the fault, i found that breathing at the area around
R63 (active attenuator) is very sensitive and offset/drift gets way
worse.Heatgunning this area resolves all errors and restores
almost-in-spec performance.
As soon as it cools down some hours, the problems all reappear.
For parts, theres a 1855-0246 (no x-ref) dual JFET, a 10p 500v ceramic
tube style cap and a 150p polystyrene (?) axial lead cap, all connected
to a wire routed on teflon standoffs, among some 'standard' parts.

Is it common for JFETs to have leakage problems at low temperature?
How to properly test the FET for leakage?
Anyone got specs on that FET?

Now does anyone have previous experience with similar problems and maybe
knows which parts are notorious to fail?

Regards, Robert

BTW:Im still looking for schematics for a Tek TAS220/250 scope...

 

[Jamie]

Robert Obermayer wrote:

Hi,

im currently troubleshooting a HP3455A for a annoying problem.
10V and smaller ranges all seem perfectly okay, as is ohms and AC
voltage, while 100 and 1000V DC dont.
Just after Power-Up the TEST mode gives me 7, 5 and 4 and readings in
the 100v range are way off (too small), increasing to almost the proper
value with every autocal click then drifting back quickly.With autocal
disabled, the reading is drifting down quickly as well.
After a short time, the error 7 disapperas and the behaviour gets a
little better but still drifting badly and way out of spec readings.
Trying to isolate the fault, i found that breathing at the area around
R63 (active attenuator) is very sensitive and offset/drift gets way
worse.Heatgunning this area resolves all errors and restores
almost-in-spec performance.
As soon as it cools down some hours, the problems all reappear.
For parts, theres a 1855-0246 (no x-ref) dual JFET, a 10p 500v ceramic
tube style cap and a 150p polystyrene (?) axial lead cap, all connected
to a wire routed on teflon standoffs, among some 'standard' parts.

Is it common for JFETs to have leakage problems at low temperature?
How to properly test the FET for leakage?
Anyone got specs on that FET?

Now does anyone have previous experience with similar problems and maybe
knows which parts are notorious to fail?

Regards, Robert

BTW:Im still looking for schematics for a Tek TAS220/250 scope...
Use component cooler on each part..

when you find the one that really goes off the deep end, You've found
the problem.
Yes, FETS can go bad. But, you may have something as simple as a
bad solder joint or, it could be a bad connection internally on one of
the components.
the chilling spray usually finds that.


--
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