[volt-nuts] Keithley 2001 Multimeter Fault - Update

[Alan Scrimgeour]

I think you're right, but the capacitors in question don't connect to the 5V 
regulator (well, maybee one) so things are more complicated. By proximity 
they appear mostly to be connected to bridge rectifiers. I guess an 
oscillating regulator could put a high ripple current through a capacitor on 
its input too.
. I've tested the overheating electrolytics and they all seem ok (ignoring 
unknown ESR), but under a couple there's corrosion. There's also possibly 
some heat damage to the pcb at one point that needs a much closer look. I 
hope it's just more corrosion.
Unfortunately my scopes dead, which I'll have to sort that out, but for now 
my meters ac range and frequency measurement mode should tell me what I need 
to know.

Alan


----- Original Message ----- 
From: "Robert Atkinson" <robert8rpi at yahoo.co.uk>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Tuesday, March 16, 2010 7:44 AM
Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault - Update


Hi Alan,
Have you put a 'scope on the 5V rail? overheating capacitors could be a sign 
of an unstable (oscillating) regulator. I'd just change the hot elecrolytics 
anyway, they have much reduced lives when they are hot. If you are testing 
them, you must include an ESR test of some sort (I've no connection to any 
vendors of ESR meters ;-)

Robert G8RPI.

--- On Mon, 15/3/10, Alan Scrimgeour <scrimgap at blueyonder.co.uk> wrote:


From: Alan Scrimgeour <scrimgap at blueyonder.co.uk>
Subject: [volt-nuts] Keithley 2001 Multimeter Fault - Update
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Date: Monday, 15 March, 2010, 21:46


Here's a progress report:
Searching for a possible short circuited tantalum capacitor I tried to 
identify if any were connected to the 5V output of the overheating 
regulator. None on the visible side of the analogue pcb were. I dived in and 
removed the pcb and did a similar search of the tants on the rear. One, 
located right next to the regulator was connected. I replaced and tested it. 
It was a bit leaky, but ok, and the replacement made no difference.
I started probing around with a thermocouple looking for overheating chips 
connected to the 5V supply, but couldn't spot any. Then I accidently 
discovered that 5 electrolytics in the vicinity of the overheating 5V 
regulator were also overheating. Unfortunately the pcb does have buried 
layers so it's not easy to tell what connects to what, and I cant get to 
connections on the rear of the pcb when it's powered up, so the whole 
process is proving rather frustrating. I think I'll try removing the 
electrolytics and if they're still ok, putting them back on long wire legs 
to allow me to access the powered up connections.
Any bright ideas welcome!

Alan
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