Keithley voltmeter = "HOT" transformer

[redrabbit]

I bought a Keithley 182, it's a 6.5 digit voltmeter that is not working properly.
Display, OK (dot matrix), but no reading is triggered.
No errors indicated on power-up test.

I have not found the problem, but may have found the source of the problem......a very HOT power transformer. In fact there are two xformers, both are hot, but one is real hot.....as in, I cannot touch it for more than 1/2 second.

What can cause a HOT transformer ?

What would I check or measure ?

Additional info:
- No schematic (typical for Keithley)
- The fuse is a 3/8 amp 250v slo-blow...it does not "blow".
- The line voltage switch is in the correct 120v position (switch working, I checked).

- Voltage checks:
"Should be"........"But measures as"
+40v..................+35v
-40v..................-35v
+15v..................+0.16v
-15v..................-8v
+5v....................+2.4v

http://www.keithley.com/support/data?asset=793



=RR=

[cocolino]

What would I check or measure ?

It seems that something in the +15V DC line draws too much current (or even produce a short) which overloads the two (in series connected) transformers and hence cause them to run so hot.
Could be in the +15V voltage regulator part itself or in the circuits which are powered from it.

I`d first try to disconnect the loads from the +/-15V power-supply board.
When the DC supply voltages then measure about right (and with reasonable low ripple voltage) the fault is somewhere after the regulator. In this case it`s probably getting difficult to repair the thing without service manual.
A fault in the power supply itself would be easier to find and fix.

[redrabbit]

YES....I temporarily cut the traces from the +15 and -15v from psu.... to the analog circuits.
Power supply is working now that it is disconnected. There must be a problem downwind of those cut traces.

Any tricks to finding a short in a network that powers about 15 opamps ?
(My continuity tester does not beep if I bridge the analog's +15v power traces "to ground", or "to each other".
(please don't tell me I need a 6.5 digit voltmeter, perhaps like a Keithley 182 )
There are 5 dipped tantalum caps, but no parts show any abuse. But one of them beeps with my meter, when bridged.

[redrabbit]

Originally posted by redrabbit
.....
There are 5 dipped tantalum caps, but no parts show any abuse.
But one of them beeps with my meter, when bridged.

I removed that cap (10uf/25v). It's shorted.
Replaced it with a new tantalum cap. (10uf/50v)
Reconnected PSU to analog circuits....now the voltages are good.

The meter now works !!!!
(paid $60 unworking "as-is"...they go for about $600-1000 used/working.)

We'll see if the bad cap is an aberration, or a symptom of a problem somewhere else.


Thanks for your tips cocolino !

=RR=

[cocolino]

We'll see if the bad cap is an aberration, or a symptom of a problem somewhere else.

Tantalum capacitors are known for short circuit failure modes so it`s not unlikely that this already was it.

Thanks for your tips cocolino !

You`re welcome (glad to see that my very humble knowledge about electronics is good for something, sometimes).