Keithley 179 Desktop Multimeter Fault

[DionneSwart]

Hey everyone.

I have a Keithley 179 Desktop meter which I know is old but I still use it on occasion alongside my various handheld meters.

The unit stopped functioning and just flashes. I have traced the issue up to the +15V rail being dead under load. When the unit is turned off the rail is fine and alive. I traced as far back as the op amp but decided that before I go and pull the op amp etc that I would ask here if anyone else has experienced such a fault with these old meters?

Cheers

Dionne

[Huluvu]

Hi Dionne,

I have exactly the same beauties and use them very often apart from my handheld meters 

Please try to rinse carefully all the switches with some mild contact cleaner first.
My proposal would be to remove the board from the housing and place the board/switches upside down to some kitchen roll paper and rinse the switches from the backside to front.
After that you should operate all switches a few times and repeat the cleaning process again.
I have good experiences with "Kontact 600" from CRC

Cheers
Arne

[DionneSwart]

Thanks for the reply Huluvu.

This meter is still neat and switches where all in order. I eventually discovered one of the tantalum caps of the +15v rail where pulling the output of the 15v regulator to ground. Upon measuring the cap in circuit it showed a short but then I lifted one leg to verify if this was truly the case as sure as nuts it was. 0.1ohm on the little blue 4.7uF 25V tantalum. Replaced with a high quality electrolytic and meter is back to full function. :-)

I really enjoy using this older meter. 20000 count for this era is awesome. Also there are no fancy MCU's etc to worry about. Bought this fella for R400.00 which is around 37USD. Nothing really.

Cheers
Dionne

[Huluvu]

Hey Dionne,

congrats to your successful fix 
I really suggest to replace all caps if possible (I usually do so)

I personally own 2 Keithley 179A and one 179 which I refurbished (repaired, all caps replaced and Calibrated)
All Meters are working smooth and reliable 

[DionneSwart]

Hi

Yes I have put an order in for all the caps and am just waiting for them to arrive. Going to replace them all for future sanity. When one of them go it can be a time consuming job trying to locate the faulty one especially since it could be any number of components that cause a power rail to go to ground. Rather reduce the risk of cap failure by replacing as you said. :-)

Do you by any chance have a service manual or such for these meters. I found one and would be happy to send to you. Although I did find it only after already repairing the fault. Good to have for any future problems.

Cheers

Dionne

[tautech]

I eventually discovered one of the tantalum caps of the +15v rail where pulling the output of the 15v regulator to ground. Upon measuring the cap in circuit it showed a short but then I lifted one leg to verify if this was truly the case as sure as nuts it was. 0.1ohm on the little blue 4.7uF 25V tantalum. Replaced with a high quality electrolytic and meter is back to full function. :-)

The dreaded bead tantalum strikes again. 
One of the first things to suspect when a V+ rail is low or dead.
The electrolytic will not filter as well as the tantalum. Plan to get a 50 V version in your next parts order for a proper replacement. Alternative could be a leaded multi-layer ceramic of the same or higher value.

[DionneSwart]

Good point Tautech.

I am planning on re-capping the entire meter as it is pretty old and some of those larger electrolytics are slightly higher on ESR than I would like as well.

Just added the little blue devils to the shopping list.

Cheers
Dionne