[Chris55000]

Hi!

OH DEAR!

I've downloaded the 175 Manual and I'm giving it a careful perusal with the intention of posting detailed fault–finding/isolation tips!

Note – the switching FETs in the input ends of meters can be a bit fussy for their pinch–off voltages, and the zero–gate bias drain current (Idss) and it may be necessary to select replacements if you can't get the types specified in the parts lists!

The gate–leakage current Igss is important as well, as if this is too high you'll have bother with erratic zeroing and unstable readings on the lower voltage ranges, and also problems with "Ohms" not returning to a true 'overflow' indication on open–circuit on high ohms or autorange.

Your description of the fault after the damage occurred suggests floating or damaged input returns at the input to the A/D converter, and that's the point to work back from. I recommend, rather than trying to work from all the drawings in one go, get yourself a cheap block of squared or graph paper, and draw out the components for one range at a time, omitting any superfluous switching relays, CMOS gates, etc., except those directly connected to the range your dealing with!

Alternatively you can achieve the same result on Keithley's OEM diagrams with a pack of multi–colour highlighter pens!

(Obviously use a PDF manual to work from as you can scribble on, cross bits out and highlight things as often as you need to, since fresh reprints are always on hand from your friendly printer!)

Generally speaking, begin with the lowest DC range first, concentrate on that first, as you need to prove no significant damage has occurred in the a/d converter and it's control circuits from the '6805 micro.

I don't think, from the sounds of it, that the '6805 has been damaged – the symptoms quoted suggest the damage occurred farther back nearest the I/P sockets!

Let us know how you're getting on, I'm keeping an eye on this thread in case you get mega–stuck!

Chris Williams