Electronics > Repair
(FIXED) Keithley 225 (current source) repair
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motocoder:
I picked up a Keithley 225 off of eBay. I probably paid too much for it given it's in need of repair, but I thought it might turn out to be one of those units where the seller just didn't know how to operate it.

There are at least two things I've found wrong with it so far. The output was pegged on -130V no matter what I set the controls to. In addition, the 100 ohm precision resistor on the main current range switch is toast.

Both output driver transistors have failed with collector shorted to emitter. I guess this is why the output was stuck on -130V. So first question I have is where to source a suitable replacement. The part seems to be a TO-66 package, which really limits the selection. I've attached a picture of it. I believe the part number is RCA 60761. This does not seem to match the number in the Keithley manual, so either they swapped parts in later generations of the product, or someone has already had a go at replacing this part.  The manual says Mfr. Code = 80164, Mfr. Desig. = 40318, Keithley Part No. = 24560A. It also has an asterisk next to it which leads to a comment about it being a specially selected transistor (selected for good hFE?).

So my first question is how to go about selecting a suitable replacement transistor. I did a search on "RCA 60761", and a page came up listing NTE124 as a replacement. I have a distinct mistrust of NTE, as they seem to sell whatever crap fell off the boat, with minimal data sheet info and no guarantee of consistency or quality.

The manual for this beast is here:

http://exodus.poly.edu/~kurt/manuals/manuals/Keithley/KEI%20225%20Instruction.pdf





johansen:
they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

you've got the schematic.. what is there to complain about?  :D
motocoder:

--- Quote from: johansen on May 22, 2015, 09:37:41 am ---they are likely matched hfe npn/pnp pairs

replacement should not require an exact match, but ymmv.

you've got the schematic.. what is there to complain about?  :D

--- End quote ---

They are both NPN, and the manual says "specially selected, order individually". There is another part where it says to order in pairs. So my guess is they are just selected for good hFE.

No complaining here. I am actually quite happy with the condition of this thing. If I can get it working, it will be a beautiful addition to the lab.

My local electronics store stocks the NTE part, so will just give those a try.
Grapsus:
The part you are looking for is "RCA 40318". I can't find the RCA datasheet, but there is one by "New Jersey Semiconductors":

http://download.njsemi.com/40318.pdf

All the characteristics seem to match the RCA part which is described here:

http://alltransistors.com/transistor.php?transistor=20627

There is even a french site selling the exact RCA part:

http://www.donberg.fr/descript/4/40318.htm
motocoder:

--- Quote from: Grapsus on May 22, 2015, 09:48:28 pm ---The part you are looking for is "RCA 40318". I can't find the RCA datasheet, but there is one by "New Jersey Semiconductors":

http://download.njsemi.com/40318.pdf

All the characteristics seem to match the RCA part which is described here:

http://alltransistors.com/transistor.php?transistor=20627

There is even a french site selling the exact RCA part:

http://www.donberg.fr/descript/4/40318.htm

--- End quote ---

Thanks, Graspus. I just noticed a bunch of toasted resistors on the range switch. I also see signs of someone that tried to make some repairs. However, in at least one case they used the wrong component value, so the whole thing is suspect and will need to be checked...
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