James, I think I have one of many places for you to look
And to all of you TDS420 and TDS460 lovers, here's another juicy tidbit of the typical "cap" damage I'm sure you have encountered once or twice or, likely, more! Anyway, it's a rainy day in Raleigh but a perfect day to do electronics repair!
I watched a Tektronix Company video and it stated that if "SPC" fails it's a "hardware" problem. I'm not sure if that is always the case or not, but that's what the video stated.
I happen to have a spare attenuator board from a TDS420 which, I imagine, is similar to a TDS460. Its part number is 671-1680-05. I focused in on the area of the group of four 33uf 10v capacitors that are prone to leak:
So I removed the old capacitors and cleaned their pads. I did continuity tests on all of their traces and I got a thumbs-up. I next turned to the resistors and tantalum capacitors that are nearby.
To the right of the 14051B Analog Multiplexer/Demultiplexer I found resistor 5621 not "connecting" to its pass-through, so I removed it:
I then tested the resistor, and it is at 5.63 kOhms:
But the funny thing was, I wasn't getting any continuity going to the left trace that sneaks under the 14051B either, so something is "open" in that direction too.
I also noticed that pin "7" on the 14051B looked burnt:
So, off goes the 14051B
What a funky-mess under that chip! And sure enough, that resistor takes a bee-line straight to pin 7! So I cleaned up the area and started to do continuity tests. Btw, look at all of the pass-throughs under that small part
The two red dots in the lower picture are where there are breaks in the traces right at the pads. All of the remaining traces and pass-throughs pass their continuity tests.
I looked up what a 14051B is and the identity of pin "7":
So James, there's one place to look
One other thing, I read that the 33uf 10v capacitors used with the LM2940CT voltage regulator on the attenuator board needs to have an ESR no greater than 1.2 Ohms based on its current. The capacitors used with the LM2990T should be under 10 Ohms. Question is, which of the four capacitors filters the LM2940CT???
I looked for a low ESR cap on Mouser and found this polymer one:
https://www.mouser.com/ProductDetail/Nichicon/PCS1A330MCL1GS?qs=sGAEpiMZZMsIwzbKW1rlgcLka6aSC%252BMP6mMfrKeBnCE%3DScratch that! I just read down the spec sheet of the LM2940CT!
So I see why Tektronix used tantalum capacitors in tandem with the standard electrolytic SMD capacitors
Anyway, I plan to practice repairing the rest of the board and then trying it out in my working TDS420 in the future. I imagine that the 14051B chips are still available?
*Also*, the TDS420/460 Quick Reference Guide I sent to Ebaman.com is now available for download.