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TDS7054 Power Supply (PSU) repair

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fitch:
awjennin,
Well, I found my issue.  Underneath one of the 68uF/35V caps that had leaked, the corrosion was so severe it ate all the way through a trace that was feeding the Vcc pin of one of the UC3844 controllers.  Once I found that, I repaired the open and the unit fired up.

Good luck with yours.

Hexley:

--- Quote from: fitch on April 14, 2020, 10:00:35 pm ---Underneath one of the 68uF/35V caps that had leaked, the corrosion was so severe it ate all the way through a trace...

--- End quote ---

fitch, be sure that you clean the entire board to neutralize any electrolyte that might be found somewhere else. Isopropyl alcohol and a distilled water rinse might be a good idea.

- Hexley

awjennin:
Fitch et al,

I'm late to the party! Work has gotten really busy as of late! i didn't even get around to checking this.   I'm thrilled that you found the issue! and I think your observation of power supply being similar is really valuable to the community. It sure makes things easier when a company reuses effort - both for them, and us!

And a huge thank you for posting the pinout of the prime power output connectors!  If that info was in the tech manual, i completely missed it!!
Now i'm really second guessing what I've done, and read TiN's comment about the 5v housekeeping supply in new light. If I can get a few moments I will check that out. What i've learned from this exercise has been great - but if it was the 5v supply all along, then i'm a dummy. ha!

However, I did order all new cap's for the power supply. Your observation (as well as Hexley's and TiN's) about Cap leakage are all supporting evidence that even if the unit is working, some caps may be close to end of life (EOL), and generally new caps every 15-20yrs will likely improve the scope because the cleaner output power rails should reduce the noise floor of the scope. Dirty power, dirty measurements?
So, I went ahead with an order. It included a few other parts which I touched with the iron a few more times  - but that aside, the entire order was ~$90 shipped. I fully intended on grabbing some ferro-flash too - forgot to finish that investigation prior to clicking 'order'!

In detailed design, caps can be picked because of bulk capacitance, equivalent series resistance(ESR), impedance at a specific freq/range, current rating, hours/lifetime, etc...  so I find it really difficult to replace these parts blindly  because i don't know what specifically was optimized/why the original caps were picked, and generally the original's have been obsoleted. Therefore I follow a person rule of thumb to replace caps
1) Replace from a similar company (United ChemiCon & Nichicon, in this case)
2) stay within a similar family, if possible
3) replace with components that are physically similar size, with same capacitance and voltage ratings as the old ones. I figure, its the physics that dictate a lot of the parameters. Capacitance eludes to foil area, voltage rating eludes to dielectric thickness, then the physical construction confines the X&Y dimensions that hopefully result in a component with similar properties in general
so, if you stay with the same manufacture (i.e. same build process), and same physical construct, the replacement part probably has similar electrical characteristics to the old one. I can't say its fool proof, or even accurate in all situations,  but that being said, here's my BOM

Capacitor replacement list
ELXY350ELL152MK40S (30x, CAP ALUM 1500UF 20% 35V RADIAL)
UPM1V680MED1TD (10x,  CAP ALUM 68UF 20% 35V RADIAL)
ELXY630ELL100MEB5D (10x,  CAP ALUM 10UF 20% 63V RADIAL)
EKZE500ELL331MJ25S (10x,  CAP ALUM 330UF 20% 50V RADIAL)
UPV1H6R8MFD1TD (10x,  CAP ALUM 6.8UF 20% 50V RADIAL)
UPV1H220MGD1TD (10x,  CAP ALUM 22UF 20% 50V RADIAL)
EKY-500ELL4R7ME11D (10x,  CAP ALUM 4.7UF 20% 50V RADIAL)
EKMH451VNN471MA50T (2x, AP ALUM 470UF 20% 450V SNAP)

Other parts (casualties to the debugging / introduction to power electronics)
FQPF4N90C (switch n-fet, associated with supply i've been debugging - cut off of CCA to preserve PWB. Old was obsolete, this appears to be very close parameters to original)
MMSZ5250BT1G (20v zener diode)
CRCW120615R0FKEA (15 ohm 1206 250mW resistor - original was bad / measured 40 ohms)
UC3842BVD1G (supply controller)
UC3844BVD1G (supply controller - not debugged, but case markings were almost unreadable - looked overheated though no pwb discoloration, worth the couple dollars just in case)


Fitch, regarding your power supply, do these voltages and currents match?




AND on a total aside...
 a little question that came about when I clipped the leads of that switch fet to remove and test it  -
 I noticed that C1 (C1:pin 2 in my schematic) is bonded to GND plane , but pin 1 is routed to the switch-fet's heatsink - which almost suggests that they expected the fet heatsink to be connected to drain of actual fet. i.e. TO-220 (non-isolated), versus the TO-220FP (isolated) version thats installed on these PSU's.  Odd, because this cap isn't doing an ounce of good, as far as i can tell. oh well!


Thanks again for everyone's input! Hopefully I will get the new caps soon, for a nice little weekend project.

fitch:
It looks like you and I have the exact same supply right down to the revision.

By the way, I just found out that my supplies would not come up with some load on them. However, they were not 0V, but like .3 or .4V. That was very different from when it did not come up at all, in that case they were all 0V. Once I saw a little movement, I put it back in my LA and it powered up.

I think the UC3842 only supplies the +5VSB.  You might check to see if that is present on J1.

awjennin:
I got the digikey box the other day, and went through a process of removing all the caps, Removing the parts around T10 where I was troubleshooting, cleaning up the PWB in all areas with 90% rubbing alcohol and a piece of cotton, reinstalling all components, then putting the board through my own version of water wash (hot water + soap, then alcohol wash, then dry) to get the gunk and flux residue off. Then after a quick power test, I put hot-glue drops between the caps to help with stressed solder joints due to vibration, over time. I even moved the stickers over from the large caps. I'm fairly happy with how it turned out.

   

My million dollar observation was this - ALL 68uF CAPACITORS ON THE POWER SUPPLY WERE LEAKING ON THE BOTTOM SIDE!!!  and while all other caps visually looked acceptable, i'm glad i replaced them all. She fired up without hesitation, and started booting!
While the leaking caps were bad, ROOT CAUSE of the failure was this: the 68uF on the input rail (shunt regulator) was bad and caused excessive resistance/ voltage-drop - to the point that the input thresholds were keeping the UC3842 SMPS to stay powered down. The 15-ohm series source resistor between the UC3842 & switching fet measured 40 ohms because the leaked capacitor ate away the resistive element on the 1206 chip.

I now need to track down a hard drive for this! It looks like a standard parallel interface laptop hard drive can be used? I will 3D print a hard-drive caddy, and then plug in the parallel cable directly in to the HDD (and remove the little parallel-to-hotswap adapter CCA inside the scope). Does anyone have experience with that? sound like an acceptable plan?

Mucho Thanks to xDevs.com because i think the SW load will be a breeze with all the resources there!

THANKS AGAIN to all those who read through and helped out with my troubleshooting. I will get a pic of it when she's back together and SW loaded

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