Products > Test Equipment

TDS7054 Power Supply (PSU) repair

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TiN:
Most likely you have shot stand-by power. There is a regulator around 3842 that is used to provide +5VStby rail.
Be sure to replace ALL electrolytic capacitors. I had those leak out nasty electrolyte and ruin the PSU. Even if they look okay, they are already doing their nasty business under the cap  :popcorn: In my case it was dead 3842 and capacitors with gunk shorting stuff on PCB.

awjennin:
I truly appreciate the input on this!

I removed the suspected zener diode and reverse biased it with 24v (current limited with a 5k resistor), and sure enough it showed about 19.5vdc across it. A little more digging on digikey revealed a SOD123 from OnSemi (seems like this PSU has lots of onSemi parts) which has a J5x case-marking for a 20v nominal zener (MMSZ5250BT1G) - This is beginning to correlate with what Hexley pointed out about the UVLO threshold... which i'm super embarrassed about misreading as a window comparator input vs the VIL/VIH that you pointed out! 15.5+ripple would absolutely be concerning if we aren't meeting the guaranteed VIH threshold! between leaky diodes, caps, pwb damage, and potentially damaged 3842, that could surely explain the voltage drop.

And to both Hexley and TiN's point, the caps do look great - but sure enough, I pulled the 68uF closest to the 3842, and while it measures close to nominal, the terminal finish of the through-holes is degraded, and if it has started leaking - that would explain the residue on the components near by!

The more I type, i'm becoming more and more convinced that the 68uF that you both called out is in fact the root cause. leakage current pulling down the VIN rail, and degraded characteristics not allowing it to keep the rail ripple-free. evidence of the residue and etched through-holes...

I will go through the effort of re-capping this PSU, point well received!!!

Side note, i'm a bit of an audio guy, and a while back I tracked down two sony DA777-ES receivers. I replaced pot's, caps, and retuned it per the service manual. While none of the caps looked bad and generally measured nominal, it made a huge difference to the audio quality!!
I will be just thrilled if i can get this scope going!!


THANKS again! I will report back when I get this SMPS cleaned up, and the PSU as a whole recapped.
Happy Easter all! stay healthy!

fitch:
Wow, I cannot believe I found this thread.  I really hope I can contribute something here, and maybe fix my power supply as well.  I have a TLA614 logic analyzer that I have had for many years.  It was plugged in and in standby essentially the whole time.  Last time I tried to power it up, nothing.  I found another thread here from another user with the exact LA and his tail of repairing his supply.  Unfortunately, my failure is different.

First, my supply has marked on the big caps:
PS2251 REV. L
119-5806-05 REV. -
12709-5102
The voltages and currents are listed, but these could be configuration dependent.

My supply appears to have the input section working.  The DC on the big caps is there, and the two switching FETs Q3 and Q5 have drive signals on their gates.  Also, my +5VSB supply works.  When I power up the supply with PS_ON# deasserted, I have a single red LED lit.  When I assert PS_ON#, the LED goes out.  I tried to look at the various outputs to see if any of them tried to come up before shutting down, but am unable to see any movement on any of them.

I find it very interesting that on my supply, I had the same cap (C807) leak all over the surrounding circuitry.  I thought that replacing that cap would fix it, but no.  I have since found 2 other of the same type of cap that also leaked and have replaced them as well.

I have been spending time in photoshop with pictures of top and bottom, tracing out the signals to build a schematic of the board so that I can make some sense of what is there.  I too am not a power supply guy, so some of this is a bit foreign to me.  But the engineer in me wants to find the cause and fix it, besides the fact that my LA is in pieces taking up space in my lab:-).

I did get a connector diagram from Tektronix, that was the only bit of information I could get.  Maybe it can help you.  The housekeeping supply you talk about may not come up until the supply is turned on.  The connection diagram shows PS_ON# location as well as POWER_FAIL#.  These might be helpful if they are traced through the circuit.  I have been trying to do that myself, although very slowly.

Since I have my +5VSB working, and at least one other 5V supply, I can take some measurements for you if they help.  Just let me know.

fitch:
By the way, I never mentioned this, but my power supply looks exactly like yours even though it was from a logic analyzer.  Perhaps it is somewhat standardized in some of the Tek units of the day.

I can confirm that indeed the controller is a UC3842.  Mine is original.

When I enable my supply, I measure slightly more than 18.3V at CR96 and 17.7V on pin 7 of the UC3842.  And, this supply is not there until I enable the supply by shorting PS_ON# to GND (I jumper J1-1C to J1-4C).

With the cap leaking in that area, is it possible some of the connections were corroded to the extent you have little to no connection left to pin 7, and therefore a lot of drop?

TiN:
You can test PSU by poking PS_ON to ground as see if it chooch. Obviously have some DC loads connected to main power rails, so you can check if all voltages correct. Maybe it's not the power supply you have problems with.

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