Products > Test Equipment
TDS7054 Power Supply (PSU) repair
awjennin:
Hello All,
I was up and feeling full of myself, and decided to purchase a non-working TDS7054 from ebay. I figured it would be a breeze to repair, but its been much more challenging than expected!
The unit I got is extremely clean. The dials, exterior, everything looks to have very little use. The insides are mostly dust free and generally look undisturbed. I know this doesn't mean that its low hours, but it looked hopeful. The HDD was missing.
What I have observed is that the unit appears to have no power. When plugging the power cord in with the toggle switch set to on, there's only a faint crackle of power making connection - which tells me theres no big inrush. When plugged in, there are no lights and the front power button is non-responsive (and led is un-lit). After running through the service manual power supply testing section (table 6-5), i measured no voltage on any of the supply rails (exposed at the Front and Rear Power Distribution Cards). I decided at this point, there was either a board-mount fuse, or housekeeping supply that was damaged on the power supply CCA.
I pulled out the power supply and got started. I observe two sections of this card - a high voltage side, and a magnetically isolated low voltage section. When I apply power, a high pitched whine slowly starts (as large electrolytic bulk caps charge), and i determined that the noise source was a high voltage film capacitor. I determined this by using paper rolled in to a cone, and placing it over parts, and listening. When I remove power, the whine slowly decreases frequency and gets quieter. Not sure if this is a clue or not.. The high voltage side of this card has 165VDC in a number of locations (i.e. at Drain of switching supply FETs, etc), but no other voltages elsewhere. I've checked the fuses, and capacitors all look to be in great condition, and resistors look to be unburned.
On the high voltage side, I see a number of switch-mode-power-supplies (SMPS) that rely on DC voltage. Based on spec sheets, it appears that this voltage needs to be between 10-20VDC. I've bugged out the board with my meter (fluke 189) and have noted all locations where this "housekeeping" voltage goes, but (and this is the humbling part), i can't figure out where its generated. I sort of suspect it comes off windings of T10, but I am much more versed in digital and analog design - not power electronics. I also don't have an available scope to take time/freq domain measurements (to look for oddities / rails trying to come up) - catch 22!
I'm shining a light through the board and trying to draw out the schematics - its a slow and error-prone exercise.
If anyone has advice / schematics / history with these units, I would greatly appreciate the assistance!!!
Thanks in advance for reading through for for any help!
For explanation of hackery near T10, please see posts 3 and 4
Hexley:
Check out the area just below T10. There is a brown electrolytic capacitor that looks like it might have bulged a bit, and there seems to be electrolyte damage around the ICs just below.
shakalnokturn:
If not signs of butchery...
awjennin:
I will pull those caps and take some measurements - thanks for reviewing!!
The IC located next to those caps is a UC3842B (SMPS controller), and I pulled the spec sheet on it (https://www.onsemi.com/pub/Collateral/UC3842B-D.PDF)
It accepts a VCC, and provides a 5v Vref - which a number of inputs (comp, fb, etc). There's an under-voltage lockout when VCC is below 10v/above 16v, or when Vref is below 3.4/above 3.6 (not sure if that is an internal measurement, or the measurement of the external pin).
I measured Vref at 1.8vdc (so we fail that condition - likely in UVLO) so was looking in to that. I removed the part and tested external to the CCA. when I powered it external (very simple setup), Vref was 5v as i expected, so i put it back in circuit
I also measured a 15-ohm series resistor at Vout (gate driver) at 45 ohms... and i don't have a 1206 1206 in 15 ohms, so I found a used 2512 and hacked it in to test.
there is an oily substance on the board which i thought was flux at first, potentially from a no-clean production process?... but you can see how nasty my rework was - so now skeptical of what it was...
When i get to root cause, I will order fresh parts, and clean things up.
I will try to get my schematics drawn out and will upload - I think my next step is to identify why Vref is being held load on this card.
Thanks again!
awjennin:
Here's a pic of the schematic, as I see it.
Via CR97, this supply does seem to be the origin / VCC housekeeping supply - but I'm still asking myself why she's not firing up. I will re-read through the datasheet again
From what I can tell, this UC3842 is powered by the 2.1mA that gets through the two series 16.6k power resistors. It appears that the diode to GND (near UC3842 pin 7, marked as 15.5v) is a Shottky but it could in fact be a 15.5v Zener making a little shunt supply - would make more sense than a shottky. It seems to be a SOD123, and has a case marking of J5L, but i'm not finding any info on that..
Not sure the point of the other diode - CR96 - is? maybe it allows the VCC/housekeeping supply (which i surmise to be 18.3v based on the V-feedback voltage divider) to provide power during a removal of prime power to assist with power down sequencing? just a guess...
As I said, the series source resistor at the gate driver / output of the UC3842 was measured to be 40 ohms - which I (poorly, as pointed out) replaced with a 15-ohm.
Now i don't think the slowed slew rate of that fet (when at 40 ohm) would have caused overheating to the point of damage, and i don't see any evidence of thermal degradation near to that fet... but something wasn't right about that. possibly another clue
i used my fluke189 (freq counter mode) but can't see output switching of the UC3842, and in Voltmeter mode i sure don't see any voltage potential, but that could also be damaged..
Some more light reading tonight, and hopefully a revelation in my dreams tonight.
thanks for any help reviewing schematic & pics. Kind regards
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