wells gardner pd279410 rev 7.3 chassis

[techoff2]

I have not seen this fault listed before.
This chassis blows the AC cct breaker when turned on.
If I remove the bridge rectifier then I can wind up the variac to 55 volts before the breaker trips.
The on board fuse is unaffected.
At one stage R506 burnt up which led me to look at T501
This is when things became even stranger.
I am working from a general schematic which may not be for rev 7.3
pins 10 to 18 are all shorted with the xfmr out of cct also shorted to pin 6.
pins 1-2  7.8 kohms
pins 1-3  6.8 kohms
pins 2-3  0.5 ohms

From the schematic this would indicate that the xfmr has internal shorts but from experience I would expect to see burning around the core.
Help appreciated.

[grantspain]

have you checked the ptc th502 and ntc th501, also the fet q501. check all diodes in primary

[lilshawn]

i would not suspect the SMPS transformer. i have never seen one fail, and i've fixed literally 100's of monitors with pretty dead SMPS's. i would think if it did fail, it would do so quite spectacularly

if you remove the bridge rectifier, you still are dumping current through other parts of the SMPS circuit since IC504 (120/240 input voltage switch) is tied on the AC side of the chassis and effectively bypasses half of the bridge rectifier. so i wouldn't discount a problem there. the SMPS controller chip monitors the AC side and the DC side to properly switch the current to make the DC voltages the proper levels and compensate for load so there is likely a problem there.

R506 will fry if SMPS controller outputs 100% duty cycle due to failure or Q501 fails shorted. so i'm pretty sure your main issue is Q501 and IC501

i would replace IC501 (since R506 was fried and it supplies power to the SMPS controller IC) and Q501 since it's probably dead too for sure since fet's don't like being done dirty like that.... maybe check D505 to make sure it hasn't been shorted by over current and i would also check all the resistors on that side of the schematic to make sure they haven't been blown open or pushed to high resistance due to the high current.

as it is right now, re-install the bridge, remove IC501 and Q501, and power the chassis... if it still results in blowing the fuse... check diodes and ceramic caps in the section for shorts to be sure... but I ultimately would replace IC504 since the caps and whatnot would probably test fine. to test for sure though, remove IC 504 and re-power. if it doesn't blow now, that's the primary issue. you'll have to either replace the switch, or bypass it and only run the monitor on 120v... (see the STR83145 datasheet for how it operates.)

if you plan to remove items to test a chassis sub-system, you have to very carefully choose what you pull out. you chance having power try to go places it's not supposed to, and some supplies rely on having a proper load applied to it to function properly. the input side of the SMPS is particularly nasty to try and isolate because it actually has DC and AC together and both has to work to test it. it's all, or nothing. having an autoswitching voltage switch makes things especially weird.

[techoff2]

progress to date--none
I have removed  Ic504, Q501 and Ic501.
Powered up the chassis and the AC cct breaker goes immediately.
The DC fuse is intact.
Removed the bridge DB501 and I can wind up the AC to full voltage without the cct breaker going.
This would eliminate any of the components to the left of the bridge.
I am still not convinced that I don't have a problem with the transformer T501.
There should not be a short between pin 6 on the primary side and all the pins on the secondary side as stated in my first entry.

CCT breakers are designed to trip when a short is detected between either active, neutral and earth in an AC cct..
So I have a 10 amp cct breaker tripping when any amount of AC is applied to the cct with the bridge in.

So with the components mentioned out of cct and the bridge in cct the only path to earth that I can see is via r575, r574, D506, R506, pin 6 and pin 15 to earth.

Have I missed something more obvious?
 

 

[lilshawn]

There should not be a short between pin 6 on the primary side and all the pins on the secondary side as stated in my first entry.

remove C507 2200p 400v (marked 222) it may be shorted causing your testing to show pin 6 which is tied to "common ground", to also show pins 9/13/14/15/16/17/18 which are tied to "chassis ground" be all shorted together.

[techoff2]

Those resistance readings were done with t501 removed from the cct board so c507 would have no influence.

[lilshawn]

okay, so was t501 in or out when the circuit breaker you have powering it faults? you didn't mention removing it at all, just that you tested it.

have you tested the rectifier?

easy peasy rectifier test... remove rectifier, place meter in diode test mode. place black negative lead on the positive power leg of the diode pack... red lead to each of the AC terminals. each AC leg should show 0.5 diode drop. place positive meter lead on the negative lead of the rectifier.... same thing... negative lead to each AC leg of the rectifier. each should show 0.5 diode drop. any of the 4 tests shows 0.000 (infinity if analog), a diode is shorted. if the meter shows OL (or 0 if analog) diode is open. if they all show 0.5 or thereabouts, diodes are fine. if diode measures out higher than about 0.6 or lower than 0.4 it's suspect and could be leaky or on it's way to failing.


transformer pin readings (this is for a TPW-E42901 transformer taken from a D9500 (basically a 19" version of the 25" 9400 but the part number on it matches the one listed in the 9400 schematic and not the one in the D9500 )  and taken with a fluke 87III in 4 1/2 digit mode and lead resistance nulled out.)

Primary side

1-3 - 0.44 ohms
5-6 - 0.04 ohms

no pins from 1-3 have any connection to pins 10-18

10-11 - 0.64 ohms
10-12 - 0.61 ohms
13-15 - 0.34 ohms
16-17 - 0.03 ohms
16-18 - 0.06 ohms
16-15 - 0.07


so it's possible the transformer is shorted internally, but not enough to catastrophically fail, but enough to trip a circuit breaker before it does.

maybe try and apply like 12 or 24vdc (even 42v if you have access) to pin 1 and measure for voltage leaking through to pins 5/6 and the other side through pins 10-18 to be extra sure.

if it is shorted internally, i suppose i got this one (if it's the same). i live in canada so it may be prohibitively expensive to ship depending where you are.

[techoff2]

I removed t501 to test its resistances and then put it back in to carry out the testing that you recommended.
The bridge is ok.
On the transformer that you have do you have any resistance readings from pin 15 to pin 6.
I have a short about 0.1 ohm.
The schematic that I have shows no electrical connection from pin 6 other than to pin 5.
pin 5 to 6 is around 1.5k ohms

[lilshawn]

no pins from 1/3 have any connection to the pins 10-18 or the 5/6

essentially 3 separate coils of wires.

1 and 3 is the primary high voltage coil.

5 and 6 is a feedback/power supply for the SMPS chip

10 to 18 is secondary side providing all the different voltages.

[techoff2]

Finally finished this beast.
The transformer was shorted from pin 6 to pin 15 causing an AC short to ground and blowing the house cct breaker.
Also make sure you use the correct UC3842b SMPS ic as other variants don't work in this chassis.