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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:
--- Quote from: techoff2 on December 12, 2023, 06:33:09 am ---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.
--- End quote ---
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.