Viewsonic G73fm - completely dead
 

Hi,
I've got a Viewsonic G73fm here that's absolutely dead - no response to
power button, power LED is off, no sign of current draw (usually you get a
little 'crack' if you plug a powered IEC into the back).

Does anyone have a service manual or schematic for this beast? Even a "The
screws are hidden here and the case latches here" type diagram would be
useful. The screws are obvious, it's the ones hidden under covers, and the
plastic latches I'm worried about...

Failing that, I'd appreciate some suggestions of things I could check -
I've done a bit of VCR repair but this is the first CRT I've tried to repair
that might actually be repairable.

Thanks,
Phil.
(valid address, but replace "despammed" with "dsl.pipex"
to bypass spam filter)

Viewsonic G73fm - completely dead
 

As bad as it is to reply to one's own emails, I've done some testing and found:
- The AC inlet fuse (2.5A 20mm ceramic cartridge fuse) is blown. And by
blown I mean completely open - infinite ohms. I can't use the "is the fuse
melted (soft blow) or metal coated inside (severe overload)" trick because
it's a ceramic-body fuse...

- All the rectifier diodes in the PSU section appear to be functional -
0.5V forward drop, "infinite" reverse drop, according to my DMM.

- The thing is absolutely full of "Luxon" electrolytic capacitors. Best
guess about 20 of them. That and the fact this thing has an mfg date of 2001
puts it square in the "counterfeit capacitors" time frame and makes me bloody
suspicious. No signs of cap leakage or bulging caps, but I'm still suspicious.

- The case is really easy to open up. No latches or hidden screws at all.
Put the CRT face down on a towel (unless you WANT to scratch the CRT that is),
remove all the screws on the back (4 around the front edge + 3 on the back
panel) and lift the back away from the front section, shaking it gently if it
gets stuck. Reassembly is just as easy - put the cover back on, then refit the
screws.

- All the PCBs are marked with a logo that looks like "JEAN" - i.e. these
guys: http://www.jean.com.tw/. So 99.9% chance it's a Viewsonic rebrand of a
JEAN monitor. Why am I not surprised? Service info appears to be available,
but only to those with a username and password, and no hint on how to apply
for access. Yay.

I've found a few of the part numbers too:
Monitor P/N: VCDTS22475-1E
CRT: Chunghwa M41AJR53X76 / CP5440BNAC1S-TC / No. B 1163581016
Mainboard:
G73FM
2202116700 VER:1.06 2001.04.20
Audio riser card:
2202117200
Barcode sticker on mainboard:
7562-00191-01050-0.0-A15025


Thanks,
Phil.
(replace 'despammed' w/ 'dsl.pipex' to bypass spamtrap)
http://www.philpem.me.uk

Viewsonic G73fm - completely dead
 

Phil:
Generally, the most common fault when the input fuse blows, is
either a shorted rectifier diode (or Bridge) or the switching FET (on
the heat sink) is shorted. Shorts on the output side of the switching
power normally cause the power supply to shut down when it starts
drawing too much current.
I would direct my time to checking the parts on the primary side of
the switching power supply. You might try checking the horizontal
output transistor and some of the larger transistors and diodes in the
secondary side if you cannot find the problem in the primary side.
Hope this helps...
John

Viewsonic G73fm - completely dead
 

Ah, the final chapter in the repair log.. :)

Buzzed out PSU with DMM from PSU side of fuse socket to N side of IEC inlet;
470KOhms, rising to a few megohms after a few seconds.

Replaced mains inlet fuse (2.5A 20mm antisurge/slow-blow) this morning,
powered on to full white screen with no retrace lines. Connected VGA tester,
loud "thunk" sound, followed by black screen and good image. Closed cover, put
back into service this afternoon.

Total cost: a 20p fuse. Some repair...

--
Phil.