Thanks for the reply. The diodes are large, probably in the amperage 
that you indicate, but the wiring harness to the video board is only 
maybe 26 gauge wire, certainly not 20 gauge. The only place I can see 
the 5 volt is to the video board to feed the video chip, and a couple of 
LEDs on the control panel. Although it looks heavy, it doesn't seem to 
go anywhere with big current. I didn't unsolder a diode to check each 
separately, but the diode function of the meter seems to indicate in the 
range expected. I figure that if a diode is bad, it would short, an if 
it was open the other would fail from overload.

Though I don't have formal bench power supply available, I could power 
the 5 volt through a small fuse say 100ma to start, and if it stayed in, 
I could then add a amp meter and see what is happening. Today was kind 
of busy around here, but tomorrow, I should be able to spend a little time.

There is a fluorescent backlight supply on board, but it seems to come 
off the 120 volt after a diode rectifier bridge. It seems that the 
problem is after those diodes, or a winding in the transformer opening 
up. but the winding seems to meter at around 0.3 ohms, and I figure 
thats OK. Between the diodes (this paragraph) and the transformer are 
the 4 components that I'm looking for info on. Without pinouts and a 
little functional description, that whole area of the PCB is a mystery. 
:(  And unless I can find data an a place to buy a few (including 
substitution), that's a dead end.  When the 5 volt does go down, it 
appears that the transformer primary goes down, but without knowing the 
winding configuration, I'm lost. I could unsolder the transformer, but 
thats 9 pins, and would rather not do that.

Dwayne Reid wrote:
> The pair of diodes in parallel and the relatively large output 
> capacitor strongly suggest this is a multi-Amp supply (2-5A would be 
> my guess).
>
> Its too bad that you don't have a bench supply available.  My next 
> step would be to substitute the bench supply for the internal 5V 
> supply and slowly crank up the current limit to see what happens (and 
> to see if anything gets hot).
>
> dwayne
>
>
> At 04:28 PM 9/5/2009, Carl Denk wrote:
>   
>> Here's the schematic, excuse my chicken tracks, but thought quick to 
>> keep thread alive was better than a fine looking drawing. There are 
>> no regulators that I could find in the 5 volt area, just off the 
>> transformer winding(s). The transformer primary seems to be a 
>> switching off the 120 VAC, with a diode rectifier bridge, and then 2 
>> MOSFETS and on to the transformer. Next back to trying to measure 
>> the ripple on the 5 volt, and try powering the video board off 
>> another supply. I have some 100 ma fast acting fuses that I can try. 
>> They are for the meters ma ranges.
>>     
>
>
>   
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