A bit more info:
1: The video board when powered by 5 volts, draws 0.46 amp when "ON" and 
0.04 amp when "standby". The manual power switch was used to switch. 
There were no components warm to the touch.

On the power supply board, appears the problem is with the transformer 
after the diode rectifier bridge that is connected to the 120 VAC line. 
With the 5 volt active or not, the transformer primary meters at :
Across primary pins 0 VAC, 164 VDC
Primary pins to PBC ground (5 vdc ground)  74 VAC, 82 VDC

The 5 volt secondary, 1.9VAC, 0 VDC. Don't understand why there is no 
potential at transformer (and the 2 diodes connected), but there is 5 
vdc at the other side of the diodes. Circuits show open to anything near. :(

The primary and 5 volt secondary show 2 ohms or less coil resistance.

The 5 volt drops out for several minutes, and the power must be down for 
it to restore. At this point I'm going to assume the transformer is 
defective, and unless someone has info on the transformer and a source, 
or sufficent info that I can chase a substitute, I'll plan recycling the 
unit at our E-scrap. :)

One more time, here's the transformer markings:

LITAI   PT-007921, TF-2820, 2650C0167, appears to be an OEM custom  

Thanks again for the help.





Carl Denk wrote:
> 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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