Sharp SI-325

[Larry]

Dead. No B+ output from PS. Checked startup voltage for IC601, STR-S5741-0.5VDC. Checked R603 and 604, both 200K. Output from bridge is 160V, 82V between R603 and 604. 0.5V to IC601. So, I changed IC 601. Startup voltage still 0.5V. C605 is not shorted. Where do I go from here?

[qrz]

just for the sake of argument , check the snubber. c607,d602,r610 and seconday side for shorts (d605,607),  don't forget r608,
q601

print here : http://www.freeinfosociety.com/electronics/schematics/video/sharpimagesi325.pdf

pin 3 of Vreg IC WILL be low as it is the base lead for the switching xistor.  .5v ok
collector output pin 1
emitter pin 2
pin 9 negative ref voltage
pin 4   .3v

[Larry]

OK, now I'm really confused.  Where does this regulator get it's start up voltage?  From the schematic, I thought it was pin 3 thru R604 and 605.

[qrz]

its a self start.
it is s'posssed to turn on upon application of pwr. then the induced voltage/current turns it OFF for a few ms,
then the cycle repeats.


years ago i had similar difficulty with a Panasonic dc/dc converter. had a bad xistor subbed to an ECG.
wouldn't start for anything . yet found NOTHING wrong. finally , went with an original xistor (NOT pana oem parts-$ ).
END of problem. there must have been an instrinsic characteristic of the xistor that was critical to operation.

bear in mind there are counterfeit semiconductors out there and your replacement might be questionable....
remember that bargin part might not be such a bargin.

[Larry]

All those parts checked OK.  The sub STRS was an original by Allegro.  I understand what you said about the STRS self starting but where does it get the voltage, current to start.  0.5 VDC to the base of a tranny is not going to start anything unless there is voltage on the collector.

[qrz]

unfamiliar with Allegro . so, cannot vouch for them...

initial start is from r604,605.
 i assume u have the same voltage drop across both res's...?

anyway, the internal xistor should then activate, drawing current through the primary side of the xformer.
the fore mentioned snubber circuit CAN interfere with circuit timing .
r608 may be a critical value for this supply . 5 watts !

[Larry]

OK, the more I understand the STRS series, the better I can troubleshoot.  Here's where I'm at.  I have the chasis on my bench hooked up to an isolation transformer.  I have lifted D607 and L602.  I have a 100W bulb hooked up between L602 and chasis ground.  There is no B+ output.  I have checked R608 with a milliohm meter 50mohm (there is another resistor soldered on the back plane).  With a VOM, I have checked R604, 605, D602, 604,605, 603 Q601. C605 and 607 are not shorted.  At power up,  I have 164VDC at pin 1 of IC601, 82.? between R604 and 605, 0.5VDC on pin 3.  I have put my 100MHz scope on pin 1 and I do not see even one pulse off IC601 after the initial power on.  Any more ideas?

[qrz]

not exactly a datasheet . but, it does show the internal schematic :
http://www.datasheet4u.com/html/S/T/R/STR-S5741_ETC.pdf.html

click " locate data sheet "

pdf file