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577 step generator troubleshoot

DW
 

Recently I had Q386 that was shorted to the frame of the 577. I have replaced both thermal pads so now there should be no short.

I reassembled this instrument and proceeded to test a transistor but it appears I get no response from the step generator when I turn it on. I probed test points 305 and counting the horizontal lines on the staircase pattern I see about 12 horizontal lines from the very bottom to the very top. I probed test point 360 and found I get similar results like 305 but the staircase pattern is either negative or positive depending on what I select.

I checked power supply voltages and they seem to be reasonable on the +-12V +-30V +200V

I notice R300 Step Zero seems to have a burn mark on it, it sparks when I fully turn it, I reset it to its default position, maybe defective

Any suggestions what I should probe or check, thanks

DW
 

My hope was replacing the thermal pad would at least have me with something I can work with. I should point out the step generator was partially working when Q386 did not have a thermal insulator pad. When I have thermal pads on both Q384 Q386 I am getting no indications of step generator function on the screen. This is weird that the step generator would show some forms of life on the screen with Q386 without a thermal pad

DW
 

I probed Q384 Q386 and found on the base of both transistors they are -1.2 volts, I see nothing on the collector or emitter of both transistors. Seems like the floating power supply maybe isn't putting out any voltage. Seeing as Q386 was accidentally shorted to the base perhaps a fuse is blown on the floating supply, well I hope at least there is not any serious extensive damage which might be the case? Yikes!

DW
 

F391, I will find and check

DW
 

F391 good

DW
 

Checking CR712 in circuit seems to be reading abnormal using my diode test feature on the multimeter, it indicates this diode is conducting both ways with a dead short. Bare to mind I have removed all the various connectors on this power supply board located at the rear of the instrument. I think I will pull it out to check it further. Perhaps if CR712 is bad that can cause some issues I'm having?

DW
 

CR712 is fine, hmm. No fuse appears to be blown, no voltage appears to be present, I hope the transformer is ok

DW
 

1.7 ohms on the secondary for the winding that supplies power for the floating supply, I think the winding is okay and there is no shorts to ground. I also checked the rectifier with the multi meter it seems to be fine.