475A with High +110 rail


Mlynch001
 

I have searched for a previous thread that was referred to in a post about a "high" 110 voltage rail on a 475 from back in 2012. I failed to find that post or any other applicable answer, perhaps I am not searching for the correct words? So I will ask for some help. I have a 475A with DM44 s/n B012228. After a thorough cleaning, I turned it on and find that for the most part it works great, no triggering issues, bright trace, all controls respond as expected, just dirty pots and switches. I supply various signals to the A and B channels and the scope displays a good trace with with amplitude matching the output of my signal generator, so calibration seems to be close enough for hobby work. The scope simply does not act as if there is anything wrong. For safety sake, I pulled the case and tested the various system voltages on the A9 interface voltage test points. I found everything to be spot on, except the +110 volt rail, Voltages as follows:
-15V = -14.998
-8V = -8.005
5V = 5.005
15V = 15.002
50V = 50.035
Unregulated 50V = 63.35
110V = 134.66

Line Voltage is 120V and the voltage calibrator on the transformer is set to the proper voltage. I pulled and tested Q1490, Q1494, Q1496 and Q1497. These all tested "good" with a transistor tester. I also replaced Q1490 and Q1494 with "Known Good" components, with no change noted in the +110V rail. I did not have exact replacements for Q1497 or Q1496, but they did both test good. I did go through to check and seat all the transistors that I could reach on all the boards. I checked the various resistors in LV +110 Supply regulator circuit (the ones I could reach) and found then to be very close to correct. I do not want to start un-soldering components without some idea of what to check next. I do not know if CR1489 has anything to do with the voltage regulation or not, but that component appears to be part of the +110 regulation system. Other than the "too high" +110 rail, the scope demonstrates no other issues, nothing smells "Hot" and no "magic smoke", The boards inside the scope are perfect, no signs of repairs, heat or damage. I just want to sort this out before I use the scope very much. Collector voltage on Q1497 is 134.5 and Q1496 at about 133.8. Almost everything agrees with the schematic EXCEPT that pesky +110V rail. Any ideas where to check next?


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Michael Lynch
Dardanelle, AR

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