On Mon, Apr 29, 2019 at 08:07 AM, Dave Hills wrote:
With 134v on Q1496 collector, you should see +61v at Q1490's base. This will
completely shut down the regulator by reverse biasing Q1494 b-e. That said it
only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483 would
have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible, but
not likely to cause these symptoms), leaving a leaky or shorted Q1496 or
CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently
fail. Also, check that CR1489 has not been installed backward.
Please forgive the caps, not shouting, just no other way to highlight for
emphasis.
Dave
Dave:
Your advice helped me find the issue! Tom Jobe suggested that I study the 110V regulator circuit description and the schematic, which I did as well. Lots of good advice from everyone got me pointed in the right direction.
Here is what I found and how I fixed the issue:
After the aforementioned study and re-reading all the comments, I decided to pull CR1489 off the board and test it. It tested good on my curve tracer, meter and component tester. Reinstalled on the board and there was no change. (of course) I also pulled all transistors, including Q1496 (again) and tested it on my little home built curve tracer and re- ran it through two additional transistor testers, all showed it to be good. (again, no Change) You mentioned that Q1490 should have a base voltage of about 61V with the 110V rail at 135V, I found it to actually be low, at 41.5V, the schematic calls for 50V, and since you said I should have about 61V; I knew that the voltage divider formed by R1486 and R1487 was not right. R1487 tested 49.9K so I moved on to R1486 and it tested "weird", meaning high resistance sometimes and then open at others. This prompted me to remove it completely from the board. After getting it off the board, I found it to be "open", so un-soldering the component probably finished it off.. I took a 68K resistor from the bench and temporarily soldered it into place of R1486, checked the 110V rail and BANG! the 110V rail was now at 119.4V (down from 135V) with no other changes. This told me I had found the issue, now I had to find a proper replacement for R1486, which was originally 60.4K (321-0364-00 - RES.,FXD,FILM: 60.4K OHM, 1%,o.125W). Doing a little math, I found that installing 100K and 150K resistors in parallel would deliver 59K, so VERY close to what I needed. Installed these two resistors parallel in place of R1486, re-tested, this brought the base of Q1490 to 50.06V and gave me exactly 110.65V on the 110V rail. Now all I have to do is find the proper replacement part and my scope in good to go! THANK YOU ALL for your advice!
Sincerely,
--
Michael Lynch
Dardanelle, AR
Mlynch001