I made another post about this Communications Monitor at one point, but I am now creating a new thread for a new problem since I've done a lot since then.
For context, I am repairing a Cushman CE-5 Communications Analyzer that has working standby power but does not run otherwise once turned on.
I narrowed the issues down to the power delivery boards, A12 and A13 that are responsible for rectification, filtering, and regulation of the three voltage busses. +20V, +5V, and -12V.
When first testing the machine before I replaced any components, both the +20V and +5V busses coming from A12 were outputting roughly 0V. The -12V buss was showing the appropriate voltage.
I have since replaced ALL of the electrolytic capacitors on both A12 and A13. This brought back power to the +20V buss which is now showing the correct voltage.
There is still no +5V buss voltage (0V)Starting from the top, the AC voltages coming from the transformer were within specifications before they enter the A13 rectifier and filter board. The voltages coming from A13 are all within ±0.5V of their required voltage but
pin 5 of A13 shows +13.3V, but should be +17.5V. Does this imply that some of the rectifier diodes are bad?
So my questions boil down to:- Should I try replacing the rectifier diodes on A13 to see if that brings the +17.5V DC output back up to spec?
- Would the out of spec voltage for pin 5 on A13 cause the +5V buss to lose output?
- What should I try troubleshooting/checking on A12 to try and figure out how to restore the +5V buss?
Thanks in advance!
I don't have much experience with voltage regulators, and this one is even more foreign to me due to its age.
I have attached all of the related schematics. The entire manual can be found at
https://bama.edebris.com/download/cushman/ce5/ce5-1973.pdf as well.
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