Posted 03 September 2019 - 09:07 PM
Based on your voltage readings, it tells me that your voltage regulator diode appears to be working properly. Your regulator itself "appears" to be working properly, however, further tests need to be conducted in order to determine if it is actually failing under load. The reason I say this is that the regulator output voltage is low. The first thing that one immediately suspects is that the regulator is bad, right? But wait, if there is a heavy current draw, or a short, this will also cause the output voltage to drop as well. So is it the regulator, or is it due to something else that is dragging down the voltage? Unfortunately, this is not a good news since you now have to find out what is causing the low voltage. Here's what I would do:
1), Most regulator and voltage rails have numerous capacitors that tethers the line to ground. The caps shouldn't actually draw current down because capacitors should only allow AC to pass, and not DC when working properly. Therefore, if you test the caps with ohmmeter, they should show infinity, after counting up. The larger the cap, the slower the count, this is normal. However, if you are getting a steady resistance reading, that is problematic, possibly a shorting capacitor. If such is the case, replace the capacitor and retest. Here are some caps that could potentially cause issues if they are shorting. They should test infinity in circuit but if not, then remove and retest. If still has resistance other than infinity, then they are bad.
C456, C303, C220, C116, C225 are possibilities.
2) If you have a transistor hfe tester, remove transistor and test. Replace if bad. Q502. Heck once the transistor is out of circuit, I would probably change it anyway but it's easy for me to say because I often have something in stock that will work as a replacement. Any NPN transistor with equal or higher current capacity should work as a suitable replacement, in other words, this part is not critical if spec'd same or higher in voltage and current capacity. However, to make installation easier, you should compare datasheets first to see if the lead layout is the same. If it is different, then the leads may crisscross or become weird. Just make sure B/C/E leads are installed in the appropriate pcb through hole.
3) Take IC201 and IC301 Vcc pins out of circuit and see if voltage comes back up. Do that by desoldering the Vcc pin from board and make sure there is no solder remaining to connect them to circuit. Verify with DMM to ensure those pins are out of circuit. If voltage on that rail comes back up with it's Vcc pin out of circuit, then replace the offending IC (do one at a time). The IC might have an internal short.
4) There's too many other possibilities to list, but essentially, the goal is to find the source of the short or load that is dragging down that regulated rail and anything connected to that rail, including coils, etc. could be the culprit. Hopefully, one of the afforementioned steps has already resolved your issue. If not, then you have more tedious work ahead of you.