Difference between revisions of "577/Repairs"
(→Symptom - Unit powers on but no CRT activity.)
Revision as of 08:23, 15 August 2019
Symptom: Unit shows signs of power but no CRT Activity. Yellow Collector caution light comes on, indicating power.
Removed Power supply from chassis for access and checked all power rails.
Found +30V at 50-100 mV.
Found 200V at 181.6V.
Found +/- 12V close @ -11.96V and +12.06V.
Found -30V at -30.66V.
It Appears that the +30 rail has a short to ground.
This process involved turning the unit on and off several times.
After about 10 on/off cycles, "Magic smoke" appears from R786.
Ohm tested transformer winding in question (orange-white-orange) 1.5ohms between each orange leg and white center tap. 2.96 ohms for complete winding (orange to orange).
Checked transformer winding output and found correct AC voltage before rectifier.
Schematic shows 3 tantalum caps in circuit, C767-772 and 781.
C767 tested as a 43 ohm resistor in my little ESR/Component tester. 576 curve tracer confirmed it as a resistor, not a capacitor.
99% sure this was my problem.
Replaced all three caps with new versions, same specs.
Replaced R786 and R766 (3.9 Ω 1/4 W 5% Carbon Comp.) 786 was open 766 had drifted to 5.16 Ω. I used metal film 1/2 W 1% as replacements, this was what I had on hand.
Powered up and fond that all rails now function with correct voltages.
Adjusted +/-30V Rails to specification and reassembled PS to Chassis.
I now have correct power at all rails and board connectors, but CRT still will not power up.
See second symptom/repair.