577/Repairs

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 (as a precaution) 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 found that all rails now function with the +30V rail well above factory specification.

Simple fix: Someone had been "adjusting" the +/-30V Trimmers to try to "fix" the +30V Power supply rail (this did not work).

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.

Symptom: CRT not coming on. Yellow Collector Warning light on. No functions on unit.

Searched the schematic and found that the Collector Caution light was controlled by relays on the Collector Supply board K115 and K125. These also shut off the rest of the machine.

K125 switches K115 and this turns the entire machine on or off (for all practical purposes).

With K125 not energized, the yellow warning comes on and not much else happens.

Tested both relays, out of circuit and they are good. They open and close when power is supplied or cut off.

Found 12V present at P122-Pin3

Found 12V present at P122-Pin4

K125 coil must be grounded to close relay and turn on K115 and the rest of the machine.

K125 coil is controlled by Q588 of the Vert/Horz Amplifier (Schematic <6>).

Pulled Q588 (TEK p/n 151-0302-00), put it in the 576 Curve Tracer, it tests as a Diode.

Why is this transistor bad?

Pulled Q544 (Same p/n as Q588) which was good to find an equivalent transistor in my stock.

Found that a 2N3904 is close enough to work as a simple switch.

Installed 2N3904 and turned the unit on.

Magic Smoke came from C587 (TEK p/n 290-0525-00). This is a 1 μF 35V Tantalum . . . . shorted!

My new 2N3904 is toast. I have plenty. I think this is the cause of the original failure of Q588.

Pulled C586 and C587 (Same item)to test. Found both testing as Resistors.

Replaced with 1 μF 50V Tantalum caps.

Installed a new 2N3904

Turn on power.

Unit switches on! Yellow Light out. . . We have a functional unit.

One very apparent problem at this time was that the dot/trace was VERY bright and the Intensity control had little effect on the brightness.

This was a simple fix. Someone had tried to "adjust" the CRT to make a trace appear (see problem described above) and had turned the Trace Intensity Pot all the way up (this did not "fix" the issue).

I adjusted this pot to the approximate mid range position and the Intensity control now works as it should.

The moral is, "Don't turn pots and trimmers for which you do not understand their function or purpose."

Did some quick tests of PNP, NPN, DIODES and Resistor Devices and found that the unit works much like the Type 576.

Next step is adjust and calibrate per the Service Manual.