I haven't been following your thread, but just noticed your latest post.? Your problem is very likely the use of 1N4004 here. The reverse recovery time of these rectifiers is much too long. They are acting as shorts for a significant part of the time they should be off, and that is undoubtedly making the power oscillator grunt hard. Transistors and transformer will overheat and eventually fail.
Get some UF4004 rectifiers instead. Remove all the 1N4004s you installed in that circuit, and replace them with their UF (for "ultrafast" reverse recovery) counterparts. I'm guessing that your overheating problems will go away. Or, at minimum, you'll have peeled off one layer of the onion.
Good luck!
-- Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 3/2/2021 11:50, jrseattle wrote:
Yes, pin 5 of transformer T710 measures as 20V DC.
I lifted R777 and noticed NO DIFFERENCE in the Waveform at the nexus of CR769 and CR776 (WF3), still at about 60V minimum. This implies that the loading occured on the left side of R777. So I replaced CR776 (I'm using 1N4004 diodes as replacements for all diodes in this area) and YES, major change. Suddenly the WF is clamped at 10V at the bottom. You were right, CR776 was leaking, even though it tested good. After reconnecting the CRT, I now have an image with working intensity control ... but only for about 25 minutes.
The current through the oscillator circuit is still increasing, though slower and oscillation stops after about 25 minutes. I also replaced CR780 and CR782 but that makes no difference.
I set the negative voltage at the anode of CR764 to -5510V and the Grid Bias at +116V. Picture quality is amazingly good and sharp (for 25 minutes).
I must say that working on this 606A is difficult because not much thought was given to maintenance by Tektronix. Adjustment locations are hard to get to sometimes (like R776) and requires pulling the HV board out of the chassis but the connections with the Power Supply board are not set up for this. There are about 8 this wires soldered to the board and they are too short to provide any slack. I already added an extension cable but the thin wires keep breaking at the solder connections on the HV board. I blew about 3 fuses in the +270V circuit because the wire broke and touched ground, until I realized what was happening. OK, enough ranting.
I'll start playing with reduced negative cathode voltage again and will focus on the "Error Amplifier" section next since the voltage at U740 pin 6 goes high (to 2V) pretty quickly, which seems wrong.
Thanks for your insight that a leaking CR776 can cause voltage to stay high (though that is counter-intuitive to me but I'm more of a software guy anyway).
