Tektronix 606A X-Y Display
Any help appreciated
My Tektronix 606A XY display, which I use with the Dutchtronix AVR Oscilloscope Clock, stopped working. I replaced several components but at this point there are still 2 problems that may or may not be related. Both are in the High Voltage section.
Note that direct supply voltages are good and stable. 2 supply voltages (+120V and +30V) are indirect, taken from T710, and they are fine until the oscillator stops.
The HV Oscillator runs fine on startup, generating -5500V on the output winding of T710. The oscillator stops running after about a minute or 2. Darlington transistor Q714 provides the current driving the HV transformer. When the oscillation stops, Q714 stays biased ON resulting in over-current. The +20V supply has a 3 amp fuse which blows or Q714 blows, whichever comes first. Q725 is the current limit transistor but it doesn't do the job. Q714 is listed as "SELECTED FROM MJE800"; I've tried several MJE800 Darlington transistors but they keep blowing up ON STARTUP (smoke). I've replaced R715, a 0.5 Ohm power resistor with a 1 Ohm power resistor to delay the stopping of the oscillator and am using a heavy duty NPN transistor for Q714 (marked 151-0606-00, Motorola SJE375; can't find any info on this device)
I've tested all components in the circuit, replaced capacitors etc. but can't figure out why the current keeps increasing resulting in the stopping of the oscillator. Of course Q714 heats up and spraying it with circuit freeze helps for a few seconds. Can someone suggest a good NPN darlington transistor to use that can easily handle 3 amps, 25V, in case the 151-0606-00 is causing the problem (which I doubt).
The base of Q725 has a voltage of 200mV and starts rising immediately. This voltage should be -0.4V.
Pin 6 of U740 has a voltage of 2.4V (should be -0.5V), stays stable for a while, then rises to 5.6V just before the oscillator collapses. The HV voltage is -5500V so there should be no error.
Follow the discussion of this issue here: https://groups.io/g/TekScopes/topic/tektronix_606a_that_resists/80868022?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,80868022
Here are the schematics of the High-Voltage Oscillator
The DC Restorer circuit is not working and, as a result, intensity is always at the max (or close to it). Please look at the DC Restorer Circuit schematics. The Anode of CR769 is set to +10V by the intensity control circuit (verified) and the cathode of CR776 is set to +110V by the Grid Bias pot (verified).
The junction of CR769 and CR776 is supposed to provide a modulation signal. This signal is a combination of the sine wave coming from the HV transformer through C768 (-250V..+250V), clamped at the bottom to a minimum of 10V (CR769 starts conducting if the signal is < 10V) and clamped at the top to a maximum of 110V (CR776 starts conducting if the signal is >110V).
Unfortunately, the signal I measure is more like +80V .. +110V which causes max intensity at all times. Changing the intensity control varies the voltage of the anode of CR769 from +10V to +80V but the cathode is unchanged (+80..+110). I replaced CR769 several times trying different diodes but to no effect. CR769 also tests good, in and out of circuit.
Please check the description of the circuit here and here ([please note that C768 and R768 are reversed on the second page).
Here are the schematics of the DC Restorer circuit
Here are some oscilloscope screenshots related to problem 2. All focus on the nexus of CR769 and CR776 (arrow with <4>):
For this measurement, R768 was lifted at the nexus point and the probe measured at the lifted end of R768. As can be seen, the sinus WF from T710 has an amplitude of about 280V, centered around 0V. AC and DC mode show the same WF. Putting the probe at the connection between C768 and R768 generates a similar WF with an amplitude of 550V
Here, R768 is soldered back in place. The probe was connected at the connection between C768 and R768. Connected to the Modulator circuit, the amplitude is still about 550V but now centered around +100V. Not sure if that is expected.
Here, the probe is connected at the nexus of CR769 and CR776, after R768. The maximum voltage is clamped at around 115V, as expected, but the lower voltage stays high around 65V. It doesn't go down to 10V, as expected (see wf 2 here). The anode of CR769 is 10V. Why doesn't that diode do it's job?
This image just combines WF2 and WF3. The top is clamped, as expected but the bottom isn't.
This image shows CH1 at the intersection of R748 and R728 (OpAmp output), which is the drive for Q714 base current and CH2 is the collector of Q714 - at startup. Both WFs are DC coupled. Ch1 swings between 1.16V and 1.74V. CH2 swings between 5V and 36V, moving about 1V every second.
Same image after about 2 minutes. Ch1 becomes irregular and the Voltage level starts to rise. CH2 doesn't change much until oscillation stops.
Same measurement as WF5 and WF6 but with the CRT disconnected, i.e. no load on the secondary winding of T710. With the CRT disconnected, oscillation continues forever.
Replaced diode CR776. WF3 now goes all the way down to the voltage set by the intensity control and is clamped. Intensity control works (after reconnecting the CRT). Even though CR776 tests good, it was leaking current, loading the WF at the nexus from CR776 and CR769, preventing it from reaching the lower voltage. I used a 1N4004 diode but was advised to replaced the diodes in this section with UF4004 since the regular 1N4004 is too slow in switching, loaded the circuit.
If I set the cathode voltage at about -5000V (or a higher value like -4700), the device works find and oscillation doesn't stop anymore. At -5000V, the image is clear but intensity control is limited. At -5500V, the specified value, the oscillation circuit eventually stops, after almost 30 minutes.
Waiting for the UF4004 diodes before doing more testing.
Suggestions on what could cause this problem are appreciated.
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