Tektronix 606A that resists being repaired


jrseattle
 

Hi, everyone.

I've been working on a Tektronix 606A XY Display but the unit resists all my repair attempts. So, I'm reaching out to this group for suggestions and assistance.

I've documented my problems here: http://dutchtronix.com/Tek606A.htm

Summary: 2 problems in the HV section, possibly related
problem 1: HV oscillator stops working after about 2 minutes
problem 2: DC restorer circuit (intensity control) not working at all

Any assistance is appreciated.

JdR


Chuck Harris <cfharris@...>
 

Take a peek at the HV transformer under the shield.
If it uses epoxy on the winding, you may have the dreaded
epoxy disease. I don't know for sure if the 606A used the
brown epoxy on the transformer, or beeswax.

-Chuck Harris

jrseattle wrote:

Hi, everyone.

I've been working on a Tektronix 606A XY Display but the unit resists all my repair attempts. So, I'm reaching out to this group for suggestions and assistance.

I've documented my problems here: http://dutchtronix.com/Tek606A.htm

Summary: 2 problems in the HV section, possibly related
problem 1: HV oscillator stops working after about 2 minutes
problem 2: DC restorer circuit (intensity control) not working at all

Any assistance is appreciated.

JdR








benj3867
 

@ jrseattle, as Chuck said, you may have a HV transformer epoxy potting issue.

To the best of my knowledge, while the 606 is much younger than the 500 series scopes that are most famous for having this problem, it may still be the case.
The symptoms you describe certainly match the classic symptoms of a bad Type 547 HV transformer: as the transformer heats up, the loses in the epoxy increase, loading the driving circuit more and more until it reaches the limits of its current pushing capabilities.

One way to pinpoint the problem is to directly manipulate the temperature of the HV transformer. For example:
1. If you direct hot air at it (not too hot!) from a hair fan, does it shorten the time it takes it to stop?
2. Once it stops working, does spraying the HV transformer with some freezing aerosol (like MG Chemicals 403C Super Cold Spray) brings it back to life until it warms up again?


jrseattle
 

Thanks Chuck for the reply. I have posted some pictures in an album called "Tektronix 606A". The exposed cloth-like material feels hard.

The transformer does NOT get hot (at least the top). Freeze spray does nothing. Pre-warming the transformer does nothing.

What would be the best way to remove the HV feedback loop from the circuit (as a test)? I don't quite understand how U740 (LM741 OpAmp) works here. It's configured as a non-inverting amplifier with a 1.83 amplification factor. With the OpAmp removed, the circuit does NOT oscillate and the voltage at pin 3 is between 11 and 15V, dependent on R736.


jrseattle
 

Thanks. See my response to Chuck.

I need to add that I need to turn the device OFF once the oscillation collapses otherwise the 3Amp fuse or Q714 will burn up since Q714 keeps conducting. The current limiter circuit does NOT work even though I tested all individual components.


Chuck Harris <cfharris@...>
 

The beeswax transformers look like they are globbed with beeswax.
The epoxy transformers either have a clear to slightly brown milky
varnish over the wires and tape, or are molded in brown epoxy.

Yours sounds like the epoxy varnish sort, but your tests are a good
sign.

Is there a manual available somewhere?

-Chuck Harris

jrseattle wrote:

Thanks Chuck for the reply. I have posted some pictures in an album called "Tektronix 606A". The exposed cloth-like material feels hard.

The transformer does NOT get hot (at least the top). Freeze spray does nothing. Pre-warming the transformer does nothing.

What would be the best way to remove the HV feedback loop from the circuit (as a test)? I don't quite understand how U740 (LM741 OpAmp) works here. It's configured as a non-inverting amplifier with a 1.83 amplification factor. With the OpAmp removed, the circuit does NOT oscillate and the voltage at pin 3 is between 11 and 15V, dependent on R736.






Ozan
 

First of all very nice description of the problems in your web page, very useful for others to understand what is happening. I don't have a good theory but I can help explaining the HV loop.

What would be the best way to remove the HV feedback loop from the circuit (as
a test)?
Feedback loop is needed for proper base bias of Q714.

I don't quite understand how U740 (LM741 OpAmp) works here. It's
configured as a non-inverting amplifier with a 1.83 amplification factor. With
Amplification factor is (100k+12k)/12k = 9.33 based on the schematic on your web page. For all practical purposes you can assume U740 is trying to drive pin 3 to close to zero volt because of this large gain from pin3 to output. At nominal output voltage of -0.5V, pin 3 voltage would be at -0.05V. Voltage coming from R730 and -5500V should balance each other to ~ zero volts by the ratio of R734A and R734B.

the OpAmp removed, the circuit does NOT oscillate
Without opamp there is no base drive to start the oscillations.

and the voltage at pin 3 is between 11 and 15V, dependent on R736.
When the circuit is not oscillating there is no -5500V to pull R734B to -5500V. R736 provides +15V to +10.99 depending on its position (ignore the loading of R734A. Pin3 is connected to voltage divider of R734A and R734B, without -5500V pin 3 voltage range you see looks reasonable.

======

Possible root causes:
- bad/heating transformer: I think you eliminated it by heating/cooling the transformer.

- It is quite possible that HV supply is working harder than expected because of some fault/load in high voltage side. Your DC restorer could be the cause, may be the tube is drawing too much current. You can try turning down the -5500V supply to see if it gives you longer time to work on the HV supply. Once you fix the DC restorer you can correct the voltages.

Just to make sure: Are +120V UNREG and +30V UNREG OK (I am using 606 schematic, not 606A in case names are different)? Is there a 606A service manual?

== DC restorer
In your web page WF4 shows CR769 cathode doesn't go below 65V. It is not because CR769 doesn't do its job but voltage never goes below 10V so that it can conduct. This is consistent with your findings that changing CR769 doesn't do anything. It is a clamp, it will only work when voltage goes below 10V.

CR776 could be leaky, when cathode of CR769 (same as anode of CR776) starts going down it may leak current, then waveform doesn't have a chance to go below 10V. Since you already probed several voltages, here is one way to find out where the extra current is going: Look at cathode of CR776. It should go slightly up when it is clamping and stay steady when its anode (WF3) is falling from clamp voltage. If it is following the falling edge of the waveform it is leaking.

Next diodes CR782 and CR780 could be leaky. This is a high voltage area and best is to disconnect the diodes and test them outside. They should be able to work with a reverse bias of 100V+ so while testing check with a large reverse bias for leakage.

Another point to keep in mind is only 27pF is driving these nodes, your scope probe capacitance is a significant load.
Ozan


Chuck Harris <cfharris@...>
 

Hi JR,

This HV section is comprised of an oscillator, and a regulator.
The oscillator is Q720, 722, and 714. The opamp biases the transistors
in the oscillator to adjust the final HV voltage.

The first thing I would look at, as always, is the quality of all of the
LV power supplies. Sometimes this sort of problem can be a capacitor
letting too much ripple happen... sometimes...

The next thing, after I determined that all of the supplies are good,
is to look at the bias voltage that drives the opamp. I would look
at the junction of R748 and R728 as a probably convenient spot. If you
have another scope, watch that voltage with one channel, and watch the
collector of Q714 with another channel... beware, the voltage could get
a little high on the collector... Not enough to damage your scope, or
probes. You want to see if there is a relationship between the oscillator
quitting, and the opamp trying to turn up the power.

If you have a HV probe, like a P6015, the next point of interest is the
-5500V at the cathode of the CRT. Is it starting to drop before the
oscillator quits?

-Chuck Harris

jrseattle wrote:

Thanks Chuck for the reply. I have posted some pictures in an album called "Tektronix 606A". The exposed cloth-like material feels hard.

The transformer does NOT get hot (at least the top). Freeze spray does nothing. Pre-warming the transformer does nothing.

What would be the best way to remove the HV feedback loop from the circuit (as a test)? I don't quite understand how U740 (LM741 OpAmp) works here. It's configured as a non-inverting amplifier with a 1.83 amplification factor. With the OpAmp removed, the circuit does NOT oscillate and the voltage at pin 3 is between 11 and 15V, dependent on R736.






jrseattle
 

Thanks for the suggestions.
1. Supply voltages check:
I remeasured the supply voltages (I repaired these earlier including rectifier diodes and a filter cap). +270V, +15V, -30V all perfect, no ripple. +120V and +30V a derived from the secondary winding of the HV transformer T710 (unusual) but look good too as long as the oscillation keeps going; +30V has a 136mV ripple (-40mV..+90mV) at the resontant frequency of 62 kHz.

2.
I added the requested measurement to http://dutchtronix.com/Tek606A.htm (see WF5, WF6). Bottom line, the OpAmp output at the junction of R748 and R728 gets distorted after about 2 minutes, then the voltage level rises quickly until the oscillator stops.

3.
The -5500V on the secondary (measured at the anode of CR764) doesn't increase more than 50V until oscillation stops.

All these measurements were taken with P820 and R596 disconnected to reduce any load from the 120V supply taken from the secondary of T710. I noticed no difference.

I did one more test: disconnect the CRT connector from the CRT so there is no load on the secondary of T710 at all. Result is WF7 on my website. Oscillation continues forever. It does mean that transformer T710 works fine under low load conditions (only 400mV drop on R714 (modified to 1R) so about 400 mA current).

I'm going to focus on the second problem (DC restorer circuit) with the CRT disconnected now that I don't have to turn the machine off every 2.5 minutes.


jrseattle
 

Thanks for your suggestions. I did the oscillator part first.
1. reducing the -5500 cathode voltage to -4400 V has no effect. Oscillation still stops at about 2 1/2 minutes
2. +120V and +30V (derived from the secondary of T710) are all good. I lifted R596 to reduce the 120V load but see no effect.

As I mentioned in my reply to Chuck, disconnecting the CRT has a very positive effect and oscillation doesn't stop anymore. I'm going to look at the DC restorer circuit with the CRT disconnected next.

Question though. You mention that CR769 doesn't clamp if the voltage never goes below 10V. I understand. But the voltage at R768 (on the side of C768) definitely goes below 10V. So something on the other side of R768 (which is the nexus of CR769, CR776 and R777) prevents that signal from going below 60V, correct?
Could this be related to the voltage shift at C768? C768 is pure AC centered around 0 when R768 is lifted but biased to 100V when R768 is connected?

JdR


Ozan
 

Your post doesn't say if +20V at pin 5 of the transformer is checked but WF5 on your web page looks OK so +20V seems to be fine. Still worth checking.

Question though. You mention that CR769 doesn't clamp if the voltage never
goes below 10V. I understand. But the voltage at R768 (on the side of C768)
definitely goes below 10V. So something on the other side of R768 (which is
the nexus of CR769, CR776 and R777) prevents that signal from going below 60V,
correct?
That is also how I see it. However, looking at the waveforms I don't think it is something hard clamping to 60V, more like there is an undesired load (pulling above 60v) that is using up the charge coming from C768 before cathode of CR769 has a chance to go below 10V.

Now that you removed the tube, you could try lifting R777 to figure out if the problem is to the right side of left side of R777.

Could this be related to the voltage shift at C768? C768 is pure AC centered
around 0 when R768 is lifted but biased to 100V when R768 is connected?
This is expected. When R768 is lifted only DC path from right side of C768 is your probe, which has a DC path to ground. So average value will be at ground (0V). When R768 is connected your clamps (and undesired leakage) are providing DC path. Let's look at right side of R768: Its maximum is limited by top clamp at 110V (from your measurement), bottom is clamped at ~65V. The waveform is not symmetric so the average is more that (110V + 65V)/2=87V. You can ask the scope to give you an average value but ~ 100V is probably reasonable (more time is spent at 110V). Since this is the average (DC) value at the right side of R768, left side is at the same DC voltage because C768 doesn't pass DC current.

Most likely one of the diodes CR776, CR782, CR780 went bad or leaky.

Ozan


Chuck Harris <cfharris@...>
 

Hi JdR,

Removing the base of the CRT eliminates the load from the
oscillator, and gives it a much larger margin for handling
overloading caused by transformer problems, or other circuitry
leakage.

If you can, measure the current the oscillator is drawing in
both conditions.

-Chuck Harris

jrseattle wrote:

Thanks for your suggestions. I did the oscillator part first.
1. reducing the -5500 cathode voltage to -4400 V has no effect. Oscillation still stops at about 2 1/2 minutes
2. +120V and +30V (derived from the secondary of T710) are all good. I lifted R596 to reduce the 120V load but see no effect.

As I mentioned in my reply to Chuck, disconnecting the CRT has a very positive effect and oscillation doesn't stop anymore. I'm going to look at the DC restorer circuit with the CRT disconnected next.

Question though. You mention that CR769 doesn't clamp if the voltage never goes below 10V. I understand. But the voltage at R768 (on the side of C768) definitely goes below 10V. So something on the other side of R768 (which is the nexus of CR769, CR776 and R777) prevents that signal from going below 60V, correct?
Could this be related to the voltage shift at C768? C768 is pure AC centered around 0 when R768 is lifted but biased to 100V when R768 is connected?

JdR






jrseattle
 

I measured the voltage drop across R715, which currently is a 1 Ohm power resistor, so the voltage drop in Volts equals the current in Amps.

Before removing the CRT connector, the drop was about 0.7V initially, constantly increasing. After about 2 minutes, it reached 2.5V, then increased very fast. I turned the machine off at 3V drop (3 Amp current). Both resistor R715 and transistor Q714 are really hot at that point. Transformer T710 is cold.

After removing the CRT connector, the voltage drop across R715 is just 400mV and it stays there.

After fixing the DC restorer circuit (more on that in my next post), the voltage drop across R715 starts at 800mV and very slowly increases. After about 20 minutes, it's up to 1.47V, then starts to increase fast and I have to turn the machine off.


jrseattle
 

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).


Ozan
 

... 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.
This could be because of not having the right power transistor in Q714. Twenty five minutes is quite a long time, probably thermal runaway. Does cooling Q714 extend the 25 minutes?

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.
Do you still have 1-ohm or did you change R715 to 0.5ohm? The output will be shifted up if there is 1-ohm.



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).
You are doing pretty good for a SW guy :).

Ozan


Tom Lee
 

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
http://www-smirc.stanford.edu

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).




jrseattle
 

Thanks. I ordered some UF4004 diodes and will install them once they arrive. Hopefully, that will remove the last circuit loading issue.


Tom Lee
 

I'm hopeful, too. I don't know if you noticed any of those diodes getting hot, but if you did, that would make the diagnosis even more definite. In any case, 1N4004s are 100x slower to recover than UF4004s, so the drive transistor will be much happier, at minimum

Keeping fingers crossed.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/2/2021 14:08, jrseattle wrote:
Thanks. I ordered some UF4004 diodes and will install them once they arrive. Hopefully, that will remove the last circuit loading issue.




jrseattle
 

Q714 is currently a Tektronix transistor marked 151-0606-00 referenced as Motorola SJE375 but I can't find any info on this device. The transistor that used to be there (broken) has no identification at all. I tried several MJE800 Darlingtons as listed in the manual but they fail because of rush-in current at startup. Can I add a small power resistor at the collector of Q714 to prevent this? Under normal operation, the current through Q714 seems to be about 1Amp (see below) so the MJE800 should work (4Amp rating)

If I set the cathode voltage at about -5000V (or a more positive value, max -4700), the device works find and oscillation doesn't stop anymore. The voltage drop across R714 in this mode is 1V or less (so 1 Amp current) and stays there. 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.

Still using 1R for R715 because my experiments show that a higher value here increases the oscillator run-time (limits current flow through Q714 is my guess, thus postponing the thermal runaway). Once I install the faster diodes (UF4004), I'll try to original 0.5 Ohm R715 again.

You say "The output will be shifted up if there is 1-ohm". I assume you are referring to U740 pin 6. Even at a limited cathode voltage of -5000V, the voltage here starts at 2V (with 1 Ohm R715).

Note that the screen image is crystal clear with -5000V cathode voltage but I would like to completely fix this device.

Should have more results in about a week.

JdR


Ozan
 

Q714 is currently a Tektronix transistor marked 151-0606-00 referenced as
Motorola SJE375 but I can't find any info on this device. The transistor that
used to be there (broken) has no identification at all. I tried several MJE800
https://w140.com/tek_common_design_parts_catalog_transistors_diodes_and_misc_may_1988.pdf
shows TIP142 for an equivalent (please double check). Digikey has them.

at startup. Can I add a small power resistor at the collector of Q714 to
prevent this? Under normal operation, the current through Q714 seems to be
about 1Amp (see below) so the MJE800 should work (4Amp rating)
TIP142 is a 10A transistor, stronger than MJE800, should work without modification unless there is still a lingering bug.

You say "The output will be shifted up if there is 1-ohm". I assume you are
referring to U740 pin 6. Even at a limited cathode voltage of -5000V, the
voltage here starts at 2V (with 1 Ohm R715).
You had a concern about higher output voltage than marked on the schematic at pin 6, and were planning to debug the error amp. I recommend delaying the debug to after placing the right power transistor, and putting back 0.5-ohm. And the new DC restorer diodes. After those changes voltage might end up at the expected value.

Ozan