Topics

Tektronix 475


Michael W. Lynch
 

All:

Just like the Energizer Bunny, the 475 is "Still Working" and the "Tilted Trace" is banished. That 82V Zener Diode must have been the intensity problem. The "new" CRT fixed the tilt. Thanks for all the help and encouragement. I just love to see one of these old scopes come back to life.

--
Michael Lynch
Dardanelle, AR


Michael W. Lynch
 

On Thu, Oct 1, 2020 at 04:03 PM, Eric wrote:


Do you have access to a curve tracer? If so I would put VR1374 across it and
check it at its zener voltage. It sounds like the zener is acting up when
warm. But if it is an 82V Zener it sounds WAY out of spec at 66 V.
Eric,

You read my mind. Yes, I actually have both a 576 and 577. See my reply to Tom below. That "suspect" diode exhibited a "normal" zener trace then suddenly broke down and I got an almost "tunnel diode" looking trace just before the trace went straight vertical. Both the 576 and 577 showed similar results after a minute or two of testing. The 475 is sitting on the bench now and looks perfectly normal after about 30 minutes of running.

--
Michael Lynch
Dardanelle, AR


Tom Lee
 

Sounds like you nailed it -- great job! It's always nice to have a hangar queen to provide substitute parts for quick troubleshooting. High-voltage Zeners can suffer from unstable surface breakdown, leading to "crazy" I-V curves. Sounds like your Zener has not aged gracefully (then again, who among us does?).

Enjoy that 475. It's a fine scope!

--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 10/1/2020 14:09, Michael W. Lynch via groups.io wrote:
On Thu, Oct 1, 2020 at 03:30 PM, Tom Lee wrote:

A not uncommon problem that causes the behavior you describe is cathode-grid1
leakage. If you preclude the usual external circuit causes, then it could very
well be a crt problem. If you still have the other crt, you could do a quick
swap back and see if the problem disappears.

There are ways to fix crts with this problem but let’s hold off on that for
now.
Tom,

Thanks for your kind reply. I may have answered my own question. After writing that post, I thought that I might actually be following or understanding Chuck's explanation and that I might have a bad 82V Zener or a bad capacitor. Since I had a parts scope (the one that the CRT came from) I went out and pulled C1373 and VR1374 from that unit and installed them into my "malfunctional" unit. I am testing now, but the issue seems to be resolved. In addition, I put the unknown or "Bad" VR1374 into my Type576 and 577 curve tracers. In both cases, the component would show a normal Zener curve, then suddenly break down into a crazy looking curve (looked almost like a tunnel diode curve). Not sure what was going on, however, both the 576 and 577 showed the same results.
Tom


Eric
 

Yep defiantly a bad zener. I had a 140V one do that at take out the negative power section on a 5200A I would imaging this would be aggravated by heat, IE cooling it off would make it behave warming it up and it goes nuts. Glad it is just a zener and not a tube.

On 10/1/2020 5:09 PM, Michael W. Lynch via groups.io wrote:
On Thu, Oct 1, 2020 at 03:30 PM, Tom Lee wrote:

A not uncommon problem that causes the behavior you describe is cathode-grid1
leakage. If you preclude the usual external circuit causes, then it could very
well be a crt problem. If you still have the other crt, you could do a quick
swap back and see if the problem disappears.

There are ways to fix crts with this problem but let’s hold off on that for
now.
Tom,

Thanks for your kind reply. I may have answered my own question. After writing that post, I thought that I might actually be following or understanding Chuck's explanation and that I might have a bad 82V Zener or a bad capacitor. Since I had a parts scope (the one that the CRT came from) I went out and pulled C1373 and VR1374 from that unit and installed them into my "malfunctional" unit. I am testing now, but the issue seems to be resolved. In addition, I put the unknown or "Bad" VR1374 into my Type576 and 577 curve tracers. In both cases, the component would show a normal Zener curve, then suddenly break down into a crazy looking curve (looked almost like a tunnel diode curve). Not sure what was going on, however, both the 576 and 577 showed the same results.
Tom


Michael W. Lynch
 

On Thu, Oct 1, 2020 at 03:30 PM, Tom Lee wrote:


A not uncommon problem that causes the behavior you describe is cathode-grid1
leakage. If you preclude the usual external circuit causes, then it could very
well be a crt problem. If you still have the other crt, you could do a quick
swap back and see if the problem disappears.

There are ways to fix crts with this problem but let’s hold off on that for
now.
Tom,

Thanks for your kind reply. I may have answered my own question. After writing that post, I thought that I might actually be following or understanding Chuck's explanation and that I might have a bad 82V Zener or a bad capacitor. Since I had a parts scope (the one that the CRT came from) I went out and pulled C1373 and VR1374 from that unit and installed them into my "malfunctional" unit. I am testing now, but the issue seems to be resolved. In addition, I put the unknown or "Bad" VR1374 into my Type576 and 577 curve tracers. In both cases, the component would show a normal Zener curve, then suddenly break down into a crazy looking curve (looked almost like a tunnel diode curve). Not sure what was going on, however, both the 576 and 577 showed the same results.
Tom

--
Michael Lynch
Dardanelle, AR


Eric
 

Do you have access to a curve tracer? If so I would put VR1374 across it and check it at its zener voltage. It sounds like the zener is acting up when warm. But if it is an 82V Zener it sounds WAY out of spec at 66 V.

On 10/1/2020 4:13 PM, Michael W. Lynch via groups.io wrote:
On Thu, Aug 13, 2020 at 09:33 AM, Chuck Harris wrote:

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
Chuck,

I know, this is an old post and an often discussed topic.

I have a similar problem. I am working on a 475. This one had a bad CRT a broken support rod inside the CRT and a tilted trace. I have located another CRT and have installed it. I have set the grid bias, which seems to agree with the manual. No more Tilted trace. The scope works fine when you first start it up, including the Z axis amp input on the back. However, after about 20-30 minutes, the CRT intensity goes "full Bright". Z-Axis input stops affecting the trace, The CRT Trace or dot is also erratic, with the intensity changing in a random fashion from off to dim to full on bright with lots of blooming (it is mostly bright during these times). Intensity control seems to effect the brightness, but will only make the trace brighter than it is at the minimum setting, so intensity control makes it either Bright or Brighter with blooming and loss of sharpness. Focus and Astig controls work as expected when cold and with a super bright trace.

I must admit, I do not fully understand the DC Restorer. I have been reading these and other posts on the subject.

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.
YES! Testing across VR1374 I get about 66VDC. This seems at odds with your statement quoted above. This is with the + lead on the band side and the - lead on the opposite end of the diode. Reversing the leads gives a negative voltage of similar potential. So I have "Low" voltage and something like you are describing below, correct?

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
I was expecting 82Volts but perhaps I am not measuring this correctly? From your statement, 60-66VDC would put me in the "blindingly bright range", correct?

Is it possible that this CRT has issues?

or

Would you say that the zener (VR1374) and related capacitor (C1373) could be breaking down?

CR1373 and VR1374 check "good" with a conventional diode check, using a DVM. Both show infinity one direction and .576 VDC in the other.

Like I stated previously, the scope works OK for the first 15-20 cold minutes of operation. After that it all goes nuts.

Your thoughts?


Tom Lee
 

A not uncommon problem that causes the behavior you describe is cathode-grid1 leakage. If you preclude the usual external circuit causes, then it could very well be a crt problem. If you still have the other crt, you could do a quick swap back and see if the problem disappears.

There are ways to fix crts with this problem but let’s hold off on that for now.

Tom

Sent from my iThing, so please forgive brevity and typos

On Oct 1, 2020, at 13:13, Michael W. Lynch via groups.io <mlynch003=yahoo.com@groups.io> wrote:

On Thu, Aug 13, 2020 at 09:33 AM, Chuck Harris wrote:


Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
Chuck,

I know, this is an old post and an often discussed topic.

I have a similar problem. I am working on a 475. This one had a bad CRT a broken support rod inside the CRT and a tilted trace. I have located another CRT and have installed it. I have set the grid bias, which seems to agree with the manual. No more Tilted trace. The scope works fine when you first start it up, including the Z axis amp input on the back. However, after about 20-30 minutes, the CRT intensity goes "full Bright". Z-Axis input stops affecting the trace, The CRT Trace or dot is also erratic, with the intensity changing in a random fashion from off to dim to full on bright with lots of blooming (it is mostly bright during these times). Intensity control seems to effect the brightness, but will only make the trace brighter than it is at the minimum setting, so intensity control makes it either Bright or Brighter with blooming and loss of sharpness. Focus and Astig controls work as expected when cold and with a super bright trace.

I must admit, I do not fully understand the DC Restorer. I have been reading these and other posts on the subject.

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.
YES! Testing across VR1374 I get about 66VDC. This seems at odds with your statement quoted above. This is with the + lead on the band side and the - lead on the opposite end of the diode. Reversing the leads gives a negative voltage of similar potential. So I have "Low" voltage and something like you are describing below, correct?

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
I was expecting 82Volts but perhaps I am not measuring this correctly? From your statement, 60-66VDC would put me in the "blindingly bright range", correct?

Is it possible that this CRT has issues?

or

Would you say that the zener (VR1374) and related capacitor (C1373) could be breaking down?

CR1373 and VR1374 check "good" with a conventional diode check, using a DVM. Both show infinity one direction and .576 VDC in the other.

Like I stated previously, the scope works OK for the first 15-20 cold minutes of operation. After that it all goes nuts.

Your thoughts?


--
Michael Lynch
Dardanelle, AR





Michael W. Lynch
 

On Thu, Aug 13, 2020 at 09:33 AM, Chuck Harris wrote:


Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
Chuck,

I know, this is an old post and an often discussed topic.

I have a similar problem. I am working on a 475. This one had a bad CRT a broken support rod inside the CRT and a tilted trace. I have located another CRT and have installed it. I have set the grid bias, which seems to agree with the manual. No more Tilted trace. The scope works fine when you first start it up, including the Z axis amp input on the back. However, after about 20-30 minutes, the CRT intensity goes "full Bright". Z-Axis input stops affecting the trace, The CRT Trace or dot is also erratic, with the intensity changing in a random fashion from off to dim to full on bright with lots of blooming (it is mostly bright during these times). Intensity control seems to effect the brightness, but will only make the trace brighter than it is at the minimum setting, so intensity control makes it either Bright or Brighter with blooming and loss of sharpness. Focus and Astig controls work as expected when cold and with a super bright trace.

I must admit, I do not fully understand the DC Restorer. I have been reading these and other posts on the subject.

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.
YES! Testing across VR1374 I get about 66VDC. This seems at odds with your statement quoted above. This is with the + lead on the band side and the - lead on the opposite end of the diode. Reversing the leads gives a negative voltage of similar potential. So I have "Low" voltage and something like you are describing below, correct?

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
I was expecting 82Volts but perhaps I am not measuring this correctly? From your statement, 60-66VDC would put me in the "blindingly bright range", correct?

Is it possible that this CRT has issues?

or

Would you say that the zener (VR1374) and related capacitor (C1373) could be breaking down?

CR1373 and VR1374 check "good" with a conventional diode check, using a DVM. Both show infinity one direction and .576 VDC in the other.

Like I stated previously, the scope works OK for the first 15-20 cold minutes of operation. After that it all goes nuts.

Your thoughts?


--
Michael Lynch
Dardanelle, AR


Simon
 

I now have it working properly, but still don’t know what the original problem was. Maybe resocketing transistors and the CRT did some good. The Y output amplifier was bad, but I found a new one on eBay. After getting a decent trace, the Y attenuators were not working properly. I brushed the leaf contacts with IPA, removed the attenuator blocks one by one, cleaned the pins, wiped them with IPA and plugged them back in. The AC-GND-DC switches also needed similar treatment. I have rational progression in Y gain on both, so all in all I think it had been in storage a long time and many contacts had gone bad or intermittent. The X position occasionally wanders to the right, but comes back after a while, maybe another poor contact to track down.
Thanks to everyone for useful advice.
Simon


Stephen
 

Thanks Chuck


Chuck Harris
 

That seems to be the standard they were trying to
achieve on everything from the 500 series through
the 2400 stuff.

In some cases, the eliminated adjustments, and let
the intensity pot come out as it may. I have seen
2465B's that had normal intensity with the pot at
the 3 or 4 O'clock position.

-Chuck Harris

Stephen wrote:

Chuck,

On any Tek scopes, or just this particular model?




Stephen
 

Chuck,

On any Tek scopes, or just this particular model?


Chuck Harris
 

That is correct.

I think of the grid bias pot as being your way to calibrate
the DC restorer and CRT to match your visual needs.

After all, how bright is bright enough, is a subjective thing.

Typically, the grid bias is adjusted so that when the intensity
control is in the "9 O'Clock" position, a spot on the screen is
just barely visible in normal room lighting.

If the DC restorer and CRT are both good, when the intensity pot
is fully CCW, the spot will be invisible, and when the pot is
at 12 O'Clock, the spot will be normal brilliance.

-Chuck Harris

tenareze32@... wrote:

I‘ll try and get to the capacitor with a smaller soldering iron, but I surmised that something might be loading the circuit as the impedance is high. It is now clipping, but I have tested just about all the components in the DC restorer.
My understanding of the grid bias circuit is that the tap on the HT transformer provides enough volts to cut off the beam (at least -80 V) when put through the voltage rectifier/doubler. The Z-axis amplifier adds enough positive volts to turn on the beam in sync with the timebase and the grid bias pot gives some more adjustment if the trace is faint (more positive volts to the grid).
Simon




Simon
 

I‘ll try and get to the capacitor with a smaller soldering iron, but I surmised that something might be loading the circuit as the impedance is high. It is now clipping, but I have tested just about all the components in the DC restorer.
My understanding of the grid bias circuit is that the tap on the HT transformer provides enough volts to cut off the beam (at least -80 V) when put through the voltage rectifier/doubler. The Z-axis amplifier adds enough positive volts to turn on the beam in sync with the timebase and the grid bias pot gives some more adjustment if the trace is faint (more positive volts to the grid).
Simon


Chuck Harris
 

The unit worked when it left the factory. If the zener
isn't leaking the current, than the capacitor is.

Leaky zeners and leaky capacitors only get worse.

-Chuck Harris

tenareze32@... wrote:

I have ordered some as I didn’t have one of that value. However, I cobbled together a chain of zeners which cut off around 83 V and tacked that in with alligator clips. I still did not have enough volts even without a zener, The tap on the HT transformer produces about 350 V at rather high impedance through the 47 pF capacitor. I thought of replacing this capacitor but it is hard to get to.
I am rather mystified at the moment, but at least the tube is good.
Simon




Simon
 

I have ordered some as I didn’t have one of that value. However, I cobbled together a chain of zeners which cut off around 83 V and tacked that in with alligator clips. I still did not have enough volts even without a zener, The tap on the HT transformer produces about 350 V at rather high impedance through the 47 pF capacitor. I thought of replacing this capacitor but it is hard to get to.
I am rather mystified at the moment, but at least the tube is good.
Simon


Chuck Harris
 

HI Simon,

Replace the zener. It isn't the voltage, it is the leakage
current that tektronix worried about. Yours is bad.

-Chuck Harris

tenareze32@... wrote:

The 82 V zener was OK, but there wasn’t enough voltage across it for it to zener. I increased the drive voltage from the HT tap through the 47 pF capacitor by reducing the value of R1326 from 390 kΩ to 47 kΩ. The brightness is fully controllable from off to plenty bright and the grid bias pot also has an effect. I now have -80 V grid to cathode. I guess this is OK despite departing from the official values. It works and I see no smoke nor smell hot components.
Simon




Simon
 

The 82 V zener was OK, but there wasn’t enough voltage across it for it to zener. I increased the drive voltage from the HT tap through the 47 pF capacitor by reducing the value of R1326 from 390 kΩ to 47 kΩ. The brightness is fully controllable from off to plenty bright and the grid bias pot also has an effect. I now have -80 V grid to cathode. I guess this is OK despite departing from the official values. It works and I see no smoke nor smell hot components.
Simon


Simon
 

That is interesting as I had a feeling that the zener might be bad, but I haven’t checked it yet. On the to-do list.
Many thanks
Simon


Chuck Harris
 

Ok, you are almost there.

There are two things at play at the same time with the intensity
grid function. The first is the DC restorer, which you have checked.
It translates the voltage output from the intensity control to a
voltage range at the control grid of the CRT.

The second thing is the grid bias control. It biases the grid so that
it is in the operating range of the CRT.

I forget, have you verified that the grid bias control has an effect?

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.

-Chuck Harris





tenareze32@... wrote:

I tested all the transistors in the Z-axis amplifier and all were OK. The amplifier puts out a square wave which varies from 0 to 35 V with the intensity control, but the beam was still too bright at all intensity settings. I started to suspect the voltage multiplier circuit (C1372, CR1377, CR1379) as the input was at least 40 V p-p. The components tested OK with a multimeter, but I replaced the diodes in case they were breaking down at circuit voltages. No change, but all controls operate as described in the manual with the scope probe on the cold side of C1372. The circuit diagram in my copy of the manual does not correspond exactly to the circuit description as it shows a separate winding on the HT transformer for grid bias, which I cannot locate (transformer on other side of the board). However, a 40-50kHz signal is coming through the 390 kΩ resistor (R1326) from somewhere. I will just live with a bright trace for the moment and concentrate on the Y output amplifier, which looks as thought it can be removed and tested on the bench.
Simon