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576 erratic brightness

Kurt Rosenfeld
 

My Tek 576 curve tracer has erratic brightness that makes the instrument almost unusable. It has two modes: normal, and uncontrollably bright. When it goes into the super-bright mode, I turn the power off so it doesn't hurt itself. I tried replacing the intensity pot, R883. That didn't help, and also the original pot measured normal. Before I start poking around with my HV probe (measuring CRT cathode and grid voltage), is this a common problem with something specific I should check?


thanks, -Kurt

 

Deliberately defocusing the beam will help protect the phosphor.

I would check or just replace D882 (140 volt zener diode) and C888
(0.0068uF high voltage capacitor) shown on schematic 14. Either
shorting out or leaking would raise the control grid voltage
increasing the intensity.

On 16 Apr 2017 18:19:35 +0000, you wrote:

My Tek 576 curve tracer has erratic brightness that makes the instrument almost unusable. It has two modes: normal, and uncontrollably bright. When it goes into the super-bright mode, I turn the power off so it doesn't hurt itself. I tried replacing the intensity pot, R883. That didn't help, and also the original pot measured normal. Before I start poking around with my HV probe (measuring CRT cathode and grid voltage), is this a common problem with something specific I should check?

thanks, -Kurt

Ed Breya
 

Bright is good - usually the problem is the opposite due to the CRT getting old or the HV sagging, or the dreaded "bad epoxy HV transformer." As I recall, the HV circuit is regulated by feedback from a winding on the HV transformer that makes +225 V (or so, I think) for the deflection circuits. The last one I worked on had different problems, but I found some of the rectifiers on that supply were shot, and marginal, rating-wise.

I'd monitor the HV cathode supply, and the +225 V to see what they look like when things go wrong, and suspect those rectifiers or any of the associated caps first if the HV regulation gets upset. It could be anything intermittent in the feedback loop, too. If the HV is solid, then look at the load/CRT bias side like the intensity and focus pot string, and the wiring from the board to CRT socket and pots. I had to change some of the big carbon composition ones that had drifted way out - if ones goes intermittent I think a lot could happen. Of course any HV caps or dipped Ta types are suspect too.

Ed

Tam Hanna
 

Hello,
well, I could say something. Are you attached to that CRT, or would you be willing to go, um, onto something else?

Tam

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With best regards
Tam Hanna
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NEW: Enjoy electronics? Like seeing oscilloscopes get repaired? Please subscribe to my new YouTube channel -> http://www.youtube.com/user/MrTamhan

Ed Breya
 

Tam wrote: "Hello,
well, I could say something. Are you attached to that CRT, or would you
be willing to go, um, onto something else?"

Tam, are you hinting at possible availability of an LCD replacement for the 576 CRT, like that for the HP8566 and other gear? If so, please tell us more. I've suggested and hoped that could be an option if someone would come up with it. Ultimately, I and I'm sure other 576 owners will need such a thing. I plan to keep my CRTs going as long as possible by limiting running time, and CRT rejuvenation, but there's only so much usable life remaining.

Ed

Kurt Rosenfeld
 

I replaced D882 and C888 in the CRT circuit of my 576 and the erratic brightness problem remains. I'll probe some voltages next.

Regarding using something other than the CRT to display the traces, I will do that if there's something wrong with the CRT or the HV transformer.


-Kurt

Kurt Rosenfeld
 

Another thing: I'm a bit confused by the 576 HV power supply schematic in one place. See the red arrow here: http://w140.com/tek_576_hvps_question.png http://w140.com/tek_576_hvps_question.png

http://w140.com/tek_576_hvps_question.png

http://w140.com/tek_576_hvps_question.png http://w140.com/tek_576_hvps_question.png


View on w140.com http://w140.com/tek_576_hvps_question.png
Preview by Yahoo





At first glance it seems like a simple voltage divider, and one would expect the pot's wiper to go to the CRT grid, but the section of wire pointed to by the red arrow changes that. Is it a mistake in the schematic?

David DiGiacomo
 

On Sat, Apr 22, 2017 at 4:34 PM, <kurt.harlem@...> wrote:

At first glance it seems like a simple voltage divider, and one would expect the pot's wiper to go to the CRT grid, but the section of wire pointed to by the red arrow changes that. Is it a mistake in the schematic?

You would prefer not to connect the CRT grid to a pot's wiper, because
when the wiper momentarily bounces open during rotation, the CRT will
go to maximum brightness.

Chuck Harris
 

I didn't mention this earlier when I saw your post, but I have
fixed two 576's thus far that had erratic brightness, and it was
cured by replacing the dead open electrolytic that filters the
+5V logic supply.

-Chuck Harris

kurt.harlem@... [TekScopes] wrote:

Another thing: I'm a bit confused by the 576 HV power supply schematic in one place. See the red arrow here: http://w140.com/tek_576_hvps_question.png http://w140.com/tek_576_hvps_question.png

http://w140.com/tek_576_hvps_question.png

http://w140.com/tek_576_hvps_question.png http://w140.com/tek_576_hvps_question.png


View on w140.com http://w140.com/tek_576_hvps_question.png
Preview by Yahoo





At first glance it seems like a simple voltage divider, and one would expect the pot's wiper to go to the CRT grid, but the section of wire pointed to by the red arrow changes that. Is it a mistake in the schematic?

Chuck Harris
 

HP very often put a large low voltage cap between the
intensity pot's wiper and ground. It was there for two
reasons, one, to make the intensity start out at zero and
slowly brighten as the CRT warms up, and two, to reduce
the effects of pot noise on the beam brightness.

-Chuck Harris


David DiGiacomo telists@... [TekScopes] wrote:

On Sat, Apr 22, 2017 at 4:34 PM, <kurt.harlem@...> wrote:

At first glance it seems like a simple voltage divider, and one would expect the
pot's wiper to go to the CRT grid, but the section of wire pointed to by the red
arrow changes that. Is it a mistake in the schematic?

You would prefer not to connect the CRT grid to a pot's wiper, because when the
wiper momentarily bounces open during rotation, the CRT will go to maximum
brightness.

Kurt Rosenfeld
 

I still don't understand. The intensity pot (R883) combined with R885 (100K in series with the wiper) just seem to form a variable shunt resistance across the 140 volt zener (D882). Assuming D882 remains in avalanche mode, won't the grid-to-cathode voltage will always be -140V? Furthermore, it looks like when the wiper of R883 is close to the top end (terminal V), R885 will be sinking enough current to pull the zener out of avalanche mode. And that looks like pulling the zener out of avalanche mode would vary the cathode voltage of the CRT, which I would expect to affect the deflection sensitivity. I am assuming that it was a high priority for the designers to keep the deflection sensitivity constant with respect to the position of the intensity control. What am I missing?

 

I still don't understand
That link does not exist in any of my paper manuals (rev 7-83 & 4-88).
Instead there is a diode (D885) with the cathode facing right. This
diode and D887 (which I haven't been able to locate on the schematic)
were only present on units with S/N below B020000. The print of D885
on the schematic is very weak so it would probably not show up good
on a B/W scan. Maybe the person who did the scan tried to improve it
and didn't realize it it was a diode and not a link.

/Håkan

Chuck Harris
 

There is a massive problem with B&W scans of tektronix
manuals from that era.

Tektronix used a blue ink to show things like the grouping of
components that were on a single board, or entity. They also
put big blue dots to show (usually labeled with letters) the
signals entering/leaving the board outline.

The big problem is, if the blue outlines show at all, on a B&W
scan, they show up as black lines.... that look for all the
world like wires...

Ok, if you look at the 576 intensity circuit schematic, you will
see a box outline that surrounds the 2M intensity pot, and the
letters V, W, and AA labeling the dots that indicate the signals
entering/leaving the pot.

They box surrounding the pot looks like wiring, but is not wiring!

The circuit has a pot with a 140V zener diode that sets the
voltage across the pot to 140V. The wiper divides that voltage,
and feeds it to a 100K resistor that goes to the control grid
of the CRT. Simple.

-Chuck Harris

kurt.harlem@... [TekScopes] wrote:

I still don't understand. The intensity pot (R883) combined with R885 (100K in
series with the wiper) just seem to form a variable shunt resistance across the
140 volt zener (D882). Assuming D882 remains in avalanche mode, won't the
grid-to-cathode voltage will always be -140V? Furthermore, it looks like when the
wiper of R883 is close to the top end (terminal V), R885 will be sinking enough
current to pull the zener out of avalanche mode. And that looks like pulling the
zener out of avalanche mode would vary the cathode voltage of the CRT, which I
would expect to affect the deflection sensitivity. I am assuming that it was a
high priority for the designers to keep the deflection sensitivity constant with
respect to the position of the intensity control. What am I missing?

Kurt Rosenfeld
 

Can somebody with a paper copy of the manual upload a scan or a photo of that schematic, or at least that part? I'm really curious what it actually looks like.

I will get a paper copy of the 576 manual and scan all of the schematics in color so this confusion ends. And then I will update the manual on Tekwiki and will share the color scans with BAMA and ko4bb.com.

Jerry Ingordo
 

In defence of the people who scanned some these Tektronix manuals, early on
color scanning was not allowed because of file size. Thats the reason for
BW. There was a time when BAMA was the only game in town and would not
accept test equipment scans and djvu format was the rule. We've come a
long way in 20 years. In time I will convert all my old scans to color.
BTW the 576 is not my scan.

Jerry
W2JI

 

Can somebody with a paper copy of the manual upload a scan or a photo
As Chuck pointed out Tek often used print in blue to indicate changes on schematics.
However in this manual the change was instead in weak grey. It is an original manual
and not a scan rev 7-83. In the newer manual I have the print is even weaker.

You can find a photo of the page here:

http://www.hakanh.com/dl/temp/576.jpg http://www.hakanh.com/dl/temp/576.jpg

/Håkan

Chuck Harris
 

I think the issue isn't the added diode, but rather the series
of manuals that didn't make the box outlines bold, and break them
around the terminal points that were denoted by a black dot.

In those few manuals, and the 576 manual found on ebaman.com is one,
the box outline was the same width line as the circuit wiring, and
the dots were slightly larger than the connection dots.

When the formerly blue, outline boxes are printed in black, they
look like some very strange, nay, insane, circuit connections. In
simple places like the intensity pot in question, one might think
that they were actual wiring, and the pot was redundant.

In other places, where the outline box crossed all of the leads on
a power supply, and made it look like all of the power supply outputs
were shorted together, one gets the idea fairly quickly...

The tektronix blue color choice used to be quite a problem back when
all copies were made on the old xerographic process copier machines,
as the copier drums weren't at all sensitive to blue light.... which
caused the blue lettering to totally disappear in the copy.

In scans where the person scanning the schematic chose "B&W Line art"
as the scanning method, a rigid threshold is used to decide whether
a point on the paper is white or black, and the blue lines either
disappear as being below the threshold, or are totally black when
they are above the threshold.

-Chuck Harris

hahi@... [TekScopes] wrote:

Can somebody with a paper copy of the manual upload a scan or a photo
As Chuck pointed out Tek often used print in blue to indicate changes on schematics.
However in this manual the change was instead in weak grey. It is an original manual
and not a scan rev 7-83. In the newer manual I have the print is even weaker.

You can find a photo of the page here:

http://www.hakanh.com/dl/temp/576.jpg http://www.hakanh.com/dl/temp/576.jpg

/Håkan

Kurt Rosenfeld
 

Thanks for the photo! That makes a lot more sense.

Ed Breya
 

BTW seeing that CRT/HV schematic again reminded me of something that may be of use for diagnostics. The signal on the cold end of the HV winding is a proxy for the output/cathode current, and is actually for another feedback loop. Its simple label "+12.5V (DCPL)" is a bit misleading, since it's not just a decoupled power supply, but an important control/feedback signal.

Ed

Kurt Rosenfeld
 

I bought a replacement (lightly used) CRT for the 576 that had the erratic brightness problem. That fixes it. It was a CRT problem, not an HV power supply problem.

The old CRT was actually usable, and the 576 was used plenty between 2017 and now. I would just steer the beam off-screen until the CRT warmed up. Then the intensity would suddenly become controllable. I suspect the CRT has grid-cathode short a room temperature, but no short at operating temperature. I'll try to measure this now that the old CRT is out of the 576.

If anyone wants the old CRT, it's yours for the price of shipping. I found it quite useful, despite its problem. Email me off-list at kurt.harlem@... if you want it. Off-list, please.