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I have searched for a previous thread that was referred to in a post about a "high" 110 voltage rail on a 475 from back in 2012. I failed to find that post or any other applicable answer, perhaps I am not searching for the correct words? So I will ask for some help. I have a 475A with DM44 s/n B012228. After a thorough cleaning, I turned it on and find that for the most part it works great, no triggering issues, bright trace, all controls respond as expected, just dirty pots and switches. I supply various signals to the A and B channels and the scope displays a good trace with with amplitude matching the output of my signal generator, so calibration seems to be close enough for hobby work. The scope simply does not act as if there is anything wrong. For safety sake, I pulled the case and tested the various system voltages on the A9 interface voltage test points. I found everything to be spot on, except the +110 volt rail, Voltages as follows:
-15V = -14.998
-8V = -8.005
5V = 5.005
15V = 15.002
50V = 50.035
Unregulated 50V = 63.35
110V = 134.66

Line Voltage is 120V and the voltage calibrator on the transformer is set to the proper voltage. I pulled and tested Q1490, Q1494, Q1496 and Q1497. These all tested "good" with a transistor tester. I also replaced Q1490 and Q1494 with "Known Good" components, with no change noted in the +110V rail. I did not have exact replacements for Q1497 or Q1496, but they did both test good. I did go through to check and seat all the transistors that I could reach on all the boards. I checked the various resistors in LV +110 Supply regulator circuit (the ones I could reach) and found then to be very close to correct. I do not want to start un-soldering components without some idea of what to check next. I do not know if CR1489 has anything to do with the voltage regulation or not, but that component appears to be part of the +110 regulation system. Other than the "too high" +110 rail, the scope demonstrates no other issues, nothing smells "Hot" and no "magic smoke", The boards inside the scope are perfect, no signs of repairs, heat or damage. I just want to sort this out before I use the scope very much. Collector voltage on Q1497 is 134.5 and Q1496 at about 133.8. Almost everything agrees with the schematic EXCEPT that pesky +110V rail. Any ideas where to check next?


--
Michael Lynch
Dardanelle, AR

Michael-
I also would check the eb voltages? of Q1496 and Q1497 make sure nothing strange and xstrs are operating in their proper regions.
I would then take a hard look at CR1489 ( protection ec junction for pass xstr), almost seems like it might be leaking badly or possibly shorted bypassing the regulation. ( I had one go bad in a different supply....took a 5kv hit). It appears you have eliminated most everything else.
those are my guesses with the info provided. Someone else may have more insight.
搁别苍é别

On the 474A L.V. schematic it shows the expected voltage at the emitter of Q1496 as being 135.6 volts, which is quite close to your 134.66 reading at your +110 test point.
It shows the expected voltage at the +110 test point as being 111 volts with only R1488 and Q1496 directly between the +110 test point and the expected 135.6 volts at the emitter of Q1496.
Maybe you can find the problem by probing around right in that area?
tom jobe...

With 134v on Q1496 collector, you should see +61v at Q1490's base. This will completely shut down the regulator by reverse biasing Q1494 b-e. That said it only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483 would have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible, but not likely to cause these symptoms), leaving a leaky or shorted Q1496 or CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently fail. Also, check that CR1489 has not been installed backward.

Please forgive the caps, not shouting, just no other way to highlight for emphasis.

Dave

Dave,

No worries on the caps! I understood why you were using them, I sometimes do the same thing and hope that people figure out my meaning. Renee, Tom and Dave, I will do some more probing and post the results. I am tempted to lift one end of CR1489 and test it, out of circuit. BTW, it appears to be mounted correctly. I think I might have found a good Q1496 and I will try that as well, once I verify CR1489. I do have a service manual for the scope, so that makes repairs much more doable for me, since I am not an EE or trained in any way. Just a self taught enthusiast. I just love these old TEKTRONIX scopes.

--
Michael Lynch
Dardanelle, AR

I forgot to say Thanks to All for such complete answers. I appreciate your patience with a very inexperienced individual.
I will post results.
--
Michael Lynch
Dardanelle, AR

This morning you got a nice reply from Dave Hills that talked about the parts of the circuit in question that limit and control the over voltage of the +110 circuit.
This led me to? search the 475A service manual and read everything it had to say about the low voltage regulator circuits.
There is a nice summary of how the +110 control low voltage circuit handles the over current and over voltage shutdowns.
After reading this, Dave Hills comments are probably right on the likely problem components. Your +110 regulator should probably shutdown with the over voltage you are seeing on the +110 test point, but it isn't!
Think about studying that +110 section on page 3-24 of my first edition August 1976 manual, it's all quite simple when you read it.
tom jobe...

Tom,

I found all the advice helpful. Any comments and advice is greatly appreciated! Your confirmation of Dave's diagnosis is a great help, going back through his comments as well. I am reading the Power Supply circuit description in the manual right now. Unfortunately, I am not that experienced with this stuff, which does complicate things somewhat. However, i am not afraid to learn and fix things. Working through the components list right now. I do appreciate your followup and your kind advice. I will post more measurements and findings as I am able to get them from the scope.

Sincerely,

--
Michael Lynch
Dardanelle, AR

On Mon, Apr 29, 2019 at 08:07 AM, Dave Hills wrote:

With 134v on Q1496 collector, you should see +61v at Q1490's base. This will
completely shut down the regulator by reverse biasing Q1494 b-e. That said it
only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483 would
have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible, but
not likely to cause these symptoms), leaving a leaky or shorted Q1496 or
CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently
fail. Also, check that CR1489 has not been installed backward.

Please forgive the caps, not shouting, just no other way to highlight for
emphasis.

Dave

Dave:

Your advice helped me find the issue! Tom Jobe suggested that I study the 110V regulator circuit description and the schematic, which I did as well. Lots of good advice from everyone got me pointed in the right direction.

Here is what I found and how I fixed the issue:

After the aforementioned study and re-reading all the comments, I decided to pull CR1489 off the board and test it. It tested good on my curve tracer, meter and component tester. Reinstalled on the board and there was no change. (of course) I also pulled all transistors, including Q1496 (again) and tested it on my little home built curve tracer and re- ran it through two additional transistor testers, all showed it to be good. (again, no Change) You mentioned that Q1490 should have a base voltage of about 61V with the 110V rail at 135V, I found it to actually be low, at 41.5V, the schematic calls for 50V, and since you said I should have about 61V; I knew that the voltage divider formed by R1486 and R1487 was not right. R1487 tested 49.9K so I moved on to R1486 and it tested "weird", meaning high resistance sometimes and then open at others. This prompted me to remove it completely from the board. After getting it off the board, I found it to be "open", so un-soldering the component probably finished it off.. I took a 68K resistor from the bench and temporarily soldered it into place of R1486, checked the 110V rail and BANG! the 110V rail was now at 119.4V (down from 135V) with no other changes. This told me I had found the issue, now I had to find a proper replacement for R1486, which was originally 60.4K (321-0364-00 - RES.,FXD,FILM: 60.4K OHM, 1%,o.125W). Doing a little math, I found that installing 100K and 150K resistors in parallel would deliver 59K, so VERY close to what I needed. Installed these two resistors parallel in place of R1486, re-tested, this brought the base of Q1490 to 50.06V and gave me exactly 110.65V on the 110V rail. Now all I have to do is find the proper replacement part and my scope in good to go! THANK YOU ALL for your advice!

Sincerely,

--
Michael Lynch
Dardanelle, AR

Hi Micheal,
Thank you for the nice repair summary!
It was good that Dave Hills brought some of his logic, reason and knowledge to your problem.
The online 'parallel resistor calculators' show that your 150k and 100k pair give you exactly 60k of resistance.
I wonder if it would be okay to leave your pair of resistors in there as a permanent fix?
Maybe some kind soul will comment on that question for us amateurs?
Those Tektronix Service Manuals are like a series of college courses on analog electronics!
tom jobe...

On Wed, May 1, 2019 at 02:46 AM, tom jobe wrote:

The online 'parallel resistor calculators' show that your 150k and 100k pair
give you exactly 60k of resistance.
I wonder if it would be okay to leave your pair of resistors in there as a
permanent fix?

Obviously, using 5% tolerance resistors of 100 kOhm and 150 kOhm in parallel will not guarantee to get 60 kOhm. The values of R1486 and R1487 as such aren't critical but their combination basically determines the 110 V: V110 = V50 * (R1486 + R1487) / R1487. I haven't checked the circuits in detail but accuracy worse than 1% for the combination may spoil scope calibration state.
You may consider using 3 resistors in a parallel/serial config for R1486 - or even slightly changing R1487 to get the correct ratio but checking with your DVM will be necessary unless you use 1% resistors. Building R1486 from a series connection of a fixed, reasonably stable (metal film) 56 kOhm resistor with a 10 kOhm trimpot in series is another possibility, giving enough resolution in trimming with a range between about 110 V plus and minus 5%, since it allows varying R1486 from about 56 kOhm to about 66 kOhm. The Ohmmeter function on almost any DVM will be accurate enough for trimming.

Raymond

On Tue, Apr 30, 2019 at 08:24 PM, Raymond Domp Frank wrote:

On Wed, May 1, 2019 at 02:46 AM, tom jobe wrote:

The online 'parallel resistor calculators' show that your 150k and 100k pair
give you exactly 60k of resistance.
I wonder if it would be okay to leave your pair of resistors in there as a
permanent fix?

Obviously, using 5% tolerance resistors of 100 kOhm and 150 kOhm in parallel
will not guarantee to get 60 kOhm. The values of R1486 and R1487 as such
aren't critical but their combination basically determines the 110 V: V110 =
V50 * (R1486 + R1487) / R1487. I haven't checked the circuits in detail but
accuracy worse than 1% for the combination may spoil scope calibration state.
You may consider using 3 resistors in a parallel/serial config for R1486 - or
even slightly changing R1487 to get the correct ratio but checking with your
DVM will be necessary unless you use 1% resistors. Building R1486 from a
series connection of a fixed, reasonably stable (metal film) 56 kOhm resistor
with a 10 kOhm trimpot in series is another possibility, giving enough
resolution in trimming with a range between about 110 V plus and minus 5%,
since it allows varying R1486 from about 56 kOhm to about 66 kOhm. The
Ohmmeter function on almost any DVM will be accurate enough for trimming.

Raymond

Raymond,

Not to worry! This is not going to be a "permanent fix". I have ordered the correct value 1% resistors for R1486 and R1487. They are cheap and easy to find exactly what is needed Tom Jode is exactly correct, the calculator indeed shows the value of the 100k/150k pair as 60k. When I installed the pair, I measured the actual value as a pair for the 5% parts I used and found that they were about 58.9K. I did not take a lot of time to "trim out" the pair, Since I was committed to obtain the correct value resistors as a permanent fix. As far as affecting the calibration, this rail does not seem to have any visible affect at all on the display on the CRT. When the 110V rail was at 135V, the scope worked perfectly, without any evidence of distortion of the signals, when I went back to 110V, the signal displayed on the CRT did not observably change, either in amplitude or frequency. I compared the 110V results to the 135V and the 119V results with identical test signals that were fed into the scope. Amazingly, all the other rails' voltages never changed, regardless of the 110V rail voltage. I was afraid that these voltages would swing one way or another and therefore checked each one as i made adjustments to the 110V rail. I am pleased to report that those other voltage rail regulators seem to have a wide input range and very tight control over their areas of responsibility. It is a testament to the careful engineering and great quality of these TEKTRONIX instruments that they can continue to work when everything is not perfect. It is evident that someone designed these circuits with just this scenario in mind.

--
Michael Lynch
Dardanelle, AR

On Tue, Apr 30, 2019 at 07:46 PM, tom jobe wrote:

Hi Micheal,
Thank you for the nice repair summary!
It was good that Dave Hills brought some of his logic, reason and
knowledge to your problem.
The online 'parallel resistor calculators' show that your 150k and 100k
pair give you exactly 60k of resistance.
I wonder if it would be okay to leave your pair of resistors in there as
a permanent fix?
Maybe some kind soul will comment on that question for us amateurs?
Those Tektronix Service Manuals are like a series of college courses on
analog electronics!
tom jobe...



On 4/30/2019 4:21 PM, Mlynch001 wrote:

On Mon, Apr 29, 2019 at 08:07 AM, Dave Hills wrote:

With 134v on Q1496 collector, you should see +61v at Q1490's base. This

will

completely shut down the regulator by reverse biasing Q1494 b-e. That said

it

only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483

would

have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible,

but

not likely to cause these symptoms), leaving a leaky or shorted Q1496 or
CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently
fail. Also, check that CR1489 has not been installed backward.

Please forgive the caps, not shouting, just no other way to highlight for
emphasis.

Dave

Dave:

Your advice helped me find the issue! Tom Jobe suggested that I study the

110V regulator circuit description and the schematic, which I did as well.
Lots of good advice from everyone got me pointed in the right direction.

Here is what I found and how I fixed the issue:

After the aforementioned study and re-reading all the comments, I decided to

pull CR1489 off the board and test it. It tested good on my curve tracer,
meter and component tester. Reinstalled on the board and there was no change.
(of course) I also pulled all transistors, including Q1496 (again) and tested
it on my little home built curve tracer and re- ran it through two additional
transistor testers, all showed it to be good. (again, no Change) You
mentioned that Q1490 should have a base voltage of about 61V with the 110V
rail at 135V, I found it to actually be low, at 41.5V, the schematic calls
for 50V, and since you said I should have about 61V; I knew that the voltage
divider formed by R1486 and R1487 was not right. R1487 tested 49.9K so I
moved on to R1486 and it tested "weird", meaning high resistance sometimes and
then open at others. This prompted me to remove it completely from the board.
After getting it off the board, I found it to be "open", so un-soldering the
component probably finished it off.. I took a 68K resistor from the bench and
temporarily soldered it into place of R1486, checked the 110V rail and BANG!
the 110V rail was now at 119.4V (down from 135V) with no other changes. This
told me I had found the issue, now I had to find a proper replacement for
R1486, which was originally 60.4K (321-0364-00 - RES.,FXD,FILM: 60.4K OHM,
1%,o.125W). Doing a little math, I found that installing 100K and 150K
resistors in parallel would deliver 59K, so VERY close to what I needed.
Installed these two resistors parallel in place of R1486, re-tested, this
brought the base of Q1490 to 50.06V and gave me exactly 110.65V on the 110V
rail. Now all I have to do is find the proper replacement part and my scope
in good to go! THANK YOU ALL for your advice!

Sincerely,

Hey Tom,

I have ordered the correct parts to replace the temporary repair. I have 25 resistors coming, so I can pick through them and find the best of the lot to make the final and permanent repair. Again, I appreciate your comments and guidance in making this repair possible.

Sincerely,

--
Michael Lynch
Dardanelle, AR

On Tue, 30 Apr 2019 17:46:08 -0700, you wrote:

Hi Micheal,
Thank you for the nice repair summary!
It was good that Dave Hills brought some of his logic, reason and
knowledge to your problem.
The online 'parallel resistor calculators' show that your 150k and 100k
pair give you exactly 60k of resistance.
I wonder if it would be okay to leave your pair of resistors in there as
a permanent fix?

They're carbon composition, and possibly will drift. The film
resistor that's the specified replacement should drift less with age
and perhaps temperature.

Harvey

Maybe some kind soul will comment on that question for us amateurs?
Those Tektronix Service Manuals are like a series of college courses on
analog electronics!
tom jobe...



On 4/30/2019 4:21 PM, Mlynch001 wrote:

On Mon, Apr 29, 2019 at 08:07 AM, Dave Hills wrote:

With 134v on Q1496 collector, you should see +61v at Q1490's base. This will
completely shut down the regulator by reverse biasing Q1494 b-e. That said it
only leaves R1483, Q1496, CR1498, and R1488 as likely suspects. R1483 would
have to have drifted VERY low, (unlikely), or R1488 VERY high, (possible, but
not likely to cause these symptoms), leaving a leaky or shorted Q1496 or
CR1489. Note that an OPEN CR1489 would likely cause Q1496 to subsequently
fail. Also, check that CR1489 has not been installed backward.

Please forgive the caps, not shouting, just no other way to highlight for
emphasis.

Dave

Dave:

Your advice helped me find the issue! Tom Jobe suggested that I study the 110V regulator circuit description and the schematic, which I did as well. Lots of good advice from everyone got me pointed in the right direction.

Here is what I found and how I fixed the issue:

After the aforementioned study and re-reading all the comments, I decided to pull CR1489 off the board and test it. It tested good on my curve tracer, meter and component tester. Reinstalled on the board and there was no change. (of course) I also pulled all transistors, including Q1496 (again) and tested it on my little home built curve tracer and re- ran it through two additional transistor testers, all showed it to be good. (again, no Change) You mentioned that Q1490 should have a base voltage of about 61V with the 110V rail at 135V, I found it to actually be low, at 41.5V, the schematic calls for 50V, and since you said I should have about 61V; I knew that the voltage divider formed by R1486 and R1487 was not right. R1487 tested 49.9K so I moved on to R1486 and it tested "weird", meaning high resistance sometimes and then open at others. This prompted me to remove it completely from the board. After getting it off the board, I found it to be "open", so un-soldering the

component probably finished it off.. I took a 68K resistor from the bench and temporarily soldered it into place of R1486, checked the 110V rail and BANG! the 110V rail was now at 119.4V (down from 135V) with no other changes. This told me I had found the issue, now I had to find a proper replacement for R1486, which was originally 60.4K (321-0364-00 - RES.,FXD,FILM: 60.4K OHM, 1%,o.125W). Doing a little math, I found that installing 100K and 150K resistors in parallel would deliver 59K, so VERY close to what I needed. Installed these two resistors parallel in place of R1486, re-tested, this brought the base of Q1490 to 50.06V and gave me exactly 110.65V on the 110V rail. Now all I have to do is find the proper replacement part and my scope in good to go! THANK YOU ALL for your advice!

Sincerely,

Harvey,

That is another critical factor, one that I took into serious consideration. I used carbon comp resistors only as a temporary measure, simply to test the rest of the system, a "proof of concept" if you will. The ones I used were 5% accuracy, so way too little precision for the job. This was evidenced by the pair having a "calculated value" of 60K and an actual resistance measured at 58.8K, so still a ways off from the desired 60.4K (+/-1%). It was simply not worth the time to try to come up with 2,3 or 4 carbon comps that theoretically could have been grouped to give the 60.4k that was specified. Much easier to just buy the right parts and fix it permanently and correctly. The repair also looks more professional as a major benefit. I have been buying Metal film resistors, whenever possible, since they do not cost that much more, they are more stable and more reliable. Thanks for your comment.

--
Michael Lynch
Dardanelle, AR

On Wed, 01 May 2019 08:31:39 -0700, you wrote:

Harvey,

That is another critical factor, one that I took into serious consideration. I used carbon comp resistors only as a temporary measure, simply to test the rest of the system, a "proof of concept" if you will. The ones I used were 5% accuracy, so way too little precision for the job. This was evidenced by the pair having a "calculated value" of 60K and an actual resistance measured at 58.8K, so still a ways off from the desired 60.4K (+/-1%).

Once you find a carbon comp resistor of the exact value, well, it's
still the exact value until it drifts.

It was simply not worth the time to try to come up with 2,3 or 4 carbon comps that theoretically could have been grouped to give the 60.4k that was specified. Much easier to just buy the right parts and fix it permanently and correctly.

Agreed. Worth the time and effort to do it right.

The repair also looks more professional as a major benefit. I have been buying Metal film resistors, whenever possible, since they do not cost that much more, they are more stable and more reliable. Thanks for your comment.

Glad to help. I use metal film for precision, and frankly, have not
replaced all that many carbon comp resistors. They generally get
replaced by film resistors since that's what I normally bought.

Harvey