Topics

Type 503 — power supply question


brian.kane@...
 

Dear Collective Wisdom,

I was given a Tektronix Type 503 oscilloscope recently. It wasn't showing a trace. I checked it out and realized that a bad diode coming off of the the second power supply transformer was the culprit. No power was getting to the heaters of the nuvistors in the vertical and horizontal amplifiers. Once the diode was replaced the scope began working again. Hurray! After running it for a while, I decided to recheck the voltages to see if it was all good, and...oof...by accident I nicked the main transformer with the lead from the multimeter. It sparked and the fuse blew.

When I replaced the fuse and started it up again the scope was kaput. The problem is in the power supply. The first transformer (T601) is putting out specified voltages. But I'm getting absolutely no voltage readings from the the second transformer (T620), which--if I understand it--is supposed to be oscillating. To be more precise, none of the various taps on the secondary of T620 are showing any voltages--all are at 0V.

The voltages are now off around the 6DQ6 tube (V620) that connects to the primary of T620. I tested the tube and it tested fine. I also stuck in a replacement, just to be sure, and no still no dice. It looks like I'm getting just under 500V across the primary winding of T620 and to the plate of the 6DQ6 tube. But I'm only getting 40V on Pin 4 of the tube, and 0V on pin 5.

I'm worried that the transformer got ruined when the thing sparked. Is that a possibility?

So here is where I need your collective wisdom. What's the best way to proceed? How can I check that the second transformer (T620) is still functional?

Here's what I've done so far:
1. I've checked resistances on the second transformer (T620) and all the resistance readings seem to check out with ones I can find on line.
2. However, I am getting ~600K ohms between the primary of T620 and ground. It that normal or does it indicate that the transformer is ruined?

Thanks ahead of time,
Brian


John Griessen
 

On 1/7/21 5:03 PM, brian.kane@yale.edu wrote:
.by accident I nicked the main transformer with the lead from the multimeter. It sparked and the fuse blew.
Can you tell by scorch marks what was connected to what during the spark?
What was the short caused by?
Two bare metal points close together shorted by the bare part of the probe?
Was other end of probe connected to a 10 megohm input multimeter?

Once you find the area to search for shorts, find the manual for the 503.

Then you can answer, " getting ~600K ohms between the primary of T620 and ground. It that normal?"


brian.kane@...
 

John,

Thank you for the response. I have a copy of the manual and spent time working through the power supply today. I understand it a lot better than I did 24 hours ago!

I was using my multimeter when I dropped one of the probes and it nicked the tab on the transformer connected directly to the 120V line. The scope was off but still plugged into the wall, and that tab is hot (I believe) even when the machine is off. I think the other probe was probably attached to chassis ground, but I'm not certain. There were no scorch marks anywhere that I could see. The fuse blew, but I didn't spot any other bad components.

It turns out that I blew up a diode on the +100V line when I nicked the main transformer. And since that line is critical to get oscillation going in the second part of the power supply, all the other lines pooped out when +100V died.

Since I was worried about whether or not I fried the transformer (T620) I ended up desoldering it today, and gave it a thorough inspection. Everything checked out fine so that thinking that the problem was elsewhere. I systematically worked through the power supply, checked values of all components and solder joints, and found the bad diode.

After replacing the diode and soldering it all back together, I double checked it, plugged it in, crossed my fingers and fired it up. It worked!

So, this question is resolved...until the next time I do something stupid!

Thanks,
Brian


Tom Lee
 

Great to hear that you found that the problem wasn't the transformer. Congrats on bringing your scope back from the dead, John.

-- 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 1/8/2021 21:17, brian.kane@yale.edu wrote:
John,

Thank you for the response. I have a copy of the manual and spent time working through the power supply today. I understand it a lot better than I did 24 hours ago!

I was using my multimeter when I dropped one of the probes and it nicked the tab on the transformer connected directly to the 120V line. The scope was off but still plugged into the wall, and that tab is hot (I believe) even when the machine is off. I think the other probe was probably attached to chassis ground, but I'm not certain. There were no scorch marks anywhere that I could see. The fuse blew, but I didn't spot any other bad components.

It turns out that I blew up a diode on the +100V line when I nicked the main transformer. And since that line is critical to get oscillation going in the second part of the power supply, all the other lines pooped out when +100V died.

Since I was worried about whether or not I fried the transformer (T620) I ended up desoldering it today, and gave it a thorough inspection. Everything checked out fine so that thinking that the problem was elsewhere. I systematically worked through the power supply, checked values of all components and solder joints, and found the bad diode.

After replacing the diode and soldering it all back together, I double checked it, plugged it in, crossed my fingers and fired it up. It worked!

So, this question is resolved...until the next time I do something stupid!

Thanks,
Brian







Daniel Koller
 

Hi Brian,

  Just a quick note - you are lucky that T620 is good.  That is unobtanium.  The 503 is a great scope, particularly for X-Y displays and low frequency stuff.    It does seem to have some common failure modes though.   One of them is the issue with the epoxy potting in T620, similar to the problems of the 547 in which the potting takes on moisture over time and gets lossy.   Eventually the circuit will no longer oscillate, or the HV winding starts to arc and burns out.  

   Another common issue is that the filament voltage for the HV rectifier is provided by a winding in the main transformer, elevated by the HV feeding the rectifier tube.  The winding insulation can break down, causing T601 to fail.  The cheap solution to this is to add a separate filament transformer just for the HV circuit.    Examples of scopes we both types of failures pop up on e-bay from time to time.

   Anyway, hopefully it won't fail for a long while, but if it does, take very careful measurements of the DC resistances of the T620 coil windings as that can help diagnose it.   And if you like this scope, keep your eyes out for parts donors that you may eventually need to keep it running.


   Dan

On Saturday, January 9, 2021, 12:20:19 AM EST, Tom Lee <tomlee@ee.stanford.edu> wrote:





Great to hear that you found that the problem wasn't the transformer.
Congrats on bringing your scope back from the dead, John.

-- 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 1/8/2021 21:17, brian.kane@yale.edu wrote:
John,

Thank you for the response. I have a copy of the manual and spent time working through the power supply today. I understand it a lot better than I did 24 hours ago!

I was using my multimeter when I dropped one of the probes and it nicked the tab on the transformer connected directly to the 120V line. The scope was off but still plugged into the wall, and that tab is hot (I believe) even when the machine is off. I think the other probe was probably attached to chassis ground, but I'm not certain. There were no scorch marks anywhere that I could see. The fuse blew, but I didn't spot any other bad components.

It turns out that I blew up a diode on the +100V line when I nicked the main transformer. And since that line is critical to get oscillation going in the second part of the power supply, all the other lines pooped out when +100V died.

Since I was worried about whether or not I fried the transformer (T620) I ended up desoldering it today, and gave it a thorough inspection. Everything checked out fine so that thinking that the problem was elsewhere. I systematically worked through the power supply, checked values of all components and solder joints, and found the bad diode.

After replacing the diode and soldering it all back together, I double checked it, plugged it in, crossed my fingers and fired it up. It worked!

So, this question is resolved...until the next time I do something stupid!

Thanks,
Brian








brian.kane@...
 

Dan,

Thank you for this. I'm never restored an oscilloscope before, so I'm learning as I go.

I'm delighted that T620 is good. Whew! I was worried that I fried it and that it would be nearly impossible to find a replacement. I wrote down all the DC resistances of T620 on the power supply schematic in the manual, so I won't lose them if I have to get back into the power supply and fuss with it.

Regarding the issues with the filament voltage for the HV rectifier, I wouldn't mind doing whatever preventative measure I can do. So, just to make sure I'm understanding the issue, I'm going to restate it and you can tell me if I've got it correct. 1) The windings between tabs 6 and 7 on T601 is for the 6.3 V filament on the CRT. When the scope is going that winding gets elevated to -3000V. That can cause the T601 to fail since its a lot of stress on that winding. 2) And the solution is to get a separate transformer, one that drops the line voltage to 6.3V, and then move the wires to the CRT filament to that new transformer.

Is that right? (Sorry, noob here...)

I love the X-Y display on this scope and the differential inputs. I don't think I'll be using it in day to day work, but I thought it would be good for aligning FM detectors, and some other audio work. The x-y on my other scope just isn't as quick and sharp as on the 503.

Thanks in advance. This is a great forum...so helpful!

All best,
Brian


Brad Thompson
 

brian.kane@yale.edu wrote on 1/9/2021 12:36 PM:

<snip>
Regarding the issues with the filament voltage for the HV rectifier, I wouldn't mind doing whatever preventative measure I can do. So, just to make sure I'm understanding the issue, I'm going to restate it and you can tell me if I've got it correct. 1) The windings between tabs 6 and 7 on T601 is for the 6.3 V filament on the CRT. When the scope is going that winding gets elevated to -3000V. That can cause the T601 to fail since its a lot of stress on that winding. 2) And the solution is to get a separate transformer, one that drops the line voltage to 6.3V, and then move the wires to the CRT filament to that new transformer.
Hello--
Regarding the replacement filament transformer, I repaired a Tek RM503 several (20!) years ago.
The filament winding on T601 had failed open-circuit, and I decided to replace it with a
120 VAC to 6.3 VAC @ 60 Hz transformer. Unfortunately, the transformers in my collection
that were satisfactory rating-wise all were spec'ed for 2500 volt insulation.

Finding a replacement with 3000 volt insulation proved difficult, and IIRC I wound up
using a "medical-rated" 10 V @1.5 A transformer rated for 4000 volts and a series
dropping resistor on the secondary side to obtain 6.3 V for the filament.

Note that the transformer's  stray magnetic  field may affect the CRT, so
you may need to move the tranny through several orientations to
limit onscreen distortion. Finding a clear mounting spot in the RM503 was not a problem.

73--

Brad  AA1IP


Leon Robinson
 

Brian
I had this same problem on a 502 50 or so years ago.I got a Stancor 6.3 volt filament transformer with
2500 volt isolation, only thing I could find that was
close. I wired it in and turned the 502 upside down to
experiment with the transformer location, it ended up
about  on the CRT centerline under the chassis with the
transformer long axis parallel to the CRT.Plenty of room and location was not too critcal.
Drilled 2 holes to mount and dressed the wiring.I looked almost factory.
I tried to get a new transformer from TEK with their
lifetime warranty. They said they had canceled the
lifetime warranty.

Leon Robinson    K5JLR

Political Correctness is a Political Disease.

Politicians and Diapers should be changed
often and for the same reasons.

On Saturday, January 9, 2021, 11:36:42 AM CST, <brian.kane@yale.edu> wrote:

Dan,

Thank you for this. I'm never restored an oscilloscope before, so I'm learning as I go.

I'm delighted that T620 is good. Whew! I was worried that I fried it and that it would be nearly impossible to find a replacement. I wrote down all the DC resistances of T620 on the power supply schematic in the manual, so I won't lose them if I have to get back into the power supply and fuss with it.

Regarding the issues with the filament voltage for the HV rectifier, I wouldn't mind doing whatever preventative measure I can do. So, just to make sure I'm understanding the issue, I'm going to restate it and you can tell me if I've got it correct. 1) The windings between tabs 6 and 7 on T601 is for the 6.3 V filament on the CRT. When the scope is going that winding gets elevated to -3000V. That can cause the T601 to fail since its a lot of stress on that winding. 2) And the solution is to get a separate transformer, one that drops the line voltage to 6.3V, and then move the wires to the CRT filament to that new transformer.

Is that right? (Sorry, noob here...)

I love the X-Y display on this scope and the differential inputs. I don't think I'll be using it in day to day work, but I thought it would be good for aligning FM detectors, and some other audio work. The x-y on my other scope just isn't as quick and sharp as on the 503.

Thanks in advance. This is a great forum...so helpful!

All best,
Brian


John Williams
 

I have faced this problem on several 310A scopes. Managed to find some wall warts with a 6 volt transformer in them. That worked fine. Most of the ones around do not have a transformer and thus are not what to look for.


Daniel Koller
 

Hi Brian,

   Welcome to Tekscopes.  I think you have come to the right place!  The collective wisdom here is immense.

    Some time ago I wrote a summary post or two of some things to look for when repairing these old (500 series) scopes, based on my personal experiences and advice from others, collected in one or two postings.   Try to look for them and see if they come up.  I am by no means the last word on any repairs on these scopes, but I have some experience (in particular with the RM503, 545 (not A or B) and 531 scopes, as well as a few others).

  Most important:  try to snag some silver-bearing solder when working on the ceramic terminals, so as not to pull the solder off them (the silver allows the solder to remain bonded to the ceramic).  There is often a small spare spool still remaining in these old scopes.

  Second, possibly first most important:   Don't electrocute yourself!  There's 500 V inside the chassis, as you already know.  be careful.  The HV supply probably won't kill you.  The 500V supply can.

   Brad's scope notwithstanding, my (limited) experience with T601, having collected a few on e-bay is that the 6.3V CRT filament winding arcs to one of the other windings internally, rather than opens up.  But, this seems less rare than T620 failing.  So, I would not worry about it.  It will be more work to pre-maturely replace the filament winding with an external transformer and it may never fail.  Just treat the scope well.   Don't run it overnight unattended by forgetting to turn it off, keep it dry and out of a damp basement, and don't run it on the hottest day of the summer.    Also, don't NOT run it.  The power supply filter capacitors can dry out and might need reforming or replacement, but if you run it every once in a while, they seem to last longer.

   But my point here is that I think T620 is more likely to fail over time than T601.   It may depend on operating environment too. But you can eventually pick up a scrap 503 to scavenge T601 out of.   And a few of us have spares.   Eventually, I may try to wind a new T620 for fun, but I gave up on that for the time being once I got a working replacement T620.   

  Dan

P.s.  What are you doing at Yale, and are you really at Yale now or there virtually?

On Friday, January 8, 2021, 04:30:09 PM EST, brian.kane@yale.edu <brian.kane@yale.edu> wrote:





Dear Collective Wisdom,

I was given a Tektronix Type 503 oscilloscope recently. It wasn't showing a trace. I checked it out and realized that a bad diode coming off of the the second power supply transformer was the culprit. No power was getting to the heaters of the nuvistors in the vertical and horizontal amplifiers. Once the diode was replaced the scope began working again. Hurray! After running it for a while, I decided to recheck the voltages to see if it was all good, and...oof...by accident I nicked the main transformer with the lead from the multimeter. It sparked and the fuse blew.

When I replaced the fuse and started it up again the scope was kaput. The problem is in the power supply. The first transformer (T601) is putting out specified voltages. But I'm getting absolutely no voltage readings from the the second transformer (T620), which--if I understand it--is supposed to be oscillating. To be more precise, none of the various taps on the secondary of T620 are showing any voltages--all are at 0V.

The voltages are now off around the 6DQ6 tube (V620) that connects to the primary of T620. I tested the tube and it tested fine. I also stuck in a replacement, just to be sure, and no still no dice. It looks like I'm getting just under 500V across the primary winding of T620 and to the plate of the 6DQ6 tube. But I'm only getting 40V on Pin 4 of the tube, and 0V on pin 5.

I'm worried that the transformer got ruined when the thing sparked. Is that a possibility?

So here is where I need your collective wisdom. What's the best way to proceed? How can I check that the second transformer (T620) is still functional?

Here's what I've done so far:
1. I've checked resistances on the second transformer (T620) and all the resistance readings seem to check out with ones I can find on line.
2. However, I am getting ~600K ohms between the primary of T620 and ground. It that normal or does it indicate that the transformer is ruined?

Thanks ahead of time,
Brian


brian.kane@...
 

Thanks to all for your advice and wisdom. My previous experience with this kind of thing has been working on restoring radios, test equipment, wire recorders, etc. This is my first oscilloscope.

I have a good sense now of what's involved in putting in a filment transformer for the CRT. I appreciate all the advice, pro and con, about whether it is worth doing.

Regarding Dan's question: I'm a professor at Yale. I teach in the Department of Music and in the Program in Film and Media Studies. I have a strong background in electronic music, but nowadays I'm mostly an historian and theorist. I'm involved in a research project that involves radio and the roots of do-it-yourself electronics. I'm very interested in the informal ways that a whole generation or two of musicians (both popular and avant-garde) learned about electrical engineering and audio engineering through tinkering with radio, hanging out at radio repair shops, reading magazines and Rider manuals, etc. etc.

This is an amazing forum. Thank you for all of your help.

--Brian


Daniel Koller
 

Ah, really cool!

Getting a wire recorder is on my list of things to do!! Also, getting one of those old tube-based portable record recorders. But I haven't got time for my active projects. Would love to hear more about the wire recorders, and I suspect others here would too, except that it's off-topic, unless you used Tek scopes to fix them!

You might also find the folks over at the funwithtubes (main@funwithtubes.groups.io) very interesting. It's also a moderated group filled with wisdom.

Also, regarding your research project, I think you will find sparkbangbuzz.com EXTREMELY interesting, particularly in light of the guy who made that website. If you could get an interview from him, I think you'd hit a goldmine.

Dan

On Sunday, January 10, 2021, 05:48:51 PM EST, <brian.kane@yale.edu> wrote:

Thanks to all for your advice and wisdom. My previous experience with this kind of thing has been working on restoring radios, test equipment, wire recorders, etc. This is my first oscilloscope.

I have a good sense now of what's involved in putting in a filment transformer for the CRT. I appreciate all the advice, pro and con, about whether it is worth doing.

Regarding Dan's question: I'm a professor at Yale. I teach in the Department of Music and in the Program in Film and Media Studies. I have a strong background in electronic music, but nowadays I'm mostly an historian and theorist. I'm involved in a research project that involves radio and the roots of do-it-yourself electronics. I'm very interested in the informal ways that a whole generation or two of musicians (both popular and avant-garde) learned about electrical engineering and audio engineering through tinkering with radio, hanging out at radio repair shops, reading magazines and Rider manuals, etc. etc.

This is an amazing forum. Thank you for all of your help.

--Brian


brian.kane@...
 

Dan,

Thanks for these links and recommendations. Fantastic stuff!

I love all the old audio equipment! Those machines are great for teaching the history of sound recording, radio, etc. with it. I've worked on both wire recorders and on a few old record cutters. I did a course on sound art where we restored a bunch of old Meissner record cutters and had the students cut their own discs. The challenge with those units is that the crystal cutting heads are all dead and have to be rebuilt. Anyway, I'm happy to talk about any of those projects if you (or anyone else) is interested. (Or just email me off list.) And I did use a Tek scope on them--my normal bench oscilloscope--but I wish I had the 503 back then!

All best,
Brian