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Tektronix 531 diode replacement
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Hello everyone, my name is Brenda and I am new to this forum. A little about me, I have a general knowledge of electronics but certainly don't know everything. I had a Tektronix 535A for almost 20 years before life got in the way. I am "collecting" old tube type scopes to use. I have a RM565, 561 and a 503 that's in working order for the most part. And a few plugins as well. I have just picked up a Tektronix 531 and it has plenty of issues, the one main issue is that most of the electrolytics caps are bad. The scope was not on for a minute and saw some smoke coming from upper section by the high voltage and shut it down. After that, I felt the electrolytics and most of them were much warmer than I am comfortable with. I downloaded a manual for the 531, but my 531 has the upgraded board to replace the selenium rectifier. From what I've read in the forum, Stan stated that the diodes were not the best and would like to replace them but I can't find what diodes would be best to use. I would like to replace the diodes and the electrolytics before moving on with bringing this oldie back to life. Any help would be appreciated. The serial number on this scope is 8121 and came with a Type M plugin unit.
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Welcome...I in a sense have a similar issue....whether to resurrect my 564.It must be 30 years since I fired it up and found no screen change (apparent 'on'). Presumably I discovered it had no filament voltage as I had two filament transformers wound to high voltage insulation, which cost a motza, Today my approach would be considerably more
exacting and circumspect, Then the complexity of Tektronix CRO's and the unwilingness of Tektronix at Nth Ryde to assist 'on that old model' other than at rates I could not afford saw me in mortal fear of the word Tektronix....Exorcism of my CRO was considered . After spending many hundreds on the transformers and finding that installing one made no difference, I resisted melancholia by putting it all away (chuckles) but the Tektronix CRO's of that type and era have a sort of character that seems to mysteriously bring out a maternal instinct. That's why I kept it!...One of the brighter and more experienced people will help you with your specific restoration as they are way ahead of me...however as a general rule ordinary diodes are much of a muchness in simple rectification or doubling. Replacing them with others should see them chosen well into and exceeding the current and piv limits of the originals, if for no more reason than personal satisfaction.. When changing capacitors... as well as that also test resistors and the actual voltage delivery of your transformer where possible and safe, In my view visual inspection, photos and notes are a wise preliminary before reaching for the soldering iron. Give close visual inspection of soldering, pcb tracks, firmness of mounting screws and connector cleanliness. A final thought...some Tektronix CRO's did not use ordinary solder but a silver-content solder. I don't know if that was the case with your models however I mention it because the heat required to melt it can damage components...so I have read. Make sure caps are discharged through using a resistor to ground rather than a screwdriver. If one thing in my comment helps, that's good. If not, others will oblige also that's a certainty. Alliteratively, 'Best of British' to you Brenda. My regards. Jack |
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Bob Albert
Once you have replaced the electrolytic capacitors, you need to evaluate the diode choice.? For most of the rectifiers, a 1N4007 will suffice.? For the higher voltages, you can put two or three of them in series.? They are very cheap so you might as well buy 100 or so just to have around.? The 1N4007 rating is 1000 Volts so any power supply section up to perhaps 300 Volts can use them.? Anything higher and you need more.? When in doubt, use more.? But the 1.0 Ampere current rating is adequate for nearly anything in that unit.? The forward drop and/or dynamic resistance might be too low for some applications, so a series resistor of perhaps 47 Ohms might be advised, about a 1 Watt rating should do.
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A good idea would be to use a Variac to bring up the power slowly.? Make sure you are using the correct fuses. With the manual, you can see that the power supply voltages are correct. At this point, you should have a working unit.? Any other problems should be simply corrected, such as calibration issues, trace quality, drift, dirty switches, burned out lamps, etc.? Now it's ready for alignment and calibration. Bob On Monday, November 5, 2018, 10:34:00 AM PST, Jack <goldmort@...> wrote:
Welcome...I in a sense have a similar issue....whether to resurrect my 564.It must be 30 years since I fired it up and found no screen change (apparent 'on'). Presumably I discovered it had no filament voltage as I had two filament transformers wound to high voltage insulation, which cost a motza, Today my approach would be considerably more exacting and circumspect, Then the complexity of Tektronix CRO's and the unwilingness of Tektronix at Nth Ryde? to assist 'on that old model' other than at rates I could not afford saw me in mortal fear of the word Tektronix....Exorcism of my CRO? was considered . After spending many hundreds on the transformers and finding that installing one made no difference, I resisted melancholia by putting it all away (chuckles)? but the Tektronix CRO's of that type and era have a sort of character that seems to mysteriously bring out a maternal instinct. That's why I kept it!...One of the brighter and more experienced? people will help you with your specific restoration as they are way ahead of me...however as a general rule ordinary diodes are much of a muchness in simple rectification or doubling. Replacing them with others should see them chosen well into and exceeding the current and piv limits of the originals, if for no more reason than personal satisfaction.. When changing capacitors... as well as that also test resistors and the actual voltage delivery of your transformer where possible and safe, In my view visual inspection, photos and notes are a wise preliminary before reaching for the soldering iron. Give close visual inspection of soldering, pcb tracks, firmness of mounting screws and connector cleanliness. A final thought...some Tektronix CRO's did not use ordinary solder? but a silver-content solder. I don't know if that was the case with your models however I mention it because the heat required to melt it can damage components...so I have read. Make sure caps are discharged through using a resistor to ground rather than a screwdriver. If one thing in my comment helps, that's good. If not, others will oblige also that's a certainty. Alliteratively, 'Best of British' to you Brenda. My regards. Jack |
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The 1N4007's peak current rating is probably adequate too, since Tek included series resistors, but does anyone know for sure?
Dave Wise ________________________________________ From: [email protected] <[email protected]> on behalf of Morris Odell <vilgotch@...> Sent: Monday, November 05, 2018 11:15 AM To: [email protected] Subject: Re: [TekScopes] Tektronix 531 diode replacement Hi Brenda, You can safely use 1N4007 diodes (1000 PV, 1Amp). If you want to increase the safety factor for a few cents more per diode you could use 1N5408s (1000 PIV 3 Amp) Morris |
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Hello Jack,
I also have a Tektronix 564! But it needs needs a lot of work. I have been able to check the voltages of the power supply and it's pretty much spot on. I can get a trace on the CRT, however the storage mode is not functional at all. I think I will probably end up replacing all the electrolytics in this scope as well. All the rails have excessive ripple and with my history of old capacitors, they tend to go with a nasty bang. You are very correct about the diodes in regards to the current and piv limits, I just wanted to ask since these old scopes have not been made in several years, I wanted to play it safe and not cause severe damage like losing a power transformer or something like that. I am aware that Tektronix scopes of this era require a silver bearing solder so I have purchased some just for my scopes only. Brenda |
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And thank you Bob, Morris and Dave Wise for your reply. I thought that the 1n4007 would be good enough, just wanted to double check. The diodes are on a PCB where the selenium rectifiers were and also has 4 power resistors mounted to the plate. I will someday soon check the forward voltage drop on the original diodes. They are most likely still good, but I just want to be careful with this 531. I know I have a LOT of work ahead of me, there are a quite a few Bumble Bee capacitors that will need changed out as well as the high voltage caps as well. The high voltage caps appears to have been replaced at one time, as they are black beauties made by Sprague with the yellow writing.
Brenda |
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On 5 Nov 2018, at 20:52, brendda75 via Groups.Io <brendda75@...> wrote:
I also have a Tektronix 564! But it needs needs a lot of work. I have been able to check the voltages of the power supply and it's pretty much spot on. I can get a trace on the CRT, however the storage mode is not functional at all. I think I will probably end up replacing all the electrolytics in this scope as well. All the rails have excessive ripple and with my history of old capacitors, they tend to go with a nasty bang.Now I wonder how many members here have a 564. I too have one, with a Type 3A3 vertical plugin and Type 2B67 and 3B3 horizontal plugins. The mainframe is notionally functional, but the upper storage portion works better than the lower ¡ª probably needs calibration ¡ª and the 2B67 free runs but won¡¯t trigger and the 3B3 won¡¯t sweep at all. The 3A3 seems to be roughly usable but in need of a tune-up. Some day, I¡¯ll get around to fixing it all up. |
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Sprague Black Beauty with yellow lettering were original equipment throughout 531 production. They are paper in oil same as the striped parts. They're almost certainly leaking enough to be a problem soon after you turn it on. The go-to replacement for the highest-voltage ones (in the multiplier) are ceramic. I've had good results with a couple of 10kV 1000pF part numbers from Murata. You have to check the specs carefully to avoid parts that lose too much capacitance at high voltage and temperature. That's why I start with 1000pF instead of 470.
Don't forget the bumblebee that resonates the primary of the HV transformer. Leakage per se doesn't matter but loss (DF) will prevent the oscillator from starting or at least make it work harder than necessary. For the medium-voltage parts (a few kV), film is easier than ceramic. I have a bag of Soviet surplus 4kV axial-lead 6.8nF parts, and you're welcome to a set for shipping plus a few bucks. When you do the HV supply, plan your lead and component dress beforehand, always seeking to minimize potential between nearby leads and bodies - including outside foil - and of course leaving enough room for the rectifiers. The output tie point needs a fresh coating of corona dope. HTH, Dave Wise ________________________________________ From: [email protected] <[email protected]> on behalf of brendda75 via Groups.Io <brendda75@...> Sent: Monday, November 05, 2018 12:59 PM To: [email protected] Subject: Re: [TekScopes] Tektronix 531 diode replacement And thank you Bob, Morris and Dave Wise for your reply. I thought that the 1n4007 would be good enough, just wanted to double check. The diodes are on a PCB where the selenium rectifiers were and also has 4 power resistors mounted to the plate. I will someday soon check the forward voltage drop on the original diodes. They are most likely still good, but I just want to be careful with this 531. I know I have a LOT of work ahead of me, there are a quite a few Bumble Bee capacitors that will need changed out as well as the high voltage caps as well. The high voltage caps appears to have been replaced at one time, as they are black beauties made by Sprague with the yellow writing. Brenda |
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Thank you very much Dave Wise, I didn't know that those capacitors would have been original. I have a lot to learn about these old scopes. I had assumed that they had been replaced at one point since when I took the covers off, it was totally spotless inside (unlike my non working 545A which is almost pure black in both HV areas) and all of the wires looked like they had been unsoldered and resoldered with all the melted insulation. I am unsure of how to remove the coating from the HV output tie point without damaging the ceramic strip. I do have quite a few HV caps from getting my Tektronix 503 up and running. Granted, the ceramic capacitors are from China, it has been working very well for the last 6 months, I am happy to say. Although the intensity is not the brightest.
As far as the bumblebee caps in the HV transformer area, all 4 are covered in oil which leads me to believe that those caps are leaking oil and will definitely have to be replaced. There is even a small pool under one of those caps. I have yet to turn the scope on its side and see how many more bumblebees are hidden in the power supply section and other hidden issues. I still have yet to find the source of the white smoke I saw when I first powered it up, I know that it came from the sweep section (most likely around the bumblebee cap around V282. I haven't done any component level testing as of yet, but no burned or scorched parts. I did not have the scope on for a minute before I shut it down, was hoping to see some life in regards to the CRT before sinking money into it. I do have 1 other question: Could one use fingernail polish for the HV output tie point? Only reason I ask is that I used fingernail polish once to stop arcing in a tesla coil..LOL Brenda |
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Hi Brenda,
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Get yourself a handful of 1N4007 rectifiers and have a ball replacing the old seleniums. A word of caution, though... the rectified DC will be a good bit higher than it would be with good seleniums. If it appears to be unreasonably high after the new diodes have been installed, (along with new electrolytics) get a 6.3 VAC/6A filament transformer, and install it to buck the line voltage on the primary side of the scope's power transformer. That will effectively reduce the AC input to the scope. Good luck with the restoration!! Dave M On Mon, Nov 5, 2018 at 11:31 AM, <brendda75@...> wrote:
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Sorry, I somehow didn't comprehend that the old seleniums had been replaced by newer rectifiers, but, the 1N4007s will still work in the scope with no problem. If the resistors on the mod board are sufficient, the filament transformer won't be necessary. I've used that trick a few times over the years, and it works beautifully when you need to reduce the line voltage safely and efficiently.
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If your scope still has the old 5642 HV rectifier tubes installed, they can be replaced with silicon rectifiers. I forget the part number. Something like 18Kv/5ma fast recovery. On the issue of damaging the ceramic terminal strips...I hope you know that silver-bearing solder is required on those strips. If the roll of that solder has been used or lost from your scope, you need to find a small supply of it. I used to have a 1/2 lb roll of it in my toolbox, but It seems to have gotten away over the years. Otherwise, I'd be glad to send you some. Again, good luck with the project!! Those old scopes were works of art in their era. Dave M On Mon, Nov 5, 2018 at 07:40 PM, David M wrote:
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Hi Brenda,
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? ? I am the owner of a 545 scope (though I've had a 531 in the past) and would have chimed in sooner but have been busy.? ?I see you've gotten some good advice though.? ?One thing I would add is that you should be a little more conservative in your replacement of parts.? Don't replace anything unless it's known bad.? The Tek circuits are very tolerant of out-of-tolerance parts, so the scope should at least work to some extent with leaaky caps, off-value resistors, etc, allowing you to trouble-shoot subsections without having to take a shot-gun approach to component replacment.? ? ? In particular, I would not replace the electrolytic filter caps unless known bad, or the power supply won't regulate and the ripple is too high.? ?They will get warm, and there is a lot of leakage in those things, but as they re-form with use, they may improve a bit.? If the main fuse didn't go, they are probably acceptable for now. ? The bumblebees are almost certainly all bad. They are either dried up and have become open circuits with no capacitance, or they leak, so are effectively resistors.? But they do not all need to be replaced right away.? ?As has been mentioned, the BB caps in the HV power supply probably do need to be replaced as they will keep the HV circuit from oscillating.? But many of them in the other circuits are bypass capacitors or frequency compensation caps across a resistor in the cathode circuit of a tube.? In that use, they decrease the AC gaien of the circuit but do nothing at DC, so for example, in the vertical circuit, the additional leakage (which is small compared to the ~few hundred ohms in parallel with it) might shift the trace position a bit but it should still be centerable on the CRT.??? ?If the cap is a coupling cap between, e.g. an output stage and the grid input of the next stage, then a leakage current will be large compared to the grid bias current of the subsequent stage, so those should be replaced.? ?? ?Finally, there is ONE that definitely needs to be replaced ASAP.? ?If I recall correctly, without the benefit of looking at the schematic, there is a BB cap coupling the output of the vertical sweep to a banana plug on the front panel, where it can potentially be touched.? ?150V +? "exposed banana plug" = "shit that hurts!", and needs to be made safe.?? ? ?I would resist using nail polish as corona dope.? It might work.? Then again, it might not, and if it arcs, you will have a harder time cleaning the carbon tracks off the ceramic insulators.? ?Again, I wouldn't disturb the HV section unless there is a problem.? I recall once that it worked fine, despite lots of dust, *until* I cleaned it.? Then I basically had to make it absolutely spotless before it worked again.? One thing to do is turn out the light and look for arcs and sparks in the dark (just being crazy careful not to touch anything).? ? ?While the lights are out, you can look for gassy tubes glowing purple.? ? ?I have written a lot of this stuff up and posted it here in the past.? If you search for my name, you might find it.? I wrote up some basic info on fixing these old tube scopes based on personal experience working on mine.? ?I also have an RM503 by the way.? Good luck! ? Dan On Monday, November 5, 2018, 9:46:07 PM EST, David M <dgminala@...> wrote:
Sorry, I somehow didn't comprehend that the old seleniums had been replaced by newer rectifiers, but, the 1N4007s will still work in the scope with no problem.? If the resistors on the mod board are sufficient, the filament transformer won't be necessary.? I've used that trick a few times over the years, and it works beautifully when you need to reduce the line voltage safely and efficiently. If your scope still has the old 5642 HV rectifier tubes installed, they can be replaced with silicon rectifiers. I forget the part number.? Something like 18Kv/5ma fast recovery. On the issue of damaging the ceramic terminal strips...I hope you know that silver-bearing solder is required on those strips.? If the roll of that solder has been used or lost from your scope, you need to find a small supply of it.? I used to have a 1/2 lb roll of it in my toolbox, but It seems to have gotten away over the years.? Otherwise, I'd be glad to send you some. Again, good luck with the project!!? Those old scopes were works of art in their era. Dave M On Mon, Nov? 5, 2018 at 07:40 PM, David M wrote:
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ps.? Save those Selenium rectifiers!!? Saw this in a very old book:? ?Take out the center screw or rivet.? Place the Se plates on a hot plate, and heat it up till the solder just melts.? Wipe it off with a rag.? Now you have a thin metal coating on a light sensitive surface with which you can play.? ?Of course those old books neglected to discuss the toxicity of Selenium, and you definitely don't want to burn it, but hey, I am still alive and healthy.? ?I didn't eat the stuff.? It's also an essential element and prevents dandruff!
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On Monday, November 5, 2018, 10:52:59 PM EST, Daniel Koller via Groups.Io <kaboomdk@...> wrote:
Hi Brenda, ? ? I am the owner of a 545 scope (though I've had a 531 in the past) and would have chimed in sooner but have been busy.? ?I see you've gotten some good advice though.? ?One thing I would add is that you should be a little more conservative in your replacement of parts.? Don't replace anything unless it's known bad.? The Tek circuits are very tolerant of out-of-tolerance parts, so the scope should at least work to some extent with leaaky caps, off-value resistors, etc, allowing you to trouble-shoot subsections without having to take a shot-gun approach to component replacment.? ? ? In particular, I would not replace the electrolytic filter caps unless known bad, or the power supply won't regulate and the ripple is too high.? ?They will get warm, and there is a lot of leakage in those things, but as they re-form with use, they may improve a bit.? If the main fuse didn't go, they are probably acceptable for now. ? The bumblebees are almost certainly all bad. They are either dried up and have become open circuits with no capacitance, or they leak, so are effectively resistors.? But they do not all need to be replaced right away.? ?As has been mentioned, the BB caps in the HV power supply probably do need to be replaced as they will keep the HV circuit from oscillating.? But many of them in the other circuits are bypass capacitors or frequency compensation caps across a resistor in the cathode circuit of a tube.? In that use, they decrease the AC gaien of the circuit but do nothing at DC, so for example, in the vertical circuit, the additional leakage (which is small compared to the ~few hundred ohms in parallel with it) might shift the trace position a bit but it should still be centerable on the CRT.??? ?If the cap is a coupling cap between, e.g. an output stage and the grid input of the next stage, then a leakage current will be large compared to the grid bias current of the subsequent stage, so those should be replaced.? ?? ?Finally, there is ONE that definitely needs to be replaced ASAP.? ?If I recall correctly, without the benefit of looking at the schematic, there is a BB cap coupling the output of the vertical sweep to a banana plug on the front panel, where it can potentially be touched.? ?150V +? "exposed banana plug" = "shit that hurts!", and needs to be made safe.?? ? ?I would resist using nail polish as corona dope.? It might work.? Then again, it might not, and if it arcs, you will have a harder time cleaning the carbon tracks off the ceramic insulators.? ?Again, I wouldn't disturb the HV section unless there is a problem.? I recall once that it worked fine, despite lots of dust, *until* I cleaned it.? Then I basically had to make it absolutely spotless before it worked again.? One thing to do is turn out the light and look for arcs and sparks in the dark (just being crazy careful not to touch anything).? ? ?While the lights are out, you can look for gassy tubes glowing purple.? ? ?I have written a lot of this stuff up and posted it here in the past.? If you search for my name, you might find it.? I wrote up some basic info on fixing these old tube scopes based on personal experience working on mine.? ?I also have an RM503 by the way.? Good luck! ? Dan ? ? On Monday, November 5, 2018, 9:46:07 PM EST, David M <dgminala@...> wrote: Sorry, I somehow didn't comprehend that the old seleniums had been replaced by newer rectifiers, but, the 1N4007s will still work in the scope with no problem.? If the resistors on the mod board are sufficient, the filament transformer won't be necessary.? I've used that trick a few times over the years, and it works beautifully when you need to reduce the line voltage safely and efficiently. If your scope still has the old 5642 HV rectifier tubes installed, they can be replaced with silicon rectifiers. I forget the part number.? Something like 18Kv/5ma fast recovery. On the issue of damaging the ceramic terminal strips...I hope you know that silver-bearing solder is required on those strips.? If the roll of that solder has been used or lost from your scope, you need to find a small supply of it.? I used to have a 1/2 lb roll of it in my toolbox, but It seems to have gotten away over the years.? Otherwise, I'd be glad to send you some. Again, good luck with the project!!? Those old scopes were works of art in their era. Dave M On Mon, Nov? 5, 2018 at 07:40 PM, David M wrote:
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Hi Dave M,
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Is the silver bearing solder used on the 4xx series scopes on the ceramic strips in the high voltage section also? If so that would explain why I had so much trouble getting the wires to release. Sorry for the change of topic but it is somewhat relevant. Jim O On November 5, 2018 at 6:45 PM David M <dgminala@... mailto:dgminala@... > wrote: |
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