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Hi all.
Looking for some help to repair an old Tek 466 which I accidentally damaged.
The story is, that I bought it non-working and upon inspection found that the scope had been stored on it's face which had pushed the red 'VAR' button back into the horizontal channel, thereby breaking the potentiometer at the end of the rod.
Unable to find a replacement (as I am in Brazil), I was fortunate enough to be able to disassemble the board and do a workable repair. Further testing on the horizontal section revealed a shorted tantalum which when replaced led to the scope firing up fine and working fine.
Before putting it back in the case, I left it running to confirm that all was ok, and all was fine except I felt that rectifiers on the -8v and +5v rails were a little hot, so feeling confident set about upgrading them.
That all seemed to go fine, except for when I switched the scope back on, there was no longer had a trace.
What followed was that at some point (and for a reason I no longer remember) I decided to test the voltages on the J5 and J6 sets of pins which connect the main A6 board to the A2 vertical pre-amp board.
Whilst doing this, I slipped and momentarily shorted two pins.
This immediately blew the 1.5A fuse on the A6 board, sending the fan off at a higher speed and most of my LV power rail voltages out.
I don't remember the sequence of events following that, as I have since pulled and tested so many components that I can no longer see the wood for the trees.
The situation now, is that when it is plugged in to the mains it immediately blows the mains fuse, so if anyone has experience in locating shorted components then it would be greatly appreciated.
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Just to add that so far have found three shorted caps.
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Hey James,
I feel for you, I've suffered the slip of death too.
For finding shorted components, you might consider powering the afflicted
rail from a current limited PSU. Be sure and set the voltage below the
nominal rail voltage, and start with a low current limit.
You can then try two methods for locating the short:
1. Temperature.
Digital probing (with a finger) may work sometimes, or you can douse any
suspect components in freeze spray or in e.g. IPA to see what warms up
and/or where evaporation occurs.
2. Voltage.
With a 5+ digit multimeter, measure your way through the components on
the board until you find the one with the lowest voltage across it. That'll
be proximate to your short.
Also, before you apply mains power again, make yourself a dim bulb tester (
), and put it in series with the scope.
Save on fuses and save on stress for the power supply components.
Siggi
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On Wed, Dec 1, 2021 at 6:23 PM James55 <james_55@...> wrote: Just to add that so far have found three shorted caps.
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Hi Siggi and thanks for the reply. Did try with a PSU on that rail but couldn't feel any heat, although the voltage did immediately drop to zero. I must have motivated myself after making the post for I was able to find another shorted capacitor and also to identify the shorted rail as the 5 volt one. Following that I disconnected one of the ac wires supplying the 5v bridge rectifier and switched it on... only for the Op-Amp U1762 to promptly explode - Doh! I'll replace that and the shorted caps tomorrow, make a dimbulb test rig before focusing on the 5v rail, and for sure I'll try the IPA trick too. My suspicion is that the failed component lies further along inside the scope and the manual isn't the clearest as to where I might be needing to look. It is in the 'Files' section if anyone is interested; /g/TekScopes/filessearch?p=name%2C%2C%2C50%2C1%2C0%2C0&q=tek+466
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On Wed, Dec 1, 2021 at 9:57 PM James55 <james_55@...> wrote: Did try with a PSU on that rail but couldn't feel any heat, although the
voltage did immediately drop to zero.
If you have a high-resolution DMM, you can play a game of warmer/colder by measuring the voltage across components or traces. If you pump 1A in, you'll have a voltage delta of 1V per Ohm. The traces carrying the current to the shorted component will make like resistors, and the voltage will be lowest next to the short. With a 6 1/2 digit DMM, especially if you zero on the input voltage you can get really, really close to the short pretty quickly.
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Apologies for the delayed response however I needed to resolve some things.
I have no access to a high resolution DMM
The only PSU I had, is an obscure Brazilian Sincler which kept (and keeps) blowing whenever it exceeds 500mA.
Due to Brazilian-level funds, I went out across to the other side of the city and bought a cheap, used, Chinese PSU only to discover it to be blown once I got indoors. After returning the next day to exchange it (and unable to test it) I was once again given a dud (for Brazil, think Nigeria). Anyway, last night it was disassembled and after finding a burnt resistor and swollen cap, the adjacent chips were replaced and finally, I have some form of working PSU.
Having said that, it has made no difference, for as soon as it is connected across the 5v rail of the scope, the PSU voltage drops to zero and a loud alarm sounds.
As for the dimtester, I also tried that but am not really sure what I should be looking for, as other than the lightbulb glowing bright, not a lot else seems to happen. No hot components etc.
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Given that you are going to do a minimalist debugging session.? I have a few suggestions, and others will modify them as needed. Starting from the basics:
1) The + 65 volt line is used as a reference voltage for the +5 volt line.? Just in general, if the +65 is not right, the +5 is not likely to be.
2) all of the supplies are linear.? While they do not have to have loads to operate, they might be happier with loads.
3) disconnect as much as you can from the power supply
4) measure the voltages.? If there's any short on another board, it shouldn't affect the power supply.? If there's still no 5 volts (and if the other PS voltages are OK), then check the parts on the power supply part of the board that makes the +5 volts.? The TEK WIKI has several copies of the service manual, make sure that you get the one that matches the serial number of your scope, it will make a difference.
5) if the power supplies are ok, then plug in board assemblies one at a time.? I'd suggest (in no particular order), horizontal amplifier, vertical amplifier, sweep, and then miscellaneous.? if at ANY time you get a very bright spot on the CRT (the HV will be plugged in), then make absolutely sure that the intensity control is turned down.? You don't need a spot on the CRT for this, and you will possibly damage the CRT.
If you don't have a current limited supply (typically set for 5 volts and about 100 ma.), then put a 50 ohm 2W resistor in series with the 5 volt lead and run the board from that.
For each capacitor, measure the voltage across it.? Assume that the input of the board goes three places (with capacitors across each "branch".
Using the schematic, measure the voltage across each capacitor. If a branch has no short, then the voltage across each capacitor would be roughly the same.
For a branch that has a short halfway down, you'd expect to see decreasing voltages down to the shorted part, and then roughly the same voltages from then to the end of the string.? This procedure will find the furthest one in the chain first.? Having more than one shorted capacitor in a branch will make things more difficult.? The lower the range you use to find the part, the better this will work.? Measure across the capacitor in question, as close to the leads as possible.
One of the more likely suspects would be a tantalum capacitor that looks like a piece of candy.? That type fails frequently.
Your 5 volt supply will work well when set to 100 ma.? I think you can try this on a per-board basis.? Depending on the board, you may not need the other voltages, but that can vary per board and I'd welcome opinions on which.
Harvey
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On 12/8/2021 7:32 PM, James55 wrote: Apologies for the delayed response however I needed to resolve some things.
I have no access to a high resolution DMM
The only PSU I had, is an obscure Brazilian Sincler which kept (and keeps) blowing whenever it exceeds 500mA.
Due to Brazilian-level funds, I went out across to the other side of the city and bought a cheap, used, Chinese PSU only to discover it to be blown once I got indoors. After returning the next day to exchange it (and unable to test it) I was once again given a dud (for Brazil, think Nigeria). Anyway, last night it was disassembled and after finding a burnt resistor and swollen cap, the adjacent chips were replaced and finally, I have some form of working PSU.
Having said that, it has made no difference, for as soon as it is connected across the 5v rail of the scope, the PSU voltage drops to zero and a loud alarm sounds.
As for the dimtester, I also tried that but am not really sure what I should be looking for, as other than the lightbulb glowing bright, not a lot else seems to happen. No hot components etc.
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Thank you Harvey for the helpful advice.
As I mentioned, none of the rails are correct as the fuse blows if it is connected to the mains.
Currently the 5v and 15v rails give a direct short with an ohm meter.
The serial number is lower than the 200000 which is were the manual was updated.
Anyway, have the 50 ohm resistor and shall have another look...
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Ok, good so far.
You may want to see if Artek Media has a manual with the appropriate serial number.
I'd disconnect everything from the supply, and then check the +5 and +15 volt rails again for shorts.? Then if you have an ESR capacitor checker, check the output capacitors.? Also check the bridge rectifier because a shorted diode in any bridge generally results in a pretty much dead short across the transformer secondary.? In fact, I'd be tempted to check the bridge rectifiers first, then the capacitors.
If the 15 volt line (for instance) reads a dead short, then check on the other side of the pass transistor.? I would not expect that reading to be a dead short.? If, on the other hand, the transformer secondary reads a dead short with the probes in both directions, then a shorted diode.? (This depends on the rectifier configuration).? I'd be checking them and not waste fuses.
Most important thing is to have the power supply disconnected from the rest of the scope.
I generally like to make a copy of the schematic and then note the readings I make.? That can often give a good idea for an overall view of the problem.
Harvey
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On 12/9/2021 4:35 PM, James55 wrote: Thank you Harvey for the helpful advice.
As I mentioned, none of the rails are correct as the fuse blows if it is connected to the mains.
Currently the 5v and 15v rails give a direct short with an ohm meter.
The serial number is lower than the 200000 which is were the manual was updated.
Anyway, have the 50 ohm resistor and shall have another look...
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I believe I have the relevant manual in .pdf form, it is just not the clearest reproduction.
Just to help fill in more details, with no power to the scope, the resistance readings across the various low voltage rails are as follows;
140v ... 24 K¦¸
65V ... 7.3 ¦¸
15v ... 13.7 ¦¸
5v ... 0.2 ¦¸
-8v ... 47.4¦¸
-15v .. 2.5 ¦¸
So connected the scope up at 100mA but nothing heated up.
When you say to test on the other side of the 'pass' transistor, are they the ones bolted under the bar at the rear of the scope? And when you say disconnect everything from the supply, are you talking about disconnecting the boards beyond the main A6 board?
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James,
It appears you have shorted tantalums. Also check CR1767. The 5V and -15V supply resistance is too low. Get 647-ULD1E470MDD1TD for the 15V supplies and 647-ULD1C470MDD for the 5V supplies. The former is due in sometime this year and latter is in stock at Mouser. These are electrolytics that will work to replace the 6.8mfd tantalums. If you want tantalums, get 25V types for the 8V and 15V supplies and 10V minimum for the 5V supply. Raising the capacitance is fine. I did this in my 466. Raising the value of decoupling is fine. These are not critical in value. The ULD types are very low ESR, long fie and high temperature. I use this series when possible. C1737 can be increased in value, e.g. 15-33mfd 100V Nichicon UHE type or 22mfd 100V ULD type. Replacing the main filters with long life high temperature long life types is advised. I found that the 1200mfd I replaced with a 2200mfd type because this value is available in the specs mentioned before. These values are used in the 46/7X types.
Testing the other side of the series pass means that you test the resistance at the TP points instead of at the collectors. You could have a bad series pass transistor.
I will suggest to change R1726 to 2W and VR1726 to 85-BZX85B9V1. There are other resistors that should be increased in wattage. The zener is highly likely open.
Mark
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Hi Mark and thank you for your advice.
Any components from Mouser shall probably have to wait until I next go to Europe, as unfortunately the postal services here simply cannot be trusted to deliver any international item. Quality components such as low ESR caps are extremely difficult to find in Brazil so my goal is to just get the scope back to a working condition. I have repaired various of this series, which have always been just fairly simple component replacement. By the tme this one is back up and running I shall certainly have a much deeper understanding on analogue scope repair. :)
Anyway, CR1767 is fine although VR1726 is indeed open. So that is progress.
I have been primarily looking for shorted tantalums and diodes, and mainly on the A6 Interface board. So far, I have found three or four but seem to have run out of tantalums to test on that board.
On the A10 Storage board however, I do find that across certain caps it reads short, only for the caps to test good with one leg removed, although across the traces still reads short. C1846 being one of these.
@ Harvey...
It has been on my mind that perhaps there is a bridge rectifier issue because some of the readings are not as they were, and they are not consistent across all rectifiers. I have just been putting it down to something further along being shorted, however I shall have a deeper look just to clarify. It would be nice to remove the rectifiers but it was such a ball-ache changing the ones I replaced that I must say, it doesn't really fill me with desire to do it again.
There was a video I saw where the tech just used a pair of pliers to literally twist the rectifier off the board before desoldering the remaining legs/pins, it's just I can no longer find the vid and don't want to risk destroying the A6 board...
As for ESR testers, I have an MK 328 which stopped working this week. Having said that I found an old Wayne Kerr 4225 ESR/LCR tester yesterday in a junk shop for under 20 USD. It switches on and a quick look inside shows it's complete, so once I clean the contacts which hold the components under test, I'll give it a spin.
Need to head into town for components.
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Just to clarify.
On the A10 Storage board, there are three tantalums which all read short when connected, yet fine with one leg disconnected.
They are, C1846 C1826 and C2097.
I am presuming that one leg of each is to ground and that the other must be to the 5v rail, so probably all ok (apart from the 5v rail short)
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James,
The 1.5A bridge rectifiers are suspect. It is recommended to use 4A types. I have done this by the advice on this group. The 4A types run cooler due to not being run near it's current capacity. The rectifiers for the lower voltage supplies are 200V. You can use a higher voltage type if you want. If you will need bridge rectifiers for other things at a higher voltages, get ten of a higher voltage at 4A to get a price break. The price break is usually ten of an item.
It sounds like you have number of shorted tantalums by your description. Keep testing each one with one leg unsoldered. Those that are bad, leave the one leg up and note what you need to replace. With one leg left up, you know that one is bad.
You may have bad transistors. If any are 151-0367-00, check C to E. Any that show leakage, replace with KSP10BU. These have a pinout of BEC.
Mark
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Hi again, and thanks for the heads-up regarding the 151-0367-00's. I'll look into those.
Just been having another look and removed the 5v bridge rectifier, which unfortunately tested fine off of the board.
I'm just gonna have to bite the bullet and remove the horizonatal boards and main filter caps to access the underside ofthe A6 board in order to remove, test and possibly replace any other bridge rectifiers. It is something I have been putting off, but seem to be getting nowhere, so I'll just disconnect everything back to the transformer and take it from there.
If I'm honest, I am thinking toward a semi conductor failure, as I have tested every single tantalum (and others) on both the A6 and A10 boards, and have run out of ideas.
James
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On Mon, Dec 13, 2021 at 02:39 AM, James55 wrote:
Hi all,
please excuse the lack of input, however I became deflated and just put a cover over the scope and walked away.
Thing is, I really do need a working scope, so today decided to give it another go.
The mains input is now running first through a dimbulb, which dutifully illuminates when the unit is switched on. All of the low voltages rails are close to zero. For example the 65v rail is up at a whopping 10.4v. The minus 15 rail is 0.5v. The minus 8 is 0.4v and the 140v rail is 5.3v with the 5v being shorted to ground at zero volts.
I removed the A5 Trigger, and A7 Timing, boards in the hope of narrowing down the search area.
Things got off to a good start with the discovery of the shorted R1377 resistor, although that made no difference to any of the readings. There is a pure short circuit on the 5v rail however for the life of me I cannot seem to find it.
I have tested and tested all over the A6 and A10 boards, testing voltages and also bell-testing for shorts with the power off. It really is at the point where I can only see trees, without a glimpse of any wood...
I have also covered the board in IPA which all just sat there wet for ages. There are no warm components, or even areas... I wish something would just explode or at least go up in smoke, as I have no real idea as to where else to look.
It did cross my mind that it might be the 5v rail 5500?f filter cap yet although I only have a DMM it still reads 7000?f, so I am assuming it is not shorted.
If anyone has any suggestions at all as to where I might check next please feel free to chip in.
Perhaps I'll just leave it plugged in, as surely a direct short has to start smoking at some point?
James
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Looking back through the earlier replies, I shall probably disassemble the verticle board in the morning. It is midnight here now.
zzzzzzzzzzzzz
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ok, the standard way of testing scope power faults is as follows:
1) determine if you have a linear or a switching supply.? For the 466, I think it's going to be linear.
2) 0 volts or fuses blowing indicates a short
3) disconnect all boards from the power supply and see what it does.? If it's a switching supply you'll have to load it down since most switching supplies need loads to operate.
4) In most tektronix designs of that time, there is one supply that has an internal reference (typically a zener).? All other supplies use that one supply as a reference voltage.? If the reference supply is off, nothing else will be ok.
5) check that the reference voltage is ok.? Check that the supply is ok (it can be, for instance, the -50 volt supply).
6) check the other supplies for proper operation.
7) plug in the other boards to see which one pulls down the supply
8) record the voltages which may show you which supply is bad.
If you have the manual, read the power supply theory of operation (ought to be one), and then see if there's a power supply troubleshooting tree.
Harvey
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On 3/1/2022 9:57 PM, James55 wrote: On Mon, Dec 13, 2021 at 02:39 AM, James55 wrote:
Hi all,
please excuse the lack of input, however I became deflated and just put a cover over the scope and walked away.
Thing is, I really do need a working scope, so today decided to give it another go.
The mains input is now running first through a dimbulb, which dutifully illuminates when the unit is switched on. All of the low voltages rails are close to zero. For example the 65v rail is up at a whopping 10.4v. The minus 15 rail is 0.5v. The minus 8 is 0.4v and the 140v rail is 5.3v with the 5v being shorted to ground at zero volts.
I removed the A5 Trigger, and A7 Timing, boards in the hope of narrowing down the search area.
Things got off to a good start with the discovery of the shorted R1377 resistor, although that made no difference to any of the readings. There is a pure short circuit on the 5v rail however for the life of me I cannot seem to find it.
I have tested and tested all over the A6 and A10 boards, testing voltages and also bell-testing for shorts with the power off. It really is at the point where I can only see trees, without a glimpse of any wood...
I have also covered the board in IPA which all just sat there wet for ages. There are no warm components, or even areas... I wish something would just explode or at least go up in smoke, as I have no real idea as to where else to look.
It did cross my mind that it might be the 5v rail 5500?f filter cap yet although I only have a DMM it still reads 7000?f, so I am assuming it is not shorted.
If anyone has any suggestions at all as to where I might check next please feel free to chip in.
Perhaps I'll just leave it plugged in, as surely a direct short has to start smoking at some point?
James
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It will almost certainly be a shorted blue-drop (or yellow-drop) tantalum capacitor.
That's what usually brings down a 465/466/475 and various 7K series and TM series plugins with the symptom of a shorted power supply rail.
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of James55 Sent: 02 March 2022 03:06 To: [email protected]Subject: Re: [TekScopes] Help required finding a Tek 466 power rail short Looking back through the earlier replies, I shall probably disassemble the verticle board in the morning. It is midnight here now. zzzzzzzzzzzzz
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Forgive me for chipping in very late in this thread, there are many good suggestions and you have to be very systematic about the process and not take short cuts or do too much work when you are very tired.
I looked at the manual on TekWiki with the file name Tek-466-early.pdf. Immediately before the schematic of the Power Supply is the diagram of power distribution between boards. This shows which of the multi-way connectors need to be removed in order to remove the +5V power to A3, A10, A7, A5 and A2 boards. If you remove all of these connectors and the +5V short remains when measured at the +5V supply then the fault is on the A6 board, this is the worst case with many places to look. Otherwise one or more of the now disconnected boards will show the short circuit on +5V and you have a much smaller area to concentrate on. If you find a short circuit on the A10 storage board and all of the capacitors measure OK with one leg removed then it is possible that one of the 74xx logic chips is the cause. I have found at least one case of this mode of failure but it is very much less common than failed capacitors.
Best wishes for your repair,
Roger
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James55