Tektronix 2232 scope.... in need of a doctor..

[Vince]

Hi every one,

OK, so I like tearing old gear apart to restore them and see how they are put together... and try fixing them when required...  I guess I should be "happy" today... ?!.... my Tektronix 2232 scope just gave up the ghost a minute ago ! Grrr...    I love that little scope, been searching for one for some time, not easy to find one in France, in a good shape, fully working, for a reasonable price, and a seller willing to bother packaging/shipping it. So... I must fix it....

Today I was fixing an issue with the big time base button on the front panel. the x10, Cal and secondary time base where behaving fine, but the main time base part of the knob was acting weirdly, it had become unusable. Expecting a complicated piece of mechanical engineering marvel at play, I wasted no time and removed the cabinet and front panel... OK so the main logic board was fully exposed, but I did not mess with it in any way, and too great care not to disturb it, not even put my greasy finger prints on anything. I left it alone.  the front panel... some of the buttons are not held by a screw, they just snap onto the shafts. The first button I pulled on... broke (the little clip inside of it, that grabs the shaft). OK, so that's bad luck, plastic got brittle over time. Next button... same luck. third.... again. Fourth.... worse : the button itself disintegrated in to a million pieces ! And I swear I was not using any pliers or any tools, I was grabbing and pulling it with two fingers, I swear !  So, so much for the front panel removal... I gave up. Then later I figured that I didn't have to remove the front panel nor even take off the cabinet/cover.... all the time base knob needed was tightening a few screws and resetting it in the correct position....

So, now I have a working time base knob... but 4 missing knobs for the vertical and horizontal position controls.... which can be found on ebay... in Greece, for like 10 or 20 buck A PIECE ! My goodness.... no other choice sadly :-(  Well at least while I am sourcing buttons, I can still use the scope just fine without theses buttons, I can just grab the shafts directly no worries.

So, I reassemble the scope, and proceed to test it to make sure everything is fine.

Yes... the time base buttons now works like a charm, and using the delayed time base feture is a delight... so after a few minutes playing with that, I switch to something else and leave the scope unattended. 10 seconds later I hear "grrr.... POOF ! ", then it shuts down abruptly.  Oh no, was working just fine ! :-(
I proceed to check the fuse... sure enough it was blown. OK it's a 30 year old fuse, maybe it decided to die... if fuses can die on their own ?!........  though the fuses was a transparent/glass type, and it clearly blew, not to mention it was pretty warm, so no doubt some energy passed through the poor little bugger.

Was a 2A slow blow, 6x32mm. only 6x32 I had was the 10 pack of 10A Fast fuses I bought for my collection of Metrix multimeters... OK I knew I was taking a risk there, but just to rule out a defective old fuse, I put it that 10A fuse in the scope anyway, thinking if the scope is fine, then 10A or whatever won't make a difference. Yes you guessed it,... the old fuse did blow for a good reason !  LOL ....  As soon as I pressed the power button, a distinct/audible "GRRRR. .. gzzzz" that laster a couple seconds tops, then followed by a big " POOF " then shut down like the first time.

Given that the scope was working just fine before hand, I take it the digital stuff is not at fault, and even if it somehow developed a big short on one of the rails... the power supply is supposed to cope with that, right ? So... I guess it's more to do with the power supply... old filter caps going bang, or something ?!

That will be my starting point anyway....

Luckily the service manual and schematics is available, have a copy of it... though I have not yet checked if the schematics for the power supply specifically, are in there... I hope it's not like my TDS 310 scope with a third party PSU where the schematics are just not available, grrrr....

Anyway, I created this topic for those who might be interested in the repair, and so I can ask for help/ideas/suggestions here and there if required.

Obviously, not being very experienced in repairing stuff, and not having that much time available, I expect this topic to be open for a few weeks at best, most probably a few months... but well, now it's been created, I can add to it as I go.... little by little.

I will try to get motivated to start work on it real soon, but most probably it will have to wait until the summer break, starting end of July, for 3 weeks.

See you in a bit for the beginning of the repair, then...

[tautech]

These run a SMPS don't they ?
For those following along it would be a good idea to post up a link to the SM so we can all play along.

Tek knobs:
http://www.sphere.bc.ca/test/tek-parts/tek-knobs.html

And from the sticky in the Repair board:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[Vince]

Hi,

Thanks so much for the links, a mine gold ! They even have the square ivory push buttons for my ancient 5000 Series rack mount Tek scope ! When I went to clean these buttons, the markings on some of them came right off as soon as the sponge barely touched them !  Will be able to make it look like new again, yahoo ! :-D

SMPS ? Nope... I just spent a few minutes looking at the schematics. No third-party SMPS in that old scope, luckily it's all Tek stuff hence documented, and it's the linear type.  Below is the relevant portion of the schematics, where one can see the transformer and all the secondary windings, rectifiers and main filter caps, which I highlighted in yellow.

I can see a couple small (33 and 270uF) filter caps related to the CRT, then a big 4,700uF one across the cooling fan, then for the main power rails I can count no less than x9 1,000uF caps. So, I will check all that when I find time...

I also just ordered a x10 pack of the appropriate 6x32mm 2A slow blow fuses, I anticipate I will go through a few of them during the repair...

[tautech]

Manual:
http://bama.edebris.com/manuals/tek/2232/
65 Mb
Waiting for download on slow connection............

I also just ordered a x10 pack of the appropriate 6x32mm 2A slow blow fuses, I anticipate I will go through a few of them during the repair...

Google dim bulb tester and make one, it will save a pile of fuses. 

The schematic snip does not confirm it's not SMPS.

[Vince]

The schematic snip does not confirm it's not SMPS.

While your download is running, I attached below the full PDF pages. As you can see it's all 100% old school linear stuff. The second page "Power Distribution" holds not much more, just simple linear 3 terminal regulators.... good old 7805/7905 would you believe it (lazy Tektronix ! LOL), and just small decoupling caps and LC filtering networks, the usual stuff. Not an inch of switching in there. Given the age of the 2232 I am not surprised... I guess all the 2000 series scope, being the same era, are linear.  I assume SMPS appeared a tad later, in the late 80's very early '90s, when the more modern, newer generation, fully digital 'TDS ' series was introduced. That was for Tek the opportunity to start a new generation of scopes from a blank sheet, hence the ideal time to update from linear to SMPS. Well, it's a reasonable assumption any how, I find.

Google dim bulb tester and make one, it will save a pile of fuses. 

Hmmm... no idea what this is, but sounds interesting, you got me curious ! 

[tautech]

SMPS ? Nope... I just spent a few minutes looking at the schematics.

Few more minutes required. 
See P276

SMPS waveforms P274

[Vince]

P276 ? on mly end it's a board view of the alternate sweep section   

Anyway, you mean you found an DC/DC converter somewhere in this scope.. but all the stuff I showed you is linear so at least 95% of the rails in the scope are generated this way... I wonder why they would add a local DC/DC somewhere, for what benefit, other than creating noise which is not really welcome on a delicate instrument like an oscilloscope...  could you give me the link to the schematics you found ? Will have a look at it, doens't seme to match the one I have. You did get the 2232 right ?   You have me intrigued...   
BTW my service manual weights only 23MB not 60+ like yours... maybe you have more stuff than I do...
Anyway, time to go to bed, give me the link to your manual and I will compare with mine... which if I remember I downloaded straight from Tektronix web site.

[tautech]

P276 ? on mly end it's a board view of the alternate sweep section   

Anyway, you mean you found an DC/DC converter somewhere in this scope.. but all the stuff I showed you is linear so at least 95% of the rails in the scope are generated this way... I wonder why they would add a local DC/DC somewhere, for what benefit, other than creating noise which is not really welcome on a delicate instrument like an oscilloscope...  could you give me the link to the schematics you found ? Will have a look at it, doens't seme to match the one I have. You did get the 2232 right ?   You have me intrigued...   
BTW my service manual weights only 23MB not 60+ like yours... maybe you have more stuff than I do...
Anyway, time to go to bed, give me the link to your manual and I will compare with mine... which if I remember I downloaded straight from Tektronix web site.

http://bama.edebris.com/manuals/tek/2232/
P276 as reported by Adobe reader.

[Vince]

OK I figured it out ! 3AM here, and must go to work... not until 13h00 luckily, but still...
Could resist the  temptation of having a quick look at the scope ! LOL

In the end... I look like a complete idiot, I owe you a big apology ! Thank you for your patience... I somehow, in some perverse way, like these moments where I look like a complete fool... make me more humble (and knowledgeable) when comes the next problem ! LOL

Sooo... took it apart, was easy, the mechanical/system engineering is excellent in this thing, thank you Tektronix.

First thing, once I got the cover off, I noticed a damaged capacitor on the main/logic board. Clearly the cap didn't quite clear the cover and it rubbed against it. You can see makrs on the top of the cap, and also it's partially lifted from the PCB. Looks ugly, but it did not look like it created any damage in the area... though obviously I will replace it any way.  No, the big "bang" I heard had to come from the hot side of the power supply, not some random short down the line, from which the PSU would be able to protect itself in a more silent way I assume.
So I removed the cover from the PSU section and performed a visual inspection..... not big transformer... the main transformer is quite small and for all the world looks very SMPS-ish to me ! Hmmm..... how comes. Then looked at the schematics and.... I was wrong as you said... sure, this transformer with all the seconddary windings, rectifier and filter caps, looks like the typical old school linear PSU.. but.... hey wait a minute, nowhere does the schematics say that the primary side is fed by the mains voltage !  So, looked some more at the scope, hey presto, found a suspicious plastic covers hiding something screwed on the rear panel, above the PSU main PCB... hmmm.. removed the cover, hey presto !  A littel board with a couple SMPS-ish transformers, fed straight from the mains !  OK then... that must be a pre-regulator stepping down the voltage going to the main transformer. So I was halkf right/half wrong... it's not quite a full blown SMPS, but a hybrid... evolved from the ancient linear design, but improved with that pre-regulator, so they can decrease the size of the main transformer. OK....
Only thing is... I could not see any integrated circuit on that regulator board, humm... there must be one somewhere though ! Then decided to removed the cooling fan to get a better view of the board in a dark corner, and hey there it was, hidden right below the cooling fan !  As soo as I read the part number on it " TL 594", I knew I had my friend ! ... Just 20 years ago at school, I remember vividly that we did experiments with that very chip, when we were taught the working principles of SMPS... well maybe not that exact part number, IIRC it was more like TL4xx something, not TL 5xx, but whatever.

So.... now that the design part is figured out, let's see what's wrong with it !
At this point I should now say that as I "worked" the scope to removed the cover (was quite stiff), I heard something moving around in the scope... like a screw or something ?! As soon as the cover was off, I agitated the scope until something came out of it.... and something did, a screw, and not a small one at that ! The kind that secures the back panel !  ... but none were missing, so no idea where that screw comes from... it's entirely possible it ahd been there from day one, when I bought it I mean, which not that long ago, and I have not used the scope much since then, so why not...

So, at least we know now that the problem is now gone away, and we jsut need to figure out what damage it caused ! :-/
The big filter caps all looked good. Here is the damage I found, cooked stuff :

- burnt 3 pin connector on a big device/package that's bolted onto the heat sink... maybe the power transistor used by the pre-regulator. The package looks hardly off-the-shelf, so I guess there will be no hope sourcing it. hopefully I can come up with a modern replacement. Probably won't be the same package, but there is some space in there, so it should be able accommodate pretty much any shape and size of package.

- just below this tranny, on the PCB, squeezed between the heat sink and the terminal of the main transformer, I noticed a 1W or so resistor that's cooked as well

- Then under the cooling fan, the TL 594 chip is cooked as well, and so is a little passive next to it, as well as little T092 tranny, and another one not too far, although not visibly cooked, is destroyed ! It's package is broken/chipped, hmmm...

The passives will be easy to replace/source I imagine, so will be the TL 594 chip, but replacing the power transistor (or whatever it is), will be less straightforward I guess.... :-/  Will look into it more closely when I have some more time.... tomorrow...

[Vince]

OK, progressing... found the schematic for the pre-régulator... page 283 in my PDF !  Go figure.

Attached below.  Sure helps ! :-)

I can see the power tranny... a MOSFET by the looks of it, Q9070 is its name.  Scrolled back 50 pages to search for it in the part list... thank god it gives all the required details ! :-)

It says it's a " MTP 6N60E ", an N-Channel MOSFET, 600V, 6A, 1.2ohms, must refer to Rdson I assume.   

Found the datasheet, attached below. Says it's obsolete... how surprising. Well at least it was an off-the-shelf component, so we have the full specs to help find a replacement...

Oh, Googled it, led me to this nice document from ST Microelectronics no less, listing replacement for many old trannies :

http://noel.feld.cvut.cz/hw/st/3650.pdf

Says a replacement could be an " STB 6NA600 ".... sadly can't find it on Farnell's website, the only big supplier we have available here in France (as far as I know anyway)... the other big names require importing from the US, prohibitive...

Well at least we have the detailed specs for this tranny, so will do a parametric search tomorrow... really must go to bed now, 4h30 here, yikes ! 

[Vince]

Woops goofed it when looking up the table.. replacement is STP 6NA60 not STB 6NA600. Well, doesn't make much difference as it's also unavailable it appears..

[tautech]

You've got some studying to do, don't hurry this repair.

A little board with a couple SMPS-ish transformers, fed straight from the mains !

This one ?

Series wound mains input filters AKA common mode chokes.

Spend some real time reading up on the PSU section and principle of operation.
The Tek Troubleshooting doc has a section on SMPS where they call it the high efficiency supply. 

[David Hess]

The shorted power transistor means doing a full rebuild of the power supply including replacement of the controller, drive circuit, aluminum electrolytic capacitors, and switching diode.

The MOSFET is not all that special.  Look for a 600 volt part with roughly the same current rating and resistance as the MTP6N60E.  Tektronix used a lot of different MOSFETs in this design over the years.

[tautech]

David, would you seriously consider a full recap on this scope too ?

[oldway]

Everyone has his own method of repair...

2236 = 2235 + multimeter/counter board
2232 = 2235 + digital board.
(more or less)

First, I don't worry about the preregulator and feed the 42V rail with my HP6296A power supply (current limited at 2A)...(without the multimeter/counter board or digital board)
I check all the power supplies for voltage and ripple and all the basic control of the scope.

If all this is ok, I check the correct working of the scope with the multimeter/counter or digital board in place.

Check the crowbar protection by increasing voltage up to 45V....

When everything is allright, then I repair the preregulator...
Replace all the four diodes 1N4007, the TL594, the Mosfet and check all the other components.

Use an isolating transformer with a bulb (100W) in serie...Check if TL594 is oscilating or not and gate signal of Mosfet.

In my opinion, you must have a good reason to replace components....So recap is a wrong decision if capacitors are of good quality and they did not failed without overvoltage.

[David Hess]

David, would you seriously consider a full recap on this scope too ?

I would absolutely replace all of the aluminum electrolytic capacitors.  Once access is gained to the power supply, it would be a waste of effort not to.

It also is not entirely clear what causes this failure and I suspect it is the aluminum electrolytic capacitors.

2236 = 2235 + multimeter/counter board
2232 = 2235 + digital board.

That is basically it.  There are minor differences in the main board but this series of oscilloscopes are all based on the 2235.  The 2213/2215 and 2225 are different series.

Use an isolating transformer with a bulb (100W) in serie...Check if TL594 is oscilating or not and gate signal of Mosfet.

Besides testing the inverter separately with a 42 volt DC power supply, I would also test the preregulator separately with a 42 volt dummy load in place of the inverter.

In my opinion, you must have a good reason to replace components....So recap is a wrong decision if capacitors are of good quality and they did not failed without overvoltage.

I would change the aluminum electrolytic capacitors once access is gained for two reasons; they are known to wear out with age and they may have been the cause of the preregulator failure.

[oldway]

I would change the aluminum electrolytic capacitors once access is gained for two reasons; they are known to wear out with age and they may have been the cause of the preregulator failure.

I don't agree...If you look at the pictures, you will see that whatkilled probably the Mosfet is a bad contact of his connector and overheating of his terminals...The Mosfet goes then in short circuit and damaged  the capacitor by overvoltage.

[Vince]

HI everyone, and thank you all for your input, much appreciated ! Looks like we have some people well versed in theses old Tek scopes, I should be covered then ! 

Yes last night was a bit of a rush... was trying to get as much done as possible befonre going to bed. Not gonna repeat that tonight... need some sleep eventually ! LOL...

So, sitting back a little tonight, digesting the information...


FET :
--------
I removed what I thought was a funny looking/custom Tek thing... and in the end that funny package was just a big plastic piece wrapping the FET to press it against the heat sink. The FET is indeed, phew, just a regular TO220 package. So that's a relief, there are no custom/impossible to find component involved in this repair, I should be able to fix it for cheap, and enjoy my scope for a long time to come.
Part number ? Indeed as one of you said, Tek doesn't stick to the BOM religiously at all : BOM said MTP 6N60E... my scope comes with 6N55.
I Googled a bit and found a replacement for the 6N60E. An IRF BC40A. Exact same specs (600V, 6A, 1.2 ohms), still available today from Farnell and only 1,5 Euros or so ! Happy I am.
The plastic connector of the FET is cooked, but the FET itself looks absolutely sparkling new ! No sign of over heating anywhere, be it on the terminal or the plastic body. However... I tested it with an ohm meter and I get either 2 ohms, or a dead short, no matter what pins I check and no matter the polarity ! OK I expect a short when the drain-Source diode is forward biased, but when in reverse I would expect an open circuit since it's an enhancement type FET...
As for the Gate-Drain or Gate-Source, of course I would not expect a short there either...
So, the FET is definitely cooked.

Thanks for the debugging tips, will proceed. The schematics are great and supply vottage and waveforms n various places, how nice from Tek...
Looks like the preregulator runs at 60kHz or so, I guess any FET should be able to cope with that...
the inverter is even more lazy, 20+ kHz.... well I guess the transformer downstream couldn't cope with more than that...

Schematics indicate 145V once the mains is rectified and fed to the FET... so that must be for the 110V mains in the USA. On my side of the pond it's 220/240, so I expect 300V+ or so.I assume the preregulator is designed for European market so as to still give the 40+Volts anyway, not 80...

OK so now that hardest part is to come : get access to th board, so I check all the components, and e able to remove and replace all the cooked components.
The heatsink is right in the way : most of the components involved in the accident, are right at its feet so to speak. It being very tall, makes it a nightmare. So I have not much choice but remove it :-/  The small three terminal regulator hooked onto it will have to hang in the air... should alright just for the time of the repair...   Fan mountng hardware, EMI board and mains socket all get in the way too, need to move them out of the way.

Not sure if the analog board,  can come off the chassis without taking the whole scope to bits... crossing fingers. Will check the service manual for info on that...

So, first get a good access to the part of the board that's hosting the PSU, then I can start working on it...

Oh, as for isolation, I do happen to have an isolation transformer, ancient thing I have never used.. that will be the opportunity, great. Rated at 100VA max, hope it will be alright...
Ah, need to check that bulb in series thing, not familiar with this trick...

Thanks again for the input, will update the thread as I go...

[tautech]

Be sure to take oldway's and David's advice, they know these scopes better than I. 

For the FET 1.2 Rds ON is nothing flash these days and a better device could be selected for much lower dissipation. You could also pick a bit higher V device for a bit more headroom over your mains voltage.
Just be sure to match Gate characteristics. 

[Vince]

Yes I thought of doing that but refrained precisely because og what you said... matching the gate specs... not quite fluent enough in FET designs to take the risk to goof it up !    So until I know better, I think it's best to stick to a direct equivalent... unless of course one of you has experience with a particular part model I could have confidence in, and just buy blindly... knowing it would work fine.

Anyway, I have made some progress again. Not going to spend the night at the workbench like yesterday, but just having a little look at the service manual.
I looked at the board view to locate and identify the components that got shot during the incident, so I can locate them on the schematics, hoping it would help me make sense of what happened. That was successful... it all seems to match the theory Oldway exposed earlier on.  Here goes.

The blown components are Q908, R907 and of course TL594.

The FET is blown, it's a fact, it's short how ever you look at it. Burnt connector indicates that it's probably the cause of the incident rather than a consequence, since the "browning" is clearly the result of years of overheating, nothing to do with, not a consequence, of the blast that happened during the incident. So, the FET goes bad, becomes a short... it dumps the rectified mains voltage to the 42 V rail...


- R902, it's 0.2 ohm shunt/current sensing resistor that lies between the FET and the middle tap of the transformer's primary winding. So, R902 blows, acting like a fuse which hopefully was fast enough to protect the transformer's primary... will have to check the windings to make sure of that though !

- Q908, the little TO92 transistor that drive the gate of the FET. It's collector is connected to the 42V rail, so here again that would explain why a big chunk of it's package has vanished into thin air....

- TL594 : connected to the poor R904 resistor obvioulsy, as it uses it to sense the current the FET is outputting. Sense pin of the TL594 is Pin #16... oh, surprise surprise, that precisely this area of the chip that got cooked... and looking more closely at the chip, I can see that pin 16 is... missing ! just blown away ! LOL
So, this confirms that....


Two other components, Q928 and R913, next to the TL594, look cooked too. However, looking at the schematics, they don't seem related to each other nor the sens pin of the chip. So most likely, that didn't get blast, but only got covered by the black smoke emanating from the TL594, simply because they happened to be there.. bad luck. So chances are they are perfectly fine... just need a good cleaning ! LOL  Will check anyway of course.... don't want to take the risk.


It seems to hold water to me... everything checks out...


Still, I am not too sure exactly how that FET's connector would cause the FET to eventually go bang... please explain, I want to know...

What kills me though, is that I only need a few bucks worth of components, and believe it or not, say maybe just an hour before the scope went bang, I placed an order on Farnell, for a 10 Euros LCD display, then scratching my head to figure out how to spend the remaining 20 Euros to get free shipping, typical !   

[oldway]

Burnt connector indicates that it's probably the cause of the incident rather than a consequence, since the "browning" is clearly the result of years of overheating, nothing to do with, not a consequence, of the blast that happened during the incident. So, the FET goes bad, becomes a short... it dumps the rectified mains voltage to the 42 V rail...

That's right.

Normally, rms current is under 2A and there are not much reasons to overheating the connector.

But I have found already 2 times a far much higher rms current in 2235 oscilloscope: 

Why can this happen ?
Pre-regulator works as a buck converter.
T906, even if it has several coils, is in fact a dc inductor with gap.
If inductance of this inductor is lower than required, then working of the buck converter become fully wrong, with high peak current and high rms value

In one case, inductance was very low because ferrite core of T906 was broken, in the other case, there was an internal short between layers or turns of one of the coils.

With such a problem, the oscilloscope works, but the Mosfet overheated and sometimes, you can hear a high pitch sound varying with brightness control.

The best way to find this problem is to check the current waveform on shunt resistor R907

Be careful, use an isolation transformer to feed the pre-regulator.

For Mosfet, don't worry, almost every single mosfet 6A 600V works well.

I like to use a TO220 isolated fullpack IRFIBC40G which is only 3.5A but never failed after replacement.

I also advise to install a fan in the 2235 because power supply run to hot  without fan.

[Vince]

Hi there,

Oldway, thanks for all the good info on that inductor/transformer ! 

Looks like it's important that I take the time to check its good health !

Did you make note of the resistance (or better yer, actual inductance if you hae an RLC meter ?) of its various coil, so I can check on mine ?

Failing that, I can at least check that the core is not broken/cracked, and that the windings aren't open-circuit or shorted, then check then current doesn't exceed 2A as you mentioned.

So, that's what I am planing to do in the next few days  :

Getting full/proper access to the board looks like a nightmare, might take time to get to it.  However, in the meantime, since we now have pretty much established that the damage occurred up stream in the preregulator, I could at least bypass it and feed the inverter with my bench power supply 2x30V 3A, putting them in series should do it. This way we will see if the scope can power up like this, and the bench PSU will tell me how much current it draws, so I can
That should be easy to test, because the shunt resistor is apparently the only link/path between the FET/preregulator and the inverter/downstream. So, since the resistor is shot, I have to remove it anyway, then I can just solder a wire on the correct pad, and connect the bench PSU there... and see what happens ! ...

Yeah, will try and do that this week, maybe not tonight, don't want to rush this...

[tautech]

However, in the meantime, since we now have pretty much established that the damage occurred up stream in the preregulator, I could at least bypass it and feed the inverter with my bench power supply 2x30V 3A, putting them in series should do it. This way we will see if the scope can power up like this, and the bench PSU will tell me how much current it draws, so I can
That should be easy to test, because the shunt resistor is apparently the only link/path between the FET/preregulator and the inverter/downstream. So, since the resistor is shot, I have to remove it anyway, then I can just solder a wire on the correct pad, and connect the bench PSU there... and see what happens ! ...

Yeah, will try and do that this week, maybe not tonight, don't want to rush this...

Wise to do this in case something has been damaged from the PSU failure.......it was suggested earlier. 

[Vince]

Well yes, it was implied that I was doing this based on the previous advice from you guys, never  claimed it was my original idea !   

[Vince]

Check the crowbar protection by increasing voltage up to 45V....

Hi Oldway,

Sorry for taking so long to react to your remark... but I didn't know what the expression "crow bar" meant... though I assumed you were talking of some clamping circuit. Looked it up on Google...  I learned something nexw again, great. Not clamping then, more like shorting. I assume you are talking about the thyristor  Q935, but the zener diode that accompanies it and drives the gate, is rated at 51V not 45.. am I missing something ? Does the SCR really start conducting as soon as 45V, the 51V zener is only there to clamp the gate voltage to a safe level ?  Bottom line is, I guess... should I be worried if the SCR turns on at 51V instead of 45V... would the few extra volts cause damage downstream ?  I guess not...
I tried checking the datasheet of the SCR to have a loot at the gate characteristics, but the parts list specifies a "SCR2217" and boy, Google just can not find any datasheet for this thing ! Nor much of a web page of any kind for that matter, this thing is like a ghost ! LOL Hmmm....

Anyway, the good news is that if this crow bar did its job, then the entire invert circuit, and the entire scope down stream... were protected during the blast :-)
So, indeed everything suggests that only the pre-regulator suffered.
I guess I would still better check that the SCR itself survived the short... might want to make sure it isn't shorted, before I apply power.

Replace all the four diodes 1N4007

Sorry for being a little slow here... first time repairing this stuff .... I see 5 diodes on the schematics, not 4, and none of them are 1N4007. I assume you are talking about the 2 diodes around the FET (CR908 and CR907) and the two across on the secondary winding of the inverter output transformer ?

Obviously these are "fast" diodes, as stated in the parts list.  For the inverter, which runs at 20kHz, thet specified MB2501 (200ns recovery time), and for the two in the pre-regulator, which runs 3 times faster at 60kHz, they specified even faster diodes, a 50ns one (MUR 460RL) and an even faster one (4ns), a FDH 9427 mated to the little bipolar tranny that drives the gate of the FET.
But the 1N4007 is a simple general purpose rectifier, it's datasheet is very short, not much specs given, and they don't even state what the recovery time is, probably because it's horrendous.

The fifth diode I see is related to the crow bar, I guess when you said "4" diodes, you were implicitly not considering that one, as it doesn't play a role in the operation of the inverter nor the preregulator...

So... I assume I must have misunderstood you ? I highligted the 5 diodes on the schematics I have.

Use an isolating transformer with a bulb (100W) in serie...

Looked that up too... these dim bulb tester seem like quite useful tool indeed ! Simple, cheap, yet effective.... what's not to love.
Will build a nice one when I get some time, but to get me going on this repair I will just make a quick hack, no proper box nor switch nor anything, just wire a bulb and that will do it. Finding a 100W bulb will probe tough... these things have been made illegal recently, you can't buy them anymore. 100W old fashioned bulbs are now replaced by 60W halogen bulbs, to save energy. I guess I can a 60W one and a 40W one in parallel...