Tek Model 464 repair

[Ero-Shan]

After all that talk in the TEA thread about their 465 scopes, I crawled into my attic and resurrected something that is somewhat close to it. Yeah, one  short.
I got that CRO about 25 years ago, still working then. But from the very beginning, this scope didn't really appeal to me: the housing was dirty, the hump on its back made it ugly, the pouch looked and felt grimy, the fan was loud, and those 'feet' make me cringe: Thinking what they might do to my desk ... 


When I switched it on again after a few years, it didn't show any trace. Owing to my dislike, I didn't care. Also, at that time TEA was just something that you drank when you're really sick. 
The world has changed quite a bit since then. Inspired by all the talk I thought it was time to reassess my appreciation for the bugger.

This means that I have to repair it first, which shall be documented here.

Please note, however: I am not on par with most of you guys (and girls?) here in my repair-ability. And I will not go out of my way to get it fixed. No expensive parts, no rewinding the HV-transformer. If I can't fix it, it won't be fixed! Which means this thread might find a sudden end. Oh, and I'm not spending all my time on it - updates may come sparingly.

That said, let's start with a view of the 'face', in its original, uncleaned state:


As you can see, the hump is a DM43. Also one short of the ubiquitous DM44. For this I couldn't find a manual or schematics. But then, it seems to work alright.

 

[bd139]

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[particleman]

After all that talk in the TEA thread about their 465 scopes, I crawled into my attic and resurrected something that is somewhat close to it. Yeah, one  short.
I got that CRO about 25 years ago, still working then. But from the very beginning, this scope didn't really appeal to me: the housing was dirty, the hump on its back made it ugly, the pouch looked and felt grimy, the fan was loud, and those 'feet' make me cringe: Thinking what they might do to my desk ... 


When I switched it on again after a few years, it didn't show any trace. Owing to my dislike, I didn't care. Also, at that time TEA was just something that you drank when you're really sick. 
The world has changed quite a bit since then. Inspired by all the talk I thought it was time to reassess my appreciation for the bugger.

This means that I have to repair it first, which shall be documented here.

Please note, however: I am not on par with most of you guys (and girls?) here in my repair-ability. And I will not go out of my way to get it fixed. No expensive parts, no rewinding the HV-transformer. If I can't fix it, it won't be fixed! Which means this thread might find a sudden end. Oh, and I'm not spending all my time on it - updates may come sparingly.

That said, let's start with a view of the 'face', in its original, uncleaned state:


As you can see, the hump is a DM43. Also one short of the ubiquitous DM44. For this I couldn't find a manual or schematics. But then, it seems to work alright.

I want either a 464 or a 466 to add to my collection. Cant wait to see how it turns out.

[David Hess]

I used a 464 long ago and did not particularly like it because of the image quality of the storage CRT.  I have a 7834 now which is much better but still not as good as a non-storage CRT.

The DM43 lacks the delta delayed sweep feature of the DM44 which most users will not miss anyway.

[oventech]

Found the wiki  page for the 464 scope. Manuals can be found here.  http://w140.com/tekwiki/wiki/464

[particleman]

I used a 464 long ago and did not particularly like it because of the image quality of the storage CRT.  I have a 7834 now which is much better but still not as good as a non-storage CRT.

The DM43 lacks the delta delayed sweep feature of the DM44 which most users will not miss anyway.

David, How about the 466 CRT image quality?  Any improvement due to reduced scanning capable CRT?

[Ero-Shan]

I used a 464 long ago and did not particularly like it because of the image quality of the storage CRT.  I have a 7834 now which is much better but still not as good as a non-storage CRT.

Thank you for reminding me - the poor image quality was my main gripe with this instrument. I knew I'd forgotten something.

Now I want to see just how bad it is. 

The DM43 lacks the delta delayed sweep feature of the DM44 which most users will not miss anyway.

I only miss the service manual.

[Ero-Shan]

Since it is customary to show some assorted Pron after stripping, I will do so. 

Tektronix is as much mechanics as electronics. Lots of rods!


My favorite, 'clutches to the left of me, clutches to the right':


The 'Pull-On' mains switch does everything right: It is fail-safe as any inadvertent actuation can only be to the safe Off state, and putting the front cover on simply pushes it in and switches the scope off. Nice. The implementation has some grandeur:


And a last one for now - a neon lamp on a throne (which is just cute):

[dzseki]

Hey, the lamp on the last picture looks incadescent to me, notheless nice!

[bd139]

Nice picture. Looks pretty clean.

I am having a personal bet there’s a blue tant in the HV osc gone short that has blown the fuse on the interface board. I think I’ve seen that on almost every 4xx I’ve had pass my hands. To the point I have spare fuses on hand just in case I come across another scope 

That bulb above is a neon. I’ve not seen one packaged like that. They are usually loose. Must be overload protection in the DM somewhere.

I do like the mains switch arrangement on these. It’s better than the wonky brittle plastic thing connected to an equally brittle latching changeover switch you get in newer instruments.

[Ero-Shan]

Nice picture. Looks pretty clean.

I am having a personal bet there’s a blue tant in the HV osc gone short that has blown the fuse on the interface board. I think I’ve seen that on almost every 4xx I’ve had pass my hands. To the point I have spare fuses on hand just in case I come across another scope 

This is the point where I have to admit something. I actually started the repair last Friday and did a few things since then. Yesterday I finally decided to start this thread, as I have a long weekend. Getting the pictures on the PC takes time, they need to be scaled, uploaded, and posts need to be written. I am slow at each of these steps, especially the writing, as I always try to write decently (and nevertheless fail miserably  ).

Please bear with me.

[Ero-Shan]

After the lengthy epilogue, let's start with the actual trouble-shooting.

First thing, of course, is to check the supply rails. While not dead on, all voltages are reasonably close to their nominal value. I try to set the +65 V exactly to 65.00 as the manual recommends, but the potentiometer isn't up to the task. No problem though, as now all rails are well within spec.

The next step according to the service manual is checking the high voltage, -1470 volts at TP1501.
Now I feel sorry for not having a HV probe. For a quick check, I use a 100 Meg resistor as probe. With 10 Meg input impedance, I'll get a 1:11 divider. I can see only a constantly changing, rather small voltage.



The +22 volts at the collector of Q1486 are OK, base is positive, however (the regulation obviously tries to get the voltages up without much success). There's about 2 volts peak-to-peak of AC at the collector.

bd139, you were referring to F1487, weren't you? It really is not blown.

There's not too much to this oscillator (I thought), what but a knackered Q1486 could it be? Instead of clearly marking the transistor's type in the schematic, you have to go back umpteen pages to find it's a ... 151-0140-00. A-ha. Next step: Get the Tektronix cross reference and look it up. It's an ordinary 2N3055.
I fetch a heat sink (more crawling in the attic) and attach a 2N3055 to it, unsolder the wires to the original one. While they're dangling in the wind, I might as well do a quick check with my transistor tester. It claims there are 2 diodes. Connect the 'new' transistor with some wires to the pads and power on: Just the same. No change whatsoever.
No more long hanging fruit, it seems.

I choose to repair this CRO in this decade and do the other things, not because it is easy, but because it is hard!

In a slightly desperate move I unsolder the cascade from pin 8 and CR1503 from pin 9 of the HV xformer. Here, of course, I make the usual blunder: while pulling CR1503 up, the feeble lead breaks right at the body of the diode.



Again I have to find out the hard way what it actually is, and finally find it in a VARO databook in the internet archive. It's a 12 kV rectifier. Bummer. I may have heaps of components, but HV stuff is markedly missing. And amazingly hard to find. My usual vendor (Reichelt) has diodes only up to 5 kV. For this, they should work, however.
With the open secondary I do a quick test wether this changed anything, which it didn't. So that is that until I get the new diode.

'It will get worse before it gets better.'

[Zucca]

Try here

hvstuff.com

when they will plug in the server again... 

[jkski]

subscribed

[bd139]

Bets off about the fuse then

Stuff a couple of 5kv ones in series if you actually need the full whack.

Can recommend hvstuff.com too. I bought caps for my D83 HT refurb from there and they arrived in 7 days even though they quoted 20-50 for "cheap ass mail"

[Ero-Shan]

I'll take a look at their site once they're back. Most likely I won't be able to pay. One fine day I will have to do something about this, but not now.

[David Hess]

I used a 464 long ago and did not particularly like it because of the image quality of the storage CRT.  I have a 7834 now which is much better but still not as good as a non-storage CRT.

David, How about the 466 CRT image quality?  Any improvement due to reduced scanning capable CRT?

I never used the 466 so could not say.  Reduced scan will definitely improve the sharpness though.  Isn't the 466 CRT just the 464 CRT with a reduced scan graticule?  I am not sure.

The 464 CRT or at least the one I used just did not even compare to the two different 7834s that I have used which were much sharper even without reduced scan.  That is not to say that the 464 was unusable, but I would not count on it for seeing fine detail.

It is possible that the 464 that I used was old or needed calibration.

The DM43 lacks the delta delayed sweep feature of the DM44 which most users will not miss anyway.

I only miss the service manual.

The only documentation I have is for the DM44 and I know I did a determined search for the DM40 and DM43 documentation.  You might have to get something from Artek Manuals.

[David Hess]

There's not too much to this oscillator (I thought), what but a knackered Q1486 could it be? Instead of clearly marking the transistor's type in the schematic, you have to go back umpteen pages to find it's a ... 151-0140-00. A-ha. Next step: Get the Tektronix cross reference and look it up. It's an ordinary 2N3055.

Tektronix used more than 3 different 2N3055s and graded some of them.  If you use the wrong one, then the high voltage oscillator may either not start or suffer from spurious oscillation.

The 151-0140-00 is the 0.3MHz low hfe variation.  The closest modern part is the 2N3772G.

Now I feel sorry for not having a HV probe. For a quick check, I use a 100 Meg resistor as probe. With 10 Meg input impedance, I'll get a 1:11 divider. I can see only a constantly changing, rather small voltage.

Checking the low voltages at the error amplifier is usually enough to tell what the inverter is trying to do and what the condition of the high voltage side is.

[Ero-Shan]

While waiting for the diode, I might as well build me a HV probe. An absolutely crappy one, to be sure. I do not intend to check voltages in the power grid. So there are no felines involved. The HV sections of CROs may generate a few kV of voltage, but no lethal current.

2 100 M resistors (totally unneccessary to split them; at this length they can easily take a few kV) and 2.53 M give, with a 10 M input impedance, a 1:100 divider.



The crappiest, ugliest, ridiculest HV probe you'll ever see, so look closely:



I might have found a better extension than that nose spray bottle, but I didn't want to waste more time searching.

Amazingly, that thing works rather nicely, accuracy seems to be better than 2 %!

The contraption must only work as a makeshift until I get a decent one (a colleague who's father had a TV repair shop to bring me his - when he finds it).

[Ero-Shan]

Diode arrived! (I expressly allowed Reichelt to give DHL my e-mail address so they could contact me before delivery, like they did all the times before. But they didn't. They seem to love leaving cards and let me fetch it at the post office. The very next day they did contact me (for a different packet): 'You're packet will arrive today'. Being at home, I was just eagerly awaiting it. Only to see the DHL van pass in front without stopping. Grrr.)

Needless to say, the new diode makes no difference (how could it?). But it feels good to at least have it.

I'm running out of ideas.

[bd139]

Firstly I would measure every voltage everywhere and mark up on schematic. Then look for anomalies.

[Zucca]

The crappiest, ugliest, ridiculest HV probe you'll ever see, so look closely.

At least not made with chinesium. For additional safety I would go ahead with this:

[Ero-Shan]

Firstly I would measure every voltage everywhere and mark up on schematic. Then look for anomalies.

I would call the missing -1470 volts an anomaly. And with all secondary voltages of T1501 missing, everything that depends on them will be anomalous, too. There's not much going into the HV generator. And what is there looks sound to me. With the HV missing, the regulator's throttle is open full.

Now comes the part where I really shine: making stupid decisions, being too lazy.

So I decided to isolate the HV generator completely by ripping out the transformer. It looked reasonably accessible after removing the fan motor. Off we go!
I try my desoldering station, solder sucker, solder wick. Everything. That damn thing just won't budge! About half an hour later I have it out, leaving an ugly mess behind. Not sure I'll ever be able to put it back in ...

So, at long last, it sits on my desk, looking rather innocent. Now I am able to check the oscillator in isolation. The few components are hooked up quickly. PSU supplies +22 volts and variable bias voltage.



At less than 3 volts bias it starts to run, and at around 5 volts the heater voltage gets close to 6.3 volts.

Voltage at collector of Q1486:



Frequency is a tad low (manual says 'about 50 kHz'), but I don't worry about 12 % in such a circuit.

I settle for a bit less than nominal voltages (5.4 volts heater, 11-10). With that I have 313 volts at 1-2.
The 'real' high voltages, however, I cannot measure with any confidence. Touching pin 8 of the transformer with a probe (connected to nothing!) capacitively loads the oscillator such that the ac voltage at the collector is roughly halved (and generates a faint 'bzzt' sound just before touching).
The best I get are 1.48 kV between 7 and 9, and 1.53 kV between 9 and 8. Between 7 and 8 there's also about 1.5 kV. These must obviously taken with a whole bucket of salt ...

Tektronix used more than 3 different 2N3055s and graded some of them.  If you use the wrong one, then the high voltage oscillator may either not start or suffer from spurious oscillation.

The 151-0140-00 is the 0.3MHz low hfe variation.  The closest modern part is the 2N3772G.

My Siemens 2N3055 seems to work all right. If they use a selected component in the high precision circuitry of a HP 3458A I'm all with them, but depending on it in a simple dc/dc converter doesn't get my blessings.

Checking the low voltages at the error amplifier is usually enough to tell what the inverter is trying to do and what the condition of the high voltage side is.

Sure. It tells me there is no high voltage. I still don't know why, however. I don't know how much my body's capacitive load was (and probe to fingers), but did it amount to more than, say 20 µA, of beam current?

Remember I isolated pins 8 and 9 already while it was still built in, and it looked totally overloaded. So I took a closer look at CR1512, C1512, CR1514 and C1514, but they all appear to be perfectly healthy. Judging from my external hookup, I'd say the transformer is also good, but with much less confidence.

I might have to partially marry it to the scope again.

[bd139]

Hmm interesting one. This is typical tek though.

If it’s not working in situ it’ll be in the feedback loop from the HV. On a couple of occasions, both 465b’s, I have broken that loop and hooked it to a bench supply to see if it had any kind of regulation. Turned out to be a resistor in one of the divider chains was out of spec so it stopped the whole oscillator dead.

“Working” is a very narrow margin between two posts of failure on these HT oscillators.

Edit: meant to say I like your HV probe. I bought a big Tenma “anal probe” out of fear in the end. I think yours is excellent for such low volume repair jobs. Maybe I need to cure the HV fear

[GeorgeOfTheJungle]

Have you got an IR camera? If so, turn it on and have a look at each and every tantalum first. If not, have a look at each and every tantalum first :-)

[bd139]

Good advice. I usually spend as much as I paid for the scope on the bloody things.

[GeorgeOfTheJungle]

[Ero-Shan]

Have you got an IR camera? If so, turn it on and have a look at each and every tantalum first. If not, have a look at each and every tantalum first :-)

It seems to me that tantalum capacitors are not held in the best esteem around here. 
I actually didn't have a single bad tant in many years. Shorted, leaky and lost-their-capacity electrolytics, yes.

When I resurrected that scope, I was actually thinking: Tek + not working = Tant. My approach to troubleshooting has always been: Don't assume too much, measure. Be systematic. Since all main supply rails are fine, there's certainly no shorted tantalum cap there. And there are no tants in the vicinity of the HV generator, either.

The tip with the IR cam is a good one (and I do currently have such a camera). On a day like today however, everything is hot.

[GeorgeOfTheJungle]

Mind you, I've only fixed one 465, and had no luck there with the "tantalums trick" either. But I've managed to fix quite a few MacMinis with this method: it's always the same tantalum (!) and you can see it very clearly with the Flir, as soon as you attempt to turn it on, it lights up very clearly on the screen.

[Ero-Shan]

That's a classic.

Wonder if it helped piling up all my components on a desk. 

Hmm interesting one. This is typical tek though.

If it’s not working in situ it’ll be in the feedback loop from the HV. On a couple of occasions, both 465b’s, I have broken that loop and hooked it to a bench supply to see if it had any kind of regulation. Turned out to be a resistor in one of the divider chains was out of spec so it stopped the whole oscillator dead.

Driving the feedback at the divider (R1525 C/D) should allow assessing the regulator. I've considered it healthy so far, as the missing HV lead to a positive bias voltage for the switching transistor (where the schematic says -3.8 volts). But I might have missed something obvious. Wouldn't be the first time. (Pulling the transformer out and still having to put it back in may be my well-deserved punishment, then. If it was so.)

“Working” is a very narrow margin between two posts of failure on these HT oscillators.

Edit: meant to say I like your HV probe. I bought a big Tenma “anal probe” out of fear in the end. I think yours is excellent for such low volume repair jobs. Maybe I need to cure the HV fear

My fear involving these 'weak' high voltages is more of inadvertently damaging things with them, or secondary damage caused by an unexpected jolt.

[GeorgeOfTheJungle]

My home made HV probe is just a long string of (IDK)MΩ resistors in series inside a heat shrink tube, and works well, helped me get the 465b back to life This 465b:

[David Hess]

Tektronix used more than 3 different 2N3055s and graded some of them.  If you use the wrong one, then the high voltage oscillator may either not start or suffer from spurious oscillation.

The 151-0140-00 is the 0.3MHz low hfe variation.  The closest modern part is the 2N3772G.

My Siemens 2N3055 seems to work all right. If they use a selected component in the high precision circuitry of a HP 3458A I'm all with them, but depending on it in a simple dc/dc converter doesn't get my blessings.

Saturation of the transformer is suppose to control the oscillation frequency but my understanding is that there are issues with LC resonance in the transformer which complicate things so the circuit can be picky about the output transistor gain and bandwidth.  Tektronix used this design for decades and finally updated it in the 455/465M oscilloscopes which use a fast high gain ring emitter type of transistor although I am not clear about what they changed to allow this without problems.

Checking the low voltages at the error amplifier is usually enough to tell what the inverter is trying to do and what the condition of the high voltage side is.

Sure. It tells me there is no high voltage. I still don't know why, however. I don't know how much my body's capacitive load was (and probe to fingers), but did it amount to more than, say 20 µA, of beam current?

Remember I isolated pins 8 and 9 already while it was still built in, and it looked totally overloaded. So I took a closer look at CR1512, C1512, CR1514 and C1514, but they all appear to be perfectly healthy. Judging from my external hookup, I'd say the transformer is also good, but with much less confidence.

The most common failure is a short in the high voltage multiplier.  Next is a shorted high voltage decoupling capacitor.  Unfortunately a low voltage test may not reveal either of these.

The high voltage multiplier can be tested simply by disconnecting either its power or ground inputs.  The CRT will work without its PDA (post deflection acceleration) voltage applied although it will be dim, fuzzy, and have about half of its deflection sensitivity.

I always get suspicious if a secondary side short does not blow the primary side fuse and one of the things I would definitely do while diagnosing an apparent short in the high voltage inverter is replace the fuse with or add a constant current limit.

[Ero-Shan]

The most common failure is a short in the high voltage multiplier.  Next is a shorted high voltage decoupling capacitor.  Unfortunately a low voltage test may not reveal either of these.

The high voltage multiplier can be tested simply by disconnecting either its power or ground inputs.  The CRT will work without its PDA (post deflection acceleration) voltage applied although it will be dim, fuzzy, and have about half of its deflection sensitivity.

I think I can preclude that. Thad had occurred to me as a likely cause, also. But disconnecting the HV multiplier and the cathode rectifier, leaving both pins 8 and 9 open, did not change anything.

I always get suspicious if a secondary side short does not blow the primary side fuse and one of the things I would definitely do while diagnosing an apparent short in the high voltage inverter is replace the fuse with or add a constant current limit.

That is a very good suggestion.

[Ero-Shan]

A quick update. I have now replaced my 2N3055 with Tek's original in the HV generator, and it works even better. Allows me to draw some 1 mm sparks from pin 8. Reminds me of the glory days when we drew several centimeters from the 25 kV of color TVs.

I think it is time to check the regulator. Maybe I shouldn't have ruled it out so quickly.

[Ero-Shan]

The regulator appears to be working just like it is supposed to, from what I understand.

So next I try to run the oscillator closer to its original environment. The only windings of the transformer used are primary and feedback (and an AC voltmeter at heater). The regulator is still driven by a PSU.
With expectations running high, I switch on the power. No oscillation. I vary the input to the regulator, which has a profound effect on the collector current of the driver: It's either in the vicinity of 0.45 amps or around 72 mA. But it does not oscillate.
Next I pull out Q1484. No change. I use the exact same 47 nF capacitor I used on the breadboard, a 5.6 k\$\Omega\$ resistor and drive it by my PSU, exactly like I did yesterday. The collector current stays around 72 mA (didn't measure explicitly yesterday, but the PSU showed 0.08 A), but it does not oscillate.
The remaining difference is the 22 volts supply, the inductor (I had only 56 µH), the 47 µF 'lytic. Added my 47 µF from the breadboard, just to make sure (I had had measured Tek's cap and it had shown good). No change.

This is ridiculous. The circuit is now exactly the same as yesterday, where I had rock solid oscillations. I better call it a day. 



With the original fan removed, I also had to create some extra wind - at the current temperatures it seemed very necessary to me. One more hindrance.

[David Hess]

If the collector current is low with no output then there has to be something wrong on the primary side; the error amplifier should be pushing the 2N3055 hard producing a high collector current.

[Ero-Shan]

If the collector current is low with no output then there has to be something wrong on the primary side; the error amplifier should be pushing the 2N3055 hard producing a high collector current.

It is. I just squinted when I saw it and drove the error amp down. Then I took it out of the loop completely and set the bias with the PSU through the 5.6 kΩ resistor. To no avail. It cannot be persuaded to oscillate any more. Tomorrow I will go back to square one and try it again on the breadboard - with the scope a few feet away.
There must be a reason for what I'm seeing. I just can't seem to find it.

[David Hess]

I hate to say it but it is not unknown for the high voltage inverter transformers in these to fail.  But your earlier test indicated that it was good.  Still, be careful about overdriving the transformer and damaging it.

[floobydust]

I say don't run the transformer without a load, unless you want to make a Tesla coil. It's too hard on the old insulation. A junkbox CCFL lamp makes a reasonable 1kV (bi-dir) zener plus a ballast resistor, for a dummy load.

We might be oversimplifying the DC-DC as a one-transistor oscillator. Would Tek really use that here?
How Tek tuned this resonant to get sine verses a noisy flyback converter, I'm not sure.

For this circuit, there might be AC (ripple) as part of the feedback signal which helps the oscillator.
So not a strictly DC error voltage going into Q1484. Any open capacitor like C1455, C1472, C1494 could cause problems too.

I'm just rambling because this blaming the transformer probably leads to a sad ending for the scope.

[David Hess]

We might be oversimplifying the DC-DC as a one-transistor oscillator. Would Tek really use that here?
How Tek tuned this resonant to get sine verses a noisy flyback converter, I'm not sure.

Tektronix used this high voltage inverter design from the beginning in their tube oscilloscopes up through the 400 series portables and the 7000 series mainframes that had linear power supplies.  Some of the 7000 series used a more well known two transistor push-pull variation.

For this circuit, there might be AC (ripple) as part of the feedback signal which helps the oscillator.
So not a strictly DC error voltage going into Q1484. Any open capacitor like C1455, C1472, C1494 could cause problems too.

AC feedback for sustaining the oscillation is exclusively through the transformer and not the error amplifier.  The oscillator operates at the resonate frequency of the transformer.

[floobydust]

I remember seeing a Tek service manual showing a shorted-turn on the HV DC-DC transformer to set the resonant frequency. I'll dig for that model number. You think that is the case here? I've never understood shorted turns, they always baffle me.

Point is OP might not be able to test his transformer here. Looking for crack in the core or a bigger airgap if the clip/adhesive moved, but an L meter on a shorted-turn transformer where we have no number as a reference, kind of a guessing game if a winding is at fault.
Could change gain with a different 2N3055, the part appears to be vanilla but antique specs of fT=0.3MHz

[David Hess]

I remember seeing a Tek service manual showing a shorted-turn on the HV DC-DC transformer to set the resonant frequency. I'll dig for that model number. You think that is the case here? I've never understood shorted turns, they always baffle me.

As far as I know the 400 series did not use any shorted turns on the high voltage transformer unlike the 500 (starting with the 535 or 545?) and 600 series tube oscilloscopes.

There are a few discussions about the purpose of those shorted turns in the TekScopes@groups.io archives.  One reason was to balance the flux through the E-cores that had asymmetrical windings for supplying heater power to the high voltage rectifiers.

[floobydust]

Ah yes, I found it- "flux balance winding" in the TM500 series (i.e. SC-502, SC-504). I'm mentioning it I thought it might be in a 464 as well, as the filament was later changed to DC power.
I think the only method left is to connect a signal generator to a winding and look at the waveforms.

Sphere has a couple 120-0909-01 transformers USD$65

[Ero-Shan]

Ah yes, I found it- "flux balance winding" in the TM500 series (i.e. SC-502, SC-504). I'm mentioning it I thought it might be in a 464 as well, as the filament was later changed to DC power.
I think the only method left is to connect a signal generator to a winding and look at the waveforms.

Sphere has a couple 120-0909-01 transformers USD$65

I'm not going to spend that amount of money on this scope that I never liked all that much. I have enough working scopes. And not just scopes. It's generally "either I can fix it cheap (mostly with components I already have) or forget it." There are exceptions, of course, like my HP3458A. Getting gear repaired is fun and satisfactory, and I'm willing to invest considerable time and energy.
But thanks anyway. If 464s all of a sudden become admired as the coolest scopes ever built, I might reconsider.

Had to change my plans for yesterday, soldering iron stayed cool (i.e. around 30°C these days). Before going back to square one, I will try one other thing: power the oscillator from my lab supply with 22 volts. I've even got new 12 kV rectifiers (2.89 a piece is OK, and they can be used for other scopes as well).

[Ero-Shan]

So I tried my interim step, not hoping much, not getting anything.

Back to square one it was. Everything exactly as on Friday. Except that the [expletive deleted] wouldn't oscillate any more. The collector current corresponds nicely with the bias voltage, showing a DC current gain of about 70. Have I somehow killed the transformer?

[David Hess]

Since you had the transformer out, I would try testing it with a function generator.

[floobydust]

I think if you ran it stuck at 0.45A for long time, it might have overheated the primary winding.

If you're sure the circuit has changed- you have the phasing correct still? Are you running it on the bench.
The feedback winding is biased at a negative voltage, so a lone 5k6 could be the problem for startup.

Transformers have several resonant frequencies and I'm surprised there's nothing to stop the leakage inductance from dominating.

[Ero-Shan]

I think if you ran it stuck at 0.45A for long time, it might have overheated the primary winding.

If you're sure the circuit has changed- you have the phasing correct still? Are you running it on the bench.
The feedback winding is biased at a negative voltage, so a lone 5k6 could be the problem for startup.

Transformers have several resonant frequencies and I'm surprised there's nothing to stop the leakage inductance from dominating.

The 0.45 A lasted all but a few seconds, then I pulled back.

I've checked the phasing several times, as that might have been the most likely blunder.
The 5.6 kΩ resistor's bias comes from a PSU, so I can easily modify it. Last week the damn thing started with robust oscillations at around 3 volts bias. Now even 6 volts don't do anything apart from increasing the DC current.

I will definitely check with a function generator also.

[Ero-Shan]

I have now driven the HV transformer with a signal generator. The resonance was at 44 kHz, and the ratio of all voltages was correct. Except the high voltage ones, which I couldn't measure - even my home-brew probe was too much of a load.

I also reversed the polarity of the feedback winding in the breadboard circuit, just to make sure. As expected, it didn't change anything. Guess I'll try with a different 2N3055 once more. All components seem to be OK, it just doesn't work.

[floobydust]

How frustrating, I would be tempted to increase gain, try a higher hFE transistor or Darlington for the experiment. Or a few 10's pF C-B if leakage inductance is a problem. I would just mess around and try things to get it to start.

Grasping at straws, I thought the 2N3055 might see avalanche if the oscillator ran into a heavy load.
I recall older power BJT's (but epitaxial vs homotaxial) could take more abuse, spikes way over the VCEO 60V rating.

In your breadboard circuit, C1483 0.05uF I think important for crisp switching and it looks like Q1484 runs about 1mA plus R1483, for bias, that looks like a couple mA.

The only other straw I have is the secondary load might be part of the resonant tuning.

[Ero-Shan]

How frustrating, I would be tempted to increase gain, try a higher hFE transistor or Darlington for the experiment. Or a few 10's pF C-B if leakage inductance is a problem. I would just mess around and try things to get it to start.

Yeah, that Darlington idea occurred to me also.

Grasping at straws, I thought the 2N3055 might see avalanche if the oscillator ran into a heavy load.
I recall older power BJT's (but epitaxial vs homotaxial) could take more abuse, spikes way over the VCEO 60V rating.

In your breadboard circuit, C1483 0.05uF I think important for crisp switching and it looks like Q1484 runs about 1mA plus R1483, for bias, that looks like a couple mA.

The only other straw I have is the secondary load might be part of the resonant tuning.

The Q1484 contribution is simulated by varying the bias for R1483.

The 44 kHz resonance is quite pronounced. (Driven by generator.) When it worked (on the breadboard all those weeks ago) it oscillated at about that frequency, though I didn't measure it explicitly.

Next weekend I'll try these things and report about the results.

Thanks for the suggestions.

[Ero-Shan]

"Next weekend" I didn't do what I wanted to. Of course that other project couldn't be finished in time ... But it is mostly completed now, so I was able to clean up my bench today and get started.

I do owe you the report (though probably no one's reading this thread anymore), so here it is.

1. Tek (RCA) transistor: as before, about 70 mA DC collector current.
2. Small cap between C and B: no change at all.
3. Siemens 2N3055: 44.3 kHz oscillation, needs more base bias for correct amplitude, about 80 mA collector current.
4. Darlington (BC107B, too much gain): hefty oscillation already at small bias, transformer makes high pitched noise, collector current around 1 Amp! I aborted this after a few seconds.
5. Back to original Tek transistor: stable 44.3 kHz oscillation, about 80 mA collector current. Transformer is slightly audible.

All these tests were done with a breadboard (hardly needed for the 5 odd components) and no transformer loading at all.

The whole setup was not mechanically sensitive, so I'm quite sure I had no bad connection. Knocking the transformer also had no influence.

I've seen my share of strange things in during the years, but what is this supposed to be? A part time transistor?

Is it time for throwing the towel?

[bd139]

Still reading

This shit happens to me all the time. Sometimes the act of taking it to bits and putting it back together fixes the problem and you will never know what it was.

Got the same thing with a Telequipment scope now. Oscillator works and doesn't drop out now after replacing ancient capacitors but the intensity control is now shot 

[Ero-Shan]

There's a time when you have to face the truth.

After having spent the better part of a day checking with a great variety of transistors to get that HV circuit to work - not a single one could be made to oscillate - I declare defeat.

I will just claim (without the slightest bit of proof) the transformer as being knackered.

As I explained in the initial posting, I never really liked this scope. But I did want to get it working again. And I feel terrible for giving up. However, I'm not throwing it out yet. Maybe in a few years I'll have a fresh look.

Thanks all for your good advice and bearing with me!

[bd139]

Worst case you can sell the tunnel diodes and have enough cash for a compensatory meal out 

(I did that when I was defeated by a 453)

Have also given up on the TQ scope I mentioned earlier.

[Ero-Shan]

Sell them? You're kidding me! 

If I'll ever gonna scrap it, I will pull out all interesting parts and keep them! You never know when you might need 'em. 

With a repair queue as long as mine, it would be plain stupid to stay stuck forever on a hopeless case. The time invested must be bounded somehow.

Let's pick some lower hanging fruit! 

[bd139]

Very sensible actually

[GeorgeOfTheJungle]

Put it on ebuy as "untested" :-)

[bd139]

Hahaha 

I actually sold a 465 parts mule on eBay as “untested. Doesn’t work. Will never work again however much you want it to”. Still got £30 for it