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]

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[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 :-)