Tektronix R556 Dual Beam Scope

[Cubdriver]

I'd mentioned the inbound 556 in Mike's 547 transformer thread, but apparently never posted the pics when it arrived in early September. Unfortunately, despite double boxing it did not arrive unscathed - took a good solid hit that bent the bottom left corner of the faceplate pretty severely, also bowing out the bottom edge of the left plug-in frame opening. Other than that (and being missing the CRT bezel; I have a replacement for that but need to find a set of nuts for it), it's in very nice physical condition.







Corner manglement detail







Finally got it onto the table last week, and started doing some preliminary body & fender work on it, straightening the bent corner partially with an adjustable wrench and gently tapping the plug-in bezel mostly back into shape, at least to the point a plug-in will seat fully. Ultimately I expect the front panel will be coming off for a proper flattening, but I suspect that will be a major undertaking.








The guy I bought it from stated that he'd been using it some time ago, then heard a 'pop' and the beam went away. He then put it aside, and eventually sold it. Got ambitions this afternoon and connected it to a variac and voltage/current metering box I have and brought it up tonight, fully expecting a HV failure and no emission from the CRT. Not quote - the HV isn't dead; instead I get both beams just fine, they just don't sweep and the brightness adjustment has no effect - they're trying tp burn through the face of the tube. Horizontal position controls move them partially on to the screen, and the vertical position adjustments on the plug-ins move them at least somewhat vertically, but I only kept it on long enough to determine this and to snap a few photos before shutting down to prevent damage to the phosphor.





More later as I find time to work on it. I did get a hard copy of the manual, so that'll be a big help. First order of business will be actual power supply voltage checks.

-Pat

[Specmaster]

Hmm, thats an interesting terminology there, I don't think I've ever seen lower and upper beam used to describe the two channels

[Cubdriver]

Hmm, thats an interesting terminology there, I don't think I've ever seen lower and upper beam used to describe the two channels

That’s because it’s literally a dual beam scope - the CRT neck is about the diameter of a soup can, and it has two separate gun/deflection plate assemblies in it. There actually are upper and lower guns. Two scopes in one CRT - horizontal scan rates for each can be totally different.

The 500 series dual beams that I know of are the 551, 555, 556, and 565. The 551 has dual beams with separate vertical deflection plates, but both guns share a single set of horizontal plates; the others have completely independent guns, each with full sets of deflection plates.

You can see that the shield barely tapers as the neck is so big:


I think there was a dual beam in the 5000 series mainframes, and the 7844 became the high end dual beam when it came out, though initially it wasn’t fast enough and apparently the 556 remained in production into the mid 70s until the 7844 was modified and caught up.

I’ll try to get some CRT neck pics, but will be limited as it’s in a shield.

-Pat

<edit to add picture>

[Zenith]

It must have been sickening to have it arrive damaged like that, especially when it sounds as if the seller had taken some trouble to pack it properly.

There must be techniques for repairing that sort of damage, used for aluminium bodied cars and aircraft. I'd be inclined to press it out with a vice. There and again, panel beaters use hammers.

That's a lot of oscilloscope.

[Zenith]

Hmm, thats an interesting terminology there, I don't think I've ever seen lower and upper beam used to describe the two channels

It was common with true dual beam scopes with plugins. The Solartron CD1400 also used "upper" and "lower". The two beams had different focus and brilliance controls as well. It wouldn't be useful to label the plugins specifically, when they might be put in either position or put in another scope.

Modern dual trace scopes are single beam and switch between the two traces, either on alternate sweeps, or in chop mode where they switch many times in the course of a sweep.

[Specmaster]

Hmm, thats an interesting terminology there, I don't think I've ever seen lower and upper beam used to describe the two channels

That’s because it’s literally a dual beam scope - the CRT neck is about the diameter of a soup can, and it has two separate gun/deflection plate assemblies in it. There actually are upper and lower guns. Two scopes in one CRT - horizontal scan rates for each can be totally different.

The 500 series dual beams that I know of are the 551, 555, 556, and 565. The 551 has dual beams with separate vertical deflection plates, but both guns share a single set of horizontal plates; the others have completely independent guns, each with full sets of deflection plates.

You can see that the shield barely tapers as the neck is so big:


I think there was a dual beam in the 5000 series mainframes, and the 7844 became the high end dual beam when it came out, though initially it wasn’t fast enough and apparently the 556 remained in production into the mid 70s until the 7844 was modified and caught up.

I’ll try to get some CRT neck pics, but will be limited as it’s in a shield.

-Pat

<edit to add picture>

Nice, I used to have a dual beam scope years ago before I gave up electronics, when I got married, never had the space any longer.

[MED6753]

Pat, is there anyway you can get a C clamp positioned to do the final straightening? Pulling that front panel would be a nightmare.

[Cubdriver]

It was painful to open the box and see the corner for sure. And I’d also like to avoid having to pull the panel if possible. Thinking maybe I’ll combine the vise suggestion with Mike's C-clamp idea and at some point use a couple of clamps and some padded steel blocks to press things flat - figure without disassembly any vise I can get into position is going to be too small and weak to exert enough force, and a clamp wouldn’t be big enough to get a nice reference to the rest of the panel.

Figure I already pressed my luck far enough gently tapping the plug-in frame back into shape - don’t want to bugger up the CRT internals with shock, and don’t want to pull it unnecessarily either, so actual hammer and dolly work is pretty much out.

-Pat

[Zenith]

I think you'd need to pad a vice with wood, which makes access in place even harder. I'm not sure how well the vice would work with metal that's been stretched. Hammer and dolly work with the panel on the scope, is completely impractical. Removing the panel and replacing it is a big job.

Maybe you can get it into good enough shape to accept the plugins, with what you can do with the panel on the scope, and leave the final sorting out as a phase 2 development.

[Cubdriver]

That’s the current plan. I got the area surrounding the plug in compartment flat enough with very gentle tapping the other day, and plan to leave it in that state until everything’s operational again, then decide how much further to go. Shame it got damaged as it was otherwise in excellent condition.

-Pat

[Cubdriver]

Put in some more time on the 556 today - started checking and attempting to reform the main filter cans. Conveniently, there is a power supply board at the bottom of the chassis that acts a a junction/distribution center for the power supply. It carries the rectifiers and a few other components, and connects the main power transformer, regulators, and filter caps, then distributes the regulated voltages out to the instrument:



The power transformer connections are on the left and top edges of the board, the regulator connections along the bottom, the filter caps in the vertical row of terminals at the center, and instrument voltage distribution at right:


Reforming filter caps:






Knock on wood, thus far the first three have reformed down to microamps of leakage.

-Pat

[MED6753]

You're brave Pat. I wouldn't trust those reformed capacitors as far as I could throw them.

[Cubdriver]

Seeing as I’d have to remove that circuit board with its gazillion connections to even access them, I’ll give reformation a shot. Knocked off another this morning; 300 uF @ 250 V; it very quickly dropped to under 10 uA of leakage. Think it’s the last of the bulk filters, but need to pore over the schematic a bit more yet.

-Pat

[Cubdriver]

Dug a little bit further into the scope - the initial issue that I find is that the 225 V rail is not present. Thus far I've only done some cursory tests of the associated power supply semiconductors with a B&K 520B transistor tester and all appear to be at least transistoring. (Initially thought the pass transistor was high leakage, then looked more closely and realized that it's shunted C-E by a pair of paralleled 260 Ω resistors that are with it on the heat sink and thus still present even when it's disconnected from the distribution board.) Unfortunately fall cleanup and house tasks have been occupying me, so I haven't gotten back to it. Need to determine if it's something shorting the 225 V rail in the scope itself, or if the issue is in the power supply. The previous owner did mention that there was a 'snap' sound when it died, so I suspect the issue may lie within the chassis (and may also have taken out the supply). Need to stare at the schematic for a bit to suss out how it works.

-Pat

[Zenith]

Depending on how many mA the 225 V line supplies (it could be a lot), a possibility that occurs, is to disconnect the 225 Volt line from the PSU and supply it the proper way, with an HV power supply, or with a bodge based on a transformer, rectifiers, a capacitor and a variac. An ammeter to head off trouble at the pass is a good idea.

Years back on a forum, someone was talking about a 32" TV he'd found thrown away, so it was almost certainly a lost cause. He said that there was a 184 V line that was absent. I suggested he make up a bodge to supply it with 184V. He did and the TV worked, much to his surprise. So clearly it was the 184 V supply itself that was at fault. At lease he had an idea of where to look. This is a bit different, because it's not a throwaway project and the 225 V line supply may require 150mA plus.

Look on the bright side. At least it isn't an SMPS.

[Cubdriver]

I still haven't done more digging into the scope yet, and while feeding it with an external supply is something that's occurred to me, I'm not yet ready to go down that path - like many Tek scopes, this one uses stacked power supplies - the +225 V supply sits atop the +100 V supply, and the +350 V supply is atop the +225 V one.

Voltages jump around a bit during startup (I'm working on getting some of my other meters that have recorder outputs operational in order to get measurements of everything simultaneously and feed the outputs into a digital scope so I can watch how they interact). I've also been reluctant to leave it on for too long given the intensity of the beams in the CRT (directed off the screen, but still...) and the fact that I don't want to fry anything further. If nothing else, it's got me working on fixing the HP 3430A DVMs that were previously languishing in the repair queue.

-Pat

[Zenith]

I really don't like those power supplies, popular until the mid 70s, that had everything referenced to one supply. I don't recall that Zener diodes were a rare and expensive part back then. In some series of scopes they did that, then used three pin regulators, then switched back, and eventually returned to three pin regulators.

Does this use a Zener or a gas filled regulator? When the gas filled ones have a lot of hours on them, they can behave very strangely. But there are other suspects, such as carbon resistors which normally go high, but can show strange behaviour until they are properly warmed up.

[mansaxel]

I really don't like those power supplies, popular until the mid 70s, that had everything referenced to one supply.

The 5440 scopes, while newer, also have this trick. Everything depends on the -30V supply. If it chooches not, no work is performed. And, it's little tantal had fried into a carbonized short, of course.

[mnementh]

...Corner manglement detail



-Pat

FWIW... having spent many, many hours ironing out tin on all manner of consumer electronics and automobiles and machinery... you're never gonna get it "not wavy" without removing it and hammering the eff out of it. Also, the only way you're going to get it flat without dinging the hell out of the face will be to hammer from the back with the front laid against a piece of wood.

If you cover the front with Kapton or masking tape, then lay it flat on a clean thick plywood... and hammer on the back only using a hardwood or metal spoon... you can def get it very close to right, even flattening out the smooshed corners to a considerable extent.

But first you have to suck it up and remove the panel. *cringe*

mnem
good luck, old friend.

[MED6753]

Pat, I have used an external power supply to troubleshoot +100V issues in a Type 547 and it proved very successful.