Tektronix 475A Display problem [Horizontal?]

[MattyMatt]

Hello everyone,

A little history:
I have been looking into purchasing a scope now for a while... not only could I use it for my audio ventures (making effects petals for my own personal amusement and education) but I am planning on repairing some of the things I have around my house (Old HAM rigs, possibly help calibrate a projection TV...etc.). Due to my lack of knowledge, I called upon a few friends for advise, and read many posts on forums. I found a great deal on a Tektronix 475A and with many blessings from quite a few around me, purchased it. It was an ebay purchase, with a fairly reputable person, the limited pics did show a few of the little flaws (some writing/a broken VAR knob, no biggy). The screen looked decent, the display a little fuzzy, but I figured that was probably someone not taking the time to adjust the focus knob.

Anywho, I got the scope today, via UPS (good old universal parcel stompers). The box was in good condition, fragile label and all (as well as my special delivery instructions on the box). The seller managed to pack the scope VERY well (which always seems to be a problem) and once i let it warm up to the air in my house, I turned it on, hoping to see a nice trace go across the screen.

It did turn on, but sadly not the fuzzy trace I saw in the ebay pics. so I checked out the ebay pics, set the scope up the way it was photographed, and still have the same picture. So I figure, that UPS knocked something loose in there (wouldn't be the first time they screwed something up for me) . So I crack the case on the scope (and I see the cause of the broken VAR control immediately (little coupler that I can purchase... glued back together for now), however, I see nothing obvious to see why the screen looks like the way it does (see pictures).

I will be honest. I do NOT know a lot about scopes, but I have no problem jumping in, as long as I am careful. I currently DO have the service manual for the scope, but it really didn't help me very much, as much of the language in there is not for someone of my limited electronics vocabulary. I probably will have someone else look at this with me, and I may be able to have it looked at professionally... but I am on a budget... I paid $150 for the scope, and I still gotta look around for some decent probes to use with it.

Sorry I can't really describe this better, but I figure a picture or two is worth a 1000 words . From what I gather, the trace is going off the screen, and bouncing around in the CRT [horizontal deflection?]. Either way, I took a shot with the beam finder on, with the scope registering that there is a trigger, un-triggered, a shot of the little coupler and associated shaft/knob combo I mentioned, and basically how clean the boards are in this piece of equipment (looks like it was treated pretty well too!).

Any advice on to properly proceed, or where in the service manual I can look for adjustments, or if I should just get rid of it (I really don't want to... I like the look and it has probably more than I will use for the majority of my little projects, and from what I hear, these are pretty much one of the nicer analog scopes for their day, and... from what I have seen/felt... its built like a tank!)

Thanks again, and if you need any specific pictures with certain settings, let me know!

 

[shrtrnd]

It's tough to diagnose these problems without putting your hands on it.
Yeah, you've got a horizontal problem. Unless you've got a problem with the 'position' knob for left/right centering of the trace.
You've got a blooming problem, either intensity is too high, or focus circuit problem.
Frankly, if you don't know a lot about scopes, somebody has to dig deeper into the scope to see what else might be wrong.
It'd cost you some bucks for a service guy to check it.
It sounds harsh, but what did the seller say about it's condition when you bought it?
If he claimed it was fully functional, you obviously didn't get it that way, and you are
entitled to your money back. If he won't cough-up the money, ebay has a policy where
you can say the thing was damaged in shipment, (which you said you suspect), and
their policy is, that you can get the full cost of your purchase back.
This does not look like a simple fix, and you'll pay as much to get the thing working,
as you did to buy it.
That's my 2-cents worth, good luck with whatever you do.

 

[davenn]

Hi

I have a 465B very similar looking, that era of Tek scopes had plug in transistors
the legs slide into a conductive foam. check to make sure none of them have 1/2 fallen out. That age of scope its possible that some / many of the electro caps have/ are failing. Many of the suspect caps are small tantalum type. on my scope several had failed dramatically, totally toasted. others you wouldnt tell just looking at them.

first thing to do would be to check the power rails in the 465B the 5 main rails all have testpoints in a close group. And if the 475A is like the 465B there's the 155V rail, that if not correct then none of the others will be. ( may be a different voltage in yours.
The Tek service manuals are very good with great flow charts for working your way through PSU and other section faults.

Dave

 

[davenn]

ok I managed to download a service manual

the main voltage rails to check are ....

+110, +50, +15, +5, -15, -8

ALL the lower voltage + and - rails rely on the +50V rail being correct else they wont regulate

cheers
Dave

 

[MattyMatt]

Thanks for the good info gents, sorry about the delay in my post... had some other issues that needed to get worked out, at any rate, within the next couple of days I'll go though the scope and get back to you on how things are going. All of the caps that I saw looked decent but I'll have some more answers on that once I get done with the rails.

I have also managed to find a "Troubleshooting your scope" document put out by tek., that seems to go to the same conclusion as I do (horizontal).... I can make some adjustments to change the display a bit, mostly with the time/div control and triggering, I can't seem to get the display to move vertically though, so I can't fully eliminate that section, but I want to see if I can get SOMETHING like a trace first....


Shrtrnd (nice name btw.... indiana jones ref?), I was able to clean the focus up, the knob is REALLY hard to turn.... its inside the POT, so i will have to figure out something for that. but I can make it look a little better.

Davenn, I'll be sure to check with a couple different MM's. I have both analog and digi, I'll double check my service manual for exact voltages on my unit, but did yours give any kind of tolerances?

Thanks Again
Matty-

 

[davenn]

yeah
+ - ~ 3V on the 110V rail and less than + - 0.5 V on the other rails quite tight

Dave

 

[MattyMatt]

Okay, just ran through all of the power supply test points, pointed out by my service manual... the 50V line was at 49.6, so I adjusted it back up to 50. I double checked it with both of my meters (analog and digital) and both meters agreed it was a little low.... double checked after my work was done, and all appears well now with the voltages.

Still having the same overall problem though, so I am moving on to see what I can do myself, with the little equipment I have.

I have still not heard anything from the person I know who can do this kind of stuff, so I will wait until I hear anything from him. I am also hoping he'll let me hang around and watch him do it (if he can) so I know what went wrong.

In the mean time thanks again!
I'll keep ya posted with updates

MattyMatt-

 

[MattyMatt]

Gonna give this one a little bump, I just got myself into the yahoo Tektronix group... WOW lots of info.

I posted there and linked to this post, and almost immediately got a response... I was advised to check the -8V supply line for ripple, suspecting one of the caps had gone... which I figure what was basically what you were starting to get at there Davenn. Using the service manual I have, looking at my block diagram, I don't see the discribed filter caps.... or is that something that will not be on the diagram?

(for info I can link to the manual for the scope, the block diagram is on 3-17 (figure 3-8)

So, quick question, I don't want to destroy anything or harm myself here. I have another scope (that I am borrowing from a friend) and I am planning on hooking this up to my scope. However, I want to make sure I am not going to damage myself or my equipment before I do so, here is what I plan to do:

I am going to power both scopes up for about 20 mins or so, while mine is apart, I am then going to find a ground to attach the probe to (which i believe there is a pin for that right around the testing area). Then I am going to clip the lead onto the -8V test point, and see if there is any ripple in the AC waveform.

Does this sound correct?

So if/when I do find a ripple, where should I be looking for the cap? on the power supply board or closer to where the test points are? I will be following the traces either way, but considering the block diagram makes no mention of the caps... I could use a little advice. I can take pictures if that would be helpful.

Thanks again
Matty-

P.S. I was also advised that I can test a cap with an ohmmeter... can someone explain this one to me? I have both a digital and analog meter, both in wonderful condition.... thanks!

P.P.S. Apparently I suck at life or at least researching properly... I found in a later section of the service manual where the diagram is more compete. I am attaching a picture of the -8V line I believe we are referring to C1468 can someone confirm for me?

Okay I'm done editing now! thanks again

 

[davenn]

hey Matt

I posted there and linked to this post, and almost immediately got a response... I was advised to check the -8V supply line for ripple, suspecting one of the caps had gone... which I figure what was basically what you were starting to get at there Davenn. Using the service manual I have, looking at my block diagram, I don't see the discribed filter caps.... or is that something that will not be on the diagram?
(for info I can link to the manual for the scope, the block diagram is on 3-17 (figure 3-8)

Yes indeed you need to see how good/bad the regulation is. As a ballpark figure I would expect good regulation would have ripple less than 200mV. 500mV - 1V would definately be bad.

So, quick question, I don't want to destroy anything or harm myself here. I have another scope (that I am borrowing from a friend) and I am planning on hooking this up to my scope. However, I want to make sure I am not going to damage myself or my equipment before I do so, here is what I plan to do:
I am going to power both scopes up for about 20 mins or so, while mine is apart, I am then going to find a ground to attach the probe to (which i believe there is a pin for that right around the testing area). Then I am going to clip the lead onto the -8V test point, and see if there is any ripple in the AC waveform.
Does this sound correct?

sounds good

So if/when I do find a ripple, where should I be looking for the cap? on the power supply board or closer to where the test points are? I will be following the traces either way, but considering the block diagram makes no mention of the caps... I could use a little advice. I can take pictures if that would be helpful.
Thanks again
Matty-

P.S. I was also advised that I can test a cap with an ohmmeter... can someone explain this one to me? I have both a digital and analog meter, both in wonderful condition.... thanks!

umm putting an Ohmmeter across a charged cap may not be good for the meter
personally not something I would do.

P.P.S. Apparently I suck at life or at least researching properly... I found in a later section of the service manual where the diagram is more compete. I am attaching a picture of the -8V line I believe we are referring to C1468 can someone confirm for me?

ok you have 2 filter caps in that circuit the one you mentioned the C1468, 33uF, and the bigger one ( the main filter cap) C1462, 5500uF.
If its as similar to the Tek 465B that I have as it seems to be, then the larger filter caps are a nightmare to remove and replace and should only be done is really necessary.

virtually ALL other electros and tantalums should be replaced as a matter of course. You have to remember they are 30 odd yrs old and have been in a relatively warm enviroment, definately well past their use by date

cheers
Dave

 

[MattyMatt]

Thanks again Dave!

I was kind of wondering about the ohmmeter thing... probably fine while its out of the circuit, but it made my eyebrow raise a tad....

I will probably be posting an update in a little while, after prepping my work area and warm up the scopes. I'll let ya know what I find... if not post a pic or 2.

By the way, other than an ESR meter (and only a few from what I understand), is there a good way to test caps in the circuit (without a scope), I have only RARELY heard of this discussed, with any certainty. I'm a little wary of doing so, but it would be nice to know for the future?

Posting again soon
Matty-

 

[MattyMatt]

Okay, so here's my update I went through after warming everything up for about 20 mins or so. Set my buddy's scope up with the following settings:

1V/DIV (I figured I could adjust as necessary later)
5ms/DIV
1X (not zoomed in)
focus and intensity as required

I went strait for the -8V line as recommended... but saw nothing... I got a trace, but it was dead flat (a nice surprise), I checked by adjusting the volts/Div so see if there was any ripple that I just wasn't seeing.... none, at all.

So then I started working my way down from the 110V line, checked all test points to find the same thing... dead flat. Next on to the 50V, same thing, dead flat on all connections. This goes on until I hit the 15V line.... that got interesting, pretty much what we expected to see, ripple... so like any other good technician, I checked all 15V Test points and all other voltage test points, and the +15V was the only one with any ripple. so it appears to be isolated to this one area of the circuit, the pictures are my result... note the settings above.

 

[MattyMatt]

Okay, I wanted to post more updates, I think I have successfully located the bad cap(s)
I am not sure if C1442 is bad, perhaps someone can suggest a test point for me... I have attached a circuit diagram of the +15V supply and pictures of the areas in question.

I tried to center C1448 to the middle of both shots to make things easier, and C1442 I believe to be opposite of my current view... I have not taken the scope apart yet, that will come soon enough, I would like to locate a replacement before I go disassembling all willy-nilly!

the area where I think C1442 is in the 4th image, is in the upper right. The overview gives a better over all view, C1448 in the lower left, C1442 more in the lower middle.

Matty-

 

[davenn]

yeah 1442 is a big can electro mounted on the other side of the board there are several side by side

 

[MattyMatt]

Okay, so I'm going to be getting ambitious soon, I am going to be heading down to my local high quality electronics store... and no, I'm not talking about The Shack. I'm not knocking them on their stuff... I worked there for 10 years, had a great time, I just don't think they have ANYWHERE near the quality of small components that they used to... but there is another place close to me that does have NICE stuff.... granted, the price is high, but worth it.

Anyway, I'm going to be grabbing the small capacitor, the big one I will have to order in. The service manual states the following for the part: (the part number is the 290...)
C1448 290-0536-00 10 uF, Elect., 25V 20%

Is there anything I should know about the specific capacitor? I know it doesn't have to necessarily look the same, but I'm not sure if its polarized or not. I'm not claiming to know a lot about components here, but I also don't want to look like an idiot when I walk into this place .

Thanks in advance.
Matty-

Oh, P.S. I "found" the big capacitors... I'm a dumbass I don't actually have to take the unit apart any further than I have it now... I didn't realize that they would be as big as they are... then I looked at the unit of measurement .

 

[(*steve*)]

Have a look at the capacitor. The specs you have are minimal (yes, it will be polarised).

Is it a tantalum, or aluminium electrolytic? If Aluminium, does it have any temperature ratings (85C or 105C)?

Which capacitor is it? Ah, I see it. But I can't find it in the photos of the board. There do seem to be a large number of dipped tantalum caps though.

 

[MattyMatt]

Its the darker blue one to the left of the ceramic disc capacitor, in the 3rd of 5 shots (Titled C1448-Side.jpg). looks like dipped tantalum... not 100% though

 

[davenn]

Its the darker blue one to the left of the ceramic disc capacitor, in the 3rd of 5 shots (Titled C1448-Side.jpg). looks like dipped tantalum... not 100% though

yeah looks like a typical tant as does th one on the other side of the disc ceramic.
replace them both with low ESR hi temp electros and they will be good for another 15 yrs

Dave

 

[(*steve*)]

Yeah, looks like a dipped tantalum. Most are yellow, but that's just the colour of the epoxy.

 

[MattyMatt]

Update:

Okay, So I have been pretty busy lately... and I have another project in the works at the moment that I am trying to see if its recoverable... but thats a story for another post.

At any rate, so I have ordered the BIG cap and a few little doodads... (couplers) from a Tek part supplier, however, they are out of the small caps... so I'll be sourcing those from someplace local perhaps. My order should be in soon, and I'll be picking up the other parts soon enough.

I was talking with some of my colleagues, had them take a look at the scope output and my readings, and it was suggested that it could also be the bridge rectifier as well (big black 4 pinned box?) part number CR1442 on the board.

Would this also give some of those symptoms... from what I gather the output looks very similar to what I was getting, but I'm honestly not sure.

Could I get some opinions on that, and how to test it, I do still have another scope at my disposal and my 2 Meters.

Thanks
Matty-

 

[MattyMatt]

More updates

Okay, so I have replaced the C1448 capacitor (dipped tant.) with one of equal specifications, and the problem has not gone away.

I have also found another unexplained test point... with a strange, half-assed saw-tooth.... which frankly I'm still trying to figure out where in the circuit this unmarked test point actually is.

I attached the pictures the red lead on the left is on the 15V line, the gray lead on the right is the unmarked test point, and the pictures are marked left and right with that reference.

so... still waiting on some other parts... I did manage to source all the CAPS, most of the resistors, and the bridge rect. on the 15V+ line, I'm looking for a little more advice before I go looking for the Op Amps and such.

Any advise is appreciated. and thanks again
Matty-


EDIT:
I think the unmarked test point, based on some friend input, and some coffee, is the 15V unregulated line... which would explain a few things.