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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Sat 20, 2014 2:22 am 
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Previously mentioned symptoms sounded like ripple on the +12 supply line, not so much dead, or un-regulated DC voltage.
But if the multimeter (analog?) showed a reduction from the DC +12 volts to something lower, it certainly may be out of regulation.

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Sat 20, 2014 2:45 am 
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PaulAm wrote:
If you get a can of duster spray from an office supply store, you can turn it upside down and use it as a freeze spray.

It would be more effective to use a can of component cooler (aka freeze mist), sold at Radio Shack.

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 30, 2014 2:54 am 
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Just got a bunch of replacement transistors, so I'm going to start digging into the power supply. I have fully checked out all other functions including the delayed sweep and everything does work until the bigger issue occurs. One thing that I did notice is that when I first power up, the traces are not stable, in other words they slowly rise upwards on the screen for a good 15 seconds or so before stabilizing. This is the oldest scope that I've ever used and I'm not sure if this is just an artifact of nuvistors warming up or if this is a sign of something else wrong. Stable as can be after that initial warm up time.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 30, 2014 4:17 am 
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Since you indicated you were unfamiliar with scope repair, you might download this Tektronix TS guide and give it a read. Makes any analog scope very understandable and easy to trouble shoot.

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 30, 2014 3:12 pm 
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I'll probably be lambasted for this for posting it again, but regardless I will reiterate: Replace all of the electrolytics in the power supply and you'll have it fixed.

You can spend time trying to find the worst offender(s) and replace only those, but all of them have been exposed to the same ravages of time and heat. IMHO, the only way to restore true reliability is to replace them all. I don't want the scope coming back in my door later for another repair so that is my procedure every time.

Speaking of heat, my flame suit is now on! :)


Last edited by PhilF on Sep Tue 30, 2014 5:49 pm, edited 1 time in total.

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 30, 2014 5:36 pm 
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Sure sounds like a high ESR filter cap.

Pete

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sat 01, 2014 2:09 pm 
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Finally got a chance to look into this some more. I decided to focus on the transformer this morning. I have every multimeter I could round up (5) connected to this thing (all digital, still waiting on my new set of probes for the 260s). All of the AC measurements are with various True RMS Flukes (87, 179, 27/FM) with the exception of one that I have an 83 on, but it's measuring about what it should and I'm watching for changes I think more than anything. Anyway, in addition I'm monitoring the +12v supply to watch for when the voltage drops. I also checked the +75V, -12V, and +150V and they look fine and pretty much unchanged by the "event" other than the +150 dropped by about 0.5V to 1V give or take. I'm going to show my measurements when good and when bad between the various points on the transformer (see schematic).

Good Bad DMM
+12V +12.06Vdc +10.87Vdc Craftsman
9-10 91.0 Vrms 88.4 Vrms Fluke 179
11-12 6.11 Vrms 5.94 Vrms Fluke 83iii
13-14 16.64 Vrms 16.12 Vrms Fluke 87V
15-16 17.22 Vrms 13.09 Vrms Fluke 27/FM

Some are off from what the schematic says, but my line voltage is like +123Vrms so I'm thinking that may be biasing these a bit high?Ports 15-16 which corresponds to the +12V supply seems to drop off steadily until it gets to a point where the +12Vdc drops to about 10V and scope goes wonky. There's a steady trend down in all of the voltages over time, but not like the port 15-16 measurement. Until it drops out, all functions on the scope are fine and it's very stable.

I'm not sure what to make of this yet, although I don't think its a transformer problem. I did swap Q1154 since I had a replacement handy, no change. I'm thinning this is transistor related, but I need to dig in more. I guess this could still be one of the big caps, but I'd like to rule some of this simpler stuff out before I dig into replacing them because it looks like a fair amount of disassembly to get them out.

The table didn't come through like I thought it would. Let my try attaching a screen shot


Attachments:
Screen Shot 2014-11-01 at 9.10.48 AM.png
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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 2:23 am 
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Location: germany
hello from germany :wink:

that looks like a sometimes overloadet +12V
Please check the rectifier bridge there, D1142. When one of them is shorting it can overload the winding like that what we see.

greetings
Martin


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 5:10 am 
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Thanks, Martin, I was actually just looking over the schematic a little while ago and I was thinking of focusing there next.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 3:14 pm 
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I took a second look this morning using another scope (B&K 1465) to see what everything looked like and a found a few interesting things. When I probe the +75, +12, or -12 with the scope working all three are clean, no ripple (that this scope can see at least). Once the +12V drops and the scope goes wonky, I see the same clean response on the +75 and -12 but now on the +12V I see a clipped sine wave at 120Hz. I'm thinking this is most definitely an issue with the rectifier.

The second thing I noticed which is independent of the above problem (I think) is that when I probed the points on the transformer I see distorted signals on all points except 11&12 which does not go through a filter cap or rectifier. Point 7&8 are also pretty clean with slight clipping on the positive peaks of the signal. I can't say that isn't something with the B+K scope either. The other three taps are all pretty distorted though for the +75, +12, and -12. The waveforms don't seem to change at all when the scope drops out other than a decrease in amplitude which is consistent with the DMM measurements I made yesterday.

At this point I believe I have a questionable rectifier for the +12v based on the findings this morning, but I'm wondering if
distorted signal measurements I'm seeing on the transformer confirm that the filter caps should be replaced, too.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 3:29 pm 
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Location: Monte Vista, CO. USA
OK, 4th time is the charm (I hope).

Replace all the electrolytic capacitors in the power supply!!!

I hope I wasn't unclear.

Really, all kidding aside, I have repaired dozens and dozens of vintage Tek scopes and not only does this need to be done anyway, but I will personally guarantee it will fix the problems you are so carefully trying to find. You do not have a rectifier problem.
-Phil


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 4:26 pm 
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Location: Parksley, VA
Phil, you win. I'm going to order the replacements this week. I don't know that I understand why the caps would cause the 12v supply issue though, and I guess I wouldn't expect them to be temperature sensitive, at least seemingly temperature sensitive.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 5:01 pm 
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Actually, old marginal caps are very temperature sensitive. That was another clue for me. And since they are barely doing the job, as temperature and load changes the symptoms become very unpredictable. Particularly on regulated outputs.

That is another reason why they have to be changed anyway. There is no way to troubleshoot any other problems the unit may have until they are ruled out.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Sun 02, 2014 6:06 pm 
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I'm learning a lot bringing this thing back, thanks for all of the help so far. I'll report back when I make some more progress on it.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Tue 04, 2014 3:36 am 
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Location: Parksley, VA
a few questions, is the best way to get to the caps to remove the A sweep board and go from below? I'm assuming I can leave the old caps in place, looks like there should be enough room below for modern electrolytics to fit. Looking through the gaps, I see ceramic strips inside, am I going to need Tek Silver loaded solder for this job?


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Nov Tue 04, 2014 4:10 am 
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Leave the old caps in place? No, not unless you can isolate them. You can't leave them connected electrically.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 19, 2017 4:29 pm 
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so, how did this turn out?

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 19, 2017 5:40 pm 
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I read about half this thread before getting lost in the details, so sorry if I'm just repeating someone else.

Work / exercise the front panel controls involved w/ problems... in fact exercise them all. They tend to self clean. I've raised a couple older teks from the dead, to basically working, by doing little else.

As you also may have other issues, I would then suggest you go back to the logical first step of confirming voltage & ripple levels for all the power supplies.

Tek put out an excellent guide to scope repair. You can download it for free from their website.

Yes, I know this thread is a couple years old; maybe the suggestions will help someone else.


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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Tue 19, 2017 9:45 pm 
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Location: Sunbury, Ohio 43074
You might be able to localize the apparent heat problem with a cold air gun (that's a heat gun with the heat off). Start by cooling sections, then once you find a section, narrow it down to a specific part of that section or even a single component.

I tend to agree with those that suspect the power supply caps. Simplest thing to check that is to just monitor the power supplies with your other scope and see which one either gets a lot of ripple as it warms up, or has it's DC change.

Someone, several I think, mentioned exercising all the switches. I have done that to both my 454 and 465 periodically and they always bounce right back. When you cleaned the switches and contacts, did you do it properly? Sometimes residue can be an issue. Rather than re-type a few volumes here, just do a search on this forum. Cleaning switches/pots has been debated many many times.

I'm not a big fan of wholescale cap replacement in these particular tek scopes. ... experience shows that the caps in them generally last a long long time. That said, if yours was operated in hostile environments, that could well be your issue.

Hope this helps

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 Post subject: Re: Tektronix 453 Restoration
PostPosted: Sep Thu 21, 2017 11:09 pm 
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Agreed, I have had 5 or 6 453's over the years and all I did was clean them and blow the dust out and all worked 100% on original parts except for one which had a stuck fan motor. I found an NOS replacement for it on eBay.

Greg


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