TDS544A failure

[daveyk]

Hello there. Pulled this TDS544A out of the company dumpster. 

It boots up with:
Fail: Aquisition
Fail: Atten/Acq Interface
Fail: TV Trigger

I replaced all the caps on the big bottom Acq board. Some were leaking but not all. Only a small group of them really tested bad on my Fluke 43b meter but that is not an ECR meter so I don't know if that means much.

As I replaced each one, I buzzed the traces out.

I've tried to let it compensate the paths thinking that might fix it, but no. That is the only time I can see any trace is at the very start of the compensation. It never gets past ch1 with a signal path failure.

I have not replaced any on the top CPU board yet but I see none leaking at all on there or any other boards.

Perhaps this is just a bad ACQ board? There's a lot more to fail other than caps.

It's a shame. The CRT is beautiful in this scope.

Thoughts please? Is this a scrapper to put back in the hazardous waist dumpster or should I keep spending time on it?

Haa, it was just at the boot failure screen with the error message as I was typing this and it power down. Now it's power cycling. Looks like scrap to me.

Dave


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[daveyk]

Hello there. Pulled this TDS544A out of the company dumpster. 

It boots up with:
Fail: Aquisition
Fail: Atten/Acq Interface
Fail: TV Trigger

I replaced all the caps on the big bottom Acq board. Some were leaking but not all. Only a small group of them really tested bad on my Fluke 43b meter but that is not an ECR meter so I don't know if that means much.

As I replaced each one, I buzzed the traces out.

I've tried to let it compensate the paths thinking that might fix it, but no. That is the only time I can see any trace is at the very start of the compensation. It never gets past ch1 with a signal path failure.

I have not replaced any on the top CPU board yet but I see none leaking at all on there or any other boards.

Perhaps this is just a bad ACQ board? There's a lot more to fail other than caps.

It's a shame. The CRT is beautiful in this scope.

Thoughts please? Is this a scrapper to put back in the hazardous waist dumpster or should I keep spending time on it?

Haa, it was just at the boot failure screen with the error message as I was typing this and it power down. Now it's power cycling. Looks like scrap to me.

Dave


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Funny, if I remove the TV Trigger too PCB, the TV Trigger test passes - lol.

The caps on the digital board and the TV trigger pcb are pristine and no leaks or corrosion visible anywhere.

I went over the acq pcb with a fine tooth comb and all caps have continuity to chips near by, both on the pos and neg sides. I did find on 33uf in what looks like the memory bank area that was not soldered. I fixed that put the lower shield back on.

It is shutting off and power cycling on its own after its been on for a while. I do keep a fan blowing on the acq board when testing. Those big chips are not overheating.

I had a good look at the power supply from above and none of this caps are swelled or leaking. I didn't pull it out.

Why is this power cycling? Would that be power supply or the digital board control of the power supply? Maybe the acq board is bad and over loading it. I did see a thermal switch in the power supply.

Still stumped and dissapointed.


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[daveyk]

I think I knew what's wrong. I think the main -15v power supply is bad. With the acq board unhooked it is only -12.5. It may be around -9 with it hooked.

Now to find schematics. The service manual doesn't have them .

Dave


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[daveyk]

I think I knew what's wrong. I think the main -15v power supply is bad. With the acq board unhooked it is only -12.5. It may be around -9 with it hooked.

Now to find schematics. The service manual doesn't have them .

Dave


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I've made progress. I got the -15v supply good again.

Now it only fails Attn/Acq Interface.



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[daveyk]

I think I knew what's wrong. I think the main -15v power supply is bad. With the acq board unhooked it is only -12.5. It may be around -9 with it hooked.

Now to find schematics. The service manual doesn't have them .

Dave


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I've made progress. I got the -15v supply good again.

Now it only fails Attn/Acq Interface.



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After fooling around and finding another poorly soldered cap, I have traced for ch1& 2. They're not right but it is more than I had before. Signal comp doesn't lock up anymore but it does error.



I may be at the end of my luck. I can't find any op amps or chip missing supply voltage.




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[daveyk]

I think I knew what's wrong. I think the main -15v power supply is bad. With the acq board unhooked it is only -12.5. It may be around -9 with it hooked.

Now to find schematics. The service manual doesn't have them .

Dave


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I've made progress. I got the -15v supply good again.

Now it only fails Attn/Acq Interface.



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After fooling around and finding another poorly soldered cap, I have traced for ch1& 2. They're not right but it is more than I had before. Signal comp doesn't lock up anymore but it does error.



I may be at the end of my luck. I can't find any op amps or chip missing supply voltage.




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Got it to pass all boot up tests but ch 3&4 will not compensate or work.





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[daveyk]

Well I spent a lot of time with this scope and I can't get any further that it no longer has boot up errors but the channels don't work. Ch1 and 2 somewhat work but the offsets drift. Channel 3 is off somewhere in la la oops land. Ch4 works sometimes but with a huge offset.

The main problem all along was probably the low voltage PS. I don't know if I caused these other problems changing all the caps or not. There had been leakages. All the traces both ground and voltage traces from the caps do go somewhere.

So I think it's time to give up and move on.  Maybe someday I'll stumble across another one with a good acq board to try. For now, I'll put it back together and throw it on the junk heap. I got other things to occupy my time.

I'm sorry this turned out to be a useless thread. If I could delete it, I would, but the forum software doesn't allow me to. Moderator: you can delete this thread please.






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[james_s]

Wow that CRT does look great, the scope is worth something just for that. These are fantastic instruments, it's a shame someone just dumped it. If you ever get tired of stepping around it I'd be happy to take it off your hands.

[daveyk]

Wow that CRT does look great, the scope is worth something just for that. These are fantastic instruments, it's a shame someone just dumped it. If you ever get tired of stepping around it I'd be happy to take it off your hands.

It's back together. I had power supply problems before I could test. It was having problems turning on. Spent three hours on that. Finally changed a 220 uf near center of ps board. Now it turns on again. Any who.... with the scrubbed acq board in place. The traces are 300% better. They are all on screen and very close to a zero baseline.

The scope still will not pass signal path compensation. Ch3 looks noisy.

Also before with a signal trigger 50% was near the top of the signal on all channels. With the board scrubbed, 50% trigger is at 50% of the signals on all channels.

Error log still recording a whole bunch of errors while trying to compensate.


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[daveyk]

Wow that CRT does look great, the scope is worth something just for that. These are fantastic instruments, it's a shame someone just dumped it. If you ever get tired of stepping around it I'd be happy to take it off your hands.

It's back together. I had power supply problems before I could test. It was having problems turning on. Spent three hours on that. Finally changed a 220 uf near center of ps board. Now it turns on again. Any who.... with the scrubbed acq board in place. The traces are 300% better. They are all on screen and very close to a zero baseline.

The scope still will not pass signal path compensation. Ch3 looks noisy.

Also before with a signal trigger 50% was near the top of the signal on all channels. With the board scrubbed, 50% trigger is at 50% of the signals on all channels.

Error log still recording a whole bunch of errors while trying to compensate.


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Spoke too soon. It was looking good. I turned it off and flipped over to its other side. When I tried to turn it on, it snapped the ground fault outlet ( it had been that too earlier ). There isn't any loose screws as far as I know.



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[james_s]

I would look for bad safety capacitors on the input side of the power supply, those can trip GFCIs. Might be worth testing for earth leakage too.

The SPC failures on these are typically caused by faulty relays in the attenuators. I had that problem on my TDS784C and replaced all 16 of the orange relays and it passes every time now. It's a fiddly job that requires very careful handling of the ceramic hybrids and you need a long slender tip on your soldering iron and a steady hand. I made a custom tip out of a piece of solid copper wire and used a combination of long slender snips and despldering to remove the old relays. Careful planning is then required to solder the four new relays onto each attenuator without painting yourself into a corner. Some polyamide tape is handy to keep from melting anything or getting solder in places you don't want to.

Fantastic instrument when working though, I absolutely love mine. My CRT is not as bright and sharp as yours but it's not bad. I really like these NuColor displays, it's such a clever way of getting full color without the grainy look of a shadow mask or aperture grill.

[nctnico]

I hate the NuColor displays (TFT replacement time!) but I really like these oscilloscope series and I had quite a few of these over the years. Getting another one would scratch a major itch of mine!

[james_s]

Well if anyone who has one of these scopes doesn't like the display and wants to retrofit it, please do contact me as I'd love to have a spare tube for mine. I'm fascinated by the very novel display technology, it was a large part of what made me choose this particular series of scope. LCDs look grainy to me, piss poor contrast ratio, there's a TDS3054 at work and the display on that doesn't come close to the NuColor CRT, which has a razor sharp image over an inky black background. The only thing I've seen with better contrast is OLED.

[daveyk]

I would look for bad safety capacitors on the input side of the power supply, those can trip GFCIs. Might be worth testing for earth leakage too.

The SPC failures on these are typically caused by faulty relays in the attenuators. I had that problem on my TDS784C and replaced all 16 of the orange relays and it passes every time now. It's a fiddly job that requires very careful handling of the ceramic hybrids and you need a long slender tip on your soldering iron and a steady hand. I made a custom tip out of a piece of solid copper wire and used a combination of long slender snips and despldering to remove the old relays. Careful planning is then required to solder the four new relays onto each attenuator without painting yourself into a corner. Some polyamide tape is handy to keep from melting anything or getting solder in places you don't want to.

Fantastic instrument when working though, I absolutely love mine. My CRT is not as bright and sharp as yours but it's not bad. I really like these NuColor displays, it's such a clever way of getting full color without the grainy look of a shadow mask or aperture grill.

Well I think I found that PS problem. I pulled it out yet again and there was a piece of RF shielding metal with fingers under the primary section of the power supply near the power entry. Removed it, reinstalled, replace fuse, and it powered back up.

Will not get through compensation. Here are the beautiful traces:


I swapped the ch1 and ch3 hybrid in front end and the problem did not move. Do there is still an AQC board problem or a problem with the front end control board.


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[daveyk]

Okay I see the final problem. I took the front panel input control board off and when I put the hybrid in for channel 3, I must not of had it flat and it is broke. She's now officially a Ka-put parts scope.






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[nctnico]

You should have posted a picture of that signal earlier. That is a typical acquisition board problem. What Tektronix did in these scopes is not use a seperate DAC for all settings but multiplex it. There are a whole bunch of TL074 FET input opamps on the board which are used as a sample&hold for stuff like offsets. This only works if there is no leakage from the storage capacitor. Your signal indicates there is some leakage in the opamp circuitry. You can see the capacitor gets charged and then slowly discharges through the excess leakage.

[james_s]

Why a parts scope? Just buy another attenuator hybrid off ebay and it looks like it'll be a working instrument. Might even be able to repair that one as it's only broken at the corner.

If you search for "tektronix attenuator" you can often find them for around $100.

[nctnico]

Why a parts scope? Just buy another attenuator hybrid off ebay and it looks like it'll be a working instrument. Might even be able to repair that one as it's only broken at the corner.

If you search for "tektronix attenuator" you can often find them for around $100.

The same amount buys you a 'for parts scope' if you have some patience. I don't know whether replacing the input hybrid requires recalibrating the scope (which is not easy due to the equipment you need).

[daveyk]

Why a parts scope? Just buy another attenuator hybrid off ebay and it looks like it'll be a working instrument. Might even be able to repair that one as it's only broken at the corner.

If you search for "tektronix attenuator" you can often find them for around $100.

Well #1: I was pxssed at myself. Early on I swapped one and three as a test. Obviously I did not have them seated properly when I screwed the board back on. I am at fault for breaking them. I was distracted. Then the last power supply problem, as I was trying to test, had me at my wits end.

#2: I searched and could not find any. I just now search for "Tektronix Attenuator" per your suggestion and found them; than you.

I don't know if that will fix it, but I don't give up easy. I know what I said, but I am stubborn and will spend more time. It took me three plus weeks to fix the Agilent E3236 power supply, but I never gave up. This scope on the other hand, I am close to needing to set it aside. The shop must get tore out and rebuilt in the next month.  I may be old fashion , but there is something sexy with that beautiful super high contrast CRT in these scopes.




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[daveyk]

Why a parts scope? Just buy another attenuator hybrid off ebay and it looks like it'll be a working instrument. Might even be able to repair that one as it's only broken at the corner.

If you search for "tektronix attenuator" you can often find them for around $100.

The same amount buys you a 'for parts scope' if you have some patience. I don't know whether replacing the input hybrid requires recalibrating the scope (which is not easy due to the equipment you need).

That's a good point; you are probably right. Most places that calibrate them only really do a "certification" and probably can not tweak the calibration values. It may be hard to find someone near me that can.




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[james_s]

Sometimes you can find a parts scope for that price range but then you usually have to pay shipping. I just hate to part out something that is in otherwise nice shape, but then I have a particular liking for these instruments.