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Good afternoon, I am a humble electronics hobbyist with some knowledge. I have repaired various household devices, but this time I decided to take on repairing my TDS744A. I bought this scope online from Japan. When I first turned it on, there was no image on the screen, though it did display through the VGA output. A few days later, I turned it on again, and only the front panel LEDs flickered.
I have recapped the entire CRT board following all recommendations, and I also replaced the RIFA capacitors in the power supply. Unfortunately, the issue persists. I don’t want to go any further on my own and am reaching out to you for assistance. It would be very helpful if you could advise me on where to look to resolve this issue.
I would be very grateful for any suggestions. I’m attaching a video showing the issue. Thank you very much in advance.
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It looks like the PSU is hiccuping. First up, I'd try unplugging the grey ribbon cable to the CRT board to isolate it from power to check it's not still bad somehow.
Have you replaced C17 in the PSU with a new low ESR cap? These are *always* in need of replacement, and can take out a few other parts if left alone..
Check a few other parts as per this image to make sure the standby power is ok:
There are a few known problems with old capacitors in these PSU's, and being an A model, it will be older and the capacitors nearer the end of their life... I have personally found a couple PSU's needing a complete set of replacement capacitors in older scopes.
Are you located in Japan? I have a bunch of spare parts for these scopes including working PSU's and CRT boards, so it wouldn't be difficult or expensive to figure out a trade etc.
Jared
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Hey Rafael, I second Jared's advice; you'll probably want to have another look at the PSU. The standby supply in these scopes is notorious for giving up, leading to dead or hiccuping supply. AFAIK there are two main versions of the standby supply, the earlier "discrete" one and the later "integrated" one. You'll find schematics for the "discrete" one here: (PS).pdf (thanks H?kan). The later version has an all-in-one switching regulator, which you'll find in the TDS520B component service manual. This is available from Tek (as well as from TekWiki, I believe). The regulation loop in the "discrete" standalone supply is non-obvious (to me) so I simulated it, and found that a high-ESR C17 wrecks it, though I don't remember the details beyond that. ISTR that the TVS diodes VR4/VR5 and Q2 get stressed when C17 is high-ESR, so you may want to replace or check the TVS diodes at least. If they fail open, Q2 will die after a short while. I should note that without e.g. a curve tracer that goes to >250V, it's not easy to test the TVS diodes, but maybe you have a suitable curve tracer at hand - you wouldn't be the only member of this group so equipped :). I think you'll also want to look for leaking electrolytic SMD capacitors elsewhere in the scope. I don't know whether the capacitor plague affects the TDS744A in particular, but it's wrecked many otherwise good TDS scopes. Siggi On Thu, Nov 14, 2024 at 7:57?PM rafael suarez via groups.io <rafik69pe= [email protected]> wrote: Good afternoon, I am a humble electronics hobbyist with some knowledge. I
have repaired various household devices, but this time I decided to take on
repairing my TDS744A. I bought this scope online from Japan. When I first
turned it on, there was no image on the screen, though it did display
through the VGA output. A few days later, I turned it on again, and only
the front panel LEDs flickered.
I have recapped the entire CRT board following all recommendations, and I
also replaced the RIFA capacitors in the power supply. Unfortunately, the
issue persists. I don’t want to go any further on my own and am reaching
out to you for assistance. It would be very helpful if you could advise me
on where to look to resolve this issue.
I would be very grateful for any suggestions. I’m attaching a video
showing the issue. Thank you very much in advance.
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Hello everyone,
Thank you for your quick responses. I will follow your suggestions as soon as I return from work. I’m afraid I don’t have a curve tracer. Perhaps there’s another way to test them?
On another note, I’ve found a few solid SMD capacitors in bad condition on the large board visible as soon as you open the scope. I’ll follow your advice and let you know what I find.
Best regards.
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Hello everyone,
Thank you for your quick responses. I will follow your suggestions as soon as I return from work. I’m afraid I don’t have a curve tracer. Perhaps there’s another way to test them?
On another note, I’ve found a few solid SMD capacitors in bad condition on the large board visible as soon as you open the scope. I’ll follow your advice and let you know what I find.
Best regards.
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On Fri, Nov 15, 2024 at 1:04?PM rafael suarez via groups.io <rafik69pe= [email protected]> wrote: I’m afraid I don’t have a curve tracer. Perhaps there’s another way to
test them?
The TVSes should measure open in both directions, until you reach somewhere beyond their threshold voltage. This is 220/250V respectively, which is why they're challenging to test. If you can somehow provide a current limited (1mA will do) ~300V DC, you can simply measure the voltage drop across them in both directions. It should be roughly 250/220V respectively. If you can't, perhaps best to leave them alone. If they've failed open, your switching transistor will fail, though it may take it a while. If it comes to that you'll want to change both TVSen and the transistor. I seem to remember replacing a BU508, which is easy enough to source. On another note, I’ve found a few solid SMD capacitors in bad condition on
the large board visible as soon as you open the scope. That would be the CPU board. Also look at the acquisition board, the front panel board and whichever other boards you have in the scope. If the electrolytic caps are the same make and vintage, the odds are that they've failed or are going to fail. There are many threads in this forum about how to clean boards that have suffered electrolyte leakage. The problem is that traces may have rotted away, and if they haven't yet, they may still rot away if the electrolyte isn't cleaned and/or neutralized. I believe in North America the recommendation used to be to wash thoroughly with; Simple Green, distilled water, IPA and then leave the boards to dry, or "bake" them at a very low temperature. Getting under components where electrolyte may have pooled can be difficult. There are many stories of people who managed to find and repair broken traces and probably as many, of people who gave up. My TDS684B(? I forget) has never yet worked, even though I cleaned it and changed the electrolytics :/. Good luck, these are very capable scopes and well worth putting some time into.
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Hi, I own a TDS694C which has a similar power supply. Worked fine but after
sitting on the shelf for a long time died on the first power up.It produced
modulated high pitch sound on standby power, and died completely on
attempt to turn main power on by the front button.
The first thing I found was a dried and blown 1000uf EC in the power
supply, s main line, C124 in TDS520B schematics. After replacement nothing
good - no standby power. Measured conductivity of integrated PWM (U12)
output transistor - open in both source-drain directions, not good. It was
TOP200YAI Replaced with a newer and more powerful TOP204YAI. Still nothing,
no input voltage on standby PWM. High voltage Zeners were ok (tested with
curve tracer, you can also do that with auto transformer and rectifier on a
breadboard, but be careful with high voltage!).
Walking along the supply line I have found a non-conducting current
limiting 1W resistor, R176. It was 2.4Ohm in my PS and *fused* 27Ohm in
TDS520B. It just cracked when the integrated PWM was blown by the voltage
spikes. Very thin crack, visible under microscope only. So it is fused in
TDS520 for a reason. Replaced with a new one and the problem was solved - I
immediately got standby power and good main power after.
Best,
-Andrei
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Good evening,
I disassembled the PSU again and checked capacitor C17, which has an ESR of 0.5 ohms—I think it’s fine. Then I tested transistor Q9, and it doesn’t show any unusual readings. Diodes VR4 and VR5 measure open in both directions, so everything seems to be fine there.
On the other hand, I rechecked the caps on the CPU board and the acquisition board and found some caps that don’t give any ESR value and others that measure out of the desired range. I tested them with my B&K 881.
I’ve made a decision to place an order with Digikey for the components in question and the solid caps. I’ll replace all the solid caps first.
Thank you very much for your advice; I’m truly grateful. I’ll update you on my progress as soon as I receive and replace the solid caps.
By the way, what would be a suitable replacement for C17? I haven’t been able to find anything similar on Digikey.
Thanks again, and good night.
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Rafel,
Mouser has the 47mfd in stock in two versions, 647-UCY2C470MPD (12,000 hour 160V) and 647-UHE2A470MPD (7000 hour). Digikey has a 56mfd UHE UHE2A560MPD (7000 hour). The UHEs are 100V. I do not know if the capacitance value is critical not seeing the power supply schematic. Even though the original checks alright, being the age it is, replacing would be best. For filtering of the B+ supplies, a higher capacitance can be used. Going up in voltage can be done.
Mark
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This would be a good choice for C17 from Digikey:
It has high ripple current capacity and very low ESR and ESL for its capacitance and voltage rating.
Lifetime is stated at 4000 hours, but that is at 105 degrees C. At a more realistic ballpark of 40-ish degrees C, you'd be looking at around maybe 200,000 hours according to the datasheet...
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Hello,
I’m truly grateful for your willingness to help and for all the suggestions regarding the repair of my TDS. I don’t want to be intrusive or take advantage of your kindness, but there’s something I can’t figure out. It’s about the solid aluminum caps of 33 ?F 10V and 10 ?F 10V. There are several series available, and I’m not sure which ones would be appropriate for the scope.
Could you please suggest one? Thank you in advance. I hope this scope comes back to life soon.
Best regards,
Rafael
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P.D. There’s something I forgot to mention, perhaps I didn’t make it clear earlier. I’m located in Lima, Peru.
Greetings from this part of the world.
Rafael
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Rafael,
The solid Al condensers can be replaced with tantalum, surface mount or radial. Go to 16V if you use tantalum as a precaution. Tantalums will have very low ESR. If these are decoupling, the same or higher capacitance values can be used.
Mark
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On Mon, Nov 18, 2024 at 01:19 AM, rafael suarez wrote:
Hello,
I’m truly grateful for your willingness to help and for all the suggestions
regarding the repair of my TDS. I don’t want to be intrusive or take
advantage of your kindness, but there’s something I can’t figure out.
It’s about the solid aluminum caps of 33 ?F 10V and 10 ?F 10V. There are
several series available, and I’m not sure which ones would be appropriate
for the scope.
Could you please suggest one? Thank you in advance. I hope this scope comes
back to life soon.
Best regards,
Rafael
When you say Tantalum, are you talking about the small metal can capacitors with the tiny black plastic base? They are standard SMD type aluminium electrolytic. They should all be replaced with equivalent types in any TDS500/600/700 series scope. IIRC only the C and D models had the electrolytic capacitors removed from the design. I usually measure the size, then select a part that has low ESR and high ripple current rating, and also I try to go up a voltage rating or two if I can. (For example, there is a cap on the front panel that is actually underrated from factory, but that's not as good as the one cap on the -ve voltage rail on the Option 05 TV Trigger board that is mounted backwards to the (incorrect) marking on the PCB!) Jared
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Hi Jared,
Thank you for your message and advice. Yes, I am referring to those caps. And I’ve decided to replace them with tantalum ones. I’ve looked on Digikey and found these: T491C336M016AT and T491C106M016AT.
Do you think they are the correct ones?
Thank you in advance for your help.
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Ah ok, I understand you now.
Those Tantalums you selected seem like a reasonable choice. Having a quick look at the spare boards I have here, it seems Tek just used the same values when they swapped to tantalum as they used for the aluminium electrolytic, so a direct replacement should be no problems.
Just double check that your selection will physically fit on the footprint on your PCB.
Jared
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Hi, thank you for the support. I will place the order in the coming days. However, I have a question since English is not my native language. Is the 1.5KE220CA - bidirectional? I want to be sure before making the purchase.
Thanks in advance!
Ah! I have taken precautions with the measurements of the tantalum caps. They fit snugly on the PCB, so I don't think there will be any issues.
Best regards.
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Yep, 1.5KE220CA is the exact part number you need. (The 'CA' at the end of the part number means bidirectional, if it were just 'A' then it would be uni-directional)
Be aware though that the two TVS diodes you need are different part numbers!
VR4 - 1.5KE220CA
VR5 - 1.5KE250CA
Be sure to get one of each type and install them in the correct locations! :)
Regards
Jared
Jared
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Hi, thank you so much for all the information provided. I’ll be placing the order in the coming days. As soon as I replace the components, I’ll share the results with you. Have a great day. Keep in touch.
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Hello everyone,
First of all, I apologize for being absent from the forum for so long.
Some time ago, I ordered the capacitors and have already replaced them. However, I made a mistake: instead of buying 10?F 35V, I purchased 10?F 16V, a big error. Well, it's only two of them, so I will find a way to fix it.
Regarding the power supply, it seems to be fine, but it clicks, which suggests a short circuit. That’s why I checked the bottom board (acquisition board) and found that it is shorted, seemingly on the 5V and 15V lines.
I am thinking of injecting 1V at 100mA to locate the short, but I don’t know which pins correspond to these voltages on the connectors.
Additionally, the rear BNC Signal Output connector is also shorted, which might be a useful clue.
I don’t have the service manual for my TDS, otherwise, I would be able to track the issue better.
Once again, I apologize for being away for so long.
73!
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Jared Cabot