TDS-320 Power Supply issue

[decoder_82]

Hello everyone,

I am trying to repair a faulty low voltage power supply on a TDS-320.
The PSU is a Zytec one (22917300) but there are no schematics available for this (even in the service manual of the TDS-320).

There is a component that clearly has burned, we can see it here: https://imgur.com/a/4AXuTxw

It is difficult to see what is written but it seems to be R17 but I am not sure.

I found that R17 on the same PSU should be 3K9 5W
https://www.electronicspoint.com/forums/threads/tektronix-tds350-psu.25635/

What bugs me as it the leg are quite close for a resistor so I am not sure.

Does anyone has idea of what the burned component was ? or has pictures of this model of PSU ?

Many thanks

[shakalnokturn]

I repaired one recently, yes it is 3.9kOhm.
If the PCB isn't too fragile I'd recommend removing the resistor and all the old solder, scratching down the pads on both sides to a shiny surface  to get a clean new solder.

The link you posted covers just about everything that usually fails on those PSU's.

There's also this that could be useful:
http://www.hakanh.com/dl/docs/troubleshooting_tips_on_tds300_ps.pdf

[decoder_82]

Thanks for the reply.

OK so it should be 3K9 5W ceramic ideally vertical mounted. Would this work ? https://www.tme.eu/en/details/crl5w-3k9/5w-resistors/sr-passives/

In my case the PCB is badly damaged so I will have to use wires as the traces are not connected anymore:

https://imgur.com/a/XB25Kp2

[shakalnokturn]

It should do the job. Maybe find a way to attach it (reliably) to the primary heatsink and run some wires.
I'm not sure what the resistor does in there, I suppose it's part of the snubber network.
It could be worth the bother checking the secondary electrolytics and TO220 rectifiers, I don't think it's normal for the resistor to get that hot.

If it hasn't been done it's worth replacing the SMD electrolytics on the front panel before they cause damage.

[decoder_82]

That's a good point some other failures might have cause the R17 to go very hot.
Will replace all the electrolytic caps. The TO220 rectifiers seem visually OK but might be worth to replace just to be sure.

I will check the SMD caps on the front panel too. Thanks for the tip :-)

[shakalnokturn]

I wouldn't bother replacing the capacitors in the PSU just for the sake of it, I checked them in my TDS 320 they were all good. The SMD electrolytics however are a threat.

The other possibility with R17 is that the value drifts up over time and it has more power to dissipate. Some ceramic resistors do drift.

[decoder_82]

Will check the ESR of the PSU caps and only replace if needed.

I will try to figure out what are the values for the front panel SMD electrolytics.

[decoder_82]

@shakalnokturn By any chance, to you know the value of C23 ? It has suffered from the heat and its value cannot be read anymore. Thanks in advance

[shakalnokturn]

I'd have to re-open the scope, I'll let you know... It could take a day or two.

[decoder_82]

This would be really awesome! Thanks for taking the time for this.

I will add to this post a full list of power board caps and their values it might be useful for someone else in the future.

[shakalnokturn]

Before I get round to it, if there's a short and precise list of things you would like me to check or measure let me know. Unless it fails again I'll only be reopening once.

[decoder_82]

Thanks for the reply. I cannot think of other information needed. My main interrogation is the value of this cap. I understand that you will not open it many times. Thanks again for your help.

[decoder_82]

One more question: for the SMD caps on the front panel, you mean C131 and C132 (both 33uF 10v) ?

https://imgur.com/a/1xWut0e

Do you suggest to replace with the same model ? For other repair, I have replaced those with ceramic caps.

[shakalnokturn]

Yes those are the ones, of course if they appear to have been previously replaced or in very good condition leave them alone.
If there are dull patches and corrosion on surrounding components or fishy smells it's time to clean-up and replace.
I've replaced with MLCC's before no problems.

[decoder_82]

Thanks for the confirmation. I will replace both as there were some greenish traces at the bottom of the caps.

[shakalnokturn]

C23 is 1µF/50V/85°C (!!)

[decoder_82]

Many thanks for checking this !

[decoder_82]

Quick update: after replacing both R17 and C23, I got correct output voltage from PSU and was able to use the scope again.

Thanks again shakalnokturn for your help.