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Old 21st April 2019, 10:47 PM   #1
kerux is offline kerux  United States
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Default Tubelab SSE blown C12 and R17 should I replace board?

I blew the C12 capacitor and R17 resistor on my Tubelab SSE after many years of use. The PCB underneath R17 is burnt and part of the this area has melted. I replaced these two parts but wondering if I should stop and replace the whole board and start over?

Any input is appreciated.

Thanks
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Old 22nd April 2019, 03:48 PM   #2
Tubelab_com is offline Tubelab_com  United States
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Quote:
part of the this area has melted.
I have seen boards, usually in high voltage circuits that got wet, where the board actually burned, but I have never seen a modern PC board melt, so I'm not sure exactly what you have. Posting a picture or two would help.

I have seen, and fixed some pretty badly damaged boards including one where a security guard who spilled Pepsi into a video monitor, the old CRT kind while it was on, despite the "no food or drink" sign at his post. High voltage can arc through the board burning a carbon track into the fiberglass. The carbon track is conductive and will short out the circuit causing more malfunction and possible further burning. The cure is to remove ALL burnt surfaces with an X-acto knife or Dremel tool even if it leaves a hole in the board. Then replace all the damaged traces with wire.......

I seriously doubt that an SSE board can get fried that bad since there should be less than 50 volts in that area. I have a couple that have turned brown under R17 and R27. One is in my industrial amp which has a chassis too small for the job and is 13 years old, and the other is a test board which has been the subjects of multiple tests and has been blown up dozens of times. They still work fine, so I'm not inclined to replace them, but I'm not concerned with the cosmetics.

The most likely failure was an aging output tube that went into red plate runaway causing the cathode voltage to rise high enough to blow C12, and possibly R17. The goo that oozes from a blown electrolytic is somewhat corrosive and can harden to a thick or solid mess over time. I have usually been able to clean it off the board with WD-40, alcohol and a toothbrush.

Once the resistor and cap are replaced be extremely cautious with the tube that was connected to them. It will likely runaway again.eventually, if not immediately. It is rare for a cap to die by itself.

If the board is discolored under the resistor, but is not charred and does not crumble when lightly scraped with a screwdriver or knife, it's probably OK. If not sure, post a picture of both sides of the R17 area.
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Old 23rd April 2019, 02:05 AM   #3
kerux is offline kerux  United States
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I will do your cure of removing all burnt surfaces and then post pictures.

Thank you for your help.
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Old 23rd April 2019, 08:07 PM   #4
kerux is offline kerux  United States
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Default pictures of Tubelab SSE after removing burnt areas at R17

Here is a picture of the top and bottom of board.

Thanks for your help.
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File Type: jpg sse_bottom.jpg (93.8 KB, 72 views)
File Type: jpg sse_topside.jpg (87.0 KB, 72 views)
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Old 23rd April 2019, 08:46 PM   #5
Tubelab_com is offline Tubelab_com  United States
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The light spots are areas of delamination. They will weaken the rigidity of the board a bit, but otherwise won't hurt anything. I wouldn't worry about it, I have seen worse. It's the black stuff that can be a problem with high voltages.
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Old 23rd April 2019, 10:24 PM   #6
kerux is offline kerux  United States
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I will replace the output tube and try playing music.

Thanks
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