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I installed another HOT. When I removed the old one which blew last night, there was goopy electrical tape around the transistor base. Maybe there was an arc.
This time around I tested +B. the TV had it set to 133VDC but turning the adjustment knob all the way to the right will bring it up to 140V. I tested by setting the multimeter to 500VDC range and putting one lead on ground and the other lead on the +B fuse. |
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I should really replace the 4.7μF cap before doing anything else, no?
Could low +B voltage be causing a problem? You said it should be around 180VDC. ...yeah, I think a total recap is the way to go Last edited by luRaichu; 04-06-2024 at 03:38 PM. |
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Should I drop the lightbulb in place of the line fuse, or the +B fuse?
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There is quite a pulse on that transistor case, you can't use layers of tape to replace the original mica insulator.
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I don't have the original mica sheet. It was glued to the old, busted HOT with some weird white substance. |
Audiokarma |
#6
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For the entire TV, is good to use a higher wattage one. Test only after aplying the correct insulator (mica) and thermal grease (+1 for correcting this first).
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So many projects, so little time... |
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That weird white substance is the silicone heat conducting grease and without it the transistor could fail from overheating. Same stuff used on processor heat sinks and should always be used. The tape itself is not very heat conducting and its breakdown of 600V is the MAX at its nominal thickness not compressed under a transistor. There can be upwards of 1000V between the transistor case and ground during the flyback period and this is why it is so important to do things the right way, HOT's are not cheap as I remember and so far I've seen about half a dozen blown in this thread.
Last edited by ARC Tech-109; 04-07-2024 at 04:30 AM. |
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But yeah this makes me feel pretty stupid. I guess I will try mounting the HOT externally just as a test to see if the previous mounting setup was bad. Last edited by luRaichu; 04-07-2024 at 08:54 AM. |
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What were the ORIGINAL HOT part numbers ?
The tape has to go ! Lets double check the ##'s then clean it up spotless & get a mica insulator & compound. On the U tubes I saw no reason for the fail looked pretty normal. Zeno |
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Audiokarma |
#11
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I'm going to buy these two things on Amazon.
Mica insulator -> https://www.amazon.com/gp/product/B07P428VZ1 Thermal grease -> https://www.amazon.com/gp/product/B005VULWLA |
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The breakdown of the insulator to ground is going to kill the transistor. If I remember the heatsink was a ground potential on those Color Pilot sets, worked on may of them as they popular computer monitors in the 80s on the Apple-II that had a composite out.
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How much thermal grease should be used? It should be applied to both sides of the mica, right?
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Yes, just a little dab is all that's needed on both sides then tighten the screws evenly allowing the thermal grease to squish out, this takes the air pockets with it.
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#15
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Just to be on the safe side, is good to check or sub the pulse H cap (or resonating/efficiency cap). Is one generally having one pin to collector of HOT and another to ground (like the ceramic cap), in the board. It have values in the vicinity of 6 to 12nF, with at least 1600V of WV. A failure with that, and the HOT shortens out.
__________________
So many projects, so little time... |
Audiokarma |
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