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Old 24th Jan 2021, 3:01 pm   #1
SnowyDog
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Default Thermal paste / compound recommendations for H.O.T.

Hello,

Any suggestions for thermal paste / compound suitable for use with a mica washer on a horizontal line output transistor, please?

These days it seems that the majority of available thermal pastes are targeted at low-voltage applications (mainly PC CPU / GPU) and these pastes contain metal.

I have found that pastes do not even have a breakdown voltage parameter mentioned in their specifications, so I can't be sure if they will be safe to use for TV repair.

Cheers
Robin
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Old 24th Jan 2021, 3:34 pm   #2
snowman_al
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Default Re: Thermal paste / compound recommendations for H.O.T.

Servisol or Dow Corning?
From Farnell, CPC, RS etc.
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Old 24th Jan 2021, 5:46 pm   #3
John_BS
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Default Re: Thermal paste / compound recommendations for H.O.T.

Any silicone-based "white" compound should be fine. Don't use too much. It should not flow out more than a tiny sliver, if that.

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Old 24th Jan 2021, 6:42 pm   #4
ChristianFletcher
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Default Re: Thermal paste / compound recommendations for H.O.T.

Not sure of your application but are thermal pad an option as they have better electrical and thermal properties than the old silicon grease
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Old 24th Jan 2021, 8:07 pm   #5
SnowyDog
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Default Re: Thermal paste / compound recommendations for H.O.T.

Thank you for your replies.

Quote:
Originally Posted by ChristianFletcher View Post
Not sure of your application but are thermal pad an option as they have better electrical and thermal properties than the old silicon grease
This is a Sony Trinitron 10" colour computer monitor from around 1984 (sold as an expensive upgrade over the basic green screen monitor with the Apricot F1,F2 and F10 computers) - the horizontal output transistor is therefore switching at around 15kHz and with voltages I think in the region of 800 to 1200V, so there is a risk of the corona effect. The PCB has air gaps to mitigate this around the transistor and the transistor is bolted to a grounded heatsink.

I did wonder about using a "modern" thermal pad instead of the mica washer and thermal grease, but I have no idea whether that would be a good option for such a high voltage & high frequency application.

Has anyone else used thermal pads instead of the original mica washer for CRT H.O.T. TV repair?

Also, whilst on the subject. The 2SD1398 transistor is in TO-3P package and when I removed the bad transistor I found that there were two mica washers between the transistor and heatsink. Is this acceptable practice? I'd be concerned that the thermal conduction would be badly impaired by using two mica washers?
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Old 24th Jan 2021, 8:14 pm   #6
G6Tanuki
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Default Re: Thermal paste / compound recommendations for H.O.T.

Could the two micas be an attempt to control stray-capacitance between the transistor and the heatsink?

Or to handle the peak voltage more reliably?

I remember using two micas to reduce collector-to-earth capacitance in an 80M HF transmitter which featured an AUY10 Germanium TO3 transistor as the RF power-amplifier.
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