I'm replacing a failed Bipolar transistor in my Marantz PM40 Special Edition. A thought struck me that perhaps I should at the same time renew the thermal paste on the other three. Is this wise or should I leave them alone?
Ian
Ian
Normal etiquette would be to replace all transistors on that channel.
That said - I replace it (thermal paste) anytime I have an amp open and making a repair in that area.
I’m sure there will be alternative points of view on this.
That said - I replace it (thermal paste) anytime I have an amp open and making a repair in that area.
I’m sure there will be alternative points of view on this.
Leave them alone unless actually replacing them or separatingb them from heat sink for any reason.
Just as preventive maintenance? ... no.
Question. If I am not really interested in maintaining galvanic insulation of a thermal grease, but I actually am interested in achieving a lower value of thermal resistance Kelvin/Watt, would it make sense to take a middle shelf thermal grease jar, say 4,8 K/W,
and mix it with nanocarbon, or at least with fine graphite dust?
...
As far as question from post #1 goes, I vote Nay. If it works - don't fix it.
and mix it with nanocarbon, or at least with fine graphite dust?
...
As far as question from post #1 goes, I vote Nay. If it works - don't fix it.
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Surely there is the risk that conductive paste of any type might creep from the device heatsink to the fixing screw which would result in a short to the amp main heatsink?
Ian
40 year old mica washers get brittle and may fail. A reason to remove and replace even if the paste is still conducting heat.
Unnecessarily removing transistors is more likely to damage a mica (and the transistor) than ageing. I vote for leave alone, except when necessary to fix a fault.
A specific CFP configuration that I am considering, actually has the collectors of all of the devices directly connected to the speaker output rail. So the idea is, to screw all the relevant transistors (driver and power transistors) to a copper thermal bus, which at the same time serves the purpose of a speaker output rail. Indeed, the whole rail would then be carrying the speaker output signal.
It would be this whole (big surface) copper rail that is actually screwed on to the heat sink proper, by means of a silicon galvanic insulation pad. Such silicon pads can be purchased as whole "sheets", from which I could cut out the necessary 38 cm x 5 cm insulation pad.
As the transistors are basically "screwed on" directly to the "speaker terminal rail" - there is no need for their galvanic separation from this rail. That is the reason why I am considering how to increase K/W, with no worries about electrical insulation.
Actually, I also was thinking about those thermal pastes that are available for mounting heat-sinks on high performance CPU's. But these "hyper-performance" pastes are somewhat controversial in terms of the price per 1 gram being offered, which annoys me.
Below - a concept of such circlotron output stage, with CFP devices that are mounted electrically direct to the speaker output rails. Not sure about the stability (as in: no oscillations) of such a CFP stage, though.
As for the thermal stability - a non issue. Overall positive thermal coefficient. I hope.
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I have done a few, in some of the older Pioneers, SX-1010 for example they used some glue crap that is hard to remove. I ended up taking the mica off and a bit of sanding, to flatten out, I used a sil-pad instead of mica/paste. Putting new mica in after that glue was applied, I thought to be worse than using a good sil-pad.
You did not say what bjts you are using to replace what was original 2sa???? / 2sc????. What package the originals are in I am assuming TO-3 metal cans. I hope you are changing all of them and not using ebay parts. If TO-3 what else to use other than MJ2119x? I guess you could you TO-3P (meant to replace TO-3), maybe even TO-264, if you do some stick handling.
You did not say what bjts you are using to replace what was original 2sa???? / 2sc????. What package the originals are in I am assuming TO-3 metal cans. I hope you are changing all of them and not using ebay parts. If TO-3 what else to use other than MJ2119x? I guess you could you TO-3P (meant to replace TO-3), maybe even TO-264, if you do some stick handling.
I did that and the transistors lasted 10 minutes !
An unplugging of my soldering sent a glitch down the mains into the amp and blew the output transistors.
I replaced them with same parts from RS Components and since then, despite trying very hard, I cant blow them.
I can only guess the ebay parts hard a very low breakdown voltage.
I did buy some old Motorola branded MJL3281A/MJL1302A from ebay polita, we shall see if they are originals, sometimes you can tell as they oxidize and that shows age. Good for using in amp destruction tests, where you push them to their limits, it might be 10 minutes if done correctly.
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