2SC2879 Finals in KPA100

[Roy Morris-6]

When I was putting in the latest mods in my KPA100 and had the PC board separated from the heat sink, I noticed that the thermal pads looked really worn.  I believe the bottom of the 2SC2879s could actually make contact with the heat sink in places.  There is a black non-conductive coating on the raised portion of the heat sink though (which I believe is to aid in heat transfer).  I don't think the mounting surface of the 2SC2879 is a part of the circuit because the two mounting screws make contact between the transistor and heat sink anyway.  The thermal pads are only used for heat transfer and not for circuit isolation.  Am I right about this?  Or are the two emitter pads on the transistor connected internally to the mounting surface of the 2SC2879?  Before I put the shield back on I checked the bases and collectors of Q1 and Q2 and they checked out fine.  The amp works just fine, but I am going to replace the thermal pads to be on the safe side.

One of the errata sheet mods recommends placing a 1K resistor across RFC3.  I presume this is for harmonic suppression.  Instead of mounting the resistor across RFC3 on top of the board, I mounted it on the bottom of the PC board making sure there was good clearance between the resistor and the heat sink.  I wonder if the mounting position of this resistor is critical.

Any comments would be appreciated.   Roy Morris  W4WFB


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[Vic K2VCO]

Roy Morris wrote:
>  I don't think the
> mounting surface of the 2SC2879 is a part of the circuit because the
> two mounting screws make contact between the transistor and heat sink
> anyway.  The thermal pads are only used for heat transfer and not for
> circuit isolation.  Am I right about this?  

Yes.

> Or are the two emitter
> pads on the transistor connected internally to the mounting surface
> of the 2SC2879?  Before I put the shield back on I checked the bases
> and collectors of Q1 and Q2 and they checked out fine.  The amp works
> just fine, but I am going to replace the thermal pads to be on the
> safe side.

The pads may *look* like they are worn away, but they are probably not!
  At least that was my experience.  I replaced them, but when I removed
the old ones I was sorry that I had bothered -- they were actually fine.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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[Stewart Baker]

Roy,

The thermal pads are probably fine. On my KPA100 I found the same, after having
ordered a new set of pads. I would keep them as spares.

The 1k resistor across RFC3 is to reduce an 80m resonance in the KPA100 T/R
switch circuit. It should be OK mounted on either side of the PCB, provided it
does not short to anything, and that the leads are kept reasonably short.

73
Stewart G3RXQ

On Sat, 23 Jul 2005 12:06:16 -0400, Roy Morris wrote:

>
>
> When I was putting in the latest mods in my KPA100 and had the PC board
> separated from the heat sink, I noticed that the thermal pads looked really
> worn.  I believe the bottom of the 2SC2879s could actually make contact with
> the heat sink in places.  There is a black non-conductive coating on the
> raised portion of the heat sink though (which I believe is to aid in heat
> transfer).  I don't think the mounting surface of the 2SC2879 is a part of
> the circuit because the two mounting screws make contact between the
> transistor and heat sink anyway.  The thermal pads are only used for heat
> transfer and not for circuit isolation.  Am I right about this?  Or are the
> two emitter pads on the transistor connected internally to the mounting
> surface of the 2SC2879?  Before I put the shield back on I checked the bases
> and collectors of Q1 and Q2 and they checked out fine.  The amp works just
> fine, but I am going to replace the thermal pads to be on the safe side.
>
> One of the errata sheet mods recommends placing a 1K resistor across RFC3.  I
> presume this is for harmonic suppression.  Instead of mounting the resistor
> across RFC3 on top of the board, I mounted it on the bottom of the PC board
> making sure there was good clearance between the resistor and the heat sink.
> I wonder if the mounting position of this resistor is critical.
>
> Any comments would be appreciated.   Roy Morris  W4WFB

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