Cooked K2/PA100 with PSK

[TI2/NA7U]

I was running my K2/100 at 40W during PSK and the heat sink was too hot to touch (ooops!). Now it has the following symptoms of sickness:

- in CW mode key down it generates either 100W or 10W when in high or low power mode respectively. In other words, always outputs maximum power.
- in SSB mode it won't put out more than 20W and only if I drive the audio pretty hard. I only do this for a second or two, not trying to make things worse =^)

I need advice on how best to organize my triage of the problem, where to look first, second, etc. I'm handy with tools, DMM, and at following instructions. :)

TIA,

Casey

[Don Wilhelm-4]

Casey,

Since it always transmits at maximum power, that alone may be your only
problem.  Look at it this way - if you had 40 watts requested and it was
putting out maximum power no matter what the power setting, it would be
operating at 100 watts or more and yes, on PSK, the heat sink would get
mighty hot.

The condition is that the K2 microprocessor is not being properly
signaled with the output level (the uP thinks there is no power), so it
ramps up the drive in an attempt to develop more power.  In SSB, the
control is on the KSB2 board which behaves a little differently, so
right now I am not very concerned with the power output level in SSB -
maybe later, but that is to be seen after you get power level control back.

The first thing to check is the condition of KPA100 diodes D16 and D17.  
You can check them in-circuit with your ohmmeter, just as you would
check any diode. low resistance one way, high resistance with the leads
the other way - if they do not measure like that, replace them.

If the diodes are both OK, then there is the possibility that there is a
problem with U5 or U6.  Let me know and I can give you some checks for
those circuits.

BTW, when things cool down (power off for more than two hours), check
the CAL TPA setting, and make certain the fan works at both low and high
speed.  It sounds suspiciously like your fan did not come on at high
speed like it should have.

73,
Don W3FPR


TI2/NA7U wrote:

> I was running my K2/100 at 40W during PSK and the heat sink was too hot to
> touch (ooops!). Now it has the following symptoms of sickness:
>
> - in CW mode key down it generates either 100W or 10W when in high or low
> power mode respectively. In other words, always outputs maximum power.
> - in SSB mode it won't put out more than 20W and only if I drive the audio
> pretty hard. I only do this for a second or two, not trying to make things
> worse =^)
>
> I need advice on how best to organize my triage of the problem, where to
> look first, second, etc. I'm handy with tools, DMM, and at following
> instructions. :)
>
> TIA,
>
> Casey
>  

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[TI2/NA7U]

D17 is about 1K ohms both directions, D18 about 2K ohms both directions. I can order these 1N5711s on eBay but it will take a couple weeks at least to receive them here in CR. Will have to re-assemble the radio to test CAL TPA settings, forgot to do it first.  PAs don't show any physical signs of cooking. I'd set the radio for 40W max on SSB and backed off the audio level so that the wattmeter showed half that. The 100W on the wattmeter was only measured in CW mode.

73,

Casey

[Don Wilhelm-4]

Casey,

If you have some 1N4148 diodes available, they will work.  The only
downside is that they do not work as well as the 1N5711s at very low
power levels, and they are not as flat with respect to frequency, but if
you can tolerate those items, 1N4148s will serve you well.

BTW - due to the way the K2 (and K3) control power, you are will achieve
better control and IMD if you drive the audio up to ust before the point
where ALC kicks in (that is no ALC bars on the K2, and the 4th bar - but
not the 5th - on the K3).  Then use the radio's power control to set the
power level.

You can do the CAL TMP without assembling everything, just put the
KPA100 board (or board and heatsink) upside down on a box or book to the
right of the K2.  You can plug the cables in and set the menu CAL TMP
properly - match the setting to the ambient air temperature.

73,
Don W3FPR

TI2/NA7U wrote:

> D17 is about 1K ohms both directions, D18 about 2K ohms both directions. I
> can order these 1N5711s on eBay but it will take a couple weeks at least to
> receive them here in CR. Will have to re-assemble the radio to test CAL TPA
> settings, forgot to do it first.  PAs don't show any physical signs of
> cooking. I'd set the radio for 40W max on SSB and backed off the audio level
> so that the wattmeter showed half that. The 100W on the wattmeter was only
> measured in CW mode.
>
> 73,
>
> Casey
>
> -----
> http://adullroar.blogspot.com Costa Rica Blog
>  

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