Elecraft

K2 No.6437

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Peter Wierach

K2 No.6437

Peter, DF8BB

Hi, since March last year my K2/100/KAT100 was working very good, but in last December I was tuning an antenna when suddenly it shut down and I could not repower it again. (I guess I was selecting the wrong antenna) After that, one of the PA transistors was damaged and I replaced it.

Now it seems to be ok but in low power the - HI CURR - warning appears and the the SSb transmision was very distorted.

So I knew i have to do some troubleshooting. I removed the KPA 100, SSB board, KAF board and the 160m board. I placed the required jumpers across J9, J10 and C167 across pin 7/12 of J11, jumper W1 left open. I did some testing but at least the warning - HI CURR - is appearing immediately when transmitting, regardless to the setting of the power controll.

I checked the transmitter and the TR-switch and replaced the entire transistors, diodes and inductors in these stages. All resitors and capacitors have been checked and they are ok. 8T and 8R switching is working. (there is only 7.6 Volt due to the voltage regulator) I rewound T2, T3 and T4.

I lifted up the output winding and supplied RF to the LPF input and there was no RF at any LPF except the selected Band.

I replaced D9 and checked the resistors at the RF detector circuit.

At this time the PA transistors Q7/Q8 are not installed, because I think there is something wrong on the control board starting at VRFDET to U6, U8, U10A, and the ALC circiut.

Before I go and kill some more PA transistors I would like to know if somebody has any suggestions how to check the contorl board VRFDET, VALC and ALC without installed Q7/Q8.

My Idea is to apply DC voltage at VRFDET to see how the ALC and the Power control are working. But I`m afraid of some more damage doing that, has anybody experiences with that?

thanks for any help...

73 Peter, DF8BB

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Don Wilhelm-4

Re: K2 No.6437

Peter,

Before you go into any troubleshooting of the power control circuits,
try this ---

Since you have the PA transistors out already, go into TUNE and measure
the DC voltage at the solder pads for the base of Q7 and Q8. If it is
not close to 0.6 volts, replace Q11 and Q13. Those transistors maintain
the base DC drive to the PA transistors. I would speculate that one of
them is either open or shorted and causes the PA transistors to conduct
too heavily (and also become non-linear).
The net result is high current and SSB distortion, and after a short
period of transmitting will kill one or both PA transistors.

73,
Don W3FPR

Peter wrote:

> Peter, DF8BB
>
> Hi, since March last year my K2/100/KAT100 was working very good, but
> in last December I was tuning an antenna when suddenly it shut down
> and I could not repower it again. (I guess I was selecting the wrong
> antenna) After that, one of the PA transistors was damaged and I
> replaced it.
> Now it seems to be ok but in low power the - HI CURR - warning appears
> and the the SSb transmision was very distorted.
>
> So I knew i have to do some troubleshooting. I removed the KPA 100,
> SSB board, KAF board and the 160m board. I placed the required jumpers
> across J9, J10 and C167 across pin 7/12 of J11, jumper W1 left open. I
> did some testing but at least the warning - HI CURR - is appearing
> immediately when transmitting, regardless to the setting of the power
> controll.
>
> I checked the transmitter and the TR-switch and replaced the entire
> transistors, diodes and inductors in these stages. All resitors and
> capacitors have been checked and they are ok. 8T and 8R switching is
> working. (there is only 7.6 Volt due to the voltage regulator) I
> rewound T2, T3 and T4.
>
> I lifted up the output winding and supplied RF to the LPF input and
> there was no RF at any LPF except the selected Band.
>
> I replaced D9 and checked the resistors at the RF detector circuit.
>
> At this time the PA transistors Q7/Q8 are not installed, because I
> think there is something wrong on the control board starting at VRFDET
> to U6, U8, U10A, and the ALC circiut.
>
> Before I go and kill some more PA transistors I would like to know if
> somebody has any suggestions how to check the contorl board VRFDET,
> VALC and ALC without installed Q7/Q8.
>
> My Idea is to apply DC voltage at VRFDET to see how the ALC and the
> Power control are working. But I`m afraid of some more damage doing
> that, has anybody experiences with that?
>
> thanks for any help...
> 73 Peter, DF8BB
>