Re: [Elecraft] K2 10W PA problem
Guido, I would like to see the base bias voltage a little bit lower - 0.63 maximum, .62 nominal. I recommend replacing Q11 and Q13. Yes, the current draw without Q7 and Q8 installed is within the normal range. 73, Don W3FPR Guido Tedeschi wrote: > I removed the PA transistors and measured the bias, which is 0.64V and > this should be fine... > Probably, it was only an heatsinking problem, unless the TX current is > wrong: without the PA transistors, with RF Power set to 5.0W (with any > value greater than 0.1W is the same), the TUNE current is 0.62A (0.24A > in RX) > Is it ok 0.62A? > Thanks and 73 > Guido, ik2bcp > > Don Wilhelm wrote: > >> Guido, >> >> Since the normal base to emitter voltage on the PA transistors self >> limits at about 0.63 volts, the best way to check the bias voltage is >> to remove the PA transistors and measure it. With the PA transistors >> out, if the base voltage during a TUNE is between 0.6 and 0.63 volts, >> then Q11 and Q13 are doing their job and you have faulty PA transistors. >> >> For proper heatsinking, yes the PA transistors must fit flat against >> the heatsink. Before soldering the PA transistors in place, I >> position them on their mounting hardware and secure them temporarily >> with a nut (no lockwasher, no thermal pad). That places the tab >> parallel with the RF board. >> Solder the transistors and remove the nuts - add the thermal pads and >> mount the heatsink. >> >> 73, >> Don W3FPR >> > > __ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
I removed the PA transistors and measured the bias, which is 0.64V and this should be fine... Probably, it was only an heatsinking problem, unless the TX current is wrong: without the PA transistors, with RF Power set to 5.0W (with any value greater than 0.1W is the same), the TUNE current is 0.62A (0.24A in RX) Is it ok 0.62A? Thanks and 73 Guido, ik2bcp Don Wilhelm wrote: > Guido, > > Since the normal base to emitter voltage on the PA transistors self > limits at about 0.63 volts, the best way to check the bias voltage is > to remove the PA transistors and measure it. With the PA transistors > out, if the base voltage during a TUNE is between 0.6 and 0.63 volts, > then Q11 and Q13 are doing their job and you have faulty PA transistors. > > For proper heatsinking, yes the PA transistors must fit flat against > the heatsink. Before soldering the PA transistors in place, I > position them on their mounting hardware and secure them temporarily > with a nut (no lockwasher, no thermal pad). That places the tab > parallel with the RF board. > Solder the transistors and remove the nuts - add the thermal pads and > mount the heatsink. > > 73, > Don W3FPR __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
Guido, Since the normal base to emitter voltage on the PA transistors self limits at about 0.63 volts, the best way to check the bias voltage is to remove the PA transistors and measure it. With the PA transistors out, if the base voltage during a TUNE is between 0.6 and 0.63 volts, then Q11 and Q13 are doing their job and you have faulty PA transistors. For proper heatsinking, yes the PA transistors must fit flat against the heatsink. Before soldering the PA transistors in place, I position them on their mounting hardware and secure them temporarily with a nut (no lockwasher, no thermal pad). That places the tab parallel with the RF board. Solder the transistors and remove the nuts - add the thermal pads and mount the heatsink. 73, Don W3FPR Guido Tedeschi wrote: > Tom W8JI wrote: > >> This is almost certainly thermal drift from heat. >> >> Poor heatsinking of transistors will cause this, so make sure he used >> the CORRECT very light amount of any heatsink compound and that the >> transistors are clean underneath and bolted to correct tightness. >> >> Also a very small upward drift in bias voltage will cause this. This >> can be because of leaky series pass bias transistors or bias pass >> transistor heating from inadequate heatsinking. >> >> > Many thanks to Tom and the others: I've noticed that PA transistors are > not installed perfectly horizontal and it is possible that the > mechanical contact with the cover is not so good. > This poor heatsinking can explain the drift but, probably, not the 1.2A > with only 0.5W... Maybe, also the two transistors used for bias voltage > are not so healty... > > __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
Tom W8JI wrote: > This is almost certainly thermal drift from heat. > > Poor heatsinking of transistors will cause this, so make sure he used > the CORRECT very light amount of any heatsink compound and that the > transistors are clean underneath and bolted to correct tightness. > > Also a very small upward drift in bias voltage will cause this. This > can be because of leaky series pass bias transistors or bias pass > transistor heating from inadequate heatsinking. > Many thanks to Tom and the others: I've noticed that PA transistors are not installed perfectly horizontal and it is possible that the mechanical contact with the cover is not so good. This poor heatsinking can explain the drift but, probably, not the 1.2A with only 0.5W... Maybe, also the two transistors used for bias voltage are not so healty... Thanks and 73 Guido, ik2bcp __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
This is almost certainly thermal drift from heat. > seem OK, the power from driver and the power output remain stable > during key-down, but the current increases... > What could be the issue? > Before spending more time on troubleshooting, has someone had and > solved the same problem? Poor heatsinking of transistors will cause this, so make sure he used the CORRECT very light amount of any heatsink compound and that the transistors are clean underneath and bolted to correct tightness. Also a very small upward drift in bias voltage will cause this. This can be because of leaky series pass bias transistors or bias pass transistor heating from inadequate heatsinking. __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
Guido, I had a similar problem with a HiCurr a couple years back working on a friends K2. The problem turned out to be R67, a 2% tolerance (blue) resister that controls feed back to the RF power control; which was burned for some reason. I replaced it and fixed the problem. If the noise blanker is installed this resistor will be hard to see because it sits under it. I did not have a problem with increasing current, but it is worth a quick look just to make sure this is not the problem. Best of luck! 73, Kevin KG4TEI On Mon, Jul 12, 2010 at 4:28 AM, Guido Tedeschi wrote: > A friend asked to me to repair his K2 10W which has a strange problem: > too high current in TX (1.2A even with 0.5W) and current that > increases during key-down until HiCurr protection. > Already changed the 2 x 2SC1969 PA transistors and re-winded the > output transformer, but nothing changed: the voltages on PA and driver > seem OK, the power from driver and the power output remain stable > during key-down, but the current increases... > What could be the issue? > Before spending more time on troubleshooting, has someone had and > solved the same problem? > Thanks and 73 > Guido, ik2bcp > __ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
Guido, Did you also change Q11 and Q13? What I suspect is that the bias is incorrect, and increases during keydown causing the current draw by Q7 and Q8 to increase. Often Q11 and Q13 are damaged by a PA transistor failure, and if they are not changed, they will in turn damage the replacement Q7 and Q8. Where did you obtain the 2SC1969 transistors? I have heard reports of Chinese transistors that do not meet specification and fail quickly. Currently, the only source of 2SC1969 transistors that I trust is those at Elecraft - order K2PAKIT, it includes the PA transistors, replacements for Q11 and Q13 as well as R50 if needed, along with mounting hardware for the PA transistors. 73, Don W3FPR Guido Tedeschi wrote: > A friend asked to me to repair his K2 10W which has a strange problem: > too high current in TX (1.2A even with 0.5W) and current that > increases during key-down until HiCurr protection. > Already changed the 2 x 2SC1969 PA transistors and re-winded the > output transformer, but nothing changed: the voltages on PA and driver > seem OK, the power from driver and the power output remain stable > during key-down, but the current increases... > What could be the issue? > Before spending more time on troubleshooting, has someone had and > solved the same problem? > Thanks and 73 > Guido, ik2bcp > > __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] K2 10W PA problem
You say the power output stays the same. Is this into a dummy load and measured with a good RF watt meter (like Bird)? If that is the case then due to power staying the same, the resistance the PA sees must be decreasing. Is this happening on all bands or just one or two? This could be related to the LPF causing it if only one or two, need to check those items in the LPF causing it. If it is all bands, I would check the items on the output side of T2 for correct value and soldering. -- View this message in context: http://elecraft.365791.n2.nabble.com/K2-10W-PA-problem-tp5282243p5282647.html Sent from the Elecraft mailing list archive at Nabble.com. __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
[Elecraft] K2 10W PA problem
A friend asked to me to repair his K2 10W which has a strange problem: too high current in TX (1.2A even with 0.5W) and current that increases during key-down until HiCurr protection. Already changed the 2 x 2SC1969 PA transistors and re-winded the output transformer, but nothing changed: the voltages on PA and driver seem OK, the power from driver and the power output remain stable during key-down, but the current increases... What could be the issue? Before spending more time on troubleshooting, has someone had and solved the same problem? Thanks and 73 Guido, ik2bcp __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
