Don, Thanks for the quick response!
I do have the 5 volts on pin 8 of U5, but a constant 0 volts on pin 7 even when tapping the Band + and Band - switches. But I'm just looking at it with a Digital Voltmeter, not a scope so maybe I'm missing it. Pin 5 of U6 is 4 volts, so at least that data point is OK. Should I suspect U5 at this point? What can I do to verify that U5 is being properly triggered at its input when I tap the Band + and Band - switches? Thanks again. -- Dave WB4JTT --- On Wed, 12/31/08, Don Wilhelm <[email protected]> wrote: > From: Don Wilhelm <[email protected]> > Subject: Re: [Elecraft] K2 PLL reference oscillator range is zero > To: [email protected] > Cc: [email protected] > Date: Wednesday, December 31, 2008, 2:56 AM > Dave, > > Yes, do check U5, but ignore the voltages on pins 1, 2 and > 3 because they are digital and the active state may not > correspond with the voltage measured with a DMM. > U5 pin 8 must have 5 volts DC for the chip to work - if > that is not true, stop (all other tests are futile). > > Next monitor the voltage at U5 pin 7 while tapping BAND+ > and BAND-. the voltage should go to 4 volts or more when > BAND+ is tapped and to nearly zero volts when BAND- is > tapped. If that is true, then U5 and its driving digital > signals are OK. > > Next thing to check is U6 - check at pin 1. The voltage > when BAND+ is tapped should be near 8 volts and when BAND- > is tapped should be near zero volts. If U5 pin 7 was OK, > but this test fails, the fault is likely with U6 or the > thermistor board. > > To give a quick check on the thermistor board, measure the > voltage at U6 pin 5 - it should be 4 volts (or more > precisely, 1/2 of the 8 volt regulator voltage. > > If the U6 pin 1 output is correct, there is not much more > to check. R19 should have a similar voltage swing (between > BAND+ and BAND-), and the voltage on varactors D16 and D17 > cathodes should be similar. > > If all checks out so far, blame X1 or L31. > > Remember that since this K2 worked properly before, there > should be only a single failure. Your challenge is to find > the source. > > 73, > Don W3FPR > > > David Johnson wrote: > > Hello and Happy New Year to everyone. > > > > I have owned K2 serial no. 3980 for about five years > and it has worked flawlessly. A couple of days ago it > stopped tuning properly. Instead of tuning across a signal, > it now acts like a crystal controlled radio that tunes in > discrete steps of approximately 10 khz as the knob is > rotated. > > This same problem was described on this list just > last month by Jon, KB1QBZ, and he said that he solved it by > redoing the alignment and test steps, part II. So I began > by redoing the 4 MHz Oscillator Calibration, which gave me a > frequency of 12.086 Mhz. > > > > The next step is the PLL Reference Oscillator Range > test, but I got no change in the 12.086 Mhz indication by > tapping either the Band + or Band - switch. > > > > So I measured the voltages on U6, and got 0 volts on > pins 2, 3 and 4 at all times, with no change as I tapped the > Band + and Band - switches. I also got a steady 2.0 volts > on pin 7. I know these voltages are supposed to change as > you tap the Band + and Band - switches, but is this > something that you can see on a digital voltmeter, or is it > something instantaneous that you need to view with an > oscilloscope? > > > > I backed up and measured voltages on U5. I got 5V > on pin 1, and 0 volts on pins 3 and 4, all opposite of what > the voltage table lists. I also got 0 volts on pin 7 > continuously even when I tapped the Band + and Band - > switches. > > > > Finally I looked at the thermistor board, but I > wasn't sure quite how to investigate it. I have 1.2 > volts at the junction of RA/Rt/RB and 0 volts at the > junction of RB and RC. > > Are my symptoms consistent with a bad thermistor, or > is this more likely to be a failure within U5 or U6? > > > > Sorry for taking up all this bandwidth but I am not > sure what my next step should be. > > > > Thank you for any and all suggestions on how to > proceed. > > > > -- Dave WB4JTT > > _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
