K2 AGC Off Problem
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Just finished alignment and test, part II of K2 #5093. All seems to be working
as it should, except for the AGC off function. This problems appears to be the same as a few others have had, the cause of which was a bad 5.068 MHz crystal (X1). Perhaps I have the same problem. When the AGC is turned off the S-meter pegs at 40 over and the receiver is muted. The AGC threshold at pin 5 of U2 measures 3.8 volts whether AGC is on or off. The voltage at pin 1 of U2 goes from 3.8 volts with the AGC on to 5.4 volts with the AGC off. All components in the AGC circuit have been inspected again for correct values and placement. All solder joints have been reheated. I cannot measure the 5.068 MHz oscillator with my frequency counter (MFJ-259) at pin 6 or 7 of U1. Perhaps this counter loads down the circuit too much for it to oscillate? Listening with a portable shortwave receiver (5 kHz steps) finds a weak carrier at 5.070 that goes on and off with the power on the K2. Before I email Elecraft support, what else should I check? Darrell VA7TO -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Hi Darrell:
Your report is very similar to what I experienced. >From an earlier email.... > Wayne suggested this too: > > "The simplest cure for this may be a different capacitor in the > oscillator that makes it too inefficient at reduced bias to even start. > > You might try 10 pF at C8 or 680 pF at C7 I installed a 10pf cap at C8 and it worked. I'm sure someone else did the same thing with success. Good luck. 73, Ken K3IU -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Darrell Bellerive Sent: Saturday, May 20, 2006 5:37 PM To: [hidden email] Subject: [Elecraft] K2 AGC Off Problem Just finished alignment and test, part II of K2 #5093. All seems to be working as it should, except for the AGC off function. This problems appears to be the same as a few others have had, the cause of which was a bad 5.068 MHz crystal (X1). Perhaps I have the same problem. When the AGC is turned off the S-meter pegs at 40 over and the receiver is muted. The AGC threshold at pin 5 of U2 measures 3.8 volts whether AGC is on or off. The voltage at pin 1 of U2 goes from 3.8 volts with the AGC on to 5.4 volts with the AGC off. All components in the AGC circuit have been inspected again for correct values and placement. All solder joints have been reheated. I cannot measure the 5.068 MHz oscillator with my frequency counter (MFJ-259) at pin 6 or 7 of U1. Perhaps this counter loads down the circuit too much for it to oscillate? Listening with a portable shortwave receiver (5 kHz steps) finds a weak carrier at 5.070 that goes on and off with the power on the K2. Before I email Elecraft support, what else should I check? Darrell VA7TO _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Ken,
Thanks for the message. A 10 pf at C8 did the trick. The receiver is now working properly in AGC off mode. What are the implications of this substitution? Does changing C8 from 39 pf to 10 pf cause any other problems? Since I robbed the 10 pf from it's future spot in the 10 meter low pass filter, a replacement part is needed in any case. Is it better to replace X1, and go back to the original 39 pf capacitor, or should I just get another 10 pf for the low pass filter and leave the 10 pf at C8? Darrell On May 20, 2006 04:16 pm, Ken K3IU wrote: > Hi Darrell: > > Your report is very similar to what I experienced. > > >From an earlier email.... > > Wayne suggested this too: > > > > "The simplest cure for this may be a different capacitor in the > > oscillator that makes it too inefficient at reduced bias to even start. > > > > You might try 10 pF at C8 or 680 pF at C7 > > I installed a 10pf cap at C8 and it worked. I'm sure someone else did the > same thing with success. > > Good luck. > > 73, Ken K3IU > > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]] On Behalf Of Darrell Bellerive > Sent: Saturday, May 20, 2006 5:37 PM > To: [hidden email] > Subject: [Elecraft] K2 AGC Off Problem > > Just finished alignment and test, part II of K2 #5093. All seems to be > working as it should, except for the AGC off function. This problems > appears to be the same as a few others have had, the cause of which was a > bad 5.068 MHz crystal (X1). Perhaps I have the same problem. > > When the AGC is turned off the S-meter pegs at 40 over and the receiver is > muted. > > The AGC threshold at pin 5 of U2 measures 3.8 volts whether AGC is on or > off. > > The voltage at pin 1 of U2 goes from 3.8 volts with the AGC on to 5.4 volts > with the AGC off. > > All components in the AGC circuit have been inspected again for correct > values and placement. All solder joints have been reheated. > > I cannot measure the 5.068 MHz oscillator with my frequency counter > (MFJ-259) at pin 6 or 7 of U1. Perhaps this counter loads down the circuit > too much for it to oscillate? > > Listening with a portable shortwave receiver (5 kHz steps) finds a weak > carrier at 5.070 that goes on and off with the power on the K2. > > Before I email Elecraft support, what else should I check? > > Darrell > VA7TO > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > 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 -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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No. Changing that cap only affects the heterodyne oscillator for the AGC and
nothing else. As long as the oscillator comes on and off on command (AGC On and Off work properly) you're FB. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Darrell Bellerive Sent: Saturday, May 20, 2006 5:50 PM To: [hidden email] Subject: Re: [Elecraft] K2 AGC Off Problem Ken, Thanks for the message. A 10 pf at C8 did the trick. The receiver is now working properly in AGC off mode. What are the implications of this substitution? Does changing C8 from 39 pf to 10 pf cause any other problems? Since I robbed the 10 pf from it's future spot in the 10 meter low pass filter, a replacement part is needed in any case. Is it better to replace X1, and go back to the original 39 pf capacitor, or should I just get another 10 pf for the low pass filter and leave the 10 pf at C8? Darrell _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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> Thanks for the message.
> A 10 pf at C8 did the trick. The receiver is now working properly in AGC off > mode. Think that's at least 5 or 6 of us on the reflector have had this issue, and this easy fix, so far. Does it deserve a builder alert on the website yet? For anyone else: If you are seeing AGC problems (hold PRE/ATT and AGC, see if your K2 goes deaf), check your junk box for a 10pF cap and change control board C8. Wayne says 680 pF at C7 may be another option. I DO NOT recommend this mod UNLESS you're seeing "AGC OFF" issues though. -- "Nosey" Nick Waterman, G7RZQ, K2 #5209. use Std::Disclaimer; [hidden email] Know what I hate most? Rhetorical questions. -- Henry N. Camp _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
Re: K2 AGC Off Problem
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If in some cases the value of C8 has to be reduced OR the value of C7 has to
be increased ( as Wayne suggests ) for proper operation of the 5.068 MHz oscillator, it would seem that the original capacitor values result in too much feedback in the oscillator. This could be a "batch" problem where some crystals have a higher Q than the norm OR the oscillator part of the NE602 (U1) has higher gain than the norm, or both. IMHO changing the value of either C8 or C7 sounds like a good fix, and any small change in frequency that results does not matter. If it is a "batch" problem I would think that it would affect a small group of K2s (by serial number) if Elecraft use the crystals / NE602s soon after delivery. Whether or not the problem deserves a Builder Alert should be suggested by a more experienced "Elecrafter" than I. 73, Geoff GM4ESD ----- Original Message ----- From: "Nick Waterman" <[hidden email]> To: <[hidden email]>; <[hidden email]> Cc: <[hidden email]> Sent: Sunday, May 21, 2006 9:36 AM Subject: Re: [Elecraft] K2 AGC Off Problem >> Thanks for the message. >> A 10 pf at C8 did the trick. The receiver is now working properly in AGC >> off >> mode. > > Think that's at least 5 or 6 of us on the reflector have had this issue, > and this easy fix, so far. Does it deserve a builder alert on the > website yet? > > For anyone else: If you are seeing AGC problems (hold PRE/ATT and AGC, > see if your K2 goes deaf), check your junk box for a 10pF cap and change > control board C8. Wayne says 680 pF at C7 may be another option. I DO > NOT recommend this mod UNLESS you're seeing "AGC OFF" issues though. > > -- > "Nosey" Nick Waterman, G7RZQ, K2 #5209. _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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