K2 signal tracing question
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I'm fault finding a K2 that is deaf and has low TX output and have a basic question regarding the sig gen I'm using. My generator will only produce 145mV pk to pk and I want to know if this should be enough to drive the AGC into saturation. I'm only measuring 4.9V on pin 1 of U2 which seems way low compared to the spec. Is this enough input as the spec is 145mV RMS from a RF probe (which I don't have - I'm using a scope). Initial checks haven't shown up anything significant but I will start from scratch again tomorrow as this week has been lost repairing my scope.
Serial no. is 2756 and it has the rev A boards BTW. 73 Paul M1PAF |
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Paul,
That is not enough signal to saturate the AGC for certain - but then if you have the Extremely Strong Signal Handling diodes in place you will likely find a similar condition because the diodes clip on those very large signals. All is not lost, you can still do the signal tracing with a lower level signal as long as you 'use your head' instead of the RF voltages indicated in the manual. You can approximate the gain/loss of each stage and check for that amount of gain/loss with the scope reading on the output vs the input of any stage. Did you really mean 4.9V signal? That is 1.75 volts RMS and is well in excess of what is required - if you meant 4.9 mV peak to peak, then fine, but if you are really measuring 4.9 volts, then something is wrong (sounds like a strong oscillation). Check the BFO input to the Product Detector at U11 pin 6 - it should be in excess of 200 mV peak to peak. Likewise check the RX VFO signal to the mixer - it must be +7dBm (5 mW) which into a 50 ohm load translates into 0.5 volts RMS or 1.4 volts peak to peak. A weak BFO signal can explain both bad receive and low transmit output. On transmit, the Transmit Mixer also needs more than 200 mV peak to peak from the VFO to U10 pin 6. Be certain the Post-Mixer Amp is amplifying, as well as the IF amplifier. If the input to the IF amplifier is low, it may be because of a KSB2 board problem - remove the KSB2 and put jumpers from pins 1 to 3 of both J9 and J10. Has this K2 worked before and suddenly failed? Or is this a new build of an older kit? In both cases, look for a solder problem, like an unsoldered connection or a poorly soldered joint. It is not unusual for a bad solder connection to work fine until enough oxidation builds up to cause a failure. 73, Don W3FPR Paul Fletcher wrote: > I'm fault finding a K2 that is deaf and has low TX output and have a basic > question regarding the sig gen I'm using. My generator will only produce > 145mV pk to pk and I want to know if this should be enough to drive the AGC > into saturation. I'm only measuring 4.9V on pin 1 of U2 which seems way low > compared to the spec. Is this enough input as the spec is 145mV RMS from a > RF probe (which I don't have - I'm using a scope). Initial checks haven't > shown up anything significant but I will start from scratch again tomorrow > as this week has been lost repairing my scope. > > Serial no. is 2756 and it has the rev A boards BTW. > > 73 Paul M1PAF > > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.5.424 / Virus Database: 270.14.49/2480 - Release Date: 11/04/09 07:37:00 > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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No - 4.9V AGC voltage which should be over 6V at saturation hence the question about the generator and it's output. Thanks for the pointers - I was looking for relative loss to the input signal anyway so will carry on tomorrow in more detail. Tonight was a quick poke around to see if I could see anything obvious. This K2 came from ebay and was like this when bought by it's present owner so I don't know if it's always been like it. I suspect a soldering problem somewhere as the insulation stripping on the toroids doesn't look very clever but don't want to jump to any conclusions just yet. 73 Paul M1PAF |
Re: K2 signal tracing question
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In reply to this post by Paul Fletcher
Paul Fletcher wrote:
> into saturation. I'm only measuring 4.9V on pin 1 of U2 which seems way low > compared to the spec. Is this enough input as the spec is 145mV RMS from a Where are you getting the specification from? The K2 doesn't operate the IF amplifier at the Vcc value given in its data sheet and the AGC operation scales with supply voltage. > RF probe (which I don't have - I'm using a scope). Initial checks haven't > shown up anything significant but I will start from scratch again tomorrow > as this week has been lost repairing my scope. > -- David Woolley "we do not overly restrict the subject matter on the list, and we encourage postings on a wide range of amateur radio related topics" List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Look at the signal tracing section of the manual. U2 is the AGC amplifier and that runs of the 8V supply. At AGC saturation the AGC amplifier should drive the AGC voltage to over 6 volts (can't remember exact value). I couldn't get anywhere near this but it seems my generator is not man enough to drive the AGC into saturation anyway as per Don's comments earlier. Regards, Paul M1PAF |
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