PLL reference oscillator failure
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PLL reference oscillator failure
 
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Hi everyone,
I've just got to the Alignment and Test Part 2 for my K2 and right off I've hit a problem. I've got the freqency counter probe plugged into TP3, and I'm in Cal Fctr, but instead of the expected 12090 I'm getting around 400. It wavers between 398 and 400. The frequency probe works all right, when I try it on the 4 MHz crystal on the control board, I get a reading as expected in the Aligntment part 1. I'll be spending more time tomorrow reviewing the PLL area but if anyone has some suggestions on likely culprits, advice would be most welcome. Thanks & 73! -Stephanie va3uxb _______________________________________________ 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: PLL reference oscillator failure
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Stephanie,
One of the high probability places for a solder failure is on the thermistor board. First thing is to check the 4 volt output - look for 4 volts on RF Board U6 pin5 - if it is not close to 4 volts, find the solder splash on the thermistor board. Sometimes you can examine the board by removing the front panel and control board, but it is no great deal to remove it completely, just clip the 8 wires and take it out - solder new wires to re-install. You are most likely to find a solder problem somewhere in the PLL area, but that thermistor board is the most likely suspect. One other common cause of PLL failure is T5 - be certain the winding sense is correct - if you have one of the windings reversed, it will not work. 73, Don W3FPR > -----Original Message----- > > I've just got to the Alignment and Test Part 2 for my K2 and right > off I've hit a problem. I've got the freqency counter probe plugged > into TP3, and I'm in Cal Fctr, but instead of the expected 12090 I'm > getting around 400. It wavers between 398 and 400. The frequency > probe works all right, when I try it on the 4 MHz crystal on the > control board, I get a reading as expected in the Aligntment part 1. > > I'll be spending more time tomorrow reviewing the PLL area but if > anyone has some suggestions on likely culprits, advice would be most > welcome. > > Thanks & 73! > > -Stephanie > va3uxb > > _______________________________________________ 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: PLL reference oscillator failure
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Hi everyone,
Thank you all so much for all your suggestions and advice! It looks like I have some kind of intermittent fault in my PLL Reference Oscillator. I've retouched all the solder points in that area incase it was a cold joint, and after poking around at it for a bit it suddenly seemed to be working. Then after a few minutes it went dead again, no oscillation. So I poked the crystal X1, just kind of tapped it, and suddenly it's oscilating again. I've verified that there are no cold solder joints, the crystal is securely soldered and the case is grounded. After working on it for a while, it seems like the slightest touch or movement to the rig can cause the reference oscillator to start and stop. Does this problem suggest a failure within the crystal itself? Thanks also for the advice and feedback about the case bottom, I've got the feet parallel now and everything seems good there. 73 de Stephanie va3uxb _______________________________________________ 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: PLL reference oscillator failure
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... or a short under the can. It's easy to get enough solder up the leads to
contact the can where you can't see it. If there's too much solder, you could get that symptom. I'd pull the crystal, clean the holes carefully with solder-wick, and reinstall with attention not to over-use the solder. Note, you can most likely leave the ground wires on the cans, just treat it/them as another lead of the (now three-legged) crystal. Dan / WG4S / K2 #2456 <snip> Does this problem suggest a failure within the crystal itself? </snip> _______________________________________________ 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: PLL reference oscillator failure
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In reply to this post by Stephanie Maks
The most sensitive part is almost always the problem part! Try gently
tapping other parts. I've often used the rubber eraser end of a pencil to make gentle pokes to determine which part is the most sensitive. Sometimes it's hard to tell! One thing that can cause this behavior is marginal feedback. Crystals oscillate by vibrating mechanically. A small change by bumping a marginal crystal can start it, or the change in capacitance caused by touching the can. Make sure C84 and C85 are the right value parts. They control the feedback. Also be sure the diodes are in the right way around: D16, D17 and D18. C86 is important to keep the source of Q19 at RF ground. And, of course, you should confirm that you have 8 volts between ground and one side of C86 that doesn't disappear if you tap things! If it's your crystal, it could be an intermittent crystal but that's really, really rare! One problem people run into installing crystals is letting too much solder flow around the pins. The solder joint look fine on the bottom, but the solder flows through the hole (they are plated all the way through) and 'puddles' around the lead under the crystal where you can't see it! If the puddle is just tiny bit too big, it'll short the crystal to the case. It's often a 'cold', intermittent joint because the case wasn't hot enough to take the solder. The blob of solder around the pin is simply resting against the bottom of the crystal case. Try checking continuity from the pins to the crystal case while tapping/wiggling it to see if there's an intermittent. Keep in mind that most DMM's are useless for this purpose because they take a while to detect the short. If it's intermittent the DMM may not detect it. Many DMM's have a "continuity" check that sounds an alarm if even a brief contact is detected. The option is to remove the crystal and look! You'll need a good solder sucker (at least a spring-loaded unit like the Soldapullet Elecraft recommends) and take care not to put more heat on the leads than you need to. Ron AC7AC -----Original Message----- Hi everyone, Thank you all so much for all your suggestions and advice! It looks like I have some kind of intermittent fault in my PLL Reference Oscillator. I've retouched all the solder points in that area incase it was a cold joint, and after poking around at it for a bit it suddenly seemed to be working. Then after a few minutes it went dead again, no oscillation. So I poked the crystal X1, just kind of tapped it, and suddenly it's oscilating again. I've verified that there are no cold solder joints, the crystal is securely soldered and the case is grounded. After working on it for a while, it seems like the slightest touch or movement to the rig can cause the reference oscillator to start and stop. Does this problem suggest a failure within the crystal itself? Thanks also for the advice and feedback about the case bottom, I've got the feet parallel now and everything seems good there. 73 de Stephanie va3uxb _______________________________________________ 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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In reply to this post by Stephanie Maks
I would check the solder connections on the toroids in the PLL section, in particular, the transformers. Check continuity thru the windings by measuring from other component leads connected to the pads the toroid leads are soldered to. If you don't get the toroid leads cleaned and tinned properly it's easy to end up with intermitant connections.
By the way, I'm impressed with the speed at which you're assembling your K2! It took me about two weeks (working an hour or two during the week and a marathon session one weekend) to get were you are in only two days! 73 de Mike, N0SO ---- Stephanie Maks <[hidden email]> wrote: > Hi everyone, > > Thank you all so much for all your suggestions and advice! It looks > like I have some kind of intermittent fault in my PLL Reference > Oscillator. I've retouched all the solder points in that area incase > it was a cold joint, and after poking around at it for a bit it > suddenly seemed to be working. Then after a few minutes it went dead > again, no oscillation. _______________________________________________ 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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In reply to this post by Stephanie Maks
Well I finally got the PLL reference oscillator to oscillate
consistently. I removed the crystal and found a tiny little sliver of metal shorting one of the pins to the case. I don't think it was solder as it wouldn't melt but I was able to pick it off with tweezers. I also re-seated D18, it was sticking up just a little and might have been shorting against the hardware that holds the 'foot' to the case bottom. After that, I was able to complete the Alignment & Test #2. Tomorrow I'll start on part 3 of the build. I did find one other thing that doesn't seem quite right. I'm having some trouble with the AGC circuit, and adjusting R1 on the control board. It says to adjust R1 to see about 3.9 volts on pin 5 of U2, but I can't get any more than 3.7 volts. Turning R1 through its full range, I go up to 3.7 with about 75% rotation, then the last 25% it just stays at 3.7 volts. I don't know if this could be a problem later. I'm reading 7.8 volts on the 8v line so it might just be that the 8v is a bit low. Thanks again for all the help everyone! 73 de Stephanie va3uxb _______________________________________________ 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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Stephanie,
Congraulations on finding the source of your PLL problem - those intermittents can be quite nasty to find. It is likely that your low 8 volt regulator is causing your lower AGC Threshold. Actually I would recommend not worrying about it at this stage - but just make a note of it. Later, you may want to optimize the AGC Threshold so there is little or no difference in the receiver gain between AGC on and AGC off (with no antenna connected) - and that condition will assure you that the AGC is not reducing the receiver gain when there is no signal present. The optimum setting in my experience is between 3.65 and 3.8 volts, but it does vary from K2 to K2. The actual voltage on the 8v line will make a significant difference on the voltage available at the AGC Threshold measurement point. BTW - with the 8 volt regulator at 7.8 volts - that is only 2.5% less then the nominal voltage, so the regulator itself is operating within its spec even though that voltage is a bit lower than usual. 73, Don W3FPR > -----Original Message----- > > Well I finally got the PLL reference oscillator to oscillate > consistently. > > I removed the crystal and found a tiny little sliver of metal > shorting one of the pins to the case. I don't think it was solder as > it wouldn't melt but I was able to pick it off with tweezers. I also > re-seated D18, it was sticking up just a little and might have been > shorting against the hardware that holds the 'foot' to the case bottom. > > After that, I was able to complete the Alignment & Test #2. Tomorrow > I'll start on part 3 of the build. > > I did find one other thing that doesn't seem quite right. I'm having > some trouble with the AGC circuit, and adjusting R1 on the control > board. It says to adjust R1 to see about 3.9 volts on pin 5 of U2, > but I can't get any more than 3.7 volts. Turning R1 through its full > range, I go up to 3.7 with about 75% rotation, then the last 25% it > just stays at 3.7 volts. I don't know if this could be a problem > later. I'm reading 7.8 volts on the 8v line so it might just be that > the 8v is a bit low. > > Thanks again for all the help everyone! > > 73 de Stephanie > va3uxb > > _______________________________________________ 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: PLL Ref. Osc. Fixed
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In reply to this post by Stephanie Maks
With normal component values, and 7.8 V on 8A, you will get 3.72 V max at
U2B pin 5, so you've done everything right. Even with 8.0 V on 8A, you shouldn't get more than 3.82 V at U2B pin 5 with RF gain set full clockwise. (The manual calls for 3.8 Volts there, not 3.9) --I did find one other thing that doesn't seem quite right. I'm having --some trouble with the AGC circuit, and adjusting R1 on the control --board. It says to adjust R1 to see about 3.9 volts on pin 5 of U2, --but I can't get any more than 3.7 volts. _______________________________________________ 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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