K2 Failure
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More on the defunct K2: I measured the DC voltage on both sides of F1,
the self-resetting fuse. With the power switch “off” the voltage is 13.8. With it on, the voltage is essentially zero on both sides of F1. Measured the resistance to ground at the collector of Q7 - it’s about 2 ohms, not the >500 it should be. But that doesn’t mean that Q7 or Q8 is the culprit. A short anywhere on the DC line could cause that reading anywhere else, yes? So, what’s the best procedure to isolate the problem — I removed the KNB2 and the KAF2; no change. Tomorrow, in the cold gray light of dawn, I will pull the front panel board and the control board off the RF board, to see if that narrows it down. Tonight, I will just have another glass of wine. That should help. Anyone know if F1 has a reset time of just under a second? If so, then the “popping” noise may be F1 trying to reset itself. Which doesn’t help locate the downstream problem, of course. But it would strongly suggest a DC short somewhere rather than some configuration glitch, yes? Ted, KN1CBR ______________________________________________________________ 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 Message delivered to [hidden email] |
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Ted,
The most common cause of low resistance on the collectors of Q7 and Q8 is failure of the thermal pads to insulate the DC voltage. Take the heatsink off and again measure the resistance between ground and the collector of Q7/Q8. If it is then OK, simply get new thermal pads. Yes, F1 trips very quickly. I don't know about the exact time, but that time is short. 73, Don W3FPR On 12/4/2015 10:56 PM, Dauer, Edward wrote: > More on the defunct K2: I measured the DC voltage on both sides of F1, > the self-resetting fuse. With the power switch “off” the voltage is 13.8. > With it on, the voltage is essentially zero on both sides of F1. > Measured the resistance to ground at the collector of Q7 - it’s about 2 > ohms, not the >500 it should be. But that doesn’t mean that Q7 or Q8 is > the culprit. A short anywhere on the DC line could cause that reading > anywhere else, yes? So, what’s the best procedure to isolate the problem > — I removed the KNB2 and the KAF2; no change. Tomorrow, in the cold gray > light of dawn, I will pull the front panel board and the control board off > the RF board, to see if that narrows it down. Tonight, I will just have > another glass of wine. That should help. > > Anyone know if F1 has a reset time of just under a second? If so, then > the “popping” noise may be F1 trying to reset itself. Which doesn’t help > locate the downstream problem, of course. But it would strongly suggest a > DC short somewhere rather than some configuration glitch, yes? > > ______________________________________________________________ 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 Message delivered to [hidden email] |
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In reply to this post by Edward A. Dauer
Good move taking a rest and a glass of wine — it is important not to panic, as that can lead to doing something silly :)
If you were entering 10000, you would have been tapping the « 0 » key next the the VFO knob multiple times, and maybe the pressure has caused the front panel and control boards to come into contact and short out. As I remember, there was not a lot of clearance there, especially with the adapter board for the encoder. I’d be looking for possible shorts around there. Just a comment on your observations with F1 — if you are applying power and measure 0V on both sides of F1, then that suggests that F1 is not open circuit. It may indicate that diode D10 has gone high resistance and is dropping the 13.8V when the radio tries to draw current. -- 73 de Matt VK2RQ On 5 décembre 2015 at 2:56:47 PM, Dauer, Edward ([hidden email]) wrote: More on the defunct K2: I measured the DC voltage on both sides of F1, the self-resetting fuse. With the power switch “off” the voltage is 13.8. With it on, the voltage is essentially zero on both sides of F1. Measured the resistance to ground at the collector of Q7 - it’s about 2 ohms, not the >500 it should be. But that doesn’t mean that Q7 or Q8 is the culprit. A short anywhere on the DC line could cause that reading anywhere else, yes? So, what’s the best procedure to isolate the problem — I removed the KNB2 and the KAF2; no change. Tomorrow, in the cold gray light of dawn, I will pull the front panel board and the control board off the RF board, to see if that narrows it down. Tonight, I will just have another glass of wine. That should help. Anyone know if F1 has a reset time of just under a second? If so, then the “popping” noise may be F1 trying to reset itself. Which doesn’t help locate the downstream problem, of course. But it would strongly suggest a DC short somewhere rather than some configuration glitch, yes? Ted, KN1CBR ______________________________________________________________ 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 Message delivered to [hidden email] ______________________________________________________________ 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 Message delivered to [hidden email] |
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Ted,
Matt is correct, if you measure 0 volts on *both* sides of F1, then the power source and D10 are suspect. It could be that the 0 volts is the result of your power source folding back or the soldering of D10 or the soldering of the power jack. In any case, cure the low resistance at the collectors of Q7/Q8 first - that low resistance indicates a direct short for the 12 volt power input. Once that is solved, you can move on to other potential problems. DO NOT attempt a TUNE or Transmit with the heat sink removed - doing that will kill the PA transistors very quickly. 73, Don W3FPR On 12/4/2015 11:36 PM, Matt Maguire wrote: > Good move taking a rest and a glass of wine — it is important not to panic, as that can lead to doing something silly :) > > If you were entering 10000, you would have been tapping the « 0 » key next the the VFO knob multiple times, and maybe the pressure has caused the front panel and control boards to come into contact and short out. As I remember, there was not a lot of clearance there, especially with the adapter board for the encoder. I’d be looking for possible shorts around there. > > Just a comment on your observations with F1 — if you are applying power and measure 0V on both sides of F1, then that suggests that F1 is not open circuit. It may indicate that diode D10 has gone high resistance and is dropping the 13.8V when the radio tries to draw current. > > -- > 73 de Matt VK2RQ > > On 5 décembre 2015 at 2:56:47 PM, Dauer, Edward ([hidden email]) wrote: > > More on the defunct K2: I measured the DC voltage on both sides of F1, > the self-resetting fuse. With the power switch “off” the voltage is 13.8. > With it on, the voltage is essentially zero on both sides of F1. > Measured the resistance to ground at the collector of Q7 - it’s about 2 > ohms, not the >500 it should be. But that doesn’t mean that Q7 or Q8 is > the culprit. A short anywhere on the DC line could cause that reading > anywhere else, yes? So, what’s the best procedure to isolate the problem > — I removed the KNB2 and the KAF2; no change. Tomorrow, in the cold gray > light of dawn, I will pull the front panel board and the control board off > the RF board, to see if that narrows it down. Tonight, I will just have > another glass of wine. That should help. > > Anyone know if F1 has a reset time of just under a second? If so, then > the “popping” noise may be F1 trying to reset itself. Which doesn’t help > locate the downstream problem, of course. But it would strongly suggest a > DC short somewhere rather than some configuration glitch, yes? > > Ted, KN1CBR > ______________________________________________________________ > 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 > Message delivered to [hidden email] > ______________________________________________________________ > 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 > Message delivered to [hidden email] ______________________________________________________________ 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 Message delivered to [hidden email] |
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In reply to this post by vk2rq
I'd agree that the voltage reading do not implicate F1.
However I would also question whether F1 would produce a popping sound, although it might through thermal effects, or by affecting the power to the audio chain. F1 isn't a circuit breaker; it is effectively a rather aggressive positive thermal coefficient (PTC) thermistor. When the current gets too high, the temperature goes up, this causes the resistance to increase, which increases the heat generation for the same current, and you have a thermal runaway. The resistance goes up so fast that it limits the current in the device severely before the device gets hot enough to self destruct (although its on resistance is permanently raised), and the actual current needed to keep it hot is quite low. Once the current demand drops below the triggered state current, it will begin to cool, and at some point, as long as the current remains low, it will cool faster than it is being heated and the resistance will drop to the point where increasing the current won't cause an immediate runaway. In the triggered state it reaches an equilibrium, letting through enough current to keep it in the high resistance state. It's not like a bi-metal circuit breaker that has no self heating once triggered and will eventually reset given just time. On the other hand, I imagine there are some load conditions in which a polyfuse would go into a slow oscillation. https://en.wikipedia.org/wiki/Resettable_fuse -- David Woolley Owner K2 06123 On 05/12/15 04:36, Matt Maguire wrote: > > Just a comment on your observations with F1 — if you are applying power and measure 0V on both sides of F1, then that suggests that F1 is not open circuit. It may indicate that diode D10 has gone high resistance and is dropping the 13.8V when the radio tries to draw current. > > On 5 décembre 2015 at 2:56:47 PM, Dauer, Edward ([hidden email]) wrote: > > Anyone know if F1 has a reset time of just under a second? If so, then > the “popping” noise may be F1 trying to reset itself. Which doesn’t help > locate the downstream problem, of course. But it would strongly suggest a > DC short somewhere rather than some configuration glitch, yes? ______________________________________________________________ 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 Message delivered to [hidden email] |
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In reply to this post by Edward A. Dauer
Two things. First, I want to say again how much I appreciate not just
this reflector, but more so the people who participate in it. Last night, within 20 minutes I had six replies from, literally, across the globe, every one of them encouraging and helpful. Thanks, guys. Second, for anyone who likes solving puzzles, I am reprinting below my recent off-list message to Don Wilhem, W3FPR, to whom the rig will be going for further investigation. Submit your diagnoses and I’ll let you know what Don discovers once he actually has the rig on his bench. No prizes available, but it might be fun . . . Here’s the latest status report: ___________________________________________ This morning the symptoms remained: No LCD light, popping noise in speaker, voltages at either side of F1 were 14 with power switch off and essentially zero with power switch on. The same on both sides of D10, and the same at the accessory DC power pins. Resistance from Q7 and Q8 cathodes to ground was about 2 ohms. 1. Removed control board and front panel board; tested RF board alone — same symptoms (low resistance and apparently a short somewhere on the 13 VDC line.) So those boards weren’t the problem 2. Reassembled everything and then removed the heat sink. Resistance measurements at Q7 and Q8 unchanged. Examined thermal pads, checked shoulder washer placement and all associated hardware. No changes – still low resistance from collectors to ground. 3. Reassembled and tried it again. No change – no lights, popping noise in speaker, voltages as in step 1 above. 4. As a last ditch effort, decided to change power supplies (had been using an MFJ rated at 40 watts.) Made up a cord to connect K2 to a known good supply with a built-in ammeter. I thought I’d be able to see a current spike if there was a short, as F1 opened and closed. The K2 came to life and the current draw was nominal (less than 200 mA.) Switched back to the MFJ supply. That worked too. Problem solved? No. See number 5. 5. Decided to see if it really was cured. Connected K2 to a known good dummy load, through an SWR meter. Keyed it with power set to 10 watts – but the forward power needle set for full scale barely nudged. Checked the power output as read by the K2’s onboard power meter. It read 0.5 watts, no matter where the power control is set. Confirmed that current maximum in the configuration menu was set at 3.5. It was. Tried the same routine with a known good antenna connected (measured SWR at less than 1.2:1) Same results. I don’t know what all this means, but I am going to take a wild guess. Last night something – maybe Q7 or Q8 but maybe something else – developed an internal short at exactly the instant I did a direct frequency entry of 10.000 Mhz (a relay clicks at that point). This morning, for whatever reason, when I changed power supplies, I am guessing that whatever had been shorted (or nearly so) burned out, creating an open circuit where there had been a low-resistance fault. Since the rig is producing 0.5 watts but not more, that makes me think it may be one of the output transistors. Note that, until last night, it happily produced as much as 15 watts. In any case, the rig came out of its coma but now it can’t speak. And maybe it can’t hear. Tthe receive function also seems to have been degraded – I heard no signals when the antenna was connected – though I can check that further by using the K3 to see if anyone is on the bands and comparing it to the K2. So, what do you think is going on? Ted, KN1CBR ______________________________________________________________ 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 Message delivered to [hidden email] |
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