17 meter QCX output issue.

Dennis Voorhees All Messages By This Member #18879 I'm having an issue with the output power varying. My QCX serial # is in the 420's (can't recall the exact serial number) B version software. I'm powering with a 20 amp good quality astron 13.8 volt switching supply. I'm using a cross needle meter on an MFJ tuner swithced into pass through mode. On Key down I will sometimes get 5 watts out initially, then key up, then key down 1 watt. Most of the time it's one watt. I have done the 10 k resistor mod. I've also looked for any cold solder joints, both visually and by tapping individual components ( inductors included) with a insulated tool in key down condition.Nothing found.? So I'm pretty much stumped on this one. Ideas? Denny AD3O
John Kirby All Messages By This Member #18888 Denny, Trouble shoot the interment problem with an insulated tool such as an 1/8 inch dowell rod. With key down and meter attached first poke around the coils for poor solder joint (lack of enamel scrapped off coil wire). Sometimes the ground side of bypass capacitors do not solder well because of the extra circuit board mass, hence more heat required to form a good joint. 72 73 John N3AAZ
Daniel Ekman SA2KNG All Messages By This Member #18889 Also, check Q6, the voltage at L4 should not be a solid ~12V. If this transistor is broken the envelope shaping does not work, the sharp rise times can lead to instability. If it's broken (and MPS2907) you should change it to the new type, it has been replaced by MPS751 in the later kits. Perhaps a smell test, or with lights off, if there's insulation problems in the LPF and arcing. This has happened when the metal tabs on the display shorted to the coil windings.
Dennis Voorhees All Messages By This Member #18891 Thanks, yeah I did poke around with a plastic tool looking for a poor or cold solder joint on the inductors and any of the transmitter parts, as mentioned above. I also did bend the tab away from the coil. However the MPS2907 sounds interesting. Looking for a substitute, but it seems although it's a general purpose PNP, the common ones I have around don't seem to cross. I assume the common 2n2907 probably won't work. Anyone try a 2n3906? I may have some of them around? toggle quoted message Show quoted text On Thu, Nov 30, 2017 at 9:04 AM, Daniel Ekman SA2KNG <knegge@...> wrote: Also, check Q6, the voltage at L4 should not be a solid ~12V. If this transistor is broken the envelope shaping does not work, the sharp rise times can lead to instability. If it's broken (and MPS2907) you should change it to the new type, it has been replaced by MPS751 in the later kits. Perhaps a smell test, or with lights off, if there's insulation problems in the LPF and arcing. This has happened when the metal tabs on the display shorted to the coil windings.
Steve All Messages By This Member #18894 Denny, if it turns out that you need an want an MPS751, I'll be glad to stick one in an envelope and mail it to you. (I bought a few extra when I replaced mine) ? Steve, N4IAG
[email protected] #18896 Dennis, The collector current limit for the 2N3906 is around 200ma. ?The suggested replacement PNP transistor MPS751 is around 2000ma. ?I think the kit draws 500ma on transmit so you need to find a PNP that will at least do 500ma. Follow the suggestions in the previous post. ?Also check the continuity from the RF connector through L1, L2 and L3 with a VOM. ?That will confirm they are connected up to C29. ? The other thought is one of the power FETs is not soldered in good. ?Reheat all three final FETs. Keep poking around. ? Bill N7EU
Dennis Voorhees All Messages By This Member #18917 Thanks for the offer Steve. I may take you up on it. Bill N7EU's comment gave me a number to work with. Heading to the somewhat local, parts distributor tomorrow. See If I can find something suitable. Let you know. I do appreciate it.? toggle quoted message Show quoted text On Thu, Nov 30, 2017 at 10:54 AM, Steve via Groups.Io <senglish222@...> wrote: Denny, if it turns out that you need an want an MPS751, I'll be glad to stick one in an envelope and mail it to you. (I bought a few extra when I replaced mine) ? Steve, N4IAG
Dennis Voorhees All Messages By This Member #18919 Steve I see the local (sorta) supplier has an NTE 159 available. Specs are somewhat close to a MPS751. At least they will do 1 amp of current. If they have one in stock, I'll give it a try. If they don't I'll take you up on the offer.? Denny toggle quoted message Show quoted text On Thu, Nov 30, 2017 at 10:54 AM, Steve via Groups.Io <senglish222@...> wrote: Denny, if it turns out that you need an want an MPS751, I'll be glad to stick one in an envelope and mail it to you. (I bought a few extra when I replaced mine) ? Steve, N4IAG
Dennis Voorhees All Messages By This Member #18972 Steve, no joy at the parts place. If the offer still stands. I would like one of your MPS751's. I am in your debt. Thanks Denny AD3O. toggle quoted message Show quoted text On Thu, Nov 30, 2017 at 10:54 AM, Steve via Groups.Io <senglish222@...> wrote: Denny, if it turns out that you need an want an MPS751, I'll be glad to stick one in an envelope and mail it to you. (I bought a few extra when I replaced mine) ? Steve, N4IAG
Dennis Voorhees All Messages By This Member #18974 My address is good on QRZ and QTH.com lookups.? toggle quoted message Show quoted text On Fri, Dec 1, 2017 at 2:53 PM, Dennis Voorhees <denny.voorhees@...> wrote: Steve, no joy at the parts place. If the offer still stands. I would like one of your MPS751's. I am in your debt. Thanks Denny AD3O. On Thu, Nov 30, 2017 at 10:54 AM, Steve via Groups.Io <senglish222=yahoo.com@groups.io> wrote: Denny, if it turns out that you need an want an MPS751, I'll be glad to stick one in an envelope and mail it to you. (I bought a few extra when I replaced mine) ? Steve, N4IAG
Steve All Messages By This Member #18976 It's in the mailbox as we speak - keep an eye out for a letter from VA. Good luck. ? Steve, N4IAG
Dennis Voorhees All Messages By This Member #18983 Thanks Steve!? toggle quoted message Show quoted text On Fri, Dec 1, 2017 at 3:43 PM, Steve via Groups.Io <senglish222@...> wrote: It's in the mailbox as we speak - keep an eye out for a letter from VA. Good luck. ? Steve, N4IAG
Guy N7BIR #18984 Dennis, i have a few MPS751’s. If you need some let me know. No charge.? 73, Guy N7BIR?
Dennis Voorhees All Messages By This Member #18991 Thanks Guy, got it covered, but it's very nice of you to make the offer.? Denny toggle quoted message Show quoted text On Sat, Dec 2, 2017 at 12:31 AM, Guy N7BIR <sgbridge@...> wrote: Dennis, i have a few MPS751’s. If you need some let me know. No charge.? 73, Guy N7BIR?
Dennis Voorhees All Messages By This Member #19361 Steve, the transistor arrived last week, and I had some other things that sucked away my time. I just did replace the transistor a few minutes ago and It's now a solid 3 watts to an antenna. Of course the band is stone dead except for some FT8 activity up the band.? Thanks so much for the 751. I owe you one.?

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17 meter QCX output issue.