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Author Topic: mfj cub problems  (Read 1654 times)
M0JHA
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Posts: 646




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« on: April 23, 2010, 07:36:37 AM »

hi, i posted a while back re my 40m cub chirping.. i had it looked at by a local ham who had it on a scope and said all looked ok. He made a little circuit which keyed the rig quite fast so he could test things. when i picked it up it sounded ok but as i say this was when the rig was being keyed quite quick.

Anyway the chirp is not as bad but still there but intermittent it tends to be more noticable when say keying a dash . i listen to the signal on another rx with a dummy load in the cub and still does it. im also hearing what sounds like electrical clicks when i keydown through my headphones which also started the same time the chirp did.

my power supply is fine ( checked it on 3 ) my antenna is fine ( res 40m loop) .

im not sure where to go now with this especially as im not really technical minded as far as electronics go.

i took the board out the case and re soldered ALL points under the board just in case there was a dry joint.. still the same. i'm suspecting a dud or iffy component(s) but don't know what to look for or even where on the board i should look.

any help on what it may be or any component(s) would cause this chirp appreciated as i really liked this rig before it decided to start chirping..

just before it started having problems i did notice even after warm up after say 4 seconds of sending the tone i heard in the headphones would drop in frequency ,im not sure if this had anything to do with the resulting chirp..

billy  uk
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AA4PB
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« Reply #1 on: April 23, 2010, 09:53:29 AM »

Billy, did you solder down those "tombstoned" capacitors?
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M0JHA
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« Reply #2 on: April 23, 2010, 10:06:34 AM »

yes, the guy who looked at it for me did this for me.. the components were a little bigger than original but of the same value.. the 3 caps that were tombstoned all went to ground ( i think i was told these were bypass caps ?) .. These caps (i imagine) would of been like this from the factory and the rig worked fine for 12 months or more..

although im not knowledgable with electronics it seems to me something has blown or gone down as i used this rig most days with no problem and the problem came on very suddenly..  im almost double certain its not a dry joint..

one thing that may give a clue is i found once when i moved my key the tx freq shifted. also picking the rig up made it do this .. im not sure if this would always be the case, its just something i fell across just before the problem was repoerted to me..

billy
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AA4PB
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« Reply #3 on: April 23, 2010, 10:44:31 AM »

Sometimes the lack of bypass caps can cause stability problems. It may have worked fine until some other component aged and changed value a little then problems started.

I would put it on a dummy load and a good regulated power supply and listen to the signal in another receiver. Check for instability with vibration or movement. Use a piece of wood or an insultated tool to apply a little pressure at different points on the PC board, looking for bad connections. It is likely to shift when pressure is applied at some points, but it should come back when pressure is removed.
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M0JHA
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« Reply #4 on: April 24, 2010, 01:23:51 AM »

just had a look, nothing springs up as obveous .. one thing i did find is that L3 ( a slug tuned coil) when i go near it does make the freq shift even if its with a plastic instrument.. C6 and C7 ( polysterene caps) also make the freq shift if touched..

the noise is not so much chirp now but just the tone shifting to more like a squeel and then back to normal..

im sure i read these components are in the vfo section of the rig but im sure its not an antenna or power supply issue ..

i don't have these 3 components but is it worth me changing them if i order the bits ??

billy

« Last Edit: April 24, 2010, 01:48:57 AM by billy talmage » Logged
KE3WD
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« Reply #5 on: April 24, 2010, 08:06:27 AM »

What does it do when there is no code key or wires involved? 

Just shorting the key jack with the shortest possible connection, such as inserting a shorted plug into the socket and then look for the chirp...


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M0JHA
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« Reply #6 on: April 24, 2010, 08:51:18 AM »

same thing.. i shorted the inside of the key socket with a piece of stiff wire to key the rig and symptoms the same..

the chirp has all but gone now and has changed to a wierd squeek almost as if its struggling to decide what tone freq to use, somethings flipping out but im just not knowledgable to know basically..

billly
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KE3WD
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« Reply #7 on: April 25, 2010, 06:57:05 AM »

A can of Freeze Spray just may be able to isolate the faulty component. 

Freeze Spray can be purchased from electronic supply houses and is used to pinpoint the bad component by applying a very small shot to just the one component and then observing operation to see if the symptom improves or not. 

Using this method, you may not have to know the full circuit operation theory in order to find the bad part. 

Good luck and 73, keep us posted. 

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WA9UAA
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« Reply #8 on: May 14, 2010, 10:41:58 PM »

Billy,
There is a mod for the Cub's AGC management. You may need some help but it was not hard to do after I built the kit. I'll see if I can find it.

One example, http://www.w1hue.us/Articles/CUB2.html

Here is the one I did, It is a little less ambitious.  http://www.w1hue.us/Articles/Cubqsk_Rick.html

73,
Rob WA9UAA
« Last Edit: May 14, 2010, 10:58:40 PM by Robert H. meyer » Logged
M0JHA
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Posts: 646




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« Reply #9 on: May 15, 2010, 01:03:46 AM »

cheers rob, yes i found that one whilst searching the web for info.. i can't find the problem although it would help if i was more knowledgable. someone mentioned C6 and C7 could be the problem so i replaced them but its still the same..

i dont know why but something is telling me its L3 ... its been put on the back burner for the last week or so as i have got myself a K1 so have been playing with that but want to get the cub up and running again ..

im going to change L3 and see if that cures it

billy
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