I cant hear anything unless my finger touches the wire on the channel selector?

[jtrouter]

Now i bet you had to look now!, This one is strange but im counting on answers from someone with a good electronic background. So I opened up my 45year old Corba 139 to use DeoxIT and to replace all the burned out lights with led's. So im not sure what i did but put it all back together turned it on and wow lights! it looked good but when i spun the dial i found that i am missing channels 3,7,11,15,19. Now i know some are going to say its a bad crystal,
EXCEPT when i touch one of the solder connections on the back of the channel selector the missing channels come back perfect. The first time it happened it was with a touch of a screwdriver and i thought it was a cold solder joint. So i redid it and no change. It only works when i touch it with the metal part of a screwdriver or if i touch it with a wet finger. Its very cold and the heater been running hard and the air is dry. So im open to ideas???
THANK YOU ALL FOR READING!

 

[144inBama]

Touching an electrical component with your finger (usually) adds capacitance and or resistance to the circuit so you may want to check the caps and resistors related to the selector.

 

[Handy Andy]

If the channel selector is "lobbing" - bent shaft, even ever so slightly - the pressure from the rotary indents and the fingers to the wafer of both can force the finger to wafer contact to drop out due to lack of spring action / tension.

The problem becomes worse as age of where this pressure from the seating occurs - if the wafer is corroded or pitted - there may not be much someone can do except take it apart and do more cleaning to it - one wafer or its wiper arm could be pitted beyond repair, causing the drop and you have to place pressure on it.

This does happen in radios that have sat in a condition prone for moisture or excessively dry - this can make the wafers copper foil "stick" to the finger that bends down to it to make contact as the wafer rotates. So if it's developed such a "pitting" condition - it can cause this missing channel condition.

The above may not be correct to the "Table" for your radio -
but the concept here is to show that the pins to wafer / slide
repeat in a certain way.​

• as @144inBama has suggested, check your main board and all the easy access stuff you can get at first.
• The condition appears to repeat on the 4-5th "detent" so it may be the shaft is still got dirt between it and the housing and the debris rolls right along with it.
• This can also occur from radios that have been "face planted" where the radio fell and landed on the channel knob - bending the shaft and your result is a repeating pattern of no-lock on specific groups of channels.

 

[Riverman]

A perfect example of why this site is so great.
A seemingly crazy, unheard of problem greeted with straightforward possible solutions from experienced operators/technicians who know what they're doing.
And in a timely manner, despite the question being posted at 3:32 a.m.
Gotta love it.

 

[Handy Andy]

Don't give out accolades just yet, this may be more of an underlying condition of broken wire conductor inside when the strands inside the wire are corroded from excessive heat/reheat of a soldering iron and the flux from the solder carboned up and now has slid under the wires' sheath and literally plugged the wire up.

Not all stranded wire 50 feet long on a spool contains strands of 50ft of continuous wire strands. A few (hundred) might overlay one or two (hundred) others...

​

The 139 - was not specific in details but the 139 is a xtal-based unit while the 139XLR uses the LSI (PLL) so to the casual observer to say crystal - might not be as far off as you think - but replace the wire or if it's in a ribbon - just use a small gauge wire and jumper the one that's affected - see if it goes away. Then you KNOW you have a bad wire in the Harness - just as bad as no receiver from a botched CB/PA switch or broken busted squelch wire....

My moneys on "This kid can do this!"

 

[jtrouter]

Again awesome thoughts, i have a lot of work to do so thank you all and i will get back here and report back!

 

[secret squirrel]

 

[DoctorAudio]

If nothing else works, you could Gorilla-glue a fake finger onto the switch..

 

[jtrouter]

If the channel selector is "lobbing" - bent shaft, even ever so slightly - the pressure from the rotary indents and the fingers to the wafer of both can force the finger to wafer contact to drop out due to lack of spring action / tension.

The problem becomes worse as age of where this pressure from the seating occurs - if the wafer is corroded or pitted - there may not be much someone can do except take it apart and do more cleaning to it - one wafer or its wiper arm could be pitted beyond repair, causing the drop and you have to place pressure on it.

This does happen in radios that have sat in a condition prone for moisture or excessively dry - this can make the wafers copper foil "stick" to the finger that bends down to it to make contact as the wafer rotates. So if it's developed such a "pitting" condition - it can cause this missing channel condition.

View attachment 56897
The above may not be correct to the "Table" for your radio -
but the concept here is to show that the pins to wafer / slide
repeat in a certain way.​

• as @144inBama has suggested, check your main board and all the easy access stuff you can get at first.
• The condition appears to repeat on the 4-5th "detent" so it may be the shaft is still got dirt between it and the housing and the debris rolls right along with it.
• This can also occur from radios that have been "face planted" where the radio fell and landed on the channel knob - bending the shaft and your result is a repeating pattern of no-lock on specific grou

 

[jtrouter]

Don't give out accolades just yet, this may be more of an underlying condition of broken wire conductor inside when the strands inside the wire are corroded from excessive heat/reheat of a soldering iron and the flux from the solder carboned up and now has slid under the wires' sheath and literally plugged the wire up.

Not all stranded wire 50 feet long on a spool contains strands of 50ft of continuous wire strands. A few (hundred) might overlay one or two (hundred) others...

View attachment 56905

​

The 139 - was not specific in details but the 139 is a xtal-based unit while the 139XLR uses the LSI (PLL) so to the casual observer to say crystal - might not be as far off as you think - but replace the wire or if it's in a ribbon - just use a small gauge wire and jumper the one that's affected - see if it goes away. Then you KNOW you have a bad wire in the Harness - just as bad as no receiver from a botched CB/PA switch or broken busted squelch wire....

My moneys on "This kid can do this!"

I tried the jumper and still no joy. When i soldered on a piece of wire about six inches long and as before the only way the channels worked was to touch the end of the wire? This is driving me crazy

Don't give out accolades just yet, this may be more of an underlying condition of broken wire conductor inside when the strands inside the wire are corroded from excessive heat/reheat of a soldering iron and the flux from the solder carboned up and now has slid under the wires' sheath and literally plugged the wire up.

Not all stranded wire 50 feet long on a spool contains strands of 50ft of continuous wire strands. A few (hundred) might overlay one or two (hundred) others...

View attachment 56905

​

The 139 - was not specific in details but the 139 is a xtal-based unit while the 139XLR uses the LSI (PLL) so to the casual observer to say crystal - might not be as far off as you think - but replace the wire or if it's in a ribbon - just use a small gauge wire and jumper the one that's affected - see if it goes away. Then you KNOW you have a bad wire in the Harness - just as bad as no receiver from a botched CB/PA switch or broken busted squelch wire....

My moneys on "This kid can do this!"

This is the older crystal 139, I do also have a 139xlr. And at this point im not sure if putting money on "this OLD kid" is a good bet.

 

[jtrouter]

Well Now i am not even sure what to say but i was just about to throw it on the pile of radios to get to later it started working just fine across all channels? Now
im just wondering if a bit of DeoxIT was not dry? I have know idea but it works now and i want to that each one of you that tried to help and thought about ways to help me. Now i have to decide do i want to tackle a tube radio or solid state?
Again you guys are awesome!

 

[Handy Andy]

Again you guys are awesome!

Give yourself a couple of atta' boys while you're at it!

 

[jtrouter]

Ok lets put the final ? to this story. So i put it all back together and was playing with it tonight and found one other strange thing about this radio. The voice lock "clarifier" on this radio is locked but on the channels that i had the problems with 3 7 11 etc when the voice lock is turned all the way CW those channels drop out? when i go CCW once i pass 12 Oclock the channels come back. This is repeated on only these channels. I think i am going to unlock the voice lock to see what happens as on the Cobra 134/138/139 its as simple as adjusting one variable potentiometer. So i thought i would throw this out here and see what you guys think. This is one strange radio