Old Cobra 29 LTD Sound Tracker No Receive until it warms up

[loosecannon]

crossbow, sp5it means well, and is very radio savvy, he just doesn't understand how to not sound like a dick when giving advice.
who knows, it may be a language thing, but he always comes across as a dick when he posts.
it's like the only things that motivate him to actually type are things that piss him off LOL.

as for the radio repair, oldtech03 has the best idea so far.
you are dealing with a thermally intermittent component, and freeze spray is the best way to find it.

if it were on my bench, the first thing i would do is isolate whether or not this is an RF problem or an audio problem. it sounds like you've already done that, but if not, use your sig gen to put in a really strong signal when the radio is 'acting up' and see if you can hear anything. im talking like 10K or 100K microvolts and see if you get any wiggle on the meter.

sounds to me like it's RF related. if so, map out the RF path into the radio starting at the antenna jack, and after the radio has warmed up and is working normally; spray each component on your list and see if you can make the radio go deaf again.
LC

 

[nomadradio]

Umm, a "lazy" 10.24 MHz crystal that's slow to start up?

Kinda far out there, but if you have a way to "sniff" for this crystal's signal, it's worth a look.

More than once I have accidentally "awakened" a lazy slow-starting crystal by poking one side of it with a 'scope probe while looking for that signal. A radio that just magically comes back to life when you poke a crystal circuit with a 'scope probe points to trouble in that circuit, whether or not the crystal itself is the culprit.

Now I just listen for it with a service monitor or any handy receiver that covers that frequency. Don't have to touch any part of the circuit to do that. The 'scope probe becomes an antenna if you hold it a half-inch from the crystal.

For that matter, checking the 5-Volt supply pin on the PLL chip would be a good idea. If that voltage is low, wacky things can happen.

73

 

[Handy Andy]

Remember Crossbow, the PLL shifts 455kHz in RX - so to do that it adjust VCO.

Were there any mods done to the Delta Tune section?

So scope R85, Tweak L22 - in RX...look for output.

You may have a bad "shift" cap.. R85 is not immune to drift either - they've changed this value from 180 ohms to 560 ohms thru the years and especially when they went to the YD (Youda) chip

IC2 is the main RX out - into L18 for TP3 and 1st IF.

IC3 is main TX out - you say you're ok to TX - that is L19...

Only other suspects in this for "shift" is IF section or L22 has a bad tuning cap - T/R Pin on the 29 is Pin 9 but the modifier in this is, that pin, for the shift to occur. Pin 10 is the "check" that looks at L22 - that cap is C94.

L23 uses 10.240 signal - and that goes to Pin 12/13 back at the PLL (Delta Tune) That is also the offset to the TX mixer from VCO TP4. You have TX - so the 10.240 seems ok - just the "Delta Tune" part gets' me.

There was once a TSB on the Cobra 19 Plus that although showed 1 cap - you have two now used in line - its is part of the loop detect "phase" section - I'm referring to C98 and C99 - should always change them for cheap insurance that you don't have one of those caps get flaky.

You would see it as a "impure" sine wave in the TP1 when you had just got RX in - and sine would improve as the caps "warmed up" and acted like they should. The change they exhibit is slight and slow (takes a while to see the difference - almost like a sawtooth) to see - but this section and the ways it acts - reminds me of my own efforts with Cobra repair, I always change these two caps.

 

[Crossbow]

First off I want to thank everyone that has chimed in here to help!

By the time I could get the readings while dead the radio came on.

Tried the can air trick but it did not put the radio back to sleep.
Even put it in a freezer bag then into the frig for an hour. Still alive!
Seems to be a cap that is draining off and slow to recharge back.

Changed out the 10.240 crystal to rule it out.
Changed out FL2 455 filter. Have seen those do weird things before so rule this out.
Changed out C94 to rule it out.

All voltages check out perfect when the radio is working.
Can not get it to stay dead long enough to do testing while dead.

Radio is working for now as again once it warms up it will stay working until the power is pulled and drained over night.

We will wait until tomorrow to see if she come on from changing these parts out.
If she is dead in the morning then I will recap the RX section.

 

[sonoma]

try changing C48 by the audio IC. I have had that that one go bad and the RX would be low. I believe that is the right one. at least that number keeps popping up in my mind. in fact I would change all the caps right in the audio IC area to rule out one of them being weak.

 

[midnight special]

i dont think i seen it any where but have ya checked the screws that hold the board to chassis?,,,, loose ones sometimes do screwy things,,,,

 

[Handy Andy]

The biggest thing you should do - when you leave this radio off...set up your test jig on TP 1 put it on say Input 1 of the Scope to R85, for AC not DC and if you have a dual trace use TP 3 to that R17 / C16 - again AC no DC on the Input 2..

When the radio powers up - the "snapshot" you get - They should be the same. Check Channel 1 and Channel 40.

IF not - retune L22 with L18 (peak one to see results to the other and equalize if needed) - to see if you can get it to sync. Also look for amplitude as well as "quality" of sine wave - compare the two TP's on the scope look for irregularities - jitters are ok, that the sync stuff, but if there is flat topping or skewed sine wave - these show up under comparison.



(NOTE: This isn't trying to insult anyone - anyone whom knows these radios already knows the parts listed above, this is for LEGACY - a memory and guide. A "How-To" for those that are not on the same level of experience we are working with...)

I hope the wax mess that they use - has not caused problems - for usually L22 is not covered in it but they smother L19 with a ton of it. This usually is not a problem - but what would you get if someone tried retuning L18 or L19? They can break a slug - like we talked about earlier - and wind up with a coil that can work, but is very limited. Check those slugs as well as "creep" from the wax on the solder side of the board.- interfering with the solder joints.

This may be a radio that someone "tuned" to TX max power, but not the RX side first. This can play a role in how the RX responds. Now you say RX is just "slow" to start, but TX is fine. Well, it means that the Varactor is working - else you would not even be able to obtain "Lock" either or - so TX works, we know the VCO seems to want to work with at least one side.

So, lets' take a look at the Lock circuit - it uses the RX power feed that uses an AGC cap - so if the AGC is slow, or has a bad cap, it can pull down the LD line to prevent the PLL to even work the RX because it can't see if RX power is even applied.

Does Pin 15 of the PLL have power when the RX side is dead?

So look at C12, see if that line even shows power going to the PLL - check ohmic resistance make sure C12 and D30 IN series goes and checks ok to Pin 15 - the Diode may not cause the problem; but C12 - if it's bad, can pull the power on that line low - and you don't get RX because the PLL can't sense it - so ohmic as well as voltage check can help you determine if Pin 15 as well as C12s' "+" side of that cap has a voltage.

The white wire in the photo is the only connection the RX side has to the Lock Detect as a means to tell the PLL - "Hello - I'm here." so if the cap, or anything in the AGC side of things - even RF Gain control - AGC "divider" resistors - even the power sourcing to that line thru R9 and RF gain control itself - will affect the ability of the PLL to even see if RX is working else it just kills the Signal.

It's not due to "because it's out of lock" - just there is no RX to work with. I gave you a list of parts in the previous message because of their influence in this chassis. This is an ST unit so the AGC is heavily used as a means to measure the level of "decompression" needed for ST's expander side of the compandor chip. It's why there is an extra stage of Audio Amplification to handle the losses from switching in or out the compandor even when they use MOSFET (CMOS) 4066 buffer to handle such losses and reduce noise injection source problems they had with earlier models.

Just so you know, the ST units did pull power from RX line C12 cap and it uses it for this Lock Detect feature - power, it's located by L1, L8 and L9 as a simple little 4.7uF Electrolytic. These ST are different than a typical LTD or even GTL BUT also used numbering of schematic of one part, but silkscreen is WHOLE other mess - D30? (Per Schematic?) May be labeled D39 - How do I know - see attached...they are a PITA but ...

See Below...

This is a FULL shot of ST mainboard, Front panel to bottom of page, but locate C12 and PLL - note WHITE wire and where it goes on the board...


In this shot, the R85 resistor is easily seen as a 180 ohm value, so you know...


This shot is the most critical of any, this one is the highlited traces backlit into top of the board for component location per traces to show routing...Note White Wire and C12 and how its' "tapped" to tie into D39 and show RX lead power good to Lock Detect on the Pin 15 of the PLL...(Note Pin 14 not used)



So if RX seems "slow" it is more than likely the age of the parts, check C12 and just simply replace it - keep it simple - you'll need to recap the radio anyways but just letting you know I'm working right beside you on these...

Good luck!

 

[Crossbow]

try changing C48 by the audio IC. I have had that that one go bad and the RX would be low. I believe that is the right one. at least that number keeps popping up in my mind. in fact I would change all the caps right in the audio IC area to rule out one of them being weak.

Yes Sir with the age on this radio I agree with recapping. Thanks for the tips and ideas!

 

[Crossbow]

i dont think i seen it any where but have ya checked the screws that hold the board to chassis?,,,, loose ones sometimes do screwy things,,,,

That is great info right there! I always check all screws right off the bat! Seen those cause many problems before. Thanks for posting this for others to get in that habit.

 

[Crossbow]

The biggest thing you should do - when you leave this radio off...set up your test jig on TP 1 put it on say Input 1 of the Scope to R85, for AC not DC and if you have a dual trace use TP 3 to that R17 / C16 - again AC no DC on the Input 2..

When the radio powers up - the "snapshot" you get - They should be the same. Check Channel 1 and Channel 40.

IF not - retune L22 with L18 (peak one to see results to the other and equalize if needed) - to see if you can get it to sync. Also look for amplitude as well as "quality" of sine wave - compare the two TP's on the scope look for irregularities - jitters are ok, that the sync stuff, but if there is flat topping or skewed sine wave - these show up under comparison.

View attachment 28012

(NOTE: This isn't trying to insult anyone - anyone whom knows these radios already knows the parts listed above, this is for LEGACY - a memory and guide. A "How-To" for those that are not on the same level of experience we are working with...)

I hope the wax mess that they use - has not caused problems - for usually L22 is not covered in it but they smother L19 with a ton of it. This usually is not a problem - but what would you get if someone tried retuning L18 or L19? They can break a slug - like we talked about earlier - and wind up with a coil that can work, but is very limited. Check those slugs as well as "creep" from the wax on the solder side of the board.- interfering with the solder joints.

This may be a radio that someone "tuned" to TX max power, but not the RX side first. This can play a role in how the RX responds. Now you say RX is just "slow" to start, but TX is fine. Well, it means that the Varactor is working - else you would not even be able to obtain "Lock" either or - so TX works, we know the VCO seems to want to work with at least one side.

So, lets' take a look at the Lock circuit - it uses the RX power feed that uses an AGC cap - so if the AGC is slow, or has a bad cap, it can pull down the LD line to prevent the PLL to even work the RX because it can't see if RX power is even applied.

Does Pin 15 of the PLL have power when the RX side is dead?

So look at C12, see if that line even shows power going to the PLL - check ohmic resistance make sure C12 and D30 IN series goes and checks ok to Pin 15 - the Diode may not cause the problem; but C12 - if it's bad, can pull the power on that line low - and you don't get RX because the PLL can't sense it - so ohmic as well as voltage check can help you determine if Pin 15 as well as C12s' "+" side of that cap has a voltage.

The white wire in the photo is the only connection the RX side has to the Lock Detect as a means to tell the PLL - "Hello - I'm here." so if the cap, or anything in the AGC side of things - even RF Gain control - AGC "divider" resistors - even the power sourcing to that line thru R9 and RF gain control itself - will affect the ability of the PLL to even see if RX is working else it just kills the Signal.

It's not due to "because it's out of lock" - just there is no RX to work with. I gave you a list of parts in the previous message because of their influence in this chassis. This is an ST unit so the AGC is heavily used as a means to measure the level of "decompression" needed for ST's expander side of the compandor chip. It's why there is an extra stage of Audio Amplification to handle the losses from switching in or out the compandor even when they use MOSFET (CMOS) 4066 buffer to handle such losses and reduce noise injection source problems they had with earlier models.

Just so you know, the ST units did pull power from RX line C12 cap and it uses it for this Lock Detect feature - power, it's located by L1, L8 and L9 as a simple little 4.7uF Electrolytic. These ST are different than a typical LTD or even GTL BUT also used numbering of schematic of one part, but silkscreen is WHOLE other mess - D30? (Per Schematic?) May be labeled D39 - How do I know - see attached...they are a PITA but ...

See Below...

This is a FULL shot of ST mainboard, Front panel to bottom of page, but locate C12 and PLL - note WHITE wire and where it goes on the board...
View attachment 28013

In this shot, the R85 resistor is easily seen as a 180 ohm value, so you know...
View attachment 28014

This shot is the most critical of any, this one is the highlited traces backlit into top of the board for component location per traces to show routing...Note White Wire and C12 and how its' "tapped" to tie into D39 and show RX lead power good to Lock Detect on the Pin 15 of the PLL...(Note Pin 14 not used)

View attachment 28015

So if RX seems "slow" it is more than likely the age of the parts, check C12 and just simply replace it - keep it simple - you'll need to recap the radio anyways but just letting you know I'm working right beside you on these...

Good luck!

Andy you always amaze me with the awesome info you post on this site to help others! You give such great info and grafts to boots. Thanks for all the help you give to everyone here buddy!

 

[Crossbow]

Update on the radio. Came in this morning and she fired right off. So far so good. I will let her sit powerless over the weekend then see what she does come Monday.

Thanks again to everyone that has chimed in here to help. Love seeing this site get back to what it use to be and that is helping others to learn. You folks are great!

 

[Captain Kilowatt]

I would also suggest switching the channel selector from channel 1 to channel 40 while it's "sleeping".

If it will receive on one or the other before the warmup period has passed, that would point to the VCO adjustment.

73

I had a mobile radio thad had a funky VCO and did not like the cold. Occasionally I could hear morse or RTTY before the receiver kicked in. It was coming from the 250Kw navy transmitter on 436 kilohertz 12 miles line of sight away. I could see their antenna system. I suspect the RF was getting into the 455 Khz IF section.