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Cobra 25ltd pll part # needed

Well, by your chart, I don't see anything "wrong" except the ones you highlight are for the Channel Selector - Are you on Channel 9 or 19?

upload_2021-9-29_8-2-40.png​

By your pinout measurements, you're running a bad channel selector - Pin 9 goes "high" to "WASH OUT" the input - this keeps the internal logic on "hold" until either it's in lock, or your have the right pins getting set so it can divide for CB channels.

Pin 7 is the LO from L15 - if it's not "ringing" it tries to force the Varactor loading higher to raise / change the capacitance to start the ring. (Your TP1/2)

Pin 8 is the EXTERNAL detect - it looks at the Microphone Jack Pin 3 - if it rises or falls - this tells the PLL to "follow" that status - so it will push power into Pin 7 - or remove it to make the PLL Shift - once the PLL sees a "lock" - Pin 9 takes over. This is where the PLL will allow you change the channels (In RX Mode) or when in TX - the 455kHz Shift takes place and you see two IF's mixing to make the 27MHz at IC2 - else TP3 - you see a 455kHz offset "change" in RX to TX and Back again.

Pin 9 "looks" at the LO VCO - but if the pins are not set right - it ignores the input -won't do anything - so no output.- sets this high to wash its input.

So your better options to even see if it could "fire" is to run the CH 9 switch ON - until you figure out the BCD encoding. At least override the BCD or even remove the cable and assembly - then set the pins manually. IF it works then you have the culprit in your hand - the selector is out of whack.
  • No radio is perfectly bullet proof. - I've seen BCD encoders - from Brand New out of the box radios - being BAD, due to someone "faceplanting" (Dropping) the radio on the channel knob - breaking the wafers inside
To set: The Channel 9 - Pin sets are simpler 10, 11 and 12 set channel 9 Channel 19 is same pins with one extra add in; Pin 15 (set these "high")
 
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Andy,
First, let me thank you for the response.

As I go through and check voltages changing channels I am getting voltage alternately with the channel selector on the pins you state are for the channels. I don't know what channel I was on during my initial voltage checks.

pin 12 channel 20-39 @7.2V 40-19 @0V
pin 13 channel 10-19, 30-40 @7.2V 1-9, 20-29 @0V
pin 14 channel 1 @0V, 2&3 @7, goes on up alternating every 2 channels or so drops
to 0V and back to 7.2V

Seems now that I am going through the channels, all the pins on the PLL change from 0V to either 7.2V until channel 17 and 18 which go to 6.4V.
From this new info, I would say the channel selector and PLL are working as they should?

When you say to set pin 9 high, since it's at 4.59V, isn't it high? How would I go about setting it high or low? Not quite understanding this point.

I also get the following
TP3 by the RX 1st mixer FET :
16.270 in RX on CH 1, or
16.710 in RX on CH 40.
I do get the 455khz shift as you describe when keying as well.

When I key the mic, the TX light lights up, but no output. The RX light is lit when not keyed, but no audio, and no audio in the PA position, just some noise when keyed. If I key another radio near by, the meter jumps to full scale even after I un-key the adjacent radio, and seems to hang until I key up with the 25, then it drops. When I key the 25, I get no deflection on the built in meter but the TX light is on.
I might also add I have switched from the internal speaker to a external to eliminate that aspect.

I took these readings as well on 2 other IC's, the audio chip and the mixer 7310. They seem to be inline with the info from CBtricks.

cobra_voltages.jpg
 
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Smokinone said:
When you say to set pin 9 high, since it's at 4.59V, isn't it high? How would I go about setting it high or low? Not quite understanding this point.

I also get the following
TP3 by the RX 1st mixer FET :
16.270 in RX on CH 1, or
16.710 in RX on CH 40.
I do get the 455khz shift as you describe when keying as well.
Click to expand...

This is the best news, the IF is working and in order to get those, you'll need to have the 10.240 working too.

You have a heartbeat...

The Pin 9 part - that is the PLL telling the VCO - the "lock" it's supposed to have is not working.

The "High" condition means the PLL isn't "stable" enough to TX - so you'll need to look back at TP3 like you did with RX - you are getting the 455kHz but something is not "releasing" (voltage on Pin 9 on the PLL is one of these indicators) - which would allow RX to get sent thru to the Audio Amp.
That may be where we focus next...

No TX could be a myriad of things...like bad caps. The radio is old enough to show this condition.

What kind of Audio Chip is in this thing? 7222AP (Single row of pins) or a TDA2005 (Dual Row)?

No PA but a TX light tells me the issue is more around the Audio Chip (amp).

When you first turn it on, do you hear any "Thunk" from the speaker?

I'll stop here and let you explore. I have some chores to do, but I'm checking in every so often today.

SIDE NOTE:
If you see a "non-Pb" symbol on the board anywhere, this thing could be Silver soldered, causing a lot of parts that will wet just fine with regular Lead-(Pb) Based solder and work just fine - but will not wet well with silver/antimony metals used in the lead free alloys for soldering. You just may need to reheat and refresh with the lead based solder to get the parts in the radio to reconnect with the traces and solder pads and to start to communicate with each other again.
 
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Handy Andy said:
This is the best news, the IF is working and in order to get those, you'll need to have the 10.240 working too.

I do have the 10.240 at the x-tal

You have a heartbeat...

The Pin 9 part - that is the PLL telling the VCO - the "lock" it's supposed to have is not working.

The "High" condition means the PLL isn't "stable" enough to TX - so you'll need to look back at TP3 like you did with RX - you are getting the 455kHz but something is not "releasing" (voltage on Pin 9 on the PLL is one of these indicators) - which would allow RX to get sent thru to the Audio Amp.
That may be where we focus next...

No TX could be a myriad of things...like bad caps. The radio is old enough to show this condition.

I see no physical signs of leakage or swollen electrolytic caps. I did change C170 and C65 as I pulled them to check, they were good, but I changed them while I had them out.

What kind of Audio Chip is in this thing? 7222AP (Single row of pins) or a TDA2005 (Dual Row)?

It has a single row bolted to the side of the radio. I believe it is the TA7222AP. The numbers are rubbed out on this one unless they are are the mounted side. I did check all the voltages on the pins and they are consistent with the voltage chart on CBTricks

No PA but a TX light tells me the issue is more around the Audio Chip (amp).

When in the PA position, I hear scratchy sounds when the mic is being keyed, but no audio out.

When you first turn it on, do you hear any "Thunk" from the speaker?

No thunk. I do hear audio when I touch the PC side with my fingers. From a squeal to some scratchy sounds as I move my fingers around the center area of the board.

I'll stop here and let you explore. I have some chores to do, but I'm checking in every so often today.

SIDE NOTE:
If you see a "non-Pb" symbol on the board anywhere, this thing could be Silver soldered, causing a lot of parts that will wet just fine with regular Lead-(Pb) Based solder and work just fine - but will not wet well with silver/antimony metals used in the lead free alloys for soldering. You just may need to reheat and refresh with the lead based solder to get the parts in the radio to reconnect with the traces and solder pads and to start to communicate with each other again.
Click to expand...

I don't see anything like that on the board. I did go over the PLL, audio, and power in areas though. It all looks pretty good through my magnifier.

Thanks so much for helping me out here.
 
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Something goofy I just ran into. I can get some audio as far as static when I hit a certain critical spot in the squelch adjustment. I can also hear SSB garble while on 38, with a strong signal from 38 LSB though very weak.. The position of the squelch changes though. I can hear SSB for an instant in one position, then I have to re-adjust the squelch again after a moment or 2. Starts at a low squelch and moves up, and actually stabilized for the moment at the higher setting, I guess as the radio warmed up.
Still no transmit though when I do get the static.
 
Hmmm...If you have time - this might help for if the SQUELCH knob running the Squelch control - opened up - it will QUENCH all signal - act like this because it's really a VERY non-linear switch that looks to a voltage across the SQUELCH control to set "trigger" - if it opened up as in a bad pot, you just need to "short" the wiper arm wire to ground and see if it pops open. Remove the wiper lead to the SQ pot and touch it to a case of one of your IF cans to see if it "pops" open - then you know you have a bad pot.

TR6 - act as the non-linear switch that ties both the AGC (to track signal strength and operate to OPEN the Squelch as the signal gets stronger) and RX voltage to set a "hysteresis" event - it sends this result to TR10 - your On - Off Switch
  • TR6 is a 2SC1815 - It can short out - due to static pokes popping the substrate when someone has a lot of static charge - another Achilles heel due to the construction of the front panel and the way it was made back then. When it blows shorted - sucks all the signal out - but shouldn't turn off Mic Audio power.
TR10 is the "Final Call" to send RX audio to the Amp - TR10 doesn't normally go out but pulling it to help troubleshoot - gives you the chance to at least hear MIC audio - removing it won't hurt the radio - again for troubleshooting only ok?
  • This is just for checking MIC Audio in TX mode only.
  • TO hear RX (weakly) - you'll need to short C54's output to (towards base of) TR10 to C56 to get any RX audio thru - a simple jumper helps here...then replace any parts to restore TR10's operation so you're not mixing RX and TX together.
In this case, the Squelch control works in RX, but in TX - no audio is still a different beast...

TR16 is a PNP - might need to check it and put in a 2SA733 - it's the TX switch - this controls the TX line, and looks at MiC Jack Pin 3.
  • - as it falls when you go into TX, TR16 is supposed to
  • - send power to the Mic amp to turn on the audio for PA to the Audio amp
    • - MIC Amp (TR13) powers up to allow Mic audio to get amplified then onto Audio Amp (R73 SENDS power to the Mic amp from the TX line)
  • - MIC Jack Pin 3 line also has power for RX when the Mic is not keyed up
  • - so when you switch to TX
    • - MIC Pin 3 Line gets grounded
    • - RX light goes out
    • - power going to RX drops on the line
    • - RX stops working
    • - if you get a TX light, then that means TR16 may not be the issue.
Might need to check traces and make sure all soldering is good and this means continuity checks - to make sure the Foil traces are not opened up.
 
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Handy Andy said:
Hmmm...If you have time - this might help for if the SQUELCH knob running the Squelch control - opened up - it will QUENCH all signal - act like this because it's really a VERY non-linear switch that looks to a voltage across the SQUELCH control to set "trigger" - if it opened up as in a bad pot, you just need to "short" the wiper arm wire to ground and see if it pops open. Remove the wiper lead to the SQ pot and touch it to a case of one of your IF cans to see if it "pops" open - then you know you have a bad pot.

Made no change when shorting the squelch

TR6 - act as the non-linear switch that ties both the AGC (to track signal strength and operate to OPEN the Squelch as the signal gets stronger) and RX voltage to set a "hysteresis" event - it sends this result to TR10 - your On - Off Switch
  • TR6 is a 2SC1815 - It can short out - due to static pokes popping the substrate when someone has a lot of static charge - another Achilles heel due to the construction of the front panel and the way it was made back then. When it blows shorted - sucks all the signal out - but shouldn't turn off Mic Audio power.
TR6 is a 2sc945 in this radio...pulled and tested OK
TR10 is the "Final Call" to send RX audio to the Amp - TR10 doesn't normally go out but pulling it to help troubleshoot - gives you the chance to at least hear MIC audio - removing it won't hurt the radio - again for troubleshooting only ok?
  • This is just for checking MIC Audio in TX mode only.
  • TO hear RX (weakly) - you'll need to short C54's output to (towards base of) TR10 to C56 to get any RX audio thru - a simple jumper helps here...then replace any parts to restore TR10's operation so you're not mixing RX and TX together.
In this case, the Squelch control works in RX, but in TX - no audio is still a different beast...

I pulled TR10, it tested OK. When pulled I lost what sound I had at the squelch point where I did have sound.

TR16 is a PNP - might need to check it and put in a 2SA733 - it's the TX switch - this controls the TX line, and looks at MiC Jack Pin 3.
  • - as it falls when you go into TX, TR16 is supposed to
  • - send power to the Mic amp to turn on the audio for PA to the Audio amp
    • - MIC Amp (TR13) powers up to allow Mic audio to get amplified then onto Audio Amp (R73 SENDS power to the Mic amp from the TX line)
  • - MIC Jack Pin 3 line also has power for RX when the Mic is not keyed up
  • - so when you switch to TX
    • - MIC Pin 3 Line gets grounded
    • - RX light goes out
    • - power going to RX drops on the line
    • - RX stops working
    • - if you get a TX light, then that means TR16 may not be the issue.
Might need to check traces and make sure all soldering is good and this means continuity checks - to make sure the Foil traces are not opened up.

TR16 pulled and tested OK
Click to expand...

While this last round of testing was going on the large transformer by the power inlet started smoking around the base and got real hot.T1 I think it's labeled.
It may be a goner.

transformer.jpg
 
Final and Driver - see if they shorted out - thought you had checked this already...

You were getting a carrier - were you using a Dummy load on the antenna jack?
 
The 2sc2078 doesn't have any readings on the Sencore Super Cricket, like it's open, the 2sc1957 shows good.
I either had it hooked up to an antenna or dummy load, depending if I was trying to receive something.

Looks like L10 and L13 both have the cores removed as well. I've never seen any without the core, but I have never dealt with these china cobra's. These are the 2 cores that look like they relate to the driver and final.
 
This is a old unit, it will have to have the power caps replaced.

Sure sign this needs a lot of caps in the power supply chain replaced.

This may also be why PA doesn't work - shorts in the power filter caps suck power away.

Be sure to keep the bench clear of debris under that PCB - nothing more needs to be said...

Be sure to keep using - if you haven't already switched over to, an in-line 2-amp fuse until you get the power supply INSIDE the radio - stable.

The Power Choke is a simple 3 amp type - so it cooks easily enough but rarer to replace - my last ones were from Digikey in 2015 and they were remaining stock from discontinued.
 
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Thanks Andy.
I don't know how much this radio means to the neighbor, or how much he would want to get into it, but it would appear the C2078 is not functioning, and as far as I can tell they aren't available any more.
We both have a lot of time and effort into this radio, and I'm not sure if it's really worth the trouble. I may pull some of the caps, I assume it's electrolytic we're concerned with. I did check and replace C170 and C65
I think he mentioned someone had tuned it up at one time, maybe that explains the missing slugs. A few caps and a little more time won't hurt. I am concerned with the transformer letting out some smoke as well.
I've junker china 148 and a 140gtl that may have usable transformers, don't know if they would fit or have the same configuration as far as inserting them into the board. Ones marked TF-151 in the 140
I do appreciate your time. I will play around a little more and report what I find.
You've been a great help to me getting this far. As you can tell I'm pretty green on these radios, and your help has brought me along.
Thanks
 
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Well, I pulled 6 caps around the power side and transformer and only 1 has been good. I'll continue.
Make that 7 with only 1 good one, which I replaced anyway. I can't believe a radio this new has bad caps already. I have an AX44 and an old Midland 77-857 that still work and haven't been re-capped...
 
Smokinone said:
Well, I pulled 6 caps around the power side and transformer and only 1 has been good. I'll continue.
Make that 7 with only 1 good one, which I replaced anyway. I can't believe a radio this new has bad caps already. I have an AX44 and an old Midland 77-857 that still work and haven't been re-capped...
Click to expand...

!8 caps bad, 5 show good but will be replaced.ACON and P_H brand

The ones on the right are bad, less 1 still in board I need to order, left pile show good.

I wonder if i can put a 0.22uf film cap in for a electrolytic 0.22uf? Works for audio equipment.



After all this, I do have audio back, I do have PA working as well. I still have the driver and final out, I will checl the 2078 with a ohm meter and see if it shows open...I think these sometimes don't respond to a trans checker.

GREAT call Andy.

This is what the 2078 looks like on my handy dandy test-all.

25_caps.jpg handy dandy-tester.jpg
 
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