Burn spot Cobra 29 LTD Chrome

[Bombardier]

Thank you, seems I always get a headache Radio, on the Regulator, I am getting voltage wise, from behind. Right leg 10v, Center 13V Left 10V again. I have Voltage Regulators coming in the Mail today. I changed Tr13 and Tr12, I am wondering if this could be the DC Transformer, the small Transformer.
I do have 10.35v at R105, and the Resistor 104 it's R104 the smaller of the 2 burn up after I replaced D8 as I discovered thar was knock out also. I will remove R104 and do what you said. For now this is where I am at. Thanks again for your reply sir.

 

[Handy Andy]

Keep us posted on progress..

 

[Bombardier]

Keep us posted on progress..

UPDATE. UPDATE. UPDATE. UPDATE.
OK I did a voltage check, I am seeing 27 volts at the center pin and disconnected( at the soldering pad). Still searching for a 27v short . I also have a 27 volt short at the driver, and with them disconnected there is still a 27 volt short at both the final and driver Center Pinn soldering tab on the board? Now I guess it's time to start pulling parts. This is where I am at.

 

[Handy Andy]

27 volts?

Yikes, is the power supply, ok?

You might be seeing a "no load" AC component.

No load? Well, your meter may not load the tested circuit down enough to load and change the measurement - not that it's supposed to, but 27 volts might be a "squeal" from the audio chip - getting amplified and sent to the driver and final - that could cause this reading - it's boosted from the AM Reactance Modulator (transformer) taking 12V and then mixing the audio AC component of the squeal - generating this.

So, for now, TR23 - remove it - might need the 880 replaced with another NPN like a 2SC1173

 

[NZ8N]

Pilot to Bombardier,
Pilot to Bombardier, ;-) Sorry, I couldn't resist.

@Handy Andy was referring to the fact that the power supply should only be at around 13.8v DC, not 27v. Check the actual voltage coming from your power supply, first. Are you measuring from board ground like from one of the metal IF transformers (cans) or from Chassis ground? I am not sure if or when they switched from floating Chassis Ground to strictly Negative Ground, but you should be applying the black (negative) lead to a "can" and measuring your voltages from that reference.

 

[Bombardier]

I think I may have found it. The large Modulation Transformer has 4 leads soldered to the board, and there are 2 that have 13+ v at the pins ( wires ). The 2 outer wires have the 13+ voltage. I did a check on another Cobra 29, that radio has Voltage on one wire. I will replace it as I have a few spares. Will update when I am finished tomorrow. Till tommow

 

[Handy Andy]

@NZ8N - you would be correct!

He is using a 2006 DTD board, so it has negative/positive ground chassis - see post #3 and #5.

Hes got the 1106 board layout, the persecutor to the LX types.

It looks to me that the "bombing run" the previous owner did - really goofed up the power system in there - as if it was intentional.

So, if it were me (IIWM) might want to break the connection until he sorts out the Audio amp and its mess.

Which to me - since we hear a thunk from the speaker - I'd unsolder all the legs of the Audio amp and the rebuild the 9V main power regulation and get the Green RX light to light, and keep lit - observing voltages...

Since he doesn't have much since CBT is gone - I'll go thru my notes to help get a chart started...

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[Bombardier]

@NZ8N - you would be correct!

He is using a 2006 DTD board, so it has negative/positive ground chassis - see post #3 and #5.

Hes got the 1106 board layout, the persecutor to the LX types.

It looks to me that the "bombing run" the previous owner did - really goofed up the power system in there - as if it was intentional.

So, if it were me (IIWM) might want to break the connection until he sorts out the Audio amp and its mess.

Which to me - since we hear a thunk from the speaker - I'd unsolder all the legs of the Audio amp and the rebuild the 9V main power regulation and get the Green RX light to light, and keep lit - observing voltages...

Since he doesn't have much since CBT is gone - I'll go thru my notes to help get a chart started...

View attachment 49732
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I have one concern, I removed a modulation mod, is there a possibility I may have missed something, and I pulled a C2029 final. I replaced 3 C2078 finals and installed a 1957 driver. I originally have this radio transmitting and going to the receive, seems during the removal of and changing the finals, to me this is when everything hit the fan? I notice when I ground the blue mike wire I can change the pitch of the Motorboat and also testing voltage on C 120 next to the Voltage Regulator I can slow down the Motorboat. Turning the Squelch back and forth I go from Motorboating to a loud farting noise. I did change the Modulation Transformer no deal there. I am stumped. I am going to go over all the transistor voltages and will let you all know as soon as I am done. Going to stick to it all day today.

 

[Handy Andy]

Make sure you keep the 2029 (2078 works too) mounting tab from shorting to the Heat sink panel - this will kill the final and damage the radio - especially the AM Modulator transformer.

​

 

[Bombardier]

Make sure you keep the 2029 (2078 works too) mounting tab from shorting to the Heat sink panel - this will kill the final and damage the radio - especially the AM Modulator transformer.

View attachment 49741
​

OK this is where I am at,Yes I have a grounding tab on the 2078 Final. I changed the SMD 39pf Capacitor on the TDA2003 Amplifier, this changed the pitch of the Motoreboat to a crawl. I up the Capacitor to a SMD 47pf and seemed to make a difference, also changed the Amp also. This is where I stopped last night. It is to have a 39pf SMD ,any suggestions on what to install between pin 2 to 4 on the Audio Amp. I rid of the 27 volts by changing the Modulation Transformer, changed the Capacitor directly to the left of the Transformer also, ( Looking from the front of the radio ) C73.
AS I am at a standstill, not having a 39pf SMD Capacitor to reinstall, as I would like to keep it at 39pf, but seem I have no choice, as this seems to be the part of or is the Motorboat issue. With the 47pf SMD installed, possibly up it or lower it, Any suggestions that I may have found a issue with the Cobra 29 LTD Classic, not Chrome.
I will take any suggestions on waht to use, I have the proper voltages.

 

[Handy Andy]

Then check and replace the INPUT caps, the ones that bright signal (audio in)

I presuming you have TDA2003 audio amp (5 pins)

If one of the caps from one of the sections that uses it, is bad, it may or may not fix the motorboat problems but due to it being 0ver 10 and nearly 15 years, the age of the unit you might want to replace some disc caps - being

C40 a 0.0022uf or 2200pF or 222 DISC - from Volume control output wire TO main PCB.

C37 - a 0.033uF or 333 DISC - might want to try a lesser value of 0.022uF (223) DISC - then once stable - see if you can raise this value up a notch back to the 333 value.

C39 is unique, it's 10uF - might have to swap in different values between as low as 1uF to as much as 10uF to see if their construction helps reduce the Motorboat effect. Caps in RF and DC and Audio - are highly subjective to these different types of realms - RF Audio and DC - with Electrolytics' especially, are not very stable and perform poorly if their values and how it's constructed are not considered.

That - being used in a area or network of feedback - in and around this amp - the ESR / ESL / ESC ratings are not always stable at all times under all events.

The Motorboat effect is from an instability in the amplifier of the Audio amp - when put in this kind of chassis - I find reducing the input capacitance helps in restoring function.

Once you have it working, you're able to make better tune-up adjustments and improve it since you can hear and listen to it - at least. Once you have it working - then work on bringing up the radio back to stock and beyond if you wish.

You got it this far from a reversed polarity - still works to some degree - good job!

 

[Bombardier]

Then check and replace the INPUT caps, the ones that bright signal (audio in)

I presuming you have TDA2003 audio amp (5 pins)

If one of the caps from one of the sections that uses it, is bad, it may or may not fix the motorboat problems but due to it being 0ver 10 and nearly 15 years, the age of the unit you might want to replace some disc caps - being

C40 a 0.0022uf or 2200pF or 222 DISC - from Volume control output wire TO main PCB.

C37 - a 0.033uF or 333 DISC - might want to try a lesser value of 0.022uF (223) DISC - then once stable - see if you can raise this value up a notch back to the 333 value.

C39 is unique, it's 10uF - might have to swap in different values between as low as 1uF to as much as 10uF to see if their construction helps reduce the Motorboat effect. Caps in RF and DC and Audio - are highly subjective to these different types of realms - RF Audio and DC - with Electrolytics' especially, are not very stable and perform poorly if their values and how it's constructed are not considered.

That - being used in a area or network of feedback - in and around this amp - the ESR / ESL / ESC ratings are not always stable at all times under all events.

The Motorboat effect is from an instability in the amplifier of the Audio amp - when put in this kind of chassis - I find reducing the input capacitance helps in restoring function.

Once you have it working, you're able to make better tune-up adjustments and improve it since you can hear and listen to it - at least. Once you have it working - then work on bringing up the radio back to stock and beyond if you wish.

You got it this far from a reversed polarity - still works to some degree - good job!

Thank you, now to go and check the Capacitors. What a day that was.

 

[Bombardier]

@NZ8N - you would be correct!

He is using a 2006 DTD board, so it has negative/positive ground chassis - see post #3 and #5.

Hes got the 1106 board layout, the persecutor to the LX types.

It looks to me that the "bombing run" the previous owner did - really goofed up the power system in there - as if it was intentional.

So, if it were me (IIWM) might want to break the connection until he sorts out the Audio amp and its mess.

Which to me - since we hear a thunk from the speaker - I'd unsolder all the legs of the Audio amp and the rebuild the 9V main power regulation and get the Green RX light to light, and keep lit - observing voltages...

Since he doesn't have much since CBT is gone - I'll go thru my notes to help get a chart started...

View attachment 49732
Not all features are available in all models...
Your Mileage May Vary.
Not Responsible For Articles Left In Clothing.​

Thank you.

 

[Bombardier]

Pilot to Bombardier,
Pilot to Bombardier, ;-) Sorry, I couldn't resist.

@Handy Andy was referring to the fact that the power supply should only be at around 13.8v DC, not 27v. Check the actual voltage coming from your power supply, first. Are you measuring from board ground like from one of the metal IF transformers (cans) or from Chassis ground? I am not sure if or when they switched from floating Chassis Ground to strictly Negative Ground, but you should be applying the black (negative) lead to a "can" and measuring your voltages from that reference.

That was a good one, made me laugh! Thanks for your help.

 

[Handy Andy]

"Bright signal in..".

Eeeww, sorry for that typo...

Have been seeing several simple problems of Disc caps having drifted from their values to the point of nearly open (no value) - happens when the robot that does the insertion - places too much torque on the part when it's leads are clipped off on the foil side

​