Cobra 89gtl/ cobra 29 Keeps Blowing finals

[Handy Andy]

Ah thanks!

So the board then we'll focus on the GTL - had to identify because I got stung a long time ago - long story...they put in a 29 board, not a bad way to go, but a lot more issues when it came to repair - nothing matched up - LOL!

So, we'll will focus on IC3 - for if and when that part heats up - we have to check voltages - PLL pin 15 (Power Good) and Pin 9 (Rx/TX Switching for SHIFT) - use BANDED side of D15 GENTLY to monitor voltages for PLL Pin15 using a DVM - a voltage rising above what is present when unkeyed (rises above 1 volt across D17's Banded side to ground - you have a leaky transistor TR17 PNP 2SA733 - if you have a spare - keep it around but you don't have to use it - there are other clues I need to know - this is the main one for now.

Another is to check to see if IC3 is "on" or you have a bad Mixer section - IF from main 10.240 will show up at TP1...

So if you have a scope put the signal side of the test lead on TP1 - R54 or R53 bare lead - by Pre-driver - look for a signal to show up and a hot part (TR10) soon afterwards...

The Pre-Driver is supposed to stay off, but if it or the circuit its in - has something wrong it can make the pre-driver turn on - but we need to see how the power to do this is getting there. To know that we need to see if a signal is arriving too...

So we look at Signal and DC power levels to see what is making TR10 hot - we have to rule out a signal emanating from the Pre-driver or the IF - then you can work on solving for TR10.

If we rule out IF leaking - this clears the way to Fix TR10 directly. Else we run the risk of going thru a lot of TR10's which you don't want...

 

[BAGEBOY]

Ah thanks!

So the board then we'll focus on the GTL - had to identify because I got stung a long time ago - long story...they put in a 29 board, not a bad way to go, but a lot more issues when it came to repair - nothing matched up - LOL!

So, we'll will focus on IC3 - for if and when that part heats up - we have to check voltages - PLL pin 15 (Power Good) and Pin 9 (Rx/TX Switching for SHIFT) - use BANDED side of D15 GENTLY to monitor voltages for PLL Pin15 using a DVM - a voltage rising above what is present when unkeyed (rises above 1 volt across D17's Banded side to ground - you have a leaky transistor TR17 PNP 2SA733 - if you have a spare - keep it around but you don't have to use it - there are other clues I need to know - this is the main one for now.

Another is to check to see if IC3 is "on" or you have a bad Mixer section - IF from main 10.240 will show up at TP1...

So if you have a scope put the signal side of the test lead on TP1 - R54 or R53 bare lead - by Pre-driver - look for a signal to show up and a hot part (TR10) soon afterwards...

The Pre-Driver is supposed to stay off, but if it or the circuit its in - has something wrong it can make the pre-driver turn on - but we need to see how the power to do this is getting there. To know that we need to see if a signal is arriving too...

So we look at Signal and DC power levels to see what is making TR10 hot - we have to rule out a signal emanating from the Pre-driver or the IF - then you can work on solving for TR10.

If we rule out IF leaking - this clears the way to Fix TR10 directly. Else we run the risk of going thru a lot of TR10's which you don't want...

Unfortunately I don't have a scope so how doni proceed

 

[Handy Andy]

Install an ammeter on the power + line and watch for power to rise up when it sits in RX mode on a dummy load.

Feel TR10 for heat when that current spike shows up.

Then look for your voltages as said earlier.

Then you may have found the problem.

We just need to know if the power is from D17 to ground (enough for IF to turn on Pre-Driver) or you have a leaky TR17 (TX Switch) or a bad diode.

Something tells me you skipped a step earlier - you need to check Diodes - did you check D20?

Locate D20 - still intact? This is the TX side for the Delta Tune that is "fixed" on TX mode. If mods have been done - suspect this area to be potentially the culprit.

If intact and seems to Key up on it's own? Usually that TX is switched thru PIN 3 MIC - which Grounds D17 the PLL's SHIFT at Pin 9 - D15 forces PLL's Pin 15 to go high - power goes to IC3 and the TX switch TR18 - IC 3 can't produce anything unless it gets power, which it's only supposed to get in TX mode - if it's on all the time - suspect a blown power steering Diode - a Shorted D20 can do this - backfeeds into TX side from Delta Tune - you'd know it by turning the Delta tune in one direction or another and it's start keying - won't show TX yet, but it can make IC 3 work from this new power source.

 

[BAGEBOY]

Will check it now

 

[Handy Andy]

We are just making sure IC 3 is not turning on sending IF even though it's not fully mixed.

It can cause parts in the TX line to fail - not from RF, but from IF - getting sent into the strip and the strip is not tuned for the IF purposely - so it doesn't pass thru into the antenna as a mixing product.

The TX strip is designed as a band pass filter for RF of 27MHz, so IF being passed can cause High SWR condition in the circuit and destroy the parts involved.

Another aspect - once the MIXER is checked out - and if it passes, you'll need to go to the Audio Amp and make sure it's filters are in place - a ringing effect caused by self-resonance can destroy the strip as well.

So as you work forward, carefully observe the DRIVER as well as the FINAL - check them Both for temperature during this time - Driver - due to the design in the strip can easily pass IF better than the Final can - which the output tank circuit after the Final is designed to remove IF as well as harmonics above CB and only pass 27MHz

 

[BAGEBOY]

We are just making sure IC 3 is not turning on sending IF even though it's not fully mixed.

It can cause parts in the TX line to fail - not from RF, but from IF - getting sent into the strip and the strip is not tuned for the IF purposely - so it doesn't pass thru into the antenna as a mixing product.

The TX strip is designed as a band pass filter for RF of 27MHz, so IF being passed can cause High SWR condition in the circuit and destroy the parts involved.

Another aspect - once the MIXER is checked out - and if it passes, you'll need to go to the Audio Amp and make sure it's filters are in place - a ringing effect caused by self-resonance can destroy the strip as well.

So as you work forward, carefully observe the DRIVER as well as the FINAL - check them Both for temperature during this time - Driver - due to the design in the strip can easily pass IF better than the Final can - which the output tank circuit after the Final is designed to remove IF as well as harmonics above CB and only pass 27MHz

I have checked D20 Tr 17 and Tr18 all are fine. What else do u want me to check..I have a scrap cobra 29 that was damage in shi0ong donu want me to change Ic3?

 

[Handy Andy]

No, as long as D17 was not showing any voltage above 1 volt

If there was a signal running rampant in there, the Diode would rectify it into a current that shows up and makes things act weirdly - but you wouldn't see it on any meter unless you have a scope or probed D17 with a DVM with the Test lead to the Banded side and the other to ground a look for voltage during the "heating up" moment - some kind of meter or scope has to be set up to detect it.

The D17 "setup" could actually provide enough power to make IC3 turn on - doesn't need much nor on all the power pins. For the Pre-driver could turn on from this rectified signal and make your day.

But now, lets look at your Audio section - your Modulation Transformer - has several filters around it - better listen to the receive, sound ok, not pinched up or low in volume?

IF you did a tune up great! But double check your squelch and RF gain make sure they operate linearly - not all scrunched up to one side - and make sure the receiver is stable - a high pitched squeal doesn't get heard very well, but can do other things - like affect the RF and Mod meter - which you may need to look at during that moment.

We're going to look at the above to help us figure out the reason why...
​

 

[Handy Andy]

Had a brain fart, D15 is the one you need to monitor - D17 is the RX/TX switch thatr looks - monitors, senses MIC Pin 3 - which on TX IS GROUNDED.

So D15's Banded side is the one you look at for any voltages above 1 volts - it can turn on the Mixer.

D20 is the TX side of the Delta Tune - can't have it flowing backwards - THEN IC3 and Predriver WILL turn on.

Also - closely monitor the audio "quality" present at the speaker - if the audio doesn't sound good and nothing you do seems to clear it up - it may not be the receiver sides fault, it may be a fault in the Audio Amp to Modulation Transformer Filter Network - there is a 10uF Tantalum - used in the 7222AP Offset pin Pin 5 from Pin 4 Mic/RX audio in - if this cap goes bad, you'll need to swap it out and use a 4.7uF Tantalum - a lower value - to start, why? Because of Tantalums as they age, and now with the newer imports and points of origin, the "Tantalum" material - can "appear differently - and act differently" in this type of circuit you may want to start small then upgrade as you get the radio to stabilize.

 

[BAGEBOY]

Will check D 15 and I replaced Ic just in case

 

[BAGEBOY]

I am still at it ..what I found was D13 BRIDGED with a small piece of wire.FRom research it is some mod which is done with the cobra 29 with shorting (D8) (which is D 13 on coba 89) .when I tomorrow this wire the short seem to go away However the final and Driver are good but I am not getting any output power. Any ideas?what the diode mod is suppose to do?

 

[TheRealPorkchop]

Change the big transformer.

 

[Handy Andy]

Uh oh...

A diode shorted out at that Modulation Transformer?

• Ouch! That can do a LOT of damage.

D13 once properly replaced with a 100PIV 3A part, should now allow you to use finals without blowing up.

You still have some work to do though.

Why?

Could be from another hidden mod you didn't catch, including caps pulled from the board, at the wrong places, now causing a high-frequency pitch "ring" or squeal you don't or can't hear, but the finals cook from it.

IT's from the sheer amount of inductance the Modulation Transformer presents to the Audio Amp - makes everything output from that amp, bass-y or muddy if you did not choose the right values of caps to use.

So to make this system work, they "match" the output on the amp side to meet the "expected" input on the transformer - so they put a string of parts in to "check" (a feedback) the output and reapply it to the input to "correct" the output to more closely meet the transformers expectations.

Below is a representation of the many feedback paths and circuit support used in an Audio Amp AM-Only radio.

​

It's why they use the values they do - they pass higher frequencies and the transformer cuts off the high-end - by acting like a Lo-pass filter - giving you an even narrower band pass that is stable enough - until you upset the stream of audio by trying to pass more band width to try and make a richer sound.

In the Graphic above, it tries to PUSH or modify the signal.
It acts upon it like a filter.
Yes, it does add a level of distortion too.​

• Not enough Bypass-capacitance applied in the loop? Don't worry - the loop forms around the Finals to Audio Transformer path instead and generates that self-induced ringing effect.

It's a simple mismatch. Like using the wrong impedance (an ohmic value) for a speaker system - you can damage the amp from it.

So they put capacitance - just enough of it - to reduce the ringing and feedback (impedance) mismatch - but at a price of fidelity. You don't get Hi-Fi - you just a narrower speech-range band pass for vocals - more for your voice than music.

Which perhaps is better because some people just can't sing...
​

So where the all the high frequencies go? The caps harness that energy and send it to ground - and in the process look like a simple resistor to those ranges of audio Tamed and put safely to ground and neutralized.

IF you don't keep those caps in there - what happens to the audio? They're (that upper higher ranges of frequencies) transformed (literally) into heat and a secondary oscillation that is a loss in the system - it's taken out on the amp - transformer and finals as extra heat they have to dissipate.

If this effect is not properly tamed, it will damage the radio even further.

 

[TheRealPorkchop]

 

[BAGEBOY]

Uh oh...

A diode shorted out at that Modulation Transformer?

• Ouch! That can do a LOT of damage.

D13 once properly replaced with a 100PIV 3A part, should now allow you to use finals without blowing up.

You still have some work to do though.

Why?

Could be from another hidden mod you didn't catch, including caps pulled from the board, at the wrong places, now causing a high-frequency pitch "ring" or squeal you don't or can't hear, but the finals cook from it.

IT's from the sheer amount of inductance the Modulation Transformer presents to the Audio Amp - makes everything output from that amp, bass-y or muddy if you did not choose the right values of caps to use.

So to make this system work, they "match" the output on the amp side to meet the "expected" input on the transformer - so they put a string of parts in to "check" (a feedback) the output and reapply it to the input to "correct" the output to more closely meet the transformers expectations.

Below is a representation of the many feedback paths and circuit support used in an Audio Amp AM-Only radio.

View attachment 46639​

It's why they use the values they do - they pass higher frequencies and the transformer cuts off the high-end - by acting like a Lo-pass filter - giving you an even narrower band pass that is stable enough - until you upset the stream of audio by trying to pass more band width to try and make a richer sound.

View attachment 46641
In the Graphic above, it tries to PUSH or modify the signal.
It acts upon it like a filter.
Yes, it does add a level of distortion too.​

• Not enough Bypass-capacitance applied in the loop? Don't worry - the loop forms around the Finals to Audio Transformer path instead and generates that self-induced ringing effect.

It's a simple mismatch. Like using the wrong impedance (an ohmic value) for a speaker system - you can damage the amp from it.

So they put capacitance - just enough of it - to reduce the ringing and feedback (impedance) mismatch - but at a price of fidelity. You don't get Hi-Fi - you just a narrower speech-range band pass for vocals - more for your voice than music.

Which perhaps is better because some people just can't sing...
​

So where the all the high frequencies go? The caps harness that energy and send it to ground - and in the process look like a simple resistor to those ranges of audio Tamed and put safely to ground and neutralized.

IF you don't keep those caps in there - what happens to the audio? They're (that upper higher ranges of frequencies) transformed (literally) into heat and a secondary oscillation that is a loss in the system - it's taken out on the amp - transformer and finals as extra heat they have to dissipate.

If this effect is not properly tamed, it will damage the radio even further.

No some one had a wire across it I believe some mod I have seen it in the cobra 29which would be d8 on the 89gtl it is d13

 

[Handy Andy]

That's a final/driver - and, if left on too long - radio killer.

Can wreck the radio to beyond its worth to repair.

Seen several with melted transformers and burnt - brittle boards from the heat.

Hopefully this one didn't suffer too much damage.