Cobra 29 GTL History




The Cobra 29 LTD is a later version of the Cobra 29 GTL. Although, the exterior is slightly different, the interior has not changed much over the years. The Cobra 29 GTL, manufactured in Hong Kong and Taiwan, was introduced in the USA in 1977 when the FCC expanded the CB band from 23 channels to 40 channels.


The Cobra 29 LTD replaced the Cobra 29 GTL shortly afterward. The changes made to the Cobra GTL and designated The Cobra 29 LTD where as follows.


1)The Red LED display was changed to Green.

2)The coil L-16 was changed from a variable to a fixed component.

3)The instant channel 9 switch was added to the Cobra 29 LTD.


The interior was not changed very much. In fact, the service manual published by Sams Photo Facts can still be used for the Cobra 29 LTD. At some point the model name was changed from the Cobra 29 LTD to The Cobra 29 LTD Classic. The service manual for Sams Photo Facts is CB-217


The Cobra 29 LTD and GTL was manufactured by Uniden for Dynascan. The Uniden 76 and the later version the Uniden 78 are the same radios as the original Cobra 29 LTD and GTL.


The Cobra 29 LTD was first manufactured in Taiwan, and then in The Philippines up until 1994. It has been manufactured in China since then. The original design remained the same for a couple of years, and then was overhauled by reducing the size of the resistors, cost, and quality of the components where ever possible.


The newest version of the Cobra 29 LTD has been changed to a greater degree. The following changes have been performed both to the exterior and to the interior in varying degrees.


1)The Cobra 29 LTD chassis now is available with Blue LEDs displays.

2)The analog faceplate is being phased out and a digital display is being introduced.

3)The original 10 pin audio chip TA-72222 was replaced first by the10 pin

      YD-1022 and finally the 5 pin TDA-2003. This was done to reduce cost.                                                                  



How it works:


Because of FCC restrictions the phase lock loop chip (2816) was required to reduce the ease of which the Cobra 29 LTD can operate outside the CB band. This PLL uses a single crystal to produce the entire mixer frequencies needed to operate. This is just the start of the operation of the PLL to prevent channel modification.


The reference crystal (10.240 MHz) is used to time the PLL and is used as a mixer frequency for the receiver. In addition the reference frequency is divided by two to produce a frequency of 5.12 MHz. This frequency is then multiplied by three to produce a frequency of 15.360 MHz.


Two phase lock loop chips may be found in the Cobra 29 LTD, the uPC2816 and the uPC2814. The factory PLL is uPC2816, but some radios may have the 2814 installed, both PLLs work interchangeably. The uPC2824 lacks the pin allowing the offset function of 455 KHz, and can be found in sideband radios, such as, the Uniden PC-122





The first early change in the Cobra 29 LTD was replacing varactor diodes D-13 and D-16 from board to surface mounted components. These varactors can be found near IC-2 the VCO mixer. The varactor D-16 (1S2687D) is used to control the Delta Tune; D-13 (1S2688EA) is used in the mixer tank circuit L-19.





The most common modification found in the Cobra 29 LTD is the disabling of the Transmit audio limiter D-11. This modification is unauthorized and is not endorsed by the author, The CB Doctor. Disabling the audio limiter may cause the carrier envelope to over modulate. Keep in mind that you can not get more than 100% of anything. A power Microphone will make the radio sound louder without exceeding the FCC part 95 regarding limiting a radio from going over 100% modulation.


It is a fallacy to believe that the radio will transmit further with the audio limiter removed. The radio will show more peak power, but this peak power reading is false. The flatten top and bottom of the carrier envelope will show up as more power because of the duration of the peak reading.


Adjusting L-14 will provide a higher carrier level; depending on the year and place of manufacturing; it will determine the gain of the carrier from the legal 4 watts to 5 to 8 watts. The carrier is needed for a receiving station to lock onto the center (Fundamental) frequency, but is a waste of power and provides no audio information.


Some of the modifications that should be avoided are cutting R-55 (1,000 ohm resistor) and jumping out D-8. R-55 is needed to help provide 50 ohm output impedance while D-8 is used to protect the Final and Driver from current flowing backward when the radio is un-keyed. If you see these modifications please restore them to factory.


Another modification is replacing the factory 2SC-2078 with a 2SC-1969. The 2SC-1969 can dissipate more heat, but has less gain than the 2SC-2078. The best combination for more power is using the parts found in the Cobra 29 GTL. The driver 2SC-2028B and the final 2SC-2029 will provide more power. The Toshiba 2SC-1969 is no longer made and can be found in limited supply at $15 a piece. I suggest saving any 2SC-1969 Finals for the purpose they were intended; the 2SC-1969 is for sideband radios.



Specific Details:


There are two mistakes on the Sams Photo Facts service manual. The first is the misnaming of the switching transistor. It is marked as TR-24 on the schematic, but is labeled on the board as TR-21. It is found near the Final


The second mistake is the voltage marked at the Final is labeled -12 volts at the collector; it is supposed to be marked +12 volts.



The PLL Circuit:


As I wrote earlier the Phase Lock Loop was required, under the FCC guidelines, to restrict the ease of expanding the Cobra 29 LTD for additional channels. The uPC-2816 uses a built in ROM (Read Only Memory). If the binary count is anything other than what is allowed the radio will go into shutdown. Another feature is the use of BCD (Binary Counter Decade). This entails using two sets of binary counters that can only add up to the number 10 for each set of binary counters. This helps limit the number of possible binary combinations to the PLL.


The other method to limit channel expansion is the feature on the uPC-2816 that shifts the radio by 455 KHz between transmitting and receiving. If you are thinking of replacing the 10.240 MHz reference crystal don’t bother. The frequency between the transmitting and receiver mixers will only increase and the two mixing frequencies will exceed 455 KHz; the result will be that you will not be on the same channel when you transmit and receive.


A method that will allow channel expansion is known as “forced signal injection.” The kit to allow for channel expansion can still be found on the market, but is slowly being phased out due to the lack of demand. The illegal export market has killed the legal domestic radio market. I know of only one distributor that still provides a version of the original Expo-Kit. This kit allows 40 channels below and above the CB band. The method used is forcing the loop mixer to “See” a mixing frequency other than the one provided by the Phase Lock Loop chip.  



The Receiver:


The receiver is a dual heterodyne receiver that drops the incoming frequency of 27 MHz to the first I.F of 10.695 MHz. This frequency is then mixed with the reference crystal 10.240 MHz; the difference is 455 KHz. This second I.F of 455 KHz is then rectified and filtered to provide just the audio from a carrier envelope.


The Transmitter:


The transmitter is a high level modulation circuit. The audio is impressed onto the carrier at the Final and the Driver. For every two watts of carrier one watt of audio is used to produce 100% modulation. This non-linear combination of 27 MHz carrier and audio at the Final and the Driver is called a carrier envelope.


The Audio Circuit:


The audio chip used in the original Cobra 29 LTD is the TA-72222AP. There have been three versions of the audio chip used over the years. The TA-72222 is the most common and the original audio chip found in the Cobra 29 GTL.


The second version was the short-lived YD-1022. The pin out is different than the TA-72222, but it is exactly the same 10 pin audio chip.


The third audio chip used today in the Cobra 29 LTD chassis and many of the new export radios is the TDA-2003. This audio chip has been around for 35 years; it lacks the frequency compensation of the former audio chips, but is a durable and rugged audio chip that may be used in a complimentary symmetry configuration or as a stand-alone audio chip. It is used as a stand alone audio chip in the newer CB radios.


The only reason the chips have been changed is to keep the cost of manufacturing low. Whichever audio chip is used the theory is the same. In the transmit mode the first winding is a center tap to a 13 volt source. This supplies the Final and Driver with 13 volts. As the audio chip is fed to the first winding the audio is imprinted on the carrier creating a carrier envelope. In the receive mode the audio is transferred to the second winding, which goes to the speaker. The DC component is isolated using a coupling capacitor.


The audio transformer is used in both the receiver and transmitter modes. It is rare, but I have come across audio transformers that have been overheated by a shorted final, and lost their inductive properties because the windings were overheated and melted the protective insulation. It is possible to have some receiver audio but no audio modulation when this happens.





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 The CB Doctor