Cobra 25LTD Classic CB Radio

All the way back to 1991 for this Cobra CB radio. Looking it up I was surprised to find that a Cobra 25LTD Classic is still sold today (This one at Walmart). The microphone input has been moved to the front, and the squelch control is no longer present ūüôā

I am pretty sure the circuitry will have  changed in the past 26 years!Cobra CB radio



On the rear of the CB radio is the power connection, speaker outputs and an N-type RF connector for the antenna. The label gives the date of manufacture – Dec 91.



Opening up the case to show the speaker and well hell of a lot of wires!

At least 20-30 wires all hand soldered between the PCB and the controls. There are also ~8 capacitors soldered between the board and the case frame (For decoupling).


No multi-layer PCB here, components on one side and solar on the rear.  (You can see a couple of the capacitors soldered to the frame at the top here.)





Removing the frame but still can’t see the board through all the wires.



Here is the board with the wires (And the big heatsink) removedCobra 25LTD PCB

The RF input (Right side, centre) is all discrete passive components with hand wound coils to make LC filters. Only one ic is clearly visible on the board. The DIP package top left that connects directly to the rotary channel selector switch.

There are actually three on ic’s on the board, two are single in-line packages one at the very bottom (centre), and one just above that orange jumper wire. All three chips on the PCB are are made by Toshiba. Even in 1991 the concept of chipsets existed. Surprisingly given its age I was able to find the schematics for this board online. As well as datasheets (Scanned copies) of the 3 chips.


Lets start with the most complex IC the 18 pin TC9106BP which is CB Transceiver PLL Frequency Synthesizer.  It takes an input from a 10.24MHz crystal oscillator and the 40 position rotary channel selector switch outputs the desired frequency band (16.725-17.165MHz in 10kHz steps).

Here is the block diagram and the die imageTC9106BP die photo

The¬†die is 2.8mm x 2.47mm and has a single layer of (Al)¬†metal with 5őľm minimum geometry tracks.

But this is an odd duck! ¬†Described in the datasheet as “C-MOS LSI” ¬†– well even in 1991 I don’t think this would have been considered Large Scale Integration, and I don’t think it is CMOS.

I looked hard for evidence of any polysilicon gates but can not find any, you can clearly see the outline of the implants/diffusions. Is it possible that they have metal gates? РI thought that was only done on really old PMOS chips, and polysilicon was needed to get the work functions matched to enable low threshold voltages for both NMOS and PMOS transistors in CMOS. Also even if the metal was used as a gate material there is no clear source and drain connections.

I think this is a Bipolar process, but even for Bipolar the transistor layout is very peculiar and it is not obvious what are the emitter, base and collector regions. Here is a close up of some circuitry and I cannot figure out how the transistors are laid out here

And this is another area, nothing is really making any sense

It really is all very odd.

The LED program converter block is quite identifiable at the bottom of the die.  Zooming into that region,  you have a resistor array, with a series of different sized horizontal diffusion resistors contacted by the vertical metal strips. So depending on the 40 position rotary switch one combination of resistors is chosen that sets up the input into the counter.


The second chip is in an uncommon 9 pin SIP mounted vertically a TA7310P which is a double balanced mixer and differential amplifier with an emitter follower circuit. Designed to be partnered with (TC9106BP) PLL Frequency Synthesizer this is how the circuits function together

A tiny Bipolar chip (1.2mm x 1.02mm) just 18 transistors a few resistors and diodes with massive metal tracks ~15őľm. This would not be out of place in early 1970’s.TA7310P die photo



The third IC is an audio amplifier TA7222AP  also in a SIP this one 10 pin with heat sink that mounted to the case




Again another small Bipolar die (3.1mm x3.02mm) also with single very large metal tracks ~14őľm. This might have been fabricated on the same process as the TA7310P.TA7222AP die photo

The large power transistor is clearly visible, with is triangular fingers. Designed to give 5.8W out (Into a 4ő© speaker load) with 53dB voltage gain.

 (Apologies for the poor quality of this die photo. It is not fully in focus in all areas, the die is solder attached to the heatsink and I could not remove it. This caused the die to not lie flat so when imaging the full field of view is not completely in focus.  When stitching the images some of the non-focus areas got included.)

There we have it, some pretty old school and interesting chips powering the CB radio in 1991.

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