Cobra 150-GTL ( cybernet PTBM121D4X )

[Steve G4WCS]

got a hankering for another SSB CB radio to play with and re learn myself cybernet.

( years ago I nearly finished a Ham international that I'd VFO'd with the G3WPO Ham Radio Today but all that went in the first iteration of "Im not playing with radio any more" along with a stack of very nice RF test kit )

Anyway, kept my eye on the usual outlets, and came across this, A Cobra 150-GTL in reasonable condition and advertised as working, and wasnt being offered for silly money.

I liked the look of the front panel with the letraset style lettering, and the solid ally knobs so went for it.

first job was a cosmetic restoration, the knob flutes were full of grime, so into the ultrasonic cleaner, case parts were trated to a dose of CIF foaming bathroom cleaner, which is excellent for lifting years of grime, and the rear panel was carefully removed and brasso'd.

the crinkle vinyl on the case panels was then waxed and buffed.

Panel surround was brasso'd

Its quite hypnotic watching an ultrasonic cleaner at work

https://youtu.be/hFLTk50-BgM

[Steve G4WCS]

before pictures

[Steve G4WCS]

during pictures

[Steve G4WCS]

after pictures.

[Steve G4WCS]

now to sort a microphone.

tried a couple of microphones I had, but although I could get it working on transmit, there was no "hiss" on receive, and no change in the noise when the mode selector was changed. screwdriver on the volume pot produced plenty of hum, so suspected no supply to the receive side.

had a look at the circuit and the stabilised supply rails are switched by a couple of transistors, checked the receive one and sure enough open circuit. this was changed and the radio burst into life.

https://youtu.be/Tb9Cy4k1Qkg

Gave it an airing on the North West Net, and the mixer crystal was found to be slightly off frequency. this was trimmed on air

https://youtu.be/yJJD1bLQ3tY

now spot on ( on mid band ) and a quick look round the shack

https://youtu.be/yJJD1bLQ3tY

[Steve G4WCS]

now for a roger beep.

there is provision on the PCB, for a roger beep circuit and relay, but without removing the board it looked far too fiddly, and I wand it to be a reversible mod anyway.

there is a circuit for a bepp int the cybernet service manual so laid one out on veroboard.

with a couple of component tweaks Ive got it to sound how I want

https://youtu.be/

https://youtu.be/5yFKtPBtcqI

https://youtu.be/6E7PqIdBcxE

[Steve G4WCS]

got as far as fitting it, but out of time so far to test properly in situ

[electronicskip]

Nice piece of kit Cobra stuff is well built and sort after.
I owned the 148GTL back in the day .

[SiriusHardware]

Actually, this is possibly the most untypical Cobra radio I have ever seen, as the chassis in higher end Cobra radios (like the 148 'skip mentions) were almost invariably of Uniden origin.

Nor does it have the typical look / styling / image of a Cobra radio, they were very fond of narrow, tapered chrome-effect knobs and more heavy 'Chroming' on the bezel surrounding the front panel - if you'd asked us to guess the brand I would have said it was a Colt of some sort, especially from the styling of the knobs.

There were a few exceptions to the Cobra Is Always "Uniden Inside"rule, for example the popular, basic Cobra 19X was based on a common Maxon (Korean made) chassis. This radio is obviously another of those unusual exceptions.

A nice restoration job anyway, well done.

[Steve G4WCS]

thanks, yes it is a nice looking radio.

After a few more nights faffing, and a relay, its all working.

it was bleeping without the relay, but coming through on the speaker, the relay cuts the speaker out on transmit and also gets rid of the annoying DC thump on the speaker when changing from reveive to treansmit. I dont know whay the original designers didnt just use a relay instead of two switching transistors, as they have put provision on the board anyway as part of the on pcb roger beep provision

[SiriusHardware]

I have a confession to make, I absolutely detest roger beeps. I used to permanently disable them in any radios which passed through my ownership.

The 'hybrid' single tone Roger Beep modules used in Ham International radios were very strident and quite troublesome - the PTT output transistor used to fail short-circuit, although you could replace the original SM transistor with a conventional transistor soldered to the pads on the module. It was easy enough to make those beeps switchable, but easier still to remove them entirely.

[Steve G4WCS]

Roger beeps are very much a marmite thing.

Handy for chatting on a quiet channel on ssb where you cant hear the carrier drop out at the other end, so you dont need to constantly be "back to you" or "over" ing

[SiriusHardware]

Agreed, and for the original purpose that they were probably conceived for - hands free 'Vox' (Voice operated PTT) they are also quite sensible.

Ones which didn't have an off switch (and that meant most radios which had roger beeps) were unforgivable though. The HyGain 'low-high-low' beep was probably the most irritating of the lot.

I didn't even restore their operation if I sold a radio on, because I didn't want to be on the receiving end of it if it ended up in my local area.

[Steve G4WCS]

Rainy afternoon in the shack, so made storage for test leads and fished the Belcom out to try and remember where I got up to with it.

[electronicskip]

Just noticed that the 150 GTL has an identical layout visually to my Harrier CBX even down to the facial look although the CBX is standard 27/81 not a multimode.

[SiriusHardware]

That happened a lot with 'high end' radios produced for the UK legal market. On a radio which really only needed a volume and squelch control it took a lot of imagination to fill up all those spaces on the front panel with something 'meaningful'.

You ended up with two or three position slide switches devoted to receiver tone control or rotary controls which dimmed the channel display (but not the meter illumination) or a three position switch which 'bent' the receive frequency up or down a bit - utterly pointless when used to communicate with other radios in the same class but sometimes unexpectedly useful if someone tried to talk to you from a nearby hi-band or superhi-band channel using a non UK radio.

Some of the extra controls were ridiculous, like Mic Gain - on an FM radio system where all channels were 10Khz spaced, what was the point of using less than the allowed maximum deviation?

Even RF gain was not as useful on an FM-only radio as it was on an A.M. radio, where a nearby signal could drive the receiver into distortion.