Cobra 148 GTL DX, not transmitting

Tricky-D · Post by **Tricky-D** » Wednesday 5th Jul 2006, 7:04

I have an old cobra 148 GTL DX and it's faulty. It seems to receive ok (well at least on uk14) but the bugger won?t transmit. The RX light goes off and the TX light lights up but the signal meter does not budge and it doesn't transmit. I have tried listening on my other box while transmitting on the cobra, also got a frequency counter connected. I have tried different mikes with no luck. The only thing I haven?t tried is transmitting on any of the "less legal" frequencies. Going to give that a try tonight.

I have came across this page http://www.satcure-focus.com/cb/page1.htm The only thing I can see that MIGHT be my problem is the Small paragraph about the Regulator Transistor. But it doesn't say what the symptoms are if this transistor fails.

Any of you guys have any ideas what might be causing this or know a good supplier of components (preferably in the UK) where I can buy said transistor?

Many Thanks

Rich

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Wednesday 5th Jul 2006, 10:28

TX/RX control information

The regulated TX/RX voltages come from TR36, TR37 and TR38.

When the mike is pressed the collector of TR38 goes high (about 8V), supplying the required TX only voltages.

In RX the collector of TR36 goes high, supplying the RX only voltages.

So I would first check that these are working as described.

Also check D86, it is a 5.1V zener diode.

I suspect that the regulator part is OK, going by the fact that the RX LED goes out and the TX LED turns on, but it is still best to check the voltages.

The other questions are whether it works on other modes, such as on SSB or AM, does the PA system work, the more you can tell us about what is working, and what is not, will help to narrow down to a area to look more closely at.

Even putting the radio on ch1 on the mid band and taking freq counter measurements on FM mode, at TP3 (bare lead of R84), when in RX and TX.

Tricky-D · Post by **Tricky-D** » Tuesday 11th Jul 2006, 22:38

Hi, sorry for the delay in posting a reply. I've been at the T in The Park music festival over the weekend up here in Scotland

Ok, tried transmitting on all bands & still no reading on the signal meter or frequency counter. There is a 5 tone bleep installed and I can hear & see the relay making & breaking when I press & de-press the mic & again, nothing on the signal meter or frequency counter.

Thank you for your reply The defpom, your advice is very much appreciated! Would you recommend replacing TR36, 37 & 38? If so what type and rating of transistor should I use? I will try and get a multimeter onto the collector of T38 & T36. If I do not get 8v would that mean the transistor being tested was faulty?
As for D86, when I?m testing it what am I looking for? Closed circuit one way and open circuit the other?

When i'm using the frequency counter on TP3, I assume I would connect the inner core to TP3 and the screen/braid to the chassis/earth of the radio? What readings should
i be looking for?

Thank you again for your Help

Rich

Rick · Post by **Rick** » Wednesday 12th Jul 2006, 0:41

Check to see if you have volts at the final if no volts check TR49 on Mk I and TR51 on Mk II. Replace with TIP42 from www.grandata.co.uk

Rick

Tricky-D · Post by **Tricky-D** » Thursday 13th Jul 2006, 3:22

Thanks for the reply Rick. what one is the final? im a little confused sorry.

Rich

Rick · Post by **Rick** » Thursday 13th Jul 2006, 3:55

The is TR43 (2SC2312) in the Mk1 and TR44 in Mk2, just check the collector voltage while you are key up (PTT de-pressed), you should have about 5.80 volts in AM/FM/CW and about 13.60 volts in SSB mode. Do not slip with meters prob you could shot the Final out.

Let me know how you get on.

Rick

Tricky-D · Post by **Tricky-D** » Tuesday 18th Jul 2006, 4:16

Hi guys,
Thats it working now. It was the 2SA1012 transistor that is bolted to the back right hand side of the casing (sorry, im unsure of the location number(does anyone know the location number?))
Cost me ?2.50 (pounds stirling) to repair. A big thanks to everyone's advice. Very much appreciated.

Thanks Again

Richard

Rick · Post by **Rick** » Tuesday 18th Jul 2006, 6:09

Its best to use transistor type TIP42.

TR49 on Mk I and TR51 on Mk II. Replace with TIP42

Rick.

Tricky-D · Post by **Tricky-D** » Friday 21st Jul 2006, 2:24

So instaed of using the 2SA1012 transistor is should of used a TIP42?

zodiac · Post by **zodiac** » Friday 21st Jul 2006, 3:51

Richard,
You have used the correct transistor but you could have bought six TIP42's (that do the same job) for the same price if you had bought them from the address that Rick gave you.
Dave.

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