Cobra 148 gtl dx SS 360 FM Mk2 no TX power

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mutley
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Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 02 Jun 2018, 09:41

Hi all,

After a bit of advice if possible?

I bought a cheap SS360FM from ebay knowing it had no TX, but thought I'd take a punt.

It's arrived and I've tested it and I've found that it does TX, but with pretty much no power. No movement on watt/SWR meter at all, but you can hear transmitted audio on another rig next to it.

Also the current draw goes from about 0.4W to 0.8W when TXing so know it's doing something :D

I could see the previous owner has had the driver and final out in the past as there's a few scorch marks etc and bad soldering.

I've checked voltages at these transistors and middle pins have the 4.8V in AM/FM and 13V on SSB.

The transistor closest to the aerial socket looking down at the solder side (Final?) has 0V on the left pin and also 0V on the right pin
The transistor closest to the chassis on the right looking down at the solder board (driver?) has 0V on the left pin and obout 0.3V on the right pin.

I've checked boh voltage regulators and they are giving out 5V and 8V

Is there anything else I can check or change that's easy? I suspect it'll be making its way down to Nick but thought I'd have a go if it's something simple.

Cheers

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ch25
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by ch25 » 02 Jun 2018, 10:55

Do you have RF probe? If not build one, than you can easily find where RF disappear.
Image
According to this schematic http://www.cbtricks.com/radios/cobra/14 ... 34_sch.jpg measure RF voltage on TX on collectors TR44, 45, 46, 47 and come back with results.
Chris
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mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 02 Jun 2018, 12:47

Thanks Chris.

Will do.

Cheers

mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 04 Jun 2018, 18:06

Ok.. so if I built it right and measured in the right place (centre pins)

On FM TX the values were:

Tr44 1.2v
Tr45 1.2v
Tr46 11v
Tr47 2v


Cheers

Nick

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ch25
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by ch25 » 04 Jun 2018, 18:17

44 and 45 seem to be dead. What is base RF voltage btw?
What is driver and final bias voltage? Are diodes D104 and D105 ok?
Chris
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mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 04 Jun 2018, 19:09

Thanks Chris.

Took out the funny diodes and they're okay.

Checked voltages at the test points with the board out. On FM TX
4.5v between tp7 and 8
5.4v between tp7 and 9.

I'll check base RF voltages shortly.

The radio looks like it's had reverse polarity at some point as there's a burnt out track and protection diode has been replaced and 1 leg used to bridge the broken trace.

Just checked receive again and it's fine. As is TX but just no power.

Cheers

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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by ch25 » 04 Jun 2018, 19:29

Measure current, should be tp7-8 50mA and 7-9 80mA USB, no modulation.
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mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 04 Jun 2018, 19:54

Tested the currents and got them as above although the vr needed turning all the way to get 7-8 to 48mA.

I noticed it was missing R267 next to the driver, so stuck in a 2.2K and I now have 1W on FM !!

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ch25
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by ch25 » 04 Jun 2018, 19:59

Set currents as I wrote and measure power. Inspect for missing/mismatched elements too :)
You can't have too many antennas...

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cb4ever104
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by cb4ever104 » 04 Jun 2018, 20:06

mutley wrote:
04 Jun 2018, 19:54
the vr needed turning all the way to get 7-8 to 48mA.

I noticed it was missing R267 next to the driver, so stuck in a 2.2K and I now have 1W on FM !!
Doesn't sound great . The VR shouldn't have to go that far . Can't think of any reason why someone would have removed R267 . It's part of the bias circuit . Also , it should have a ferrite bead on the lead of it as well . The MV1Y might still be bad . I presume you replaced R267 and then checked the current , as you would have got 0mA without it .
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mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 04 Jun 2018, 20:14

Yeah currents are after replacing the resistor. I think I have some ferrite beads out in the garage so will check tomorrow.

I'll keep checking for any missing components as its still not right. Currently I can get just under 1 Watt on FM and needle barely moves on SSB.

Cheers all :D

mutley
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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by mutley » 05 Jun 2018, 21:46

I'm disappointed no one spotted my deliberate mistake ;)

R267 is 2.2 ohm not 2.2K :lol:

It's firing on all cylinders now. One pesky resistor was all it was.

Thanks for the advice though. I've learned a little bit more about fault finding.

Cheers

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Re: Cobra 148 gtl dx SS 360 FM Mk2 no TX power

Post by cb4ever104 » 06 Jun 2018, 07:31

mutley wrote:
05 Jun 2018, 21:46
I'm disappointed no one spotted my deliberate mistake ;)

R267 is 2.2 ohm not 2.2K :lol:
I did notice it , but I thought I'd leave it to see if you could figure it out for yourself :)

Glad you got it fixed .

S.
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Paul aka "Sammy". (108) PY60 Old Paisley DX Group (RIP Rab Markie)
Ex GM0 , now EA5 .
Handle "Fortune Hunter" back in "the day"
Radios ....? Anything that says "TAIWAN R.O.C" on the back.

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