Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

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Phuture
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Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by Phuture »

Hi,

Can anyone suggest a reason for instability around the target frequency only when trying to set it, according to the service manual, via L42 on the 16.35 MHz xtal variable inductor row? On the MK1 radio I have, the reference frequency for LSB, channel 40, HI band is 17.1575 MHz in order to obtain the channel 40 frequency of 27.855 MHz. However, every time I get within about 2.5kHz of 17.1575 MHz the signal stability starts to deteriorate and if I manage to leave L42 around 15.75 MHz it jumps around within +/-1kHz. If I tune the frequency to 17.60 (or a bit lower) its happy, but of course the channel frequency is off at 27.8575 MHz. If I tune L42 about the same amount below 17.1575 MHz, again, stability returns and then becomes unstable again around 17.05, so I guess there is some pattern there. I get this on HI, LSB only when tuning channel 40 but stability improves as I turn to lower channels. Would this be because the VCO is on the edge of its range and perhaps I need to give it a quarter turn CW? I already set the VCO to 5.4V as the service manual advises but the crystal reference frequencies it describes are different to my radio's.

Thanks
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cb4ever104
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by cb4ever104 »

The service manual for the mk1 isn't right . It says ch40 for the adjustments , but the readings they quote are for ch19 . So . on TP3 ch 19

LOW AM 16.040
LOW USB 16.0425
LOW LSB 16.0375

MID AM 16.490
MID USB 16.4925
MID LSB 16.4875

HI AM 16.940
HI USB 16.9425
HI LSB 16.9375

Sammy
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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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Phuture
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by Phuture »

That's useful information - thanks Sammy. I tried aligning the LSB on CH19 with 16.9375, but it's still unstable. I thought it might be some kind of detuning effect so I used a 100pF cap between my hook probe and TP3 but it makes no difference. I also tried adjusting the VCO a little to see if it would "move" the range of instability off but I don't think it made much difference. Really baffled.
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by cb4ever104 »

The few times that I've had that issue it's always been the little 470uH inductor that's been the issue . L45 in your case .
Capture.JPG

Don't know if you have one spare ? If not , then try moving the LSB one with the USB one and see if the fault moves with it . It might also be the main inductor L42 . You could swap them over too if it's not L45 to see what happens .

Sammy
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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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Phuture
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by Phuture »

Thanks Sammy, I'll give that a try. Let's say one or more of the inductors is faulty, are we talking donor only spares or have you been able to source the same or similar inductors from somewhere?
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by cb4ever104 »

Phuture wrote: 09 Jul 2021, 17:53 Thanks Sammy, I'll give that a try. Let's say one or more of the inductors is faulty, are we talking donor only spares or have you been able to source the same or similar inductors from somewhere?
I just take one from a scrap radio . The mk2 uses the same part , and there's a few in the Ranger pcbs too . You could always look for some on Ebay as well.
30TM060
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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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by Phuture »

Thanks Sammy. I caught a break! Before trying to swap round the cans I took a look for dry joints with a magnifying eyepiece. I found a tiny splatter of solder shorting one of L42's pins to another track. Levered it off, as it wasn't stuck well, and problem solved! So relieved I don't have to do the swap and go hunting for a spare.

Thanks for the reference frequencies - spot on with the radio and noted them on my print out. :thumbup:
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Re: Cobra 148 GTL DX MK1 LSB reference frequency for HI band on LSB

Post by cb4ever104 »

Great ! Glad you got it sorted .
30TM060
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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