Cobra - 148GTL - Malaysia - PCB EPT014811Z

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Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 05 Feb 2018, 15:52

Hello, friends.
I need some advice of any of you.

Cobra - 148GTL - Malaysia - PCB EPT014811Z
You can get its schematics here: http://www.cbtricks.com/radios/rci/tr_2 ... sm_sch.pdf.

I got a 148GTL, dead on arrival, with the following characteristics:
  • PLL MB8719
  • Crystal X3 is 11.1125MHz (Someone may have changed it to achieve channel -79)
  • CB/PA key is unconnected. Radio is hardwired to CB (This key must have been used to extra channels)
  • HI/LOW key is unconnected. Radio is hardwired to HI (This key must have been used to extra channels)
  • Almost everithing else is unchanged.
At this moment:
  • I have the trippler circuit working (after replace TR20) and I can read 33.3375Mhz at the TR20 emitter.
  • I have VCO Unit (UHIC070) working and I can set valid frequencies (34765Mhz to 35205Mhz) variating voltage on its pin #5.
  • I have plenty of sure PLL is working good, since I tested it on another radio and it works as expected.
  • Frequencies at the FI (PLL Pin #17) are difference between the frequencies present in TR20 emmiter and base, as expected, and are about 1.5Mhz (if my memory serves me well at this time).
Unfortunately, the PLL does not lock. LD (PLL pin # 6) does not change, and so, the transmition will not turn on (TR35). When I change channels in the rotary key, the frequency will not change. The radio is stucked in an uncommon frequency (outside the 40ties) no matter what channel is displayed.

Since I was away from analogic electronics for about 20 years, this error is beating the hell out of me.

So, I ask you for guidance. I can perform any tests you like, if requested. But since I'm doing it in my spare time, it can take a day or two for the results.

Thanks in avance.
... and keep the tubes glowing!

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 05 Feb 2018, 16:02

Hi .

Have any traces been cut from the channel selector to the PLL ? Also . Check the binary inputs from the channel selector to the PLL . They're listed in the service manual .

Sammy
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 13 Feb 2018, 19:50

Hello, @cb4ever104
Sorry being late. Life's beating me those days!
cb4ever104 wrote:
05 Feb 2018, 16:02
Have any traces been cut from the channel selector to the PLL ?
No. Checked it twice and tracks seems to be original. Tested for continuity and all seems to be ok!
cb4ever104 wrote:
05 Feb 2018, 16:02
Check the binary inputs from the channel selector to the PLL . They're listed in the service manual .
We have a problem here: A logic level "1' (High) in the rotary key comes from PLL pin #6 (Loop Detect), and since the radio's PLL cannot - at this moment and I do not know why - detect the loop, this line is always in a low level. So, there is no high levels available at rotary key/programming inputs.

Any help is appreciated.
Thanks once more.
... and keep the tubes glowing!

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by sonoma » 14 Feb 2018, 02:57

you said the radio has a 11.1125 crystal in it. look to see if pin 10 of the PLL is soldered to ground. when they have a 11.3258 crystal pin 10 has to be unsoldered.
just a thought.

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 14 Feb 2018, 09:18

sonoma wrote:
14 Feb 2018, 02:57
you said the radio has a 11.1125 crystal in it. look to see if pin 10 of the PLL is soldered to ground. when they have a 11.3258 crystal pin 10 has to be unsoldered.
just a thought.
Good point .
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 14 Feb 2018, 11:51

sonoma wrote:
14 Feb 2018, 02:57
you said the radio has a 11.1125 crystal in it. look to see if pin 10 of the PLL is soldered to ground.
cb4ever104 wrote:
14 Feb 2018, 09:18
Good point .
Hi sonoma and cb4ever104 ! Thanks for the tip!
Pin 10 is currently open (no high, no low), as seen in the schematics.

Does anyone of you have a better schematics for MB8719 than the "two page" one available in the internet?
I'm lookingfor some theory about Phase Detector of that PLL. {bnghd}

Thanks once more!
... and keep the tubes glowing!

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 14 Feb 2018, 12:18

Is the 10.240 input ok ?
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 14 Feb 2018, 12:42

cb4ever104 wrote:
14 Feb 2018, 12:18
Is the 10.240 input ok ?
Yes, it is. Reading 10.238MHz. I believe we can disconsider the missig 0.002 Mhz.
... and keep the tubes glowing!

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 14 Feb 2018, 13:01

OK . I'm going to have a look at the schematic .
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by sonoma » 14 Feb 2018, 13:45

if you have the 11.1125 crystal in the radio pin 10 should be soldered. you said it is open. do you mean it is cut at pin 10 ? this makes the freq very flaky.
here is a link to the 148 radios from Cobra. are you looking at the ones in cbtricks? those are pretty good.

http://downloads.cobra.com/CB/Models/148-SERIES/

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 14 Feb 2018, 13:57

Especialista wrote:
14 Feb 2018, 11:51
sonoma wrote:
14 Feb 2018, 02:57
you said the radio has a 11.1125 crystal in it. look to see if pin 10 of the PLL is soldered to ground.
cb4ever104 wrote:
14 Feb 2018, 09:18
Good point .
Hi sonoma and cb4ever104 ! Thanks for the tip!
Pin 10 is currently open (no high, no low), as seen in the schematics.
Pin 10 should only be open when there's a 11.3258 crystal fitted . If you have a 11.1125 then pin 10 needs to be grounded .

Sammy
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by cb4ever104 » 14 Feb 2018, 14:03

sonoma wrote:
14 Feb 2018, 13:45
if you have the 11.1125 crystal in the radio pin 10 should be soldered. you said it is open. do you mean it is cut at pin 10 ? this makes the freq very flaky.
here is a link to the 148 radios from Cobra. are you looking at the ones in cbtricks? those are pretty good.

http://downloads.cobra.com/CB/Models/148-SERIES/
Yeah . I was looking at the diagram and his posts when you posted sonoma .

Thanks for the link .

Sammy
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 - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 15 Feb 2018, 21:08

cb4ever104 wrote:
14 Feb 2018, 13:57
Pin 10 should only be open when there's a 11.3258 crystal fitted . If you have a 11.1125 then pin 10 needs to be grounded .
Bull's eye!!, cb4ever104!!! :clap: :clap:
I've grounded pin #10 and the PLL is finally on a good loop. I can switch channels via rotary switch and TP1 shows frequencies changing, as expected.
I was also able to align AM/LSB/USB deviation frequencies to stock ones.

I just cannot guarantee the correct channels frequencies... Seems to be out of order... I'll check it later.

Now, after the storm, seeing schematics for an ol' Cobra 140 GTL (that uses a 11.1125MHz crystal) I can really see pin 10 is grounded:
Image

Another thing I've learned today is that there are differences between RCI8710 and MB819. Since the schematic I was using was found by PCB number, it was a Ranger PLL, not Fujitsu. On Ranger ones, pin 10 is useless. That's why it's open:
Image

Well, I'll check the rest of the radio this weekend... I'll keep the thread going. Please, stand by: I'm sure I'll need further help!

By the time, I thank you all very much!
... and keep the tubes glowing!

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by sonoma » 15 Feb 2018, 23:30

when you get above channel 23 the freq are out of order. take a look at the assigned freq for cb radios. a lot of charts on the web for that.
glad you finally soldered in pin 10. as I mentioned earlier it can make a radio rather flaky when not soldered as it should be.

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Re: Cobra - 148GTL - Malaysia - PCB EPT014811Z

Post by Especialista » 16 Feb 2018, 00:18

sonoma wrote:
15 Feb 2018, 23:30
when you get above channel 23 the freq are out of order. take a look at the assigned freq for cb radios. a lot of charts on the web for that.
Thanks for the tip. I'll look into it.
sonoma wrote:
15 Feb 2018, 23:30
glad you finally soldered in pin 10. as I mentioned earlier it can make a radio rather flaky when not soldered as it should be.
That was driving me nuts. I need to pay more attention to schematics! {bnghd}
Thanks oce more!
... and keep the tubes glowing!

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