Help needed for Cobra 148 NW ST

[CTStallion]

Okay,

Apparently, when this rig went back to Cobra to get Factory Reconditioned, instead of changing out R130 like they should've, it appears the "geniuses" at Cobra installed a Mod consisting of a board labeled: KEPC-635-A, and says: "for 148nw latchup reset."

The board consists of 2 transistors, 4 resistors, a diode, and an electrolytic cap. There are 4 wires coming from the board, all colored differently and connected (I 'THINK') to the main PC board as follows:

Yellow wire goes to the (+) of where C115 is supposed to be
Red and Blue wires go across where I think R130 is supposed to be
Black wire connects to the (-) of C8

I am so frustrated at this point, I wonder if I can just swap out this PLL for a known GOOD MB8719 instead of bothering with this ridiculous jury rig by Cobra.

Or, does this KEPC-635-A board serve some OTHER purpose?

 

[ExitThirteen]

R130 is going to be inline with a wire, it won't actually be located on the PCB, the resistor will be shrink wrapped with a wire and soldered into the PCB. All the 148NW/ST's I've worked on have had it this way, I think the wire is either blue or black, if memory serves.

(edit) You could probably swap in an MB8719 PLL, but you have to make sure that the control voltage is +8V. The newer MMB8719's and the RCI8719's use +5V.


Hope this helps!


~Cheers~

 

[CTStallion]

R130 is going to be inline with a wire, it won't actually be located on the PCB, the resistor will be shrink wrapped with a wire and soldered into the PCB. All the 148NW/ST's I've worked on have had it this way, I think the wire is either blue or black, if memory serves.

(edit) You could probably swap in an MB8719 PLL, but you have to make sure that the control voltage is +8V. The newer MMB8719's and the RCI8719's use +5V.


Hope this helps!


~Cheers~

Sure does... I bet it's in the BLUE wire to the latchup board.

Thanks AGAIN!

 

[Robb]

R130 is going to be inline with a wire, it won't actually be located on the PCB, the resistor will be shrink wrapped with a wire and soldered into the PCB. All the 148NW/ST's I've worked on have had it this way, I think the wire is either blue or black, if memory serves.

(edit) You could probably swap in an MB8719 PLL, but you have to make sure that the control voltage is +8V. The newer MMB8719's and the RCI8719's use +5V.


Hope this helps!


~Cheers~

Yup, it is a blue wire. It is right on the very edge of the board. If you find the blue wire that has the black shrink wrap at the bottom, take off the shrink wrap after lifting the wire from the board, it is there. Not on the board but at the end of the blue wire and very small - easy to miss!

Now I got to find a scrap 10 ohm/1/2 watt resistor . . .

 

[Robb]

Houston . . . . we have LIFTOFF!!!

That got the VCO & Loop Osc working again.

Thanx a meelyun, Xit13!!!

 

[sonoma]

glad you found it Robb, that little resister is hard to find on the side of the board. it sure makes the radio act up when it is going bad.

 

[CTStallion]

Okay.... same here R130 was bad, replaced it with a new 1/2 watt 10 ohm, and now I have been able to restore VCO voltage.

Using service manual on CB Tricks for the NW ST, I was able to set the VCO voltages on Channels 1 and 40 to within specs, and that restored the radio to TX/RX on MOST channels. (Funny thing, IF the voltage is NOT precisely adjusted to specs, the rig transmits off freq on channels 1-3, but that's okay now.)

Now I am back to SQUARE 1: radio works properly on all channels EXCEPT 31, 32 and 33, where there is NO PLL LOCK there.

What I have done so far is:

#1 - Changed the channel selector thinking that was at fault (it wasn't)
and
#2 - Replaced the R-130 10 ohm resistor.

While the service manual doesn't have the PLL voltage charts for the NW ST's MMB8719 PLL, (only for the 148 GTL and ST's MB8719), I checked the voltages (on channel 9 as the manual dictates) and got the following on the PLL pins:

1 = 3.63 V
2 = 3.78
3 = 3.93
4 = 3.93
5 = 3.20
6 = 8.00
7 = 3.79
8 = 3.53
9 = 8.0
10 = 7.90
11 = 0
12 = 7.89
13 = 7.89
14 = 0
15 = 0
16 = 7.89
17 = 3.72
18 = 0

 

[CTStallion]

Shouldn't I be seeing voltage on PLL pin 11 I wonder?

 

[Robb]

Matrix diode or pll?

 

[ExitThirteen]

CTStallion, go to a diffferent channel and check voltage on pin 11 again. On some channels, pin 11 may not be in use, and will show no voltage, as these particular PLLs use a binary method to create the frequencies. Since you're getting everything but 31-33, I'd suspect the problem is elsewhere, perhaps a bad switching diode that's keeping voltage on one of the pins or a bad bypass cap that's grounding out a pin. There is an outside chance that your PLL chip has a bad pin, but that's not real common.

(edit) If you're seeing +8V on pin 9 of the PLL you have an early MMB8719, it seems I had forgotten that there are 2 versions of that PLL, the early versions runs +8V and the later ones run +5V.

Hope this helps!


Oh, and BTW Robb, my check had better be in the mail.

~Cheers~

 

[sonoma]

CTstallion here is some 148 info that is more current than cbtricks. Bennie has not updated the info there in years and this info is from cobras web sight. might help you some also.

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

 

[Robb]

One of the other major issues with this radio is that you cannot turn up the ALC w/o it getting 'warble' modulation during TX. Seems like I can always tell when someone is using a NW/ST for this very reason. Must be a common complaint; is there a fix? Is RF getting back into the PLL circuit? Not enough current being delivered to the finals? Any special biasing instructions for the driver/final?

Got it all working fine now otherwise, except for that TX warbling problem in SSB mode.

 

[CTStallion]

CTStallion, go to a diffferent channel and check voltage on pin 11 again. On some channels, pin 11 may not be in use, and will show no voltage, as these particular PLLs use a binary method to create the frequencies. Since you're getting everything but 31-33, I'd suspect the problem is elsewhere, perhaps a bad switching diode that's keeping voltage on one of the pins or a bad bypass cap that's grounding out a pin. There is an outside chance that your PLL chip has a bad pin, but that's not real common.

(edit) If you're seeing +8V on pin 9 of the PLL you have an early MMB8719, it seems I had forgotten that there are 2 versions of that PLL, the early versions runs +8V and the later ones run +5V.

Hope this helps!


Oh, and BTW Robb, my check had better be in the mail.

~Cheers~

Hmmm, so what should be my NEXT step in the troubleshooting process? I have a PLL truth chart (Thanks SONOMA!) for a 148 (although not a Night Watch). I checked pin 11 on different channels and it DOES change voltage on a few of the lower channels as you said it would.

I also have a known good MB8719, should I change it to that?

Where should I be looking for these switching diodes at?

 

[ExitThirteen]

Since you have a good MB8719, and you get +8V at pin 9, I'd say to go ahead and swap it in, and see if you get 31-33, if not then you know to look elsewhere. Let us know how you get on.



~Cheers~

 

[Robb]

http://www.cbtricks.com/radios/cobra/service_bulletins/graphics/tsb_10031.pdf