148gtl blown d44 and mic Jack conversion

[338_MtRushmore]

I have a 148gtl (8719/412) that I have no history on. It is obvious that d44 is blown. I desoldered the 1969 and 2166, and they tested ok using the diode setting on a dvm. I also found 8 volts at the sideband pot, so I assume the voltage regulator is good. Is there anything else I should check before replacing the diode? Also, do I need to keep the board on the 5 pin jack in circuit when I install a 4 pin jack, or can I hooked the wires directly to the 4 pin? Thanks

 

[loosecannon]

you should test the voltages on the MB3756 regulator to make sure they are correct.
if they are, you can proceed on.
use this schematic as it has the voltages listed. remember that pin 6 should be receive only 8 volts, and pin 8 is transmit only 8 volts.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf

as for the mic jack, if you choose not to use the little PC board, then you should install your own bypass caps and chokes on the pins, just like they are on that circuit board.
these parts are very important for filtering and keeping the mic from squealing.
LC

 

[338_MtRushmore]

you should test the voltages on the MB3756 regulator to make sure they are correct.
if they are, you can proceed on.
use this schematic as it has the voltages listed. remember that pin 6 should be receive only 8 volts, and pin 8 is transmit only 8 volts.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf

as for the mic jack, if you choose not to use the little PC board, then you should install your own bypass caps and chokes on the pins, just like they are on that circuit board.
these parts are very important for filtering and keeping the mic from squealing.
LC

Thanks for the info. I have read many places that the mic jacks swap is easy, just swap the wires. I kinda figured they were full of it, those components were put there for a reason. I will try to check voltages tonight.

 

[357]

you can swap the 4 pin, just solder the wires from pin 2 and pin 4 together on the 4 pin where you're putting ground. Cobra 4pin you'd put the wires from pin 4 and 2 to pin 1 on the new jack.

 

[338_MtRushmore]

The voltage regulator was good, but I didn't record the actual results. I only have my phone for Internet right now, so the pic has the actual voltages on the left, and the specified on the right. It is close, but maybe not close enough.
I found out the the frequencies are supposedly off by the previous owner. There is no rx light, but the static increases when a mic is plugged in. There is a tx light, and a 5 watt carrier, but no swing. TX and rx do not work with a known good radio. Not too sure how to proceed.

 

[sonoma]

put a speaker on the pa jack and switch to PA mode and see if you have the PA function. the chip you are showing is not the raeulator. it looks more like it is the pLL. should have 8719 stamped on it if it is the PLL. make sure you have a good mic. it can cause some problems with audio out if it is bad.

here is a link to the service manual for this radio.

http://www.cbtricks.com/radios/cobra/148gtl/index.htm

 

[338_MtRushmore]

put a speaker on the pa jack and switch to PA mode and see if you have the PA function. the chip you are showing is not the raeulator. it looks more like it is the pLL. should have 8719 stamped on it if it is the PLL. make sure you have a good mic. it can cause some problems with audio out if it is bad.

here is a link to the service manual for this radio.

http://www.cbtricks.com/radios/cobra/148gtl/index.htm

Correct, that is the mb8719. It doesn't seem to be as specified, but I don't know if that means it is bad, or if that could be caused by something else. I know the mic is good. It has a resistor for talkback, and I don't hear any audio.

 

[338_MtRushmore]

I have a grant xl that the pLL tests ok. I guess I will swap them out and see if that works. Thanks for the help.

 

[Robb]

If this a new type 148 - the kind with the mic plug on the front of the radio - then the pll voltage is different.

I'd check the voltage going to the pll first to make sure it is stable. IIRC, the newer 148's have their own regulator; 5v is it? Don't recall ATM . . .

 

[338_MtRushmore]

It is a side mic. The board (412) seems to match the schematic loosecannon posted. I guess I just don't really know what to do. For instance, pin 1 is reading high. The only thing between pin 1 and 8 volts from the regulator is R95. It would seem to me that the resistor has lost its value, and that is raising the voltage through it. I could also be completely wrong, as I have no idea what the chip is actually doing with pin 1.

 

[338_MtRushmore]

I decided to revisit this 148gtl ebay special. I robbed a 5 pin mic jack from a grant xl and installed it. I got static, but no actual receive or rx light. It did deadkey, but no swing or PA. I randomly started scanning channels on a monitor, and it would break squelch around 28.8xxx Long story short, I swapped the pll from the grant xl, and I had receive on frequency! After lots of repairing hacks, I decided the echo board was faulty, and the TX is good. I am happy that I beat it finally. I guess it is a good thing it was the pll, and not a shorted ceramic or something hard to find.