Cobra 148 GTL problem

[eldorado303]

Hi guys. I got the PLL in the mail today. I replaced the old one
with the new. Then I measured voltage at TP9, and adjusted L19
for 3.2V on CH40. I changed to CH1 and the voltage remained very
similar, around 3V. I pressed the PTT ( with antenna connected ),
and the RX/TX went red for a couple of times, but the S-Meter
didnt rise, on AM Mode. Then it stopped working again, no more
RX/TX red light. I measured voltage at TP9 again, and it was
6,8V both on CH40 and CH1. I guess the PLL fried again.
I guess I'll sell the rig for parts or something like that.
Anyway, thanks for all the help.

Thanks,

João G.

 

[338_MtRushmore]

If the voltage at the mb3756 was good, what else could be responsible for frying the PLL again? Should one recap before replacing voltage regulator and/or PLL?

 

[adrian davies]

If it has a mb3756 in it, it is old and worth fixing.
Restoring as many 148's as I can get from Hamfests - lol.

Here is where I would start:

1) Check pin 9 on the 8719/pll chip; it should be ~8v all the time. If you aren't seeing 8v; then the regulator has failed or there is a shorted electrolytic cap(s) on that 8v rail. Old radio; then old, dried out caps are a commonplace issue.

2) BTW - Replace all 10v caps in the radio using a 16v or 25v with the same capacitance value, as a rule.

3) Check to see if the Cobra Service Bulletin #1226 was done to it, which adds a 18v zener diode near the voltage regulator. That diode may be faulty or probably not even present. I do that SB mod to all 148/142 and all Uniden chassis that have the mb3756 regulator. This doesn't fix anything, except keeping that regulator from failing. That part is becoming more hard to find, so it would be wise to keep it working.

4) Then check the VCO voltage as the Service Manual describes.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf

WAW, checked back a few times to see if any one had reppled to this post but no answers and now loads of help,
first of it ia a cobra 148 mk2 PB-010AA BOARD. here is a update on closer inspection this has been fitted with a eprom board and wier'd up a bit strange, from the board one wire goes vol-sq power i asume ?, second wire goes to mike gain-rf, could be variable power ?
then there is a wire going to low mid and hi this is a 5 position switch now, the wire from here goes to a horse shoe adjuster on back of the board lower side not speaker side, then two wire's from the same area go to the ch9 switch , and there are two wires from the speaker side of board going from the hi-low switch to LCC2 AND d92+ on the board.
also notice when i key the mike to tx the light flickers and the light on the power meter does the same and the radio seams to vibrate, is it possible to host pics here would be much simpler, only my second post and just learning, many thanks

 

[adrian davies]

WAW, checked back a few times to see if any one had reppled to this post but no answers and now loads of help,
first of it ia a cobra 148 mk2 PB-010AA BOARD. here is a update on closer inspection this has been fitted with a eprom board and wier'd up a bit strange, from the board one wire goes vol-sq power i asume ?, second wire goes to mike gain-rf, could be variable power ?
then there is a wire going to low mid and hi this is a 5 position switch now, the wire from here goes to a horse shoe adjuster on back of the board lower side not speaker side, then two wire's from the same area go to the ch9 switch , and there are two wires from the speaker side of board going from the hi-low switch to LCC2 AND d92+ on the board.
also notice when i key the mike to tx the light flickers and the light on the power meter does the same and the radio seams to vibrate, is it possible to host pics here would be much simpler, only my second post and just learning, many thanks

just realised i have replied to the wrong post , simmelar fault on mine, sorry

 

[222DBFL]

Has no one noticed he is using an antenna to do this? Could be the antenna is the issue. Have you used a dummy load or are you mentioning the antenna as the antenna port of the radio and that hopefully you are using a 50ohm dummy load rated for about 20 watts? If you are please disregard my post. If you are using your antenna, this may he where the issue is and not in the radio. Just some food for thought if this is the case. Please don't flame me if I missed someting. It's early and the way your post was worded it seemed as if you were using an antenna. Again. Sorry if that is not the case. Have a good one

 

[kaos513]

Hi guys. I got the PLL in the mail today. I replaced the old one
with the new. Then I measured voltage at TP9, and adjusted L19
for 3.2V on CH40. I changed to CH1 and the voltage remained very
similar, around 3V. I pressed the PTT ( with antenna connected ),
and the RX/TX went red for a couple of times, but the S-Meter
didnt rise, on AM Mode. Then it stopped working again, no more
RX/TX red light. I measured voltage at TP9 again, and it was
6,8V both on CH40 and CH1. I guess the PLL fried again.
I guess I'll sell the rig for parts or something like that.
Anyway, thanks for all the help.

Thanks,

João G.

Has no one noticed he is using an antenna to do this? Could be the antenna is the issue. Have you used a dummy load or are you mentioning the antenna as the antenna port of the radio and that hopefully you are using a 50ohm dummy load rated for about 20 watts? If you are please disregard my post. If you are using your antenna, this may he where the issue is and not in the radio. Just some food for thought if this is the case. Please don't flame me if I missed someting. It's early and the way your post was worded it seemed as if you were using an antenna. Again. Sorry if that is not the case. Have a good one

He may have bad solder joints as well