Cobra 148GTL This issue has me stumped

[Handy Andy]

It's not fixed yet!

But thanks though...

My ramblings - it's hard to see thru someone's eyes, let alone how to tell them what you see - we're working on finding a solution based upon the clues I've gleaned and my own experience with failed diodes. When they blow shorted - a lot of sections stay active and even worse - fight each other for contention. It's hard.

The weakest link, all the way back to the power feeds, sometimes they can blow like fuses and you still have no solution - but then, the after effects - it's sometimes easier to then DVM the trace branches and select modes to find the lowest readings or biggest drop - then you may have something. Shorts show up as ohmic values tested both ways from the switch - but from each of the modes themselves in trace routings - not to ground - it's harder when the diodes fail open - you see less ohmic effect on that particular mode - but which diode?

:+> Andy <+:

 

[Robalo]

mode switch is ok 3hrs off work cero result. where is the rabbit ? lol

 

[Handy Andy]

Read this thru, because of we've can't determine the Mode switch is bad, then let's return back to what you found when the PLL was hooked up with - because BOTH conversations are in here....

Option 1 - One Line At A Time...And It Put You In A Mode...
(apologies to Johnny Cash)

Well, as a suggestion - because of what you're up against, I'd disconnect all the route lines going to the board from the switch. Not the power ones from the board - leave those connected to the rotary switch..

I'm serious! I've had guts of that radio dragged out onto the bench so I can test jig the modes. to find two bad diodes from a character that got goaded by his late night drunks that told him he was off and wound up screwing up the radio.

Reconnect only the USB - test, disconnect, go to LSB - test, disconnect then AM - test - leave (AM) in. Then connect USB - test - does problem return - then the two sections are related. THEN YOU KNOW - you'll have to search out and look for the short causing this.

OPTION 2 - Then All Else Fails - Pull The Pin and RUN...

ELSE IF THE SHIFT still is in there. then that mode is traceable back to the PLL - it's the only thing we haven't changed.

Your "extra pin" left powered - has me concerned that the PLL will lock but the IF (frequency) it generates as part of a loop is not locked or has lost programming - for the N-shift. IT should not do this, but if the PLL is being sent something - or a line from it is being held high (Pin 6 ok? - goes to TR35, and TR33 for AM mode - it indirectly affects TR47 and TR53) the system will not shift because it thinks it's in a mode where it needs to lock in on a frequency based upon the status of the channel changer line (yes, jiggle it- the channel knob- if the detent can force the shift to return in the PLL we may have something) TR35 helps to tell the MB3756 how to toggle, while TR47 and TR53 handed the mute function when the PLL is not locked per Pin 6 line (S/B High have a voltage about 4V or so - low is down to about 0.4.~ 0.2V) AND STATUS OF TR33 (In AM MODE)

If work has been done to the PLL and the Modes seem to be fine - then the PLL and it's associated LOCK line from Pin 6 - has to be checked...Means all the Bases of two transistors are connected to it. If any one of hem blow shorted - there's another headache - power flows into the line from the bad one you don't need. Telling the other Pin 6 is ok...

:+> Andy <+:

 

[OldTech03]

I must say the Tech info on here as of late is astounding ! Although @ my age I'm still learning . But Wow ! Too bad you guy's don't get paid for this info . This forum just gets better an better JMO

Careful you might get us thinking it's time to go on strike.

 

[Tolteca]

Guys. Have been too busy to take up this project again however I am very curious as to what the solution will end up being so please stand by. For the moment I've taken up another of what i thought would be a simple alignment job only to find that I have yet another one of these "weird" issues.

BTW to answer the question as to how I'm doing the alignment. In order to keep the impedance of my frequency counter from loading down the oscillators, I hook up my frequency counter behind my oscilliscope. In other words I probe with my oscilliscope and use the output jack at the back of the oscilliscope that spits out what's on channel B and then run that into the frequency counter. This way there is no loading and no shift whatsoever on any of the oscillators while they are being tested.

I've done lots of radio alignments it's just this one turned weird.

 

[nomadradio]

One incredibly-rare kind of fault comes to mind.

The quick test is to simply tune in a carrier from any AM station on the air, or from a signal generator.

The three frequency-set slugs in L22, L23 and L59 should change the radio's receive frequency ONLY for one mode. Turning that slug when either of the other two modes is selected should show almost no change in frequency.

If any of those three slugs IS active on more than one mode, this becomes a major clue. The slug in L59 should change the pitch of the received signal in USB mode only.

If you hear a difference in LSB while turning L59, that's a problem. You'll need to key the mike and read a frequency counter in line with the coax to see if it changes the AM transmit frequency. It should NOT.

Same deal for L22. Should change the LSB frequency, but not USB or AM transmit carrier.

And with L23, same rule. Should change only the AM transmit frequency when turned, and should have almost no effect in either sideband mode.

If it flunks this test, and one of those three adjustments is active in more than one mode, this points to a failed diode in the circuit that selects just one of these slugs at one time.

73

 

[jon666]

send it to snake radio customs he will jack it up good

 

[Robalo]

ok I pulled TR35,TR33,TR47,TR53 and tested D41,D43 are ok.Now TR35 normal channel 1-40 ECB 7.9V-13.8V-7.9V switch to to upper TR35 ECB 1.3V-13.8V-1.3V

 

[kaos513]

Hey guys,

I have an interesting one for you. I've never gotten a radio with this fault before and frankly it's driving me nuts. I have a Cobra 148 that after aligning everything, side band is always still 1.5khz off frequency. When I put it on LSB I have to turn the clarifier about 15% low and when I have it on USB I have to turn it about 15% high. I measured the 34Mhz oscillator to see how much I was compensating and it is indeeda bout 1.5khz.

It's almost as if the shift in the 7.8Mhz oscillator isn't getting out. I can see the 7.8Mhz oscillator changing when I switch to side band and I can also hear the side band change when I tweak the 7.8Mhz while receiving a signal, so it's not like the oscillator isn't working or it's signal isn't going anywhere. It definately has an effect on the radio and also properly shifts. Regardless the amount I'm off frequency on side band corresponds exactly with the amount of shift that this oscillator is supposed to provide when it changes from 7.8Mhz to 7.985 to 7.8010.

This radio had a high frequency mod where PIN 10 was being sent about 4V through a 3.3k resistor sourced from L21, this mod is unfamiliar to me and it seemed to cause a lot of drift so I undid it. Instead I replaced the mixer crystal with an 11.325 and did the usual logic switch on pin 10 and 8. This fixed the stability but not the bizarre issue with the shift.

Sideband reception also seems to be low as I was not able to properly calibrate the side band receive meter.

Any ideas? Can't seem to figure this one out.

check for any corrosion on all crystal pins.

 

[Handy Andy]

Ok, keep it simple...

Do you get a Red TX and Green RX light in all modes?

Because the upper mode is showing fail on TR35 - AM mode Base 7 volts, in USB it's 1.3 - so, kinda low - so USB is out of lock...to Verify LSB should show same as AM on TR 35 Base

Means L59 failed...Check for voltage across L59 - and D33

:+> Andy <+:

 

[Robalo]

Here we go again remove R95,replaceall this parts for fresh one c88,89 ,c227,228,229.L24,25,26. TR29,TR30.D32,D33,D34,D39,D40,D41. Now yes TX RED LIGHT. VOLTAGE ON DIODE ARE OK BUT D41 IS NOT LOCKING NO VOLTAGE MAYBE TR31? THANK YOU.

 

[Handy Andy]

Remember, you said you checked TR31 already. It's the selector that routes - that when you're in AM mode, aka the BASE is on in AM mode, which if it has power from TX MODE (aka lock back to TR35 and the MB3756) R112, R113 and TR21 Base shows voltage - they will show 3.7 to 5V. D41 shows power 5V ~ 7V - R150 shows power and you get 7.8MHz on X4

When not in TX mode - what is NOT supposed to have power IN AM MODE - is TR31 - so that kills TR21 power AND D41 - no power in RX Mode.

One thing I'm seeing and I'm trying to tell you this, and you're frustrated - by what was posted... you seem to have EXTRA power from somewhere still sending - to where? Not sure. It is acting like your RX IF for AM mode is still on no matter what mode - which brings us back to TR31 because of it's "AND" functions. It turns on AM 7.8MHz ONLY FOR TX - not RX. So D41 should be off in RX but ON in TX.

The AND diodes (power steering) are D36 (LSB) , D37 (USB) and D38 - BUT D38 has a CONDITION that is set on it's UNBANDED SIDE - It SHOWS NO POWER IN AM MODE ONLY IN TX.

What I'm trying to lead you to, is the 7.1MHz IF in AM. Is that sending/working? That power routing seems to be skewing your readings. That takes me back to TR33 - AM mode RX - but only in AM Mode, It's that darn IF 2nd conversion stuff that is making this harder to track down.

And you say Receive for SSB is attenuated, or lower - that is on Pin 4 of the TA7222P - it's the audio pin that turns off using TR53 when the PLL is out of lock - it can't do that if the cap that it uses - to suck all the power off the audio line - C232 2.2uF - is bad or TR53 is not on when the PLL fails to lock - hence TR33 and TR47 work together to power TR53.

Pin 6 of the PLL should be +6V or so when in lock - D31 makes this directional - as in, when it's out of lock, it sinks current INTO the PLL there is a pull up resistor of 22K (R163 off of Pin 1 PLL power) on the line back at the PLL to "float" the line about 2.V or less - just nothing that goes below 0V or above 8V so it doesn't go poof...and helps in recovery.

Then there is a major fail mode, the channel selector does that - it GROUNDS the line all the way to zero if it works right.

There is a DETENT on the channel selector switch, that CLOSES when the detent is not set right - this helps the PLL FORCE DOWN and NEGATE the effects of R163 pull up. It sets this line LOW (knob not set right) and turns ON TR47 (grounds base lead it's a PNP) thru it to TR53 (NPN via splitter R223 and R224 off of TR33) to mute Pin 4 of audio chip and OFF TR35 (an NPN) It sets ground, pulling power off the line, turning off the TR35 and PREVENTS MB3756 toggle RX and TX - while keeping 8 volts available for mode selector - back to TR33 (NPN) being told to stay on if in AM mode. Just TR35 keeps TX from working while the BASE, Pin 6 of the PLL or the channel selector DETENT are LOW.and reserve power from TR33 goes thru into the splitter and into TR47 to turn on TR53 as a mute.

So, does the channel selector seem to work ok? Does Pin 6 at the PLL float at 2V or less, seems to (1.3) so the PLL out of lock signal means it's not getting something right. and only in specific areas - so perhaps the Channels "programming" is not right - hence that special pin getting special treatment.
WHEW

:+> Andy <+:

 

[sonoma]

WheW is right Andy. I got winded reading the post you just did. you sure did a lot of note taking posting that portion.

 

[Handy Andy]

Thanks, but I feel just as frustrated too...

I guess - when all else fails - replace the PLL...

But t raises so many questions...

BCD from the channel selector - is it correct? (Jiggle the knob)

AM Mode - TR33 and TR31 - these two, with the D36, D37 an D38 - all NEED to send power to X4 in ALL TX but not all (AM not) RX modes...

PLL Pin 6 - man I remember a convo or two on CB tricks about that 22K resistor - affects all PLLs' Cobra and Uniden - they do use on the BCD/LED Display encoders...the Detent in one, grounds when good, the other POWERS when good...

WOW - it's trying to keep it all straight....

:+> Andy <+:

 

[Robalo]

I have pll socket and replaced 3 times same results. thank you ill get to the bottom off this warranty.I appreciate you help. i go with channel selector now .it is good learning leasson LOL.