In every combination adjusting L32, L37, L7,L8, L39 the highest frequency I could get this to go was 23.500 MHz. Notice L30 also has a effect.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Old President Grant same cobra 138XLR and Trc449 odd problem not sure where to start.
- Thread starter Low_Boy
- Start date
Hmmm. 7.8 MHz times three is 23.4 MHz.
Really sounds as if there is no local-oscillator injection from L16 reaching the transmit mixer FET7.
Did you say "L30"? Or did you mean L39?
If the only signal reaching FET7 is coming out of L39, peaking it should have some effect. Just not the effect you want.
L7 should have some effect, but peaking it on receive tends to work best. L8 should NOT make a difference in transmit, only receive.
Can you hear even the tiniest trace of a signal on the channel frequency?
At all?
Either an input is missing from FET7 or the part itself is bad.
Maybe.
If you can't even "sniff" a 27 MHz signal with a receiver set to the same channel, the transmit mixer is MIA. Just can't say why. A coax jumper with the shell backed off the far end makes a good 'sniffing' antenna.
73
Really sounds as if there is no local-oscillator injection from L16 reaching the transmit mixer FET7.
Did you say "L30"? Or did you mean L39?
If the only signal reaching FET7 is coming out of L39, peaking it should have some effect. Just not the effect you want.
L7 should have some effect, but peaking it on receive tends to work best. L8 should NOT make a difference in transmit, only receive.
Can you hear even the tiniest trace of a signal on the channel frequency?
At all?
Either an input is missing from FET7 or the part itself is bad.
Maybe.
If you can't even "sniff" a 27 MHz signal with a receiver set to the same channel, the transmit mixer is MIA. Just can't say why. A coax jumper with the shell backed off the far end makes a good 'sniffing' antenna.
73
Both seam to have a effect. L30 was cranked and and if it is too far one way or the other it limits my adjustment and if cranked one way or the other too far the output signal is gone. L39 has the same effect.
It does receive on the right channel though, so 27.185 Ch19 there is receive from my signal generator. but it transmits on 23.500 MHz.
So next I will check FET 7 and sniff for a signal. This one is very confusing.
I set up a sniffer. Put both radios on channel 4 that seems to be the closest to 27MHz. Fired up ( Keyed )the Grant and absolutely nothing. Sniffed around the whole radio. Then I checked FET 7 mixer and it tested good. I do not know what to check next. It could be a bad VCO IC5 or PLL IC7. All voltages are right on. I am stumped.
If it receives on channel like it should, I would next check to see that the 7.8 MHz signal is in fact reaching FET7. That particular mixer circuit is generally pretty foolproof. But if you can't hear the tiniest hint of 27 MHz, this means the mixer isn't mixing.
73
73
I just put a frequency counter on FET 7 an if I am looking at this right the top two legs are source and gate 1. I am getting around 34.98 MHz on both the gate two and drain show no frequency.
I'm stuck at TX osc x3 (23.5 /3)
or x3,4,6, x2 (22.675 /2)
These are two different symptoms.
The first is TX osc tripling
The second is the tripler doubling
Either something goofy around TR40 or work back from FET17-16 to TR 27-28...
Just a thought , has L37 been "broad banded" ?
From memory until I get back to the bench :-/
or x3,4,6, x2 (22.675 /2)
These are two different symptoms.
The first is TX osc tripling
The second is the tripler doubling
Either something goofy around TR40 or work back from FET17-16 to TR 27-28...
Just a thought , has L37 been "broad banded" ?
From memory until I get back to the bench :-/
Well Kop you may have possibly hit the nail on the head. I thought this was all factory revisions. I never knew you could broadband L27. Now I will have to get out my old Cobra 138 and look at this and possibly redoo.
...37
easy to see on the bottom of the board. The trace to the center input pin has been cut and moved to the outside. usually the same on the output.
check L24 while you're at it
Last edited:
Well I sure missed that at first glance. Should decisively shut down the transmit. I would wire a jumper to the center pin, across the visible cut mark. The real benefit from that mod is on the other side of L37 IMHO. That connection HAS been made. Changing the 'tap' on this side of the coil where it connects to FET7's drain is far less rewarding.
73
73
I think that when the problem is finally found it will be something so stupid it has been overlooked. I can not find a thing that tests bad....... So Far.