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Cobra 148 GTL
- Thread starter Probate
- Start date
Schematic says D75 and D52 are both KB262, which is a dual series diode. Voltage drop through each should be about 1.4v. Are they both (D75+D52) gone? You could get away with either of them, and put a jumper in place of the missing one, then readjust VR3, the TX Freq Adj control. Could use 2 1N4148 diodes in series for each if both are missing.
If your plan is to "lock" the transmit frequency so it can never be adjusted to line up with everyone else on the channel, be sure to check the trimmer pot VR5. This is where the "stock" transmit frequency is adjusted when the clarifier does not control it. If it has become "scratchy" over the years, this could cause your transmit frequency to become terribly random from one keyup to the next. A drop of cleaner, and a few excursions from end to end will usually clear this up. Just listening to the sideband transmit on a second radio will reveal if it adjusts smoothly when turned.
Never underestimate the importance of hearing the result of any adjustment to a sideband radio's transmit side. Way too many ways to accidentally make a SSB radio sound like dog doo on the air. Hearing is believing.
73
Never underestimate the importance of hearing the result of any adjustment to a sideband radio's transmit side. Way too many ways to accidentally make a SSB radio sound like dog doo on the air. Hearing is believing.
73
When people talk about unlocked clarifiers, do they mean increasing the frequency shift achievable or do they mean allowing the clarifier to also control TX instead of just RX? Or do they mean both?
My galaxy 959 is modified so the clarifier controls both the TX and RX and also expands the frequency range. However, I find it difficult to tune stations in. With such a span, I have to go back and forth a couple times to get it right on. The problem with that is that it will likely occur near the same spot every time and will test the rotational life of the potentiometer. I plan to revert mine back to the original frequency span so tuning is easier but keep it controlling the TX. If that radio had a frequency counter, i would keep RX and TX clarifier separate, but with how temperature sensitive and drifty that radio is in my truck, I like the ability to assume the other person's TX is close to accurate so I can tune to that by ear and hope for the best.
My galaxy 959 is modified so the clarifier controls both the TX and RX and also expands the frequency range. However, I find it difficult to tune stations in. With such a span, I have to go back and forth a couple times to get it right on. The problem with that is that it will likely occur near the same spot every time and will test the rotational life of the potentiometer. I plan to revert mine back to the original frequency span so tuning is easier but keep it controlling the TX. If that radio had a frequency counter, i would keep RX and TX clarifier separate, but with how temperature sensitive and drifty that radio is in my truck, I like the ability to assume the other person's TX is close to accurate so I can tune to that by ear and hope for the best.
By unlocked they do mean shifting rx and tx together. I myself prefer clarifiers unlocked. You could try a 10 turn potentiometer if you have any room in the radio. I put one in my daily driver 148GTL and I love it.
Correct both are missing didn't have any info on diodes, want to place radio back to stock. Is there a single diode replacement out there?
Thank you for your detailed response, I also have a question on my 148f that it squeals on am after 11 o'clock position only on am with multiple amplified microphones. Just happened out of the blue and can't figured out what happened. If you have any advise please advise
Regards
I would start checking the mounting screws for the board to the chassis to make sure they are tight and have a clean ground connection.
The mike amp has its own filter cap on the power for that circuit. It's C18, originally 47uf. Got in the habit of using 470 or 220uf to replace it. Usually clears up this issue.
73
73
D52 and D75 are KB262 diodes, they are known as "stabistor" diodes which are made to have a very constant forward voltage regardless of current or temperature.
they are also known as "forward reference diodes".
those parts aren't made anymore and you're going to have a bit of a hard time finding them. they cross reference to a Panasonic MA27T but they will probably be almost just as hard to find.
two 1N4148 diodes in series is your best bet unless you just have to have that factory look.
as for your 148F, Nomad's advice is sage and should be taken.
the only thing i might add is that power mics tend to squeal a lot easier if the modulation limiter is removed from the radio.
the usual suspect in your radio would be TR24. if this is removed, replace it with a 2SC945 transistor as it will make SSB sound bad when gone.
you might also find R131 gone, or TR26 gone.
up to you if you want to replace them but the radio will be prone to overmodulate if they are gone.
LC
they are also known as "forward reference diodes".
those parts aren't made anymore and you're going to have a bit of a hard time finding them. they cross reference to a Panasonic MA27T but they will probably be almost just as hard to find.
two 1N4148 diodes in series is your best bet unless you just have to have that factory look.
as for your 148F, Nomad's advice is sage and should be taken.
the only thing i might add is that power mics tend to squeal a lot easier if the modulation limiter is removed from the radio.
the usual suspect in your radio would be TR24. if this is removed, replace it with a 2SC945 transistor as it will make SSB sound bad when gone.
you might also find R131 gone, or TR26 gone.
up to you if you want to replace them but the radio will be prone to overmodulate if they are gone.
LC