PTBM070cox board rx improvement

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rumblefish
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PTBM070cox board rx improvement

Post by rumblefish » Saturday 8th Jan 2011, 5:10

This is a general electric model 3-5869a 40 channel base station. the tx audio on the radio is excellent. the problem is the rx performance. the sensitivity and selectivity dont seem to be on par with most unidens or cobra radios for that matter. :banghead: Is there anything to be done internally to improve receiver performance. I've noticed with this board there are many spots for components that are not installed? I love this little bases tx audio, I get many complements on the audio with just an rk-56 noise cancelling mic. =D> the noise floor on this radio is terrible. :cry: are there any upgrade transistors for this available or capacitor changes I could make? It looks like they"general electric" really cheaped out on the 455khz filter! Winter is here and I'd like to turn this into a project.any help or ideas would be vey much appreciated. Hey rick! just want to say hello and thanks so much for your tech help in the past. your knowledge has helped memore than I can say!! super site you guys! :!: :!: Yours sincerely, rumblefish, 428 from the garden state wavin' a hand... threes....... :dance:
the only stupid question is the one that didn't get asked!!!

lbcomms
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Re: PTBM070cox board rx improvement

Post by lbcomms » Sunday 9th Jan 2011, 2:26

Is there anything to be done internally to improve receiver performance
First of all, make sure it is operating correctly.
You are looking at a 35 year old radio - many components (especially capacitors) degrade over time.
If you haven't got the test gear needed or the experience needed to "do it by ear" it's going to be difficult, or expensive(it you pay someone else to do it).
the noise floor on this radio is terrible
They aren't too bad when they are working correctly, you could have a noisy component (capacitor / diode / transistor / resistor) causing your troubles.
changes I could make?
Before doing mods, make sure the original circuit is operating correctly.
Modifying a faulty circuit to increase its performance is like putting a turbocharger on a clapped out 1986 ford that uses more oil than gasoline.
The turbo might increase performance, true, but you would get better results fixing the original issue that caused the lack of performance in the first place.
It looks like they"general electric" really cheaped out on the 455khz filter!
That's the least likely component to cause a high noise floor.
The 455khz filter is there to remove adjacent channel interference, not to reduce noise.

Without details such as sensitivity measurements, can't help any more than that.

Cheers

rumblefish
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Re: PTBM070cox board rx improvement

Post by rumblefish » Tuesday 11th Jan 2011, 3:27

thank you for the reply, I have replaced all of the electrolytic capacitors. I'm in the process of replacing the smaller disc capacitors. I plan on replacing all of the transistors with new ones.I was searching online for a replacement for the 2sc710;s in this rig. there are a total of 6 2sc710 transistors. no one seems to have a direct replacement for this transistor. just curious if the 2sc2999 will work? looks like I would have to turn the 2sc2999 180 degrees compared to 2sc710.bce for the 710 and ecb for the 2999. I dont have the schematic for the radio.the only info on rx sensitivity I can find is .5uV for 500mW. thnx again, rumblefish
the only stupid question is the one that didn't get asked!!!

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