M1225 VHF,no recieve

n5tbu · Post by **n5tbu** » Fri Feb 16, 2007 6:31 am

No schematics here,radio transmitts but no receive,has audio.
Any common problems to look for?
mod

Post by **wavetar** » Sun Feb 18, 2007 12:21 pm

Won't receive at all even when blasting a large signal in? Almost certainly the 1st local oscillator crystal is bad.

n5tbu · Post by **n5tbu** » Mon Feb 19, 2007 3:26 am

Thanks,I should have mentioned I had changed that crystal but still no receive.
mod

jackhackett · Post by **jackhackett** » Mon Feb 19, 2007 5:52 am

Which crystal did you change? What kind of test equipment do you have available?

n5tbu · Post by **n5tbu** » Mon Feb 19, 2007 7:48 am

I don't have it in front of me at the moment,but it was the crystal behind the vol control layed on it's side and the case soldered to the board.
I have typical small shop equipment.
mod

kb0nly · Post by **kb0nly** » Mon Feb 19, 2007 1:31 pm

Does it transmit ok, that is does it pass audio ok on transmit? Just curious, because if you cant get it fixed i'll buy the dumb thing, i need another 1225 for a repeater project.

n5tbu · Post by **n5tbu** » Mon Feb 19, 2007 1:52 pm

Yes,it does transmit fine......it is for a customer,so if I can't reair it,I need to keep it for spare parts for them.
mod

jackhackett · Post by **jackhackett** » Mon Feb 19, 2007 2:03 pm

n5tbu wrote:I don't have it in front of me at the moment,but it was the crystal behind the vol control layed on it's side and the case soldered to the board.
I have typical small shop equipment.
mod

The 16.8MHz one? That's the reference oscillator crystal. The one you'll want to check is Y52 behind the speaker(closest to the 455KHz filters), it's a 44.395MHz crystal, if you have a service monitor or spectrum analyser you can sniff around it for the signal.

n5tbu · Post by **n5tbu** » Mon Feb 19, 2007 3:12 pm

Thanks Jack,I will look into that wednesday morn,44.394??? weired IF! No wonder i MISSED IT!.
MOD

n5tbu · Post by **n5tbu** » Wed Feb 21, 2007 7:08 am

The 44.395 xtal is oscillating rather strongly. Still no receive with the IFR on
highest setting.
mod

jackhackett · Post by **jackhackett** » Wed Feb 21, 2007 7:26 am

I would try injecting the IF freqs, 455KHz and 44.85MHz, not sure where to tell you to inject it, maybe you can couple enough signal around the filters for it to work.

n5tbu · Post by **n5tbu** » Wed Feb 21, 2007 7:37 am

I tried that also....nothing!
mod

Post by **wavetar** » Wed Feb 21, 2007 12:13 pm

You're not trying to feed the I.F. frequencies into the radio's antenna port, are you? If you're getting nothing even when injecting 455KHz in the area of the small, square, plastic covered ceramic filters, then I'd say your receiver I.C. is bad.

n5tbu · Post by **n5tbu** » Wed Feb 21, 2007 1:33 pm

No,I injected in the area of the 455's.
Does the book give typical voltages of the reciever IC pins?
I am suspecting an open trace or plated thru hole,do to the vibration from the tractor it was on.
I do appreciate all the help!
Is the rec IC the 14 pin with "14066b" on it?

mod

jackhackett · Post by **jackhackett** » Wed Feb 21, 2007 2:00 pm

No, a 4066 is a CMOS switch, it's used to switch between the narrow and wide filters. You could try changing the bandwidth, maybe one of the 455 filters is bad, switching bandwidth would use the others.. it's a long shot though..

The receive IC is the one next to that, 32 pins,probably a little harder to deal with

n5tbu · Post by **n5tbu** » Wed Feb 21, 2007 3:05 pm

I tried programming narrow and wide for that test first......not the 455's.
I did see that larger IC but was hoping it was something else! I wish I had some voltages to compare it to before I attempt to change that IC. I really hate to be a "parts changer"tech!
mod