Copper Talk » Ask The Tech » Radios BASE » Old Lafayette no AM rx-- help « Previous Next »

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Semi
Junior Member
Username: Semi

Post Number: 21
Registered: 7-2005
Posted on Wednesday, November 08, 2006 - 11:08 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Found an Telsat ssb140 at a thrift real cheap.
It was working tx and rx both modes. While doing some electrolytic changes and tuning it decided to stop rx on AM. SSB tx and rx are fine although power out on ssb is low[ 8-10W]pep, rx is excellent. Am tx is great. Audio excellent power 20-22pep but no recieve. No meter movement no audio on rx. I've changed the am detector diode and mixer diodes but no changes. I don't think it''s the rf amp cause ssb ok.

I like the radio cause it's an old cybernet chassis but it uses 2sc710 that I can't seem to find on any of my old radios. Any suggestions for more available replacements?

Anyone have any ideas on what to check for the rx?

Thanks
Semi
Top of pagePrevious messageNext messageBottom of page Link to this message

Kid_vicious
Senior Member
Username: Kid_vicious

Post Number: 2246
Registered: 9-2004
Posted on Thursday, November 09, 2006 - 12:26 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

the 2sc710 can be replaced with a 2SC945.

since you said it was working, and now after some "fiddling" inside, its not; im going to guess that a wire has become loose or has all but broken off somewhere.
i would start with the mode switch, spray it with contact cleaner and rotate back and forth a bit.
then wiggle all the wires connected to it, looking for one to break off in your hand.
you might also try reheating all the solder joints on the mode switch.
good luck,
matt
Top of pagePrevious messageNext messageBottom of page Link to this message

Semi
Junior Member
Username: Semi

Post Number: 22
Registered: 7-2005
Posted on Thursday, November 09, 2006 - 5:26 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Thanks for your reply Kid.
I lurk alot and notice you always try to help.
I'll fiddle around with the mode sw. but what confuses me is that it switches to AM[tx ok still] but won't recieve on am. SSB rx is excellent.
I think the leads are different on the 945, backwards so I have to turn it around, but that's good to know as I have tons of 945's lying around.

Again all help is appreciated.
Semi
Top of pagePrevious messageNext messageBottom of page Link to this message

Pogo12
Intermediate Member
Username: Pogo12

Post Number: 107
Registered: 3-2005
Posted on Thursday, November 09, 2006 - 7:33 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Its most likely going to be something simple such as you mistuned something.look for any solder you may have droped.Did you put in the caps properly.Retrace what you did and put back what you took out if it is possible.Is there any audio hash on AM at all?By this I mean the sissle sound you hear when there is no signals that tells you the receiver is active.Im trying to jog your thinking a little.
Top of pagePrevious messageNext messageBottom of page Link to this message

Tecnicoloco
Member
Username: Tecnicoloco

Post Number: 99
Registered: 4-2003


Posted on Thursday, November 09, 2006 - 8:07 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Double check the polarity of the caps that you replaced,maybe you have one cap reverse or the wrong value,maybe bad capacitor even though is brand new.
Also I will check the mode switch for any broken wire.

Good Luck
Tecnicoloco
CEF-117
Sonora,Mexico
73's.
Top of pagePrevious messageNext messageBottom of page Link to this message

Kid_vicious
Senior Member
Username: Kid_vicious

Post Number: 2250
Registered: 9-2004
Posted on Thursday, November 09, 2006 - 10:37 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

thanks semi, "try" is the operative word here. im no tech by a long shot, but if i think i have an insight, i try to post something.
whether i actually help or not, who knows.
lots of times, after i give some advice on something to try, the person asking the question never posts the result, and the thread dies.
i figure they either gave up completely, or fixed the radio and forgot all about the forum. LOL

pogo and tecnicoloco both have good ideas.
if a cap was put in backwards it could be giving you weird problems. it might also blow! so definitely re check those.

only problem with old CB gear; they have ghosts!
matt
Top of pagePrevious messageNext messageBottom of page Link to this message

Semi
Junior Member
Username: Semi

Post Number: 23
Registered: 7-2005
Posted on Thursday, November 09, 2006 - 11:06 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Appreciate all the suggestions.
I've pretty much rechecked all my work but I'll go over it again. I've made simple mistakes before!!!!!LOL
Pogo
When in am rx I get no volume whatsoever, no hiss or sizzle, and no meter movement. SSB rx and tx normal.

Thanks
Top of pagePrevious messageNext messageBottom of page Link to this message

Pogo12
Intermediate Member
Username: Pogo12

Post Number: 108
Registered: 3-2005
Posted on Friday, November 10, 2006 - 1:34 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

It sounds like you have a loose voltage wire on the mode switch or somewheres just like kid had said. This is the most likely.Ive seen mistuned oscilators do this but its not likely.If you put in a cap backwards it could block the rec. voltage.Shake all the wires a little while in AM mode and see what happens.Look for a loose crystal.There should be 8.18volts at the junction of c29 and r101 in AM receive only.If its not there you will not have AM receive.Since the ssb rx works no AM VOLTAGE is my best guess.
Top of pagePrevious messageNext messageBottom of page Link to this message

Pogo12
Intermediate Member
Username: Pogo12

Post Number: 109
Registered: 3-2005
Posted on Friday, November 10, 2006 - 1:51 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

By the way a 2SC710 can be replaced with a radio shack 276-2009 or a nte85.A very common npn transistor.
Top of pagePrevious messageNext messageBottom of page Link to this message

Semi
Junior Member
Username: Semi

Post Number: 24
Registered: 7-2005
Posted on Friday, November 10, 2006 - 9:29 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I haven't had the time to get back inside the radio but I really appreciate all the help and hopefully I can get to it today and I'll keep you all posted with progress.

Chow
Top of pagePrevious messageNext messageBottom of page Link to this message

Semi
Junior Member
Username: Semi

Post Number: 25
Registered: 7-2005
Posted on Friday, November 10, 2006 - 10:28 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Pogo
I was leaning in the voltage direction so thanks for the idea.
As luck would have it[good for a change] Lou Franklin's book has the SAMS for the power supply on pg. 295.
I checked the voltage on tr 42 and 43 and both seem to be operating correctly. Changing voltage at key up and unkey, and voltage is there on rx AM.
I think he mentions that branch 5-8 are rec only so I'll check those components but this is where I kind of get lost.
Seems like the caps I replaced were correct.

Thanks and if any other ideas please let me know.

Semi

Add Your Message Here
Post:
Username: Posting Information:
This is a private posting area. Only registered users and moderators may post messages here.
Password:
Options: Enable HTML code in message
Automatically activate URLs in message
Action: