Title: Central Electronics 20-A no audio on AM Post by: N7AND on March 26, 2023, 09:29:37 PM Hi all I could use some help with my 20a it was working fine and then
one morning it still had a carrier but no audio on AM only. All other modes work as they should. I have checked everything I can think of and still no audio on AM. Any help would be much appreciated. Thank you Kevin Title: Re: Central Electronics 20-A no audio on AM Post by: Pete, WA2CWA on March 27, 2023, 03:10:24 AM Hi all I could use some help with my 20a it was working fine and then You could explain what it is that you "checked". Tubes? Components? Voltages? etc.?one morning it still had a carrier but no audio on AM only. All other modes work as they should. I have checked everything I can think of and still no audio on AM. Any help would be much appreciated. Thank you Kevin How about a schematic? Hard to talk about something when you don't have a diagram in front of you. Title: Re: Central Electronics 20-A no audio on AM Post by: N7AND on March 27, 2023, 06:11:44 AM Hi Pete
Well I have check the tubes, caps and I don't see anything out of spec. Here is the schematic. I must be missing something because It had great modulation on AM before. Thank you for looking. 73s Kevin chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://frank.pocnet.net/instruments/CentralElectronics/20A/PDF2198-01.pdf Title: Re: Central Electronics 20-A no audio on AM Post by: K8DI on March 27, 2023, 08:21:08 AM If, as you say, it works correctly in all other modes, upper and lower sideband, but no am, then look hard at the mode switch. Since they’re simple to check, check the modulator diodes too. Pretty much everything needs to be working for the sideband modes to work, the unit does not have separate audio or rf sections for am vs ssb. If the ssb modes aren’t properly working, then there’s much more to look at…
Ed Title: Re: Central Electronics 20-A no audio on AM Post by: WD5JKO on March 27, 2023, 10:37:59 AM Hi all,
I wager that modulation transformer TR3 is bad (likely an open primary), and that when running SB1 or SB2, the opposite sideband rejection has been lost. Jim Wd5JKO Title: Re: Central Electronics 20-A no audio on AM Post by: AJ1G on March 28, 2023, 05:48:18 AM Be sure you haven’t inadvertently left the INDICATOR switch in CARRIER NULL instead of SET MAX LEVEL.
In CARRIER NULL audio is removed from the balanced modulator to permit the audio signal from interfering with finding the deepest null, both audibly in a receiver, and visibly on the RF INDiCATOR cat’s eye tube when adjusting the CARRIER NULL A and B controls. IIRC, this also removes the audio in the SB1 and SB2 sideband positions of the MODULATION mode switch. Forgetting to take the INDICATOR switch out of CARRIER NULL has bitten me more than a few times in “Oh crap, where did my audio disappear to?” moments. Don’t want to sound like Captain Obvious, but figure it’s worth mentioning since I’ve frequently been bitten by this operator error. BTW, don’t think I’m shouting at you by using all caps when referring to control names and positions. Standard Navy technical manual convention to use all caps in operating, testing, alignment, and troubleshoot procedures. Old habits die hard, even in retirement. Title: Re: Central Electronics 20-A no audio on AM Post by: N7AND on March 28, 2023, 11:14:51 AM Thank you Chris I don't think I forgot to switch to SET MAX LEVEL
but i will check. I saw on the net that a good replacement for TR-2 and TR-3 is a Hammond 166E12 could I get your guy's opinion on using this transformer? Thank you Kevin Title: Re: Central Electronics 20-A no audio on AM Post by: wa2tak on March 29, 2023, 09:25:55 AM Last I heard...Mr. Nick Tusa @ CE-Multiphase has them for sale.
Contact: nick.tusa@ce-multiphase.com Title: Re: Central Electronics 20-A no audio on AM Post by: WD5JKO on April 03, 2023, 12:43:08 PM Some old but good info here: http://amfone.net/Amforum/index.php?topic=27305.msg241482#msg241482 Jim Wd5JKO |