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Author Topic: Realistic (Radio Shack) HTX-100 with no transmit power on SSB or CW  (Read 5143 times)
WA4ZYN
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Posts: 39




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« on: February 05, 2014, 03:00:35 PM »

Hello.  I recently acquired a Realistic HTX-100 that does not transmit.  Receive is perfect, and the radio looks good inside (no obvious damage).  When keyed in SSB or CW, "TX" is indicated on the LCD, but there is no power output, power indicator LED's do not light, AND I notice that receive sound is not muted.  I even tuned to a clear signal and found that I still hear it despite keying microphone (SSB) or key (CW).  My thought is that this is a TX/RCV switching issue, but I am sure having no luck finding what component is not functioning.  I have a copy of the Service Manual, and have been going through the troubleshooting page for "no transmit."  Everything seems to check out OK until I get to IC3, which has no measurable voltage at any pin.  I'm scratching my head.  Any suggestions or words of wisdom out there?  Thank you in advance for any and all suggestions.

73,
Francis  WA4ZYN
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K9YLI
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Posts: 850




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« Reply #1 on: February 06, 2014, 07:51:15 AM »

google  the  number on IC3.. get the  data sheet  if you cannot  determin which pin is the
vcc    power.  there should be  5 or  maybe 8 volts.
this should be an easy  volt meter  chase..
also  if you can figure out which pin is the out put  it can be jumpered to and  "active" state
to cause following circuits to work,,, as in  pick the  transmit relay..
I dont have the schematic,  but   PTT circuits are usually not   complicated..
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WA4ZYN
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Posts: 39




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« Reply #2 on: February 06, 2014, 08:46:57 AM »

Thanks for the helpful suggestions.  I'll check it out in a few days (when I finally get some free time) and will get back to you.  From the block diagram it looks to me that IC3 is the electronic equivalent of the TX/RCV relays in my older equipment.  I just didn't think of "jumpering" across it. 

Tnx again.
73,
Francis  WA4ZYN
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K9YLI
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Posts: 850




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« Reply #3 on: February 07, 2014, 08:19:12 AM »

   DONT jumper accross it........
jumper the output  to  "active"  level...

if the relay  has the other side to  + power (likely) then  jumper the output of the chip to ground..
as  transistors  switch to ground,,   not to   power  like most  toggle switches..
 
if the  'active'  output were   'high'  then you would tie the output to  + collector voltage.. to simulate
correct output..

 a little  computer  binary  logic  here...
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WA4ZYN
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Posts: 39




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« Reply #4 on: February 07, 2014, 09:30:04 AM »

Tnx.  I appreciate the computer logic.  I grew up in the non-computer age, and have been dragged kicking and screaming into the solid-state world (I like tubes).  But, I will follow your advice.  I'm going to do some more poking around and checking of voltages, as I am curious about things like Q30, Q32, and Q35, which all seem involved in the switching.  The lack of voltage at IC3 makes me curious (now that I am pondering it) about whether there is any +8V in that section. 

Thanks. 
73,
Francis  WA4ZYN
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K9YLI
Member

Posts: 850




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« Reply #5 on: February 08, 2014, 07:50:52 AM »

http://www.cbradio.nl/realistic/Service_Manual_Realistic_HTX100_Homer_ENG.pdf


i found this  book..
In lower right corner of  schematic,    PTT line  suppllies  collector voltage to the first transistor in the  sequence.
that is  plate voltage to a old guy like me.
think of transistors as    simple  triodes.
or  switches..
if they are biased plus on the  base  the conduct to saturatin and are a switch..
reverse biase and the shut off.  same as triodes.

I didnt read thru the book too  much. and didnt down load.

it looked like it may have simplified explainations of the circuiits.

make a silmple switch  for  CW mode and  try each and compare things
 through out the circuits. as  CW is also just  PTT  ..
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WA4ZYN
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Posts: 39




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« Reply #6 on: February 08, 2014, 06:16:36 PM »

Tnx, my friend.  I have a copy of the Service Manual, and already had planned to spend my next attempt doing exactly what you suggest: start with the beginning of PTT line there and work my way down line. 

Will post what I learn from it.

73,
Francis
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WA4ZYN
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Posts: 39




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« Reply #7 on: February 09, 2014, 05:07:21 PM »

Hello.  I'm still working to troubleshoot this HTX-100.  I am following the block diagram, and want to check Q401.  I cannot locate Q401 in the transceiver or any of the diagrams in the Service Manual.  Does anyone know where Q401 might be?  I'd really appreciate any insight!!

Tnx.
73,
Francis  WA4ZYN
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WA4ZYN
Member

Posts: 39




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« Reply #8 on: February 12, 2014, 08:06:50 PM »

Well, tonight I have success to report!  I found and replaced a shorted electrolytic capacitor connected to IC3 and immediately all voltages normalized.  I have since tested the HTX-100 with dummy load and wattmeter, and have full output on CW (full 25 watts).  I had no luck with SSB until I swapped out microphones, and subsequently found full output on sideband too.  Now to check out the microphone!  Thanks and any and all who offered so much advice and encouragement.  I couldn't have done it without you.

73, and thanks again!
Francis  WA4ZYN
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N8AUC
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Posts: 75




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« Reply #9 on: February 20, 2014, 10:03:24 AM »

I love it when a story like this has a happy ending!

Nice job, Francis!

73 de N8AUC
Eric
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N4NYY
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Posts: 4742




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« Reply #10 on: February 20, 2014, 03:24:13 PM »

If you want to test the driver and final, just unsolder the legs and diode test them with a DMM. That is a quick and dirty to see if there is an outright failure there.
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