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Realistic TRC-453 no PA audio and no modualtion in transmit mode

ausman1000

ausman1000

New Member
Aug 13, 2023
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Looking at the schematic it seems that the PA section consists of TR30 and IC5 the audio amp chip. I have no problem with audio so I assume IC5 is good. Removed TR30 and tested it on a Atlas DCA55 and it checks good. Next I check the voltages on TR30 in the PA mode and they do not correspond to what the service manual shows. I have 2.5v on the base, 7.8v on the collector, and 7.8v on the emitter. The manual calls for 2.4v 4.6v 1.8v.

At this point I need a little help please!

Cheers, ausman1000
 

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  • TRC-453 Service Manual.pdf
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  • realistic_trc_453_sm_pg44_45_sch.pdf
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  • cb radio.jpg
    cb radio.jpg
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Brandon, the light bulb in my head just turned on. The voltages in the manual are in the keyed mode. They all test spot on when keyed. Hmm... then I wonder what is going on.
 
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The mic I am using is this more modern RadioShack 5 pin din and not the original mic that would have been supplied.
 

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Can you confirm a couple of things for me?

first, does the radio receive other stations at the expected volume level?
I don't mean "i can hear static", i mean "i can hear people talking without the volume cranked up".

next, you say you have no TX modulation. can you please tell us how you are determining that?
can you confirm that you do indeed have a carrier by setting up a second radio on the same channel?

last, have you tried different mics and different PA speakers to make sure that isn't the issue?
 
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loosecannon said:
Can you confirm a couple of things for me?

first, does the radio receive other stations at the expected volume level?
I don't mean "i can hear static", i mean "i can hear people talking without the volume cranked up".

next, you say you have no TX modulation. can you please tell us how you are determining that?
can you confirm that you do indeed have a carrier by setting up a second radio on the same channel?

last, have you tried different mics and different PA speakers to make sure that isn't the issue?
Click to expand...
Yes I can hear people talking and yes I believe it is at expected volume levels.

I have a carrier.

The trc-453 shows no modulation on the meter pictured. Another known good radio does.

I can hear cb audio on the PA speaker when in PA mode but when I key up and talk into the mic I hear nothing from the PA speaker.
 

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DoctorAudio said:
That condenser stock mic will only work on a radio that has phantom power voltage on the mic audio pin. The 453 doesn't have that voltage. You need the dynamic RS stock mic. (Or any other working mic)
Click to expand...
Thank you Doctor! and thank you Brandon!

Doctor, before I saw your message I placed an order for the oem mic that would have been supplied with this radio. Picked up a very nice looker on ebay just about 30 minutes ago.
 
DoctorAudio said:
You can use a dynamic stock mic on any Radio Shack radio but the condenser stock mic only works on a few of the newest models. (Like that one you pictured)
Click to expand...
Thanks for the education it is very much appreciated! I spent some time recapping this unit and tuning the receive section per the manual. Once the correct mic arrives I can move onto the transmit alignment.
 
Here is a pic of the top side. Every electrolytic cap has been replaced with new. I put a black dot on each one to keep track. The only cap not yet done is one on the front panel pcb, I think it is a 1uf working from memory. Surprisingly I did not find any old caps that were way off the value mark. Some had high ESR readings at 10 but nothing shorted. All solder connections on the main board have been reflowed so hopefully it should perform better than new after that alignment.
 

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glad to see someone else already found your problem, i guess i should have looked at your pics!

be sure to come back to this thread after your TX alignment and let us know how it all went.
LC
 
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