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Blacksmith
New member
Username: Blacksmith
Post Number: 7
Registered: 2-2005
| | Posted on Friday, April 07, 2006 - 12:12 pm: | 
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Hello,My freind has a Realistic TRC-457. It has been moded and has variable power .At 4 watts Deadkey AM every thing looks good but turn the power down and the VSWR rises from flat to 2or more and the audio sounds over modulated (Nasty).If I turn the power past 4 watts the same thing accures.The modulation control is very touchy so my thoughts ar that the limiter is cut.On SSB the modulation has to be turned way down close to off just to be uderstood.Can any body tell me how to fix this?Or what the problem might be.He desires clean modulation and a dead key of a watt or so swinging forword to 5 watts AM. He has an old trucker 400 amp that he wants to drive with this thing and he doesent want to blow up the amp.Thanks for any help on this nightmare!! |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1386
Registered: 9-2004
| | Posted on Friday, April 07, 2006 - 5:46 pm: |
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with a radio that is aging such as the one your friend has; the problems are not always as straight forward as with a newer radio.
things have gotten hot repeatedly, caps are drying out, pots are all gunked up with carbon deposits, etc...
so, MY first step when dealing with a radio that has been around a long time, is to clean everything thoroughly, and replace a few of the parts that are known to go bad, even if they havent yet.
on this chassis in particular, there are a few spots that have become known trouble spots.
first is the AM regulator transistor.
it is a 2SC1419 from the factory, and this part has long been known to be junk.
you will find three or four of them in your radio. (one is on the power supply board)
replace all of them with NTE152's, and move the one near the PLL chip to a bigger heatsink somewhere else in the radio, and run wires from its original spot to its new home, bolted to a big piece of aluminum. (also, look behind this transistor at the big resistor with teflon covering it.
pull off the teflon and see if its burnt, if so, replace it with a 47-56 ohm 2 watt resistor)
C179 near the finals has been known to cause problems.
replace it with a 2.2uf ELECTROLYTIC cap. its a tantalum cap from the factory.(observe polarity!)
also, replace any electrolytic cap in the radio that is 1,000uf or bigger.
replace the one on the main PC board with a 2200uf cap. (its the biggest one on the main PC board)
replace the big cap on the power supply board with a 4700 or 6800uf cap.
both of these should be rated at 35v or more.
thats about $20 worth of parts that are readily available, and easy to replace.
(as long as you know how to properly change out a power device)
try the transistor changes.
check the radio.
try the C179 change.
check the radio.
replace the caps.
check the radio.
if it still wont work, the problem needs to be traced down by a tech with the proper equip. to do it. even if you have to go this route; the radio you get back will be ready to rock with the new parts.
OK, having said all that; the FIRST thing you should do is get some contact cleaner (not WD40!)
and spray it in the variable power pot while turning it back and forth quickly.
see if that does the trick.
now do all the other pots on the radio in the same fashion.
take the cover off the relay inside the radio and spray the contacts.
also, all the switches should be sprayed while pushing the button over and over.
careful not to spray the meters, or plastic, as some contact cleaners will dull the finish.
try some of these things, and post the results.
good luck,
matt |
Blacksmith
New member
Username: Blacksmith
Post Number: 9
Registered: 2-2005
| | Posted on Friday, April 07, 2006 - 9:15 pm: |
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Thanks alot I will try that. |
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