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Realistic TRC 457 heats up, drifts, un adjustable
- Thread starter vswr
- Start date
Done! No kidding! Who needs a power mic when you have mod like this. I had to tone it right down its a screamer! Thanks for that tip!
Sadly, I am still having constant issues with ct2, ct3, and AM. Turn one and it offsets the other. Then adjusting AM through it all out of alignment again. If I go by the sams adjustments, my center is at 2 oclock on lsb. Ive never had a radio so tough to align like this. I tried using a mini muffin fan to cool down the TO3 transistor but I can hear the fan through the speaker especially loud on AM. Might have to figure out a way to isolate it because somehow the motor rotation sound is feeding back into the DC and then working its way into the audio. Maybe using shielded wiring or using a voltage regulator like a 7812 or both. There has to be some method to keep it from drifting but I'm lost.
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VSWR,
something you said in one of your posts prompted me to ask, what color is the heat sink compound you are using?
you said "top of the line CPU compound" and many times that type of compound is conductive.
if the compound you are using on that TO3 transistor is grey/silver, then i would say thats your issue with the heating.
you need to use the white silicon non-conductive grease.
just a guess but i thought i'd mention it.
LC
something you said in one of your posts prompted me to ask, what color is the heat sink compound you are using?
you said "top of the line CPU compound" and many times that type of compound is conductive.
if the compound you are using on that TO3 transistor is grey/silver, then i would say thats your issue with the heating.
you need to use the white silicon non-conductive grease.
just a guess but i thought i'd mention it.
LC
Good point! I havent done it yet but yeah it would be conductive. I'll stick with the white stuff. Thanks!
ok well then that's not the problem.
I apologize if this seems pedantic but have you done a continuity test from the body of that transistor to the metal chassis of the radio?
LC
I apologize if this seems pedantic but have you done a continuity test from the body of that transistor to the metal chassis of the radio?
LC
Yes. No conductivity and I do get an adjustable voltage now since I replaced the resistive shorted disc cap.
If that disk cap shorted, it would have created a path around the regulator's collector to its base putting unregulated DC-0.7v into the radio. This could have killed a cap or something that is now drawing excessive current. A bad cap wouldn't necessarily shut a radio down and it might explain why you hear the fan noise in the radios audio. Whats on the main rail apart from the secondary regulators? The audio IC (I didn't look)? Check that cap out!
Edit, I didn't see one at risk by the audio IC, but there are a couple caps worth checking. C107, C94, C135. Those are by the DC input.
Strange problem, hope you get it figured out. For those following along, I posted a patched together schematic here last summer. https://www.worldwidedx.com/attachments/trc-458-schematic-jpg.73265/
Edit, I didn't see one at risk by the audio IC, but there are a couple caps worth checking. C107, C94, C135. Those are by the DC input.
Strange problem, hope you get it figured out. For those following along, I posted a patched together schematic here last summer. https://www.worldwidedx.com/attachments/trc-458-schematic-jpg.73265/
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Its been recapped.I tested every cap that I put in because I've had bad caps sold as new to me in the past and that created some serious power supply issues. I only discovered this bad disc once I started working on it, and the first step was to check to ensure 13.8V. I found 17 volts which put me into full fledge test mode and thats when I found that bad 2k ohm disc short. Definitely going to figure out why the fan is being heard. Its not getting coupled onto the RF or mod but the wiring harness I put in would probably do better with a shielded wire, then a .01 disc across the line at the motor. Great to be getting some good brainstorming help. My most confusing issue is the drifting. Its a real mess tune. Once I get it just right as per sams, the center moves off to the 10 oclock position and then thec2 oclock. Then I put the cover back on and the small amount of heat generated within the PLL about 85°, is enough to throw the upper and lower sideband off by 50 to 100hz. So,,, now Im adjusting it based on the difference I find between the cover on and cover off. Once I get that figured out, I will unlock the clarifier. I moved anything I could away from the 3 crystals like the coils, resistors and transistors that may generate just enough heat to change the crystal oscillation. Maybe I'm barkin up the wrong tree. Maybe the cover is causing interference with those circuits. Never seen anything like this in 45 years as a tech. What a mess. This radio was a virgin too!
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I would start with the power supply problem 1st. Once you figure that out then move to the alignment issues. I'm not saying the alignment issue comes from the power supply issue but because the alignment issue is for AM, USB and LSB I would think that the issue would be voltage or heat. Both are current challenges with the power supply...
Hmmmm..... interesting. There are 2 sources of hookup for the 457. Its a base but has a mobile hookup. Completely independent. I'll hopk up a power supply and then see if it holds an alignment whats really odd about the alignment is that once you align the ssb, the AM goes off freq by as much as half a khz.then you line up the am and it throws the ssb off. Ive had this radio in a tote for over 30 years since Ive been using my cobra 2000gtl. It was working perfectly and my daily radio until I got into the greatness of the 2000. I just recently decided to get into it to recap it. After bringing it up on the variac, it looked ok but sounded a bit high on ssb. I recapped it and no change. Then when I went to the alignment procedure, I found the voltage was high. Found that fault and remedied it but noticed that the to3 was getting really hot on ssb. Not too bad on AM but still hot warm enough on rx I dare say normal. The muffin fan I put in keeps the t03 cool. The alignment goes well but the clarifier sounds way too high when Im dialed in as per sams. I find using the freqs given on the truth table start to slowly misalign as I increase the channel dial. 3 straight 8 hour says and still can't figure out why it just shifts like that. Im starting to think the crystals are all done. The wierd thing too is that sams has it wrong for ct2 and ct3. Its actually swapped vice versa. I might put my sledgehammer into it. That will fix it good.
many times when your alignment get more and more off the more you turn the channel dial, its the 10.240 crystal that is the issue.
LC
LC