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Realistic TRC 457 heats up, drifts, un adjustable

vswr

vswr

Disinterested observer
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Aug 20, 2023
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Ive been working on a 457 here and there. Recapped it including the tantalums. Started to do the alignment and found there to be 16 volts out rather than 13.8. Pulled the regulator and it tested fine. Ended up being a disc capacitor that had a 2k ohm short across the 2sc1618 base to collector. Replaced it, tested out the transistor on 2 of my testers. Multimeter and transistor tester. Came out showing good. Assembled it back together and adjusted my 13.8v with the new disc cap installed. There is a horrible problem of heat from the 2sc1618 that is creating so much heat that the box just drifts on all modes forever. If I align it with the cover off, its fine. Put the cover back on, it drifts. As much as 100hz on lsb, usb and am. Anyone have some idea as to whats going on here? I hate to have to put in a clarifier mod to cheat.
 

Are you talking about the TO-220 transistor that is mounted to the shield wall of the PLL section?
if so, there is a TSB on this. It involves moving that transistor (TR36) and a resistor (R153) to another area where it can be connected to a heatsink.
move the resistor as well and solder it to the transistor leg and then wire out to the board.

you may have to re-adjust your SSB offset trimmer caps. CT2 and CT3.
LC
 
loosecannon said:
Are you talking about the TO-220 transistor that is mounted to the shield wall of the PLL section?
if so, there is a TSB on this. It involves moving that transistor (TR36) and a resistor (R153) to another area where it can be connected to a heatsink.
move the resistor as well and solder it to the transistor leg and then wire out to the board.

you may have to re-adjust your SSB offset trimmer caps. CT2 and CT3.
LC
Click to expand...
Its the T03. I never noticed a to220 mounted within the PLL. I'll recheck tomorrow. I'll keep the post informed. Thanks for the tip.

73s
 
ok that's the power supply.

Something is wired wrong, or a bad component, or maybe that transistor was mounted improperly and is making contact with the chassis?
(i would figure that would pop it but maybe not)
LC
 
The transistor gets really hot. The heat sink too is really hot. Im gonna redo the heat sink compound and see how it goes. I will remove the load from the power supply section by lifting the orange wire. Then do a current draw test to see the flow. I think the transistor itself could be the problem since the disc cap had a 2k ohm short from base to collector. It could be partially damaged having thrown that transistor into saturation. The radio works fine with an external supply. The heat within the box is whats causing the drifting. The shipping is expensive and there is only 1 source that I found. I could replace the entire supply with a chinese switching power supply. A helluva lot cheaper than the costnof the transistor, tax and shipping.
 
A switchmode brick is cheaper than the labor to repair an original linear-regulated supply. Has better overload/overvoltage protection as a rule. Most factory power supplies did not have either, beyond a fuse.

Any idea the schematic callout number of the transistor that's getting hot?

73
 
No not off hand. I can get that number off the sams today. Its the big to3 c1618 left back corner. Ive heard talk of people saying its just better to swap it for a brick switching power supply. I do have one here. 30 amp. I just need to take an AC tap off of it for the clock. Im using a 100 amp switching supply already on a 667 but I do find them to introduce some line noise. I suppose I could decouple that.
 
Tr401, cct trace is 27. Seems to normalize at about 90° when in rx. Ssb goes up to 120. Im gonna replace the paste with a top of the line CPU paste.also add a 2 inch muffin fan that will expel the heat fom the rear. See what that will do.
 
I recommend making sure there's a 4-Amp fuse between a replacement power supply and the radio's power switch. A 5-Amp switcher will fit inside the radio and have more than enough current capacity for that radio.

73
 
nomadradio said:
I recommend making sure there's a 4-Amp fuse between a replacement power supply and the radio's power switch. A 5-Amp switcher will fit inside the radio and have more than enough current capacity for that radio.

73
Click to expand...
I picked up a buck power converter. I think it willwork great and its small enough to fit exactly imside the transistors heat sink space. I can use all the components and it will replace only the transistor. Problem is max draw is 3amps. Schematic says the radio uses 2.6amps full ssb mod. I'll give it a shot. It was 10 bucks for 5 of them so if I blow it up it's no bid deal. Whete can i find the 5 amp you talk of?
 
nomadradio said:
I recommend making sure there's a 4-Amp fuse between a replacement power supply and the radio's power switch. A 5-Amp switcher will fit inside the radio and have more than enough current capacity for that radio.

73
Click to expand...
Also, i never run full 12 watts out. I tune it at about 9.5 to 10 ssb and 3.5 am.
 
loosecannon said:
Are you talking about the TO-220 transistor that is mounted to the shield wall of the PLL section?
if so, there is a TSB on this. It involves moving that transistor (TR36) and a resistor (R153) to another area where it can be connected to a heatsink.
move the resistor as well and solder it to the transistor leg and then wire out to the board.

you may have to re-adjust your SSB offset trimmer caps. CT2 and CT3.
LC
Click to expand...
No transistors mounted to the shield wall.
 
loosecannon said:
Are you talking about the TO-220 transistor that is mounted to the shield wall of the PLL section?
if so, there is a TSB on this. It involves moving that transistor (TR36) and a resistor (R153) to another area where it can be connected to a heatsink.
move the resistor as well and solder it to the transistor leg and then wire out to the board.

you may have to re-adjust your SSB offset trimmer caps. CT2 and CT3.
LC
Click to expand...
That was in the Cobras. The Presidents and Realistics already have it bolted to the heat sink.
 
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I also have another annoying issue with my audio. People say it sounds pinched. Even if its cranked lower than specs. I dont use AM do I like clean audio. Is there a mod or trick I can do to clean up the audio and get rid of the pinch sound? Its like the amc kicks in slightly after I modulate. Cant stand it.
 
From 399's (sadly defunct) site:

Locate D23 (next to VR7). In the Realistic rigs a 220 ohm resistor is in the D23 location. Replace this resistor with a 1N914 or 1N4148 diode. (observe polarity). This change will make the Realistic 858 mod limiter the same as all of the other brands. Really opens up the audio. Replace C87 with a 10uF 16v electrolytic cap.
 

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