Thought I would put the dx500v back in the pickup today. Hooked it up with radio off, and of course it started smoking when the amp was powered on. What causes these resistors to overheat? 
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Dx500v smoking
- Thread starter 338_MtRushmore
- Start date
That is the result of blown up DEI output transistors. When the base junction burns open on the transistor, the drive power that would normally feed it, now burns the 10 ohm resistor open since the low impedance base, is no longer in parallel with that resistor to take the drive.
It have 7 volts on that pad with the power off. Should the transistor even see power without the amp switched on?
I just noticed you said these parts started smoking with the radio off? If that is really the case, it would appear the grounded resistor in the bias voltage divider has opened and may be applying 12 volts to the bases of the transistors. That would be bad because it can burn all 4 output transistors up.
There should never be more than about 0.6 volts on the bottom tab of these transistors. Any that have 7 volts on them, are burned up now. Sorry to be the bearer of bad news.
Not that it's worth fixing anyways, but where is the 7 volts coming from? Keyed, or even powered up I could understand, but powered off seems odd to me.
Well, since the relay needs to be keyed before bias can be applied, it is likely the 7 volts you see now is the result of the blown output transistors, "leaking" in the reverse direction. That will allow the collector voltage that is always applied, to leak back into the base circuit. If I'm right, removing the defective transistors will also remove the 7 volts. If I were going to fix it, I would remove the transistors and key the amp to insure the correct .6 volts is being applied, prior to installing new transistors.
And you will be replacing all 4 transistors since dei pills are no longer available. What a pain in the ass...
I was told for years that cb stuff was junk. I refused to believe it. I know now that I was an idiot for not listening. I'm not throwing good money after bad.
In the picture I posted of the bias resistor, you can also see the 25 ohm, 5 watt brick resistor. Check the end towards the front unkeyed and see if there is close to 12 volts there. If so, the main relay has stuck contacts