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Dx500v smoking
- Thread starter 338_MtRushmore
- Start date
Yeah, there should only be ~12 volts on the front end of the 25 ohm when the relay is keyed. It should be zero when it's unkeyed. Either something got under the PC board and shorted together some traces or the main relay has one of it's NO contacts stuck shut.
I removed the keying transistor and it unkeyed. E to C shows .006 both ways which I'm pretty sure isn't good? There is 12 volts at the b&c pads.
That still doesn't explain 1.9v at the 2879
That still doesn't explain 1.9v at the 2879
DId you check the pills for collector base short circuit ?
When the 2879's go base collector short it smokes the 10ohm resistors & on texas stars it will smoke the divider resistors that Doctor Audio pointed out,
The TS bias circuit is the worst circuit i have ever seen in any cb amp, its a disgrace,
they have no clamping diodes to stop voltage going over .6-.7v & voltage on the base pads is dependant on the divider resistors and the 10ohms been in spec,
TS only got away with the shitty bias circuit becasue they used genuine Toshibas that are tough as old boots.
When the 2879's go base collector short it smokes the 10ohm resistors & on texas stars it will smoke the divider resistors that Doctor Audio pointed out,
The TS bias circuit is the worst circuit i have ever seen in any cb amp, its a disgrace,
they have no clamping diodes to stop voltage going over .6-.7v & voltage on the base pads is dependant on the divider resistors and the 10ohms been in spec,
TS only got away with the shitty bias circuit becasue they used genuine Toshibas that are tough as old boots.
The power pole connector only goes 1 way, so I highly doubt it. That would make some sense to me though if it happened. I'd honestly be happier if it was my mistake vs the apparent self destruct mode it set itself into.
The pills are shorted every way. My biggest question is the cause of this. Even if it was keyed with no drive, I wouldn't think it would blow the pills. To pull it from use, set it on the shelf for a year or 2, then it self destructs with zero drive in a few seconds bothers me.
In theory if I replaced the four 10ohm resistors along with the two (i think) divider resistors it should restore correct bias?
In theory if I replaced the four 10ohm resistors along with the two (i think) divider resistors it should restore correct bias?
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In theory yes,
but i would add a couple of big diodes in parallel with the 2 divider resistors so voltage can't go over ,6-.7v,
then you may need to increase the value of the divider resistor a little if voltage is too low.
did TS change the value of the divider resistors to accomodate the different brand 2879's ? schematic shows them to be 1.8ohm
but i would add a couple of big diodes in parallel with the 2 divider resistors so voltage can't go over ,6-.7v,
then you may need to increase the value of the divider resistor a little if voltage is too low.
did TS change the value of the divider resistors to accomodate the different brand 2879's ? schematic shows them to be 1.8ohm
i would check bias without the transistors in to make sure its a little over .6v
when the transistors are in it will drop as you drive the amp and the 2879's start drawing more current from the bias circuit,
measure it at the junction of the wirewound & divider at that side of the choke to keep rf out of the meter.
when the transistors are in it will drop as you drive the amp and the 2879's start drawing more current from the bias circuit,
measure it at the junction of the wirewound & divider at that side of the choke to keep rf out of the meter.
That was with burnt resistors. I'd have to order parts to replace anything, and I'm not too interested in that anytime soon. Who would I talk to with the conditions right now.
What is the actual cause of the failures, and why. That's the part I dont understand.
What is the actual cause of the failures, and why. That's the part I dont understand.
One of the four RF transistors was already damaged and on the verge of failure before you applied power. Turning on the amplifier simply nudged it over the edge of the cliff.
The DEI transistors have a reputation for failure without a clear cause.
And for failures with cause.
The 25-ohm 5 Watt resistor feeding the bias circuit of each circuit board each has a 'hot' side that would have full power-supply voltage on it if the relay is 'leaking' power in receive mode.
73
The DEI transistors have a reputation for failure without a clear cause.
And for failures with cause.
The 25-ohm 5 Watt resistor feeding the bias circuit of each circuit board each has a 'hot' side that would have full power-supply voltage on it if the relay is 'leaking' power in receive mode.
73
