Re: It stopped transmitting

Soundararajan Ra.

As Ft is 300 Mhz maximum gain possible is 300/7=43 the same gain can be get at 10 ma with 3904 .where as if we use 2n22222a  for same  gain 150ma is required .
About bc 48 series ft is to be taken as minimum 150 mhz max 300 mhz may not be suitable. 
we may change R132 to 180 ohms so that current wille be 3.1/180=17 ma

soundar VU2RJW

On Thu, Jan 5, 2017 at 12:19 PM, Raj vu2zap@... [BITX20] <BITX20@...> wrote:


My Q13 is running uncomfortably hot. The static dissipation without RF is greater than the rated 225mW.
Farhan is aware of this and a change of one or two resistors are in order.

Raj vu2zap

At 05-01-2017, you wrote:


I think John may have nailed it.

Quiescent voltage at the emitter of Q13 is around 3.1 volts, so current is 3.1v/100ohms = 31ma, which is ok
Quiescent power dissipation is (12-3.1)*.031 = 276 mW, which is an awful lot for a Sot23 package.

If mine blew, I'd replace it with something in a bigger package capable of dissipating the heat.
A 2n3904 in the standard TO-92 through hole, or perhaps even better in the Sot-223 that Fairchild has.
A 2n2222a could work too if that's what you got.

When hacking in a leaded part, I'd just solder the legs to the nearby R's and C's rather than try to use the Sot23 pads.

Alternately, might be able to adjust that 100 ohm emitter resistor as John suggested, to maybe 150 ohms.  I'm not sure how that would impact performance here.

Jerry, KE7ER

---In BITX20@..., <iam74@...> wrote :
Note that you still haven't figured out what caused the problem in the first place. It may be that there is too much current flowing through the transistor and the emitter resistor may have to be adjusted. This can happen with too long a hold on the PTT. Maybe a heavier transistor is called for...

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