Re: SDR40 v6.1 TX/RX stopped working
- At 3:11 am ((PDT)) Wed Aug 1, 2007, in Digest 1864, ian mcphedran wrote:
>Hi, I have replaced Q9 and no improvement resulted.Looking at the circuit diagram, with nothing connected to
/PTT OUT then Q9 should be having an easy life thermally,
even if it were partially on or defective.
>It is running hot too and the '5V' rail is 4.1V.But it's getting hot, so is the 5V regulator U4 and the
> I'm still searching for the fault that's taking all that 110mA on the
> 5v line.
voltages are out, so there is excess supply current
somehow, even though R35 measures ok so it
can't be passing through that.
Other candidates are an invisible and perhaps
partial short to 5V, i.e. U4 pins 15-16 or a faulty U4.
Was that 4.98K an in-circuit measurement?
If so, then that should prove no short to the
5V rail unless it's a voltage-sensitive one.
It's time for some very careful inspection around the
U9 collector circuit node, perhaps some resistance
measurements to nearby voltage traces...
...and if all else fails, some judicious track-cutting
or IC-lead lifting to isolate the possible sources.
>U8 86 FAt 2:44 am ((PDT)) Thu Aug 2, 2007, in Digest 1865 Tony Parks wrote:
>U8 pin 7 reads 4.2V
>Sure looks like U8 is a problem.Perhaps, but output near the +ve rail could be:
a) inputs driven to output saturation
b) output pulled high externally (i.e. by a pins 7-8 short)
c) device failure which also emulates (a) or (b).
Both (b) and (c) can explain the U8 heating
but only (c) is that device's fault.
(There's no source for the heating current in case (a).)
Meanwhile, none of these explain why:
d) Q9 ran hot and
e) Q9 replacement ran hot
This doesn't seem to be in a related part of the circuit,
so was Ian just unlucky with two rogue 2N3904?
[BTW, my apologies for saying that Q9 collector pad voltage
3.63V with Q9 removed seemed too low. I recalculated as
3.64V - within the DVM last digit uncertainty.
(Vcc-Q8vbe) * R40/(R35+R40) + Q8vbe =
(4.32-0.65) * 22.1/(4.99+22.1) + 0.65 = 3.64V ]
>Let me know what components you need to restore your boardI continue to admire your outstanding support for this project;
>and I will send them right away at no cost.
"May Great Leader Tony live ten thousand years!"