"Frank Gilliland" wrote in message
...
On Wed, 18 Aug 2004 23:12:09 -0400, "Dr.X" wrote
in :

"Frank Gilliland" wrote in message
.. .
On Wed, 18 Aug 2004 21:10:10 -0400, "Dr.X" wrote
in :

I set the dead key on AM to one watt and when the radio gets hot, it
goes
back up to almost full wattage and modulation sounds "pinchy"
according
to
reports. I have to then cool it off before it will act normal again.
Does
anyone else see this with theirs? Mine is in my vehicle so the
temperatures
do get extreme. I find this happens a lot after it's been parked in
the
sun
for a while. Today I tested my temperature theory by taking the cover
off
and holding the radio in front of the ac vent so it would cool off
fast.
It
went back to normal in just under a minute. Sounds like a weak
regulator.
I
hope some one has seen this before and knows the solution. I don't
have a
schematic for it. :-(

tia,
Dr.X


What you are experiencing is called "thermal runaway" and is most
likely caused by either high quiescient collector current or a failure
of the heatsink coupling (loose power transistor). You need to take it
to a competent tech.

Hi Frank. Thanks for answering.

I thought it was thermal runaway also, but I don't think so now. Although
the results are the same. Thermal runaway happens when the component
generates enough of its own heat to send it on its way. The issue I'm
having
starts at 160 degrees F. The interior of the van gets to 168 sitting in
the
Florida sun. Transmitting a dead key at 10 watts with an swr of 2:1 only
gets me 91 F and never goes into runaway. The heat sink compound is doing
it's job as is the sink itself. The final rf area never even gets warm.

The only hot spot in the radio is what appears to be a regulator mounted
at
the left side about 2 in. behind the mode selection switch. I can't say
what
that does since I don't have a schematic and I really haven't started
poking
around in this radio yet (other than a quick crystal swap).

As for having a competent tech look at it, well I've been a component
level
technician for about 20 years but haven't messed with radio in about 10
years. A competent tech with a 10 year gap would want a schematic. ;-)


Then what you are asking for is a schematic. Sorry, I don't have one
for your radio.

Yes, I guess I am asking for a schematic. But I was also hoping that some
one had already experienced this and would save me some time. That's ok,
I'll just keep looking for a schematic or just use some common sense to
track it down. I'll be getting into it this weekend anyway to tinker around.
I'll fix it then.

Thanks again Frank.
-Dr.X