"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. ;-)

Thanks again for the reply.
-Dr.X