I installed the repaired cruise control amplifier. Then I removed the
solar battery charger that I'd hard-wired in to the car, my wife had
been
complaining about it sitting on her seat!
Next, I removed the Hirschmann 6000EL antenna. It looked OK inside,
and the belt was intact. Removing the motor connectors from the PCB I
could apply power direct to the motor and it ran. I removed the mast,
and found it to be intact, but dirty and sticky. I spent considerable
time sluicing it out with acetone, trying to free it up. It's
significantly better now, but I may not be done with this process.
The antenna's circuit board is built with three IC's: a CMOS 40106
Schmitt trigger hex inverter, a CMOS 4060 14-stage ripple counter, and
a 555 timer. There are four power transistors, and six signal
transistors as well, as well as the expected crop of diodes,
resistors, and capacitors. There is one power resistor that is
probably for motor current sensing.
On the way to church the cruise performed properly. But on the way
back it would work, and then get into this neck-snapping throttle
yanking cycle. I suspect that its internal gain resistor(s) are wrong
for a stick shift, it was too regular (and intermittent) for it to be
still bad soldering, etc. Need to research this I guess.
I dug into my box of spare cruise amplifiers, and didn't find what I
needed. I have one manny-tranny vacuum box, but no slushbox models
with the same PCB layout, so inferring the differences from the vacuum
circuit is no good. And, of course, no manny-tranny servomotor boxes.
(If I had one, I'd just use it.)
I tried the antenna out, and it's completely dead. The paint stripper
I use for cruise amplifiers is methylene-chloride-free, and did not
even dent the RTV coating that's on the antenna board. I did some
searching, and it appears that methylene chloride itself will attack
RTV. Got to find some, I guess.
But as I don't _have_ any methylene chloride, and whereas I
_do_ have some other solvents, I thought I'd try some of the
others first. I placed the board in a glass dish and put a puddle of
Xylene in it just deep enough to cover the bottom of the board.
(Xylene can be used to thin RTV on application.) I then covered the
dish to retard evaporation. I let it sit for an hour and then
scrubbed it with a wire brush. The Xylene softened up the RTV enough
that a lot of it scrubbed off the board. Another two dips and scrubs
finished the job. Then I washed off the board with running water and
a scrub brush, and dried it.
Unfortunately resoldering had no effect on the board. What's worse,
it was difficult to do, since it is a tightly-packed double-sided
(at least) board.
So I started probing around with the 'scope. The 4060 counter gets a
665 Hz clock on it for several seconds after power-up. No doubt
this is part of the initial retraction logic (since the switch was
disconnected at that time). When the switch is (virtually) set to UP,
the counter again got a clock for a time. Also at this time the 555
timer pin 2 (trigger) voltage begins to sink, and after a few seconds
it fires. When the switch is set to DOWN the clock again runs for a
time. In other words the circuit appears to be reacting to stimulus
OK. The two big heat sinks are home to the motor's 'H' drive, and
each sink is common to the collectors of a complementary NPN/PNP pair
of BD437/438 power transistors, and to one of the two motor pins. Two
of the transistors have a small base voltage on them which goes away
when the motor is supposed to be running (at least in one direction).
None of the transistors is getting power, however, and for an H drive
the PNP transistors' emitters should be very positive. The big
resistor in the center of the board is intact, as are the two
mid-sized 39-ohm units. Because of the density of the board, the
encapsulation, the presence of the looming heat sinks, and its
multi-sidedness it is extremely difficult to trace out the circuit. I
wonder if I have a working antenna to compare with in the parts box?
Something is definitely wrong with the heavy drive power supply on the
board. I know that the board senses motor current draw to stop
running when the antenna mast hits its stops, no doubt this is what is
broken. More later, I hope.
-- Jim
