MinnKota mystery
I picked up this beatup looking trolling motor at an estate sale. I tested it and it didn't work
so we negotiated a deal and I've finally gotten around to disassembling it to see what's
what.
It's a MinnKota 55A and the armature was in amazingly good shape considering it's age.
In fact not too much was wrong but I've come to repairing the variable motor speed
control and I'm stumped.
The circuit is not complex, it's dirt simple.
Basically it's a TO-3 npn transistor with the collector connected to the battery + thru the
motor. You could say it''s 'sinking' the motor LOL.
The emitter is at the ground or negative battery potential.
In series with the base is a 1N4148 or similar with the band towards the xistor.
Next is an 11 ohm 10 watt which I guess is current limiting.
After that is a wire wound pot of 10w - 120 ohm design that connects to the battery +.
I thought, Ah ha it's obviously the wire wound pot since it is up top and those kid of pots
always go intermittent anyhow. I took it apart and cleaned it since it actually was pretty
spotty as I turned the shaft. Interesting design. Since it's emulating the throttle control on
an outboard motor, when you turn it all the way CCW it disconnects, or opens,
the base circuit terminating any drain on the battery (while you're casting away, I guess).
Okay, here's the problem. When I switch to the variable mode (which connects the
emitter to ground) the motor goes to some in between speed and wont vary no matter
where I set the pot (except when I turn it to off). I've tested all the parts three times in
frustration and everything looks good. I have a VOM and a Heath IT-27 so I'm fairly sure
of my measurements. Unfortunately I dont know enough to really do a proper SPICE simulation of how it's supposed to work and I cant find a cross ref for the power xistor.
It measures as a low beta device, but not as low as the type found in a TV horizontal
oscillator, more like a typical 2N3055 or 2N3771. This pass transistor has a 'house' marking on it of S39316. ECG, SK, NTE show nada. I looked at the Fairchild database
and some exclusive silicon valley service I managed to con a trial membership on.
But still no joy.
The whole machine was made in 1973 according to the date stamped on the motor
armature and the datecode on the xistor. It's a typical Fairchild device of the period,
using the same low profile aluminum package that Motorola was using at the time.
Was it a darlington that drifted? I dont know so I'm hoping for some enlightenment.
Everything tests as working yet it doesn't perform.
Perhaps the manner of implementation caused a problem?
I dont think it's the motor. It was almost pristine, showing hardly any carbon buildup
from the brushes.
The way the speed control is set up, the select switch selects between Hi or Var with
a center position of 'off'. So it's definitely 'break before make'.
In the circuit that switch connects the battery - (minus) either directly to the motor
or engages the xistor emitter (which is left floating otherwise).
Suggestions, detections or selections. I need some help.
Oh yeah. I have this neat old German meter my dad brought home from the war that
has various shunts and it has a high amperage one that does 12A AC/DC max.
According to that meter the motor draws approximately 4A running with a surge of
perhaps 8 to 11 amps when first started. Difficult to tell surge exactly on an analog meter.
I picked up this beatup looking trolling motor at an estate sale. I tested it and it didn't work
so we negotiated a deal and I've finally gotten around to disassembling it to see what's
what.
It's a MinnKota 55A and the armature was in amazingly good shape considering it's age.
In fact not too much was wrong but I've come to repairing the variable motor speed
control and I'm stumped.
The circuit is not complex, it's dirt simple.
Basically it's a TO-3 npn transistor with the collector connected to the battery + thru the
motor. You could say it''s 'sinking' the motor LOL.
The emitter is at the ground or negative battery potential.
In series with the base is a 1N4148 or similar with the band towards the xistor.
Next is an 11 ohm 10 watt which I guess is current limiting.
After that is a wire wound pot of 10w - 120 ohm design that connects to the battery +.
I thought, Ah ha it's obviously the wire wound pot since it is up top and those kid of pots
always go intermittent anyhow. I took it apart and cleaned it since it actually was pretty
spotty as I turned the shaft. Interesting design. Since it's emulating the throttle control on
an outboard motor, when you turn it all the way CCW it disconnects, or opens,
the base circuit terminating any drain on the battery (while you're casting away, I guess).
Okay, here's the problem. When I switch to the variable mode (which connects the
emitter to ground) the motor goes to some in between speed and wont vary no matter
where I set the pot (except when I turn it to off). I've tested all the parts three times in
frustration and everything looks good. I have a VOM and a Heath IT-27 so I'm fairly sure
of my measurements. Unfortunately I dont know enough to really do a proper SPICE simulation of how it's supposed to work and I cant find a cross ref for the power xistor.
It measures as a low beta device, but not as low as the type found in a TV horizontal
oscillator, more like a typical 2N3055 or 2N3771. This pass transistor has a 'house' marking on it of S39316. ECG, SK, NTE show nada. I looked at the Fairchild database
and some exclusive silicon valley service I managed to con a trial membership on.
But still no joy.
The whole machine was made in 1973 according to the date stamped on the motor
armature and the datecode on the xistor. It's a typical Fairchild device of the period,
using the same low profile aluminum package that Motorola was using at the time.
Was it a darlington that drifted? I dont know so I'm hoping for some enlightenment.
Everything tests as working yet it doesn't perform.
Perhaps the manner of implementation caused a problem?
I dont think it's the motor. It was almost pristine, showing hardly any carbon buildup
from the brushes.
The way the speed control is set up, the select switch selects between Hi or Var with
a center position of 'off'. So it's definitely 'break before make'.
In the circuit that switch connects the battery - (minus) either directly to the motor
or engages the xistor emitter (which is left floating otherwise).
Suggestions, detections or selections. I need some help.
Oh yeah. I have this neat old German meter my dad brought home from the war that
has various shunts and it has a high amperage one that does 12A AC/DC max.
According to that meter the motor draws approximately 4A running with a surge of
perhaps 8 to 11 amps when first started. Difficult to tell surge exactly on an analog meter.