|Re: Transistor Needed for HP-67 Charger|
Message #3 Posted by Don Jennings on 3 Nov 2008, 12:05 p.m.,
in response to message #1 by David McClellan
I found a nice thread about the 67 charger, here is one post:
How are you measuring the output voltage? The HP67 charger has a 3 pin connector. The middle pin is logic ground. One of the outside pins is a voltage-regulated 4V (or so) supply for the calculator's logic chips. The other outside pin is a 50mA (or so) constant current source to charge the NiCd. The open-circuit voltage of that is around 17V. You should check these 2 outputs sepatately, with the -ve lead of your voltmeter on the centre contact of the connector and the +ve lead on each outside one in turn. If it still seems bad, then you need to look inside. Unplug the charger from the mains. There is live mains on the PCB inside when it's plugged in, but there's no stored high voltage, so it's safe to take apart when unplugged. Take out the 4 little screws on the bottom and separate the case. If you have one of the dual-voltage (115V/230V) models, then the voltage selector switch will fall apart at this point. Don't lose the parts, and note how they go. In fact clean the contacts of the swtich now. The circuitry is not complicated. There's a mains transformer, bridge rectifier and smoothing cap (the tall electrolytic can) giving around 17V across the cap. In one corner of the PCB is a circuit consisting of a zener diode, resistor and transistor to give the +4.2V supply. In the other corner is a 2 transistor circuit to give the constant current supply. Some versions have a picofuse soldered to the PCB in series with the transformer secondary winding. Common failures are the fuse (if fitted), the output cable (check for continuity between the holes in the connector and the solder pads on the PCB -- bend/stretch the cable looking for problems). If those are OK, then solder a couple of wires onto the terminals of the electrolytic capacitor, put the case back together (it's essential to do this if you have the dual-voltage model since you need to have the voltage selector switch assembled, and it's a good idea on all models to avoid a possibly lethal mains shock). Connect your voltmeter to the 2 wires you connected to the cap and plug the charger into the mains. If you don't get around 17V DC there, then test the transformer, rectifier diodes, smoothing cap, mains wiring, etc. If you do, then check the components in the 2 regulator circuits.
I have an HP-67 Charger, Model 82002A, marked "Made in Singapore," with no voltage on the line that would charge an installed battery.
I find an apparent bad transistor on the circuit board. The transistor is marked with two lines on one side with "M356" on one line and "3-338" on a second line.
I cannot find a current equivalent transistor. Has anyone had any success in replacing this transistor in such a charger and, if so, what did you use that was an equivalent?
I thank you, David