Transistor Needed for HP-67 Charger



#9

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


#10

Quote:
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


What you mainly need to know is the voltage output of the charger, 12 volts? 5 volts? It should say, HP could tell you. If you don't have a weird connector you could just by a universal charger at radioshack. If you knew the voltage and current rating of the charger, they are not too expensive. Is the transistor a small 3 pin job with pins sticking out the bottom, or is it a larger TO3 or T05 can? You also need to know if it is a PNP or an NPN, probably NPN but if the guy is dead it will be hard to figure out. I would try to find out the voltage and current rating and get a replacement. If you can't find the proper plug, you can always salvage the old one, chop it off with a few feet of wire sticking out and just solder the wires, making sure you have the right polarity. The polarity should be on the HP67 side and may say the voltage too on a tiny symbol next to the 67 plug. You can find those specs at HP I am sure. Good luck. The problem with finding that transistor is even if you find one, you won't know if that is the only fault till you get one and solder it in place, only to find the input transformer is dead or some such.
#11

Quote:
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


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.
#12

Quote:
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


Also found this:
HP chargers are not simple as they appear!. For all but the 'classic' series, the charger is just a transformer. But it's a transformer with a carefully-controlled output characteristic. The voltage is designed to drop on-load, and to drop in a defined way. The official testing procedure for chargers (I believe) involves testing them open-circuit and then again with a specified load resistor across the output. These types will show a higher-than-expected output voltage off-load. Last year I built a charger tester. A box containing relays and resistors that plugs into an HP3421 data logger. Under program (HP71) control, it connects different loads to the charger and records the output voltage. And when some HPCC memebers find some other chargers for me to test, I'll hopefully be able to give the experimental specs for all of them. FWIW, for the AC input chargers it is possible to specify them just by the open-circuit voltage and internal resistance -- the V-I curve is _very_ linear. As I said, that applies to all chargers apart from the 'classic' one. And we're discussing a 'classic' one here. That charger contains a lot more than just a transformer. 3 trnasistors, 4 diodes, a zener, and a handful of R's and C's. This unit has an internal voltage regulator to run the machine and an internal current regulator to charge the NiCd battery. That's why the output voltage of the 'classic' charger should not be much higher off-load than when loaded.
#13

Here's the schematic.

I don't think that the transistor characteristics are at all critical. The calculator can work off a reasonable range of voltages (3.6 - 4.5) and the current needed to charge nicads covers quite a range (30ma - 80ma at least). You can probably use a 2N2222 for the logic supply. For the charging supply, you might be able to get away with using a 2N2907 for both transistors. Experimenting with this should be pretty easy to do.

Edited: 3 Nov 2008, 12:44 p.m.


#14

Quote:
Here's the schematic.

I don't think that the transistor characteristics are at all critical. The calculator can work off a reasonable range of voltages (3.6 - 4.5) and the current needed to charge nicads covers quite a range (30ma - 80ma at least). You can probably use a 2N2222 for the logic supply. For the charging supply, you might be able to get away with using a 2N2907 for both transistors. Experimenting with this should be pretty easy to do.


So it is an NPN. Was wondering about that. good old 2n2222! or th 2907
I must have a hundred of them in my junk box:)
"http://www.hpmuseum.org/batts/battery.htm

#15

Don, Katie:

Thank you for the wealth of information that you have provided. I know it takes time.

I had also looked at this site:

http://www.jacques-laporte.org/HP35%20power%20unit.htm

by Jaques La Porte and well down the page is a photo of the circuit board including the unnumbered side of the transistor. The transistor is marked "Q3".

I'll take a look at the transistor options presented and see if either one will work.

Thank you again. I'll let you know the outcome.

Very sincerely, David

#16

Hi David,

The 3-338 marking on the power transistor means its full HP part-number is 1853-0338. The M356 is the Motorola logo and the date code 356. This transistor was probably specified by HP and may not have an exact equivalent, however I think you can replace it with an MPS-U51, -U52, or -U55 if you can find one of these. Other close equivalents may exist, but be careful that the original transistor has a non-standard pin layout!

The smaller transistor is marked 3-015, its full HP part-number is 1853-0015. This one is a standard 2N3640. A 2N3906 or 2N4403 would work equally well here.

I would recommend replacing the two transistors at once, just in case a defective transistor would fry the other one. Of course you should test the charger after it's repaired, before connecting it to your priceless HP calculator!

Joel Setton


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