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HP46 Printer Repair Tip
Posted by Alex Knight on 26 Feb 2001, 3:43 p.m.
I recently debugged a problem with a HP46 desktop calculator printing incorrectly and thought I would share this information. The symptom was that while the calculator would occasionally print a number correctly, more often than not the machine would print the wrong digits, or double-strike, or skip digits - it was not at all predictable as to what the machine would print. Attempting to do a LIST command to dump all the registers resulted in mostly garbage being printed.
I isolated the problem to the +15 Volt power supply. There is a 3000 microfarad, 30 Volt DC rated filter capacitor in the calculator's +15 Volt power supply that appears to provide the printer with power. In my unit, this capacitor had failed, causing a large amount of ripple on the +15V supply, which was confusing the printer and causing it to print incorrectly. Replacing this capacitor completely fixed the problem. Unfortunately, checking for this problem requires disassembly of the calculator. If you decide to attempt to repair the machine, do so at your own risk, I specifically disclaim any responsibility (and so does the MoHPC, I'm sure - sorry but my lawyer always makes me say this!). Also, please take precautions that you don't damage the machine with static electricity since you will have the covers off.
In order to get to the test point for the +15 Volt supply, you have to remove five recessed phillips-head screws on the bottom of the calculator, and then ease the top cover off of the machine. It is also necessary to remove the bottom metal cover, there are a bunch of phillips-head screws holding it on. Next, there is a brown plastic rectangular "cup" to the right of the printer mechanism (I think this is a place that a service card, or maybe even a quick reference card, could be stowed). You need to remove this cup to access the test point. There are two small phillips-head screws on the bottom of the machine that hold it in place (which is really the only reason you had to remove the bottom cover to start with).
The only way to test the power supply is to now power up the machine with all these covers removed, so be very, very careful as there is 115 Volts AC running around inside, as well as other DC voltages. It is best, I think, to have the machine sitting on a non-metal bench to do this part of the test (just make sure you don't zap the machine with static electricity). Power up the machine, and look for the test point (a small gold plated post) marked +15V in the area from which you removed the rectangular cup. You'll need to have a digital volt meter set to the appropriate scale to measure 15 Volts. If possible, use test clips instead of probes to free your hands to operate the machine. Clip the positive lead of the voltmeter to the +15V test point. Clip the negative lead of the voltmeter to a ground - the easiest place I found was on the opposite side of the small silver cylindrical tantalum capacitor whose positive side is connected to a PC track immediately to the right of the +15V test point.
With the digital volt meter hooked up, you should read something close to +15 Volts when the printer is running but not printing. Press the LIST button on the machine, and the voltage should not change much if at all. If, however, you read something a volt or so below +15 Volts when the printer is not printing, and the voltage sags while the machine is printing, then the filter capacitor may need to be replaced. On my machine, the voltmeter was reading about +13.5 Volts before printing, and sagging to as little as +11.5 Volts while printing. I then hooked up an oscilloscope and noticed that the filter capacitor was hardly providing any filtering. The 3000 microfarad, 30 Volt capacitor is the tall, silver-colored one that is immediately in front of the big power transformer in the right rear corner. If you need to replace the capacitor, try and find one that is closely matched. Don't go any lower than a 30 Volt rated capacitor, or lower than 3000 microfarads. It isn't good practice to use a capacitor with a voltage rating that is a lot higher than necessary. I used a 3300 microfarad, 35 Volt capacitor in mine, but your Mileage may Vary.
I suspect that the capacitor went bad because this particular machine was government surplus and may have been sitting in a warehouse unused for many years. Electrolytic capacitors can go bad from nonuse, and I believe this to be the case here. Once I replaced this capacitor, the +15 Volt supply was very stable even with the printer dumping all of the registers of the machine.
I currently have a photo on my website (look in the HP section) of a HP46 with the top cover removed, you can see the transformer and the filter capacitor location in that photo. I currently don't have a view that shows the printed circuit board with the printer "cup" removed, but hope to have that available in the near future.
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