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Dave Britten · 09-18-2022, 12:07 AM

I picked up a Radio Shack version of the Sharp CE-122 printer for my PC-1211. I've got it running with a fresh battery pack, but it seems like the printer drive motor is running too fast. If I'm reading the service manual correctly, the head should make about 10 sweeps when printing a line of text (7 pixel rows and three blank rows to advance the paper). But I'm counting roughly 19 sweeps. The printed characters end up being stretched horizontally, and some of the dots at the right end of each of the four printhead columns get wrapped down to the next pixel row. This makes me think the printer motor is running too fast, and the computer is only picking up every 2nd timing signal from the printer, as they're coming much faster than the computer is able to handle.

Anybody ever seen this before? Is it as simple as the motor drive voltage being wrong? And if so, what sort of aging component(s) might be causing it?

Jeff_Birt · 09-18-2022, 01:37 AM

I have been working on a few of these recently too. From this paragraph it sounds like that PCU is counting timing pulses the printer itself generates while in motion. I think I might take a look at this signal as a fist stop.

Dave Britten · 09-18-2022, 12:26 PM

Yeah, I get the feeling that the motor is moving too fast, causing the printer to generate timing pulses faster than the computer/PCU can deal with them. It's very consistent, i.e. not just an occasional timing pulse that's missed randomly.

I need to find a good oscilloscope at a hamfest... Smile

Jeff_Birt · 09-18-2022, 07:33 PM

The motor is ON or OFF, there is no speed control. There is no way to make it faster except increasing the voltage. How are you powering the CE-122? While it takes a 6V charger, the battery pack is 4.8V. If the battery pack is not present and 6V is plugged into it, the board will see 6V instead the 4.8V it expects. The battery is part of the voltage regulation when the 6V external power is connected.

Dave Britten · 09-18-2022, 08:33 PM

(09-18-2022 07:33 PM)Jeff_Birt Wrote: The motor is ON or OFF, there is no speed control. There is no way to make it faster except increasing the voltage. How are you powering the CE-122? While it takes a 6V charger, the battery pack is 4.8V. If the battery pack is not present and 6V is plugged into it, the board will see 6V instead the 4.8V it expects. The battery is part of the voltage regulation when the 6V external power is connected.

Right, that's why I was wondering if the voltage was wildly out of spec somehow. You never know with aging caps! I'm using a new 4xAA NiCd pack, plus the standard AC adapter that came with it, so I'm not pumping 12V into it or anything crazy like that. So either the motor is running too fast and causing the PCU to miss every other timing signal, or something else is impeding every other timing signal. Hmm...

Jeff_Birt · 09-18-2022, 08:42 PM

Bad capacitors are not going to increase voltage. The wall wart is for charging not for running. Charge the pack up and then remove the charger. If you keep it plugged in all the time it will boil the battery pack dry. The only charge control on these things is the time you have the charger plugged in. This is true for most rechargeable stuff of this era.

Dave Britten · 09-18-2022, 08:45 PM

(09-18-2022 08:42 PM)Jeff_Birt Wrote: Bad capacitors are not going to increase voltage. The wall wart is for charging not for running. Charge the pack up and then remove the charger. If you keep it plugged in all the time it will boil the battery pack dry. The only charge control on these things is the time you have the charger plugged in. This is true for most rechargeable stuff of this era.

Yup, I couldn't find any electrolytic caps in the schematic for the printer control circuitry anyway, so it kind of rules that out! The wall wart is wired basically in parallel with the battery, and I was getting the same results running from the (charged) battery, or with the printer plugged in too. I probably need to do some deeper disassembly of the printer mechanism and see if anything looks amiss. Not sure what exactly is generating the timing signals to the PCU. I don't think it's any kind of optical encoder, but I could be wrong.

Dave Britten · 09-18-2022, 10:22 PM

Fixed it!

The timing mechanism is just a simple magnetic speed sensor on the end of the drive shaft, with two timing pins on the ribbon cable. I'm assuming it creates an AC waveform across these two pins as the motor runs - I'm no EE though. Smile

If you look at the circuit diagram, you'll see that each side of the timing signal has a 330 pF (filter?) cap to ground. The one on the right had some white gunk around its legs, though I don't have any idea where it came from. There wasn't any sign of leakage from any components - old crusty flux residue of some sort maybe? Anyway, I cleaned up around the legs of the two caps, and it seems to be running fine now. That certainly explains why the PCU was only seeing half of the timing signals if one side was partially shorted to ground!

I need to find some fresh(er) ink ribbons though. 40-year-old new-old-stock ink leaves something to be desired.

https://i.imgur.com/8qswbKI.jpg

Jeff_Birt · 09-18-2022, 11:41 PM

Nice, glad you got it going. I picked up some NOS Epson ERC-05 ribbons from eBay a while back. They were still sealed but I've only had about 50% success with them out of the box.

Dave Britten · 09-18-2022, 11:44 PM

I put a few minuscule drops of distilled water onto the ink roller to rejuvenate it a bit, and it's a slight improvement.

I found some ERC-05s on Amazon, and ordered one for only $6. We'll see how it is when it gets here in a week or so.

I wonder if I can re-ink the rollers in the other two I have that are almost totally dried out. One was still new in the Radio Shack packaging, so I would think the ribbon itself is still okay.

Jeff_Birt · 09-18-2022, 11:47 PM

I have been thinking that stamp pad ink might work for reinking ribbons. It seems to have the similar properties in that it does not dry out quickly on the pad but dries on paper quickly. I have not tried it yet.

Dave Britten · 09-23-2022, 05:23 PM

I got the new ribbon in the mail from Amazon today. Looks terrific! Print quality is just like new. Knowing that they dry out over time (e.g. 40 years), I'm not sure it would make sense to stock up on tons of them, but I might order 2 or 3 more so I've got some spares.