ALPS plotter problem - UK Vintage Radio Repair and Restoration Discussion Forum

mole42uk · 24th Jun 2016, 5:31 pm

Hi,
Does anyone remember anything about the ALPS DPG1302 plotter - used on the Atari 1020, Commodore 1520, Oric MCP40, Tandy CGP115 etc?
I have one that sets the four pen barrel unit 45 degrees out of alignment when switched on, so that it doesn't plot anything visible - there should be a pen at the top but there is a space at the top and two of the pens at 45 degrees either side of the top.
I can't find an obvious adjustment for it, can anyone help?

TonyDuell · 24th Jun 2016, 6:04 pm

I know them well, in their various sizes (the common ones being about 2" and 4" wide, but I've seen an electronic typewriter that uses that mechanism but wide enough to take A4 paper!).

Does it do the re-alignment at power-on, where it clicks the pn holder round until the black pen is on top? OK, so it won't put the black pen on top, but it should at least shuffle the carriage back and forth. If it doesn't manage that, suspect a cracked pinion on the stepper motor spindle. A very common problem (but the replacement pinions are like hen's teeth!)

If it does do the shuffle, then with the unit off, try twiddling the gears to do that movement and thus rotate the pen holder (the holder is rotated by a springy metal pawl on the chassis fitting into ratchet teeth on the holder). If you can get a pen on top doing that, then the mechanism is basically working. If not, look at said pawl, and at the ratchet pawl on the carriage that prevents the holder from turning backwards and holds it in position when it is not being turned.

If the mechanism seems to be OK, the 'black' position of the holder is detected by a thin rod magnet in one of the webs of the holder and a reed switch on the chassis. Check that next.

JohnBHanson · 25th Jun 2016, 4:30 am

Sometimes this can be caused by a pen that is not quite inserted correctly. I have a silver reed typewriter that uses the same mechanism.

mole42uk · 25th Jun 2016, 10:01 am

Hello Tony,
Yes the head does it's little re-alignment dance at switch-on and the black pen nearly gets to the top. I can get a pen on top if I alter the setting manually and then it will draw a line.
I have examined the gears, none are broken (I fortunately have a spare plotter, and that has good pinions too). I also think the magnet and reed switch are working (it tries to get the black pen on top).
I wonder if the gears have jumped a bit?

TonyDuell · 25th Jun 2016, 5:12 pm

The gears jumping wouldn't matter. There is no position feedback other than the fact that it runs the horizontal motion (pen carriage) into the end stop at power up (that's the nasty buzz) and then uses that as a reference.

It might be damage to the ratchet pawls (there is one on the frame one on the carriage) or to the 'teeth' on the pen holder. I assume that with the carriage away from the left side frame you can rotate the pen holder by hand in one direction only (do not force it!) and that it clicks into position (I can't remember if it's two or three clicks between the pen positions).

I would check the magnet and reed swtich. With machine turned off you can move things by hand and see what is happening. And try a continuity tester across the reed switch,

24th Jun 2016, 5:31 pm	#1
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	ALPS plotter problem Hi, Does anyone remember anything about the ALPS DPG1302 plotter - used on the Atari 1020, Commodore 1520, Oric MCP40, Tandy CGP115 etc? I have one that sets the four pen barrel unit 45 degrees out of alignment when switched on, so that it doesn't plot anything visible - there should be a pen at the top but there is a space at the top and two of the pens at 45 degrees either side of the top. I can't find an obvious adjustment for it, can anyone help? __________________ Richard Index: recursive loop: see recursive loop

24th Jun 2016, 6:04 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: ALPS plotter problem I know them well, in their various sizes (the common ones being about 2" and 4" wide, but I've seen an electronic typewriter that uses that mechanism but wide enough to take A4 paper!). Does it do the re-alignment at power-on, where it clicks the pn holder round until the black pen is on top? OK, so it won't put the black pen on top, but it should at least shuffle the carriage back and forth. If it doesn't manage that, suspect a cracked pinion on the stepper motor spindle. A very common problem (but the replacement pinions are like hen's teeth!) If it does do the shuffle, then with the unit off, try twiddling the gears to do that movement and thus rotate the pen holder (the holder is rotated by a springy metal pawl on the chassis fitting into ratchet teeth on the holder). If you can get a pen on top doing that, then the mechanism is basically working. If not, look at said pawl, and at the ratchet pawl on the carriage that prevents the holder from turning backwards and holds it in position when it is not being turned. If the mechanism seems to be OK, the 'black' position of the holder is detected by a thin rod magnet in one of the webs of the holder and a reed switch on the chassis. Check that next.

25th Jun 2016, 4:30 am	#3
JohnBHanson Heptode Join Date: Aug 2009 Location: Worthing, Sussex, UK. Posts: 661	Re: ALPS plotter problem Sometimes this can be caused by a pen that is not quite inserted correctly. I have a silver reed typewriter that uses the same mechanism.

25th Jun 2016, 10:01 am	#4
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	Re: ALPS plotter problem Hello Tony, Yes the head does it's little re-alignment dance at switch-on and the black pen nearly gets to the top. I can get a pen on top if I alter the setting manually and then it will draw a line. I have examined the gears, none are broken (I fortunately have a spare plotter, and that has good pinions too). I also think the magnet and reed switch are working (it tries to get the black pen on top). I wonder if the gears have jumped a bit? __________________ Richard Index: recursive loop: see recursive loop

25th Jun 2016, 5:12 pm	#5
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: ALPS plotter problem The gears jumping wouldn't matter. There is no position feedback other than the fact that it runs the horizontal motion (pen carriage) into the end stop at power up (that's the nasty buzz) and then uses that as a reference. It might be damage to the ratchet pawls (there is one on the frame one on the carriage) or to the 'teeth' on the pen holder. I assume that with the carriage away from the left side frame you can rotate the pen holder by hand in one direction only (do not force it!) and that it clicks into position (I can't remember if it's two or three clicks between the pen positions). I would check the magnet and reed swtich. With machine turned off you can move things by hand and see what is happening. And try a continuity tester across the reed switch,