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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 4th May 2024, 1:54 pm   #321
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

It would be interesting to see just what it does to the carriage, which needs the gear to be good I think. It may be that if it takes too long to move and change the state of the sensor then something in the firmware times out.

The fact that the Olympia unit has the same fault is a warning about buying a second one 'for spares'. If there's a weak part it's likely to be defective in the second one too.
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Old 4th May 2024, 4:22 pm   #322
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Yes - but I had expected at least some attempt from the motor to home the carriage to the left hand side at power on.

Colin.
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Old 4th May 2024, 8:37 pm   #323
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Question for Tony - the fact that the motor rotates smoothly in the direction needed to 'home' the carriage from far left suggests that the carriage motor drive circuit (four outputs from U9, passing through four of the six gates of the 7404, passing to driver transistors Q7 to Q10 - is working OK. Is there any way at all that that arrangement could be working enough to drive the motor in one direction but not in the other direction? I seem to recall that Colin found it was sometimes trying to drive the carriage from the centre position to the 'home' position, albeit very weakly.

As I understand it, making it rotate one way or the other is just a matter of cycling the drive output signals in either forward or reverse order so if the drivers can make it drive one way they should be able to make it drive the other way?

For Colin: With power off, and assuming that suspect diode has now been replaced, repeat the forward and reverse 'Diode' tests on all four diodes - we expect to see the same forward and reverse values for all four now, please verify that is the case.

With power on and the carriage in the 'homed' position, is the 10R resistor associated with Q3 still getting very hot?

That broken gear, what does it drive, the paper drive roller?
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Old 4th May 2024, 8:44 pm   #324
Mark1960
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Default Re: Commodore 8026 printer / typewriter

Is there a case cover detect switch that needs to be overridden? This used to be quite common on printers to avoid injury or damage to the printer.
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Old 4th May 2024, 8:49 pm   #325
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Diodes in circuit with no connectors connected are now various readings between 0.580V and 0.613V and overload in the reverse direction.

10R resistor still noticeably hot to the touch.

Colin.


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Originally Posted by SiriusHardware View Post
Question for Tony - the fact that the motor rotates smoothly in the direction needed to 'home' the carriage from far left suggests that the carriage motor drive circuit (four outputs from U9, passing through four of the six gates of the 7404, passing to driver transistors Q7 to Q10 - is working OK. Is there any way at all that that arrangement could be working enough to drive the motor in one direction but not in the other direction? I seem to recall that Colin found it was sometimes trying to drive the carriage from the centre position to the 'home' position, albeit very weakly.

As I understand it, making it rotate one way or the other is just a matter of cycling the drive output signals in either forward or reverse order so if the drivers can make it drive one way they should be able to make it drive the other way?

For Colin: With power off, and assuming that suspect diode has now been replaced, repeat the forward and reverse 'Diode' tests on all four diodes - we expect to see the same forward and reverse values for all four now, please verify that is the case.

With power on and the carriage in the 'homed' position, is the 10R resistor associated with Q3 still getting very hot?
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Old 4th May 2024, 8:52 pm   #326
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Pin 38 of U9 - when the carriage is not home reads 1.9V. When the carriage is homed, -0.87V.

I'll get my scope out.

Colin.



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Originally Posted by SiriusHardware View Post
Checks for today:

If possible, re-test that diode out-of-circuit or at least desoldered / disconnected at one end. If it then measures the same as the others (0.64V / 'OV'), reinstate it and we will consider other possible reasons for that different measurement when in-circuit. If it still measures differently out of circuit, stop and let us know.

Check what happens on the INT pin (pin 38) of U9, first with the carriage NOT in the 'homed' position and then with the carriage in the 'homed' position. If the INT pin shows one state when homed and another when not homed, that is what we would expect. If you don't see that behaviour, report back.

With the carriage centred (not homed), for each of U9 pins 33, 32, 31, 30, scope what happens on those pins immediately following switch-on. If the drive signals (digital waveforms) are present there try following them through the relevant gates of U1 to the bases of Q7, Q8, Q9, Q10.
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Old 4th May 2024, 9:06 pm   #327
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Quote:
10R resistor still noticeably hot to the touch.
Where is the current which is heating that resistor going to?

With the unit off and the carriage motor disconnected from the PCB, do the same sort of diode test on the diode which has one end connected to Q3. If the readings aren't similar to the ones you got from the others, stop and report back.

With the carriage motor reconnected, the power on, and the carriage homed, please measure and report the voltages (with respect to 0V) on the following points:

Q6 base
Q6 collector
Q3 emitter
Q3 base
Q3 collector

Q7 collector
Q8 collector
Q9 collector
Q10 collector
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Old 4th May 2024, 9:11 pm   #328
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Quote:
Pin 38 of U9 - when the carriage is not home reads 1.9V. When the carriage is homed, -0.87V. I'll get my scope out.
Yes, we think there should be a waveform when the home sensor is in one state, probably no waveform, but possibly a different waveform, when the home sensor is in the other state.
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Old 4th May 2024, 9:29 pm   #329
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

This may be progress but I cannot be sure. I have fiddled/freed up some of the gears (there's quite a few) on the print carriage and now at switch on, the motor spins for c.6 seconds and then beeps consistently.

I'm printing a replacement gear right now but I'll put everything back and see if that's the printer trying to home itself.

The 10R resistor is no longer getting hot.

Colin.
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Old 4th May 2024, 9:55 pm   #330
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

OK, so if you refit the carriage motor and switch the machine on with the carriage initially at the mid position does it now successfully drive to the home position and beep once? (maybe it only beeps when it tries to do something, and fails).

Edit: sorry, not sure if I missed an edit from you but I see you are about to try that.
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Old 4th May 2024, 10:00 pm   #331
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

No movement on the broken gear, although it's trying - I now need to fix that.

Colin.
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Old 5th May 2024, 12:47 am   #332
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

To answer some of the questions :

There is nothing in the hardware that is only used for one direction of rotation. It is just a matter of which order the signals are sequenced.

There doesn't seem to be a cover interlock switch. I guessed much earlier that one of the microswitches connected to J6 was for that, but no, both are for the paper loading system.

I wonder if it has to find the home position of the daisywheel at power-up too. That's the other sensor on the carriage, the wheel is driven by the stepper motor there. Maybe fiddling/freeing the gearing has allowed it to do that.

There are only 2 mechanism sensors -- carriage home and daisywheel home. There is nothing associated with the ribbon drive (the other motor on the carriage, the unit of 4 solenoids, and a lot of gears/clutches) or the paper feed (stepper motor on the main chassis connected to J2 on the main PCB)
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Old 5th May 2024, 10:32 am   #333
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Thanks Tony. Let's see if Colin can make a working gear, that may be the next breakthrough that's needed.
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Old 3rd Jun 2024, 9:05 pm   #334
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Apologies for the silence; other priorities I'm afraid (that I expect will carry on for a while).

I've not made much progress, but here's what I have done. I've made a new gear and shaft to replace the broken one.

When trying to test it by powering on the printer, the main motor gear that meshes with the new one moves a little bit trying to home the print head, but doesn't move nearly enough to get the print head homed.

I removed the new gear/shaft so the main motor is not meshed with any other gears and I get the same problem (ie the gear on the main motor spins a very small amount) so I don't believe the fault lies with the replacement gear.

I then took the same motor from the other Olympia and plugeed that into the pcb in place of the original Commodore one and get the same outcome (ie a small amount of movement at power on but not enough).

At all times, when I move the print head manually to the home position, I can see the main motor spinning when it hits the home sensor to move it into the correct position so I think that sensor is working ok.

Any good ideas?

Thanks.

Colin.
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Old 4th Jun 2024, 1:30 pm   #335
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

It's been a while so I think we need a re-cap.

In #329 you appeared to have made a breakthrough after freeing up some of the mechanism, reporting that after having done so the (dismounted) carriage drive motor would consistently spin in the correct direction to home the carriage for about 6 seconds and then the unit would beep and stop the motor, presumably due to failure to home the carriage within that time because of the non-working gear.

Is that still the case, or has something regressed again?

With the motor dismounted, are you able to make the carriage move in either direction by turning whatever the motor turns - or more correctly by hand-turning whatever the broken gear would normally turn, equally easily in both directions?

Just thinking about why the gear would have broken in the first place - simple plastic fatigue is one possibility but maybe the carriage slides were prone to going stiff, requiring more and more force to be transmitted through the gear, until eventually something had to give.

Last edited by SiriusHardware; 4th Jun 2024 at 1:36 pm.
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