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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 22nd Apr 2024, 6:39 pm   #241
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

I don't believe that here is any sensor triggering on the right hand side of the carriage.

I did try to apply some gentle pressure to the print head mechanism to push it towards the left hand side and the carriage does move a few inches towards the left on its own when the printer is powered on. However it does not complete the movement and also the resistor at the bottom left of the board gets very hot

I suppose it's possible that the cracked cog isn't helping but I don't think that's the current issue.

Colin.
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Old 22nd Apr 2024, 7:41 pm   #242
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

I'd remove the carriage motor mechanically but leave it connected to the logic board. Power up and now try moving the carriage by hand. Does the motor do anything? Does the motor seem to be 'locked' when the power is on?

I don't think there are any interlocks. The only possibility is the microswitches connected to J6, but those seem to be related to paper loading.

The beep is a good sign. It shows the processor is doing something, it's not being held in the reset state.
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Old 22nd Apr 2024, 9:09 pm   #243
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Removing the carriage motor done; when moving the carriage by hand the motor does nothing until the optical sensor is triggered then it 'judders' until the sensor is no longer triggered.

The motor remains locked unless the optical sensor is triggered.

I have done some judicious fiddling with the wiring/J1 connector and I can get the motor to spin a little sometimes at power-on which makes me think that the connectors are no making good connections (too many connectors).

I've tried a bit of deoxit and wiggling but I may think of replacing the J1 connector completely. Is there something I need to measure up to ensure I get the right connector? I have tried putting some DuPont jumper wires on and they fit perfectly if that helps.

Colin.
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Old 22nd Apr 2024, 10:49 pm   #244
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

What sort of connectors are we talking about - IDC (pin row) connectors, either single or double row?

If you've only got a ruler or other measuring instrument with a resolution of no better than 1mm, measure the distance between the centres of the two outermost pins in a row and divide that measurement by (number of pins in a row -1). (Where you have 'n' pins in a row there are (n-1) spaces between the 'n' pins, and it's the size of the space between two adjacent pins that you actually want).

Once you've arrived at the spacing between adjacent pins (= pin pitch), then you can go looking for actual connectors which have a pin pitch equal to or very close to the pitch of your original connector.

I would tend to try to investigate the connections by attempting to measure the through resistance of each connector connection - you might be able to identify just one connection which is bad and try to do something about it, for example by temporarily bypassing it with a jump wire as you did when you had that problem with the power supply connector on one of your PETs.
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Old 22nd Apr 2024, 11:06 pm   #245
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Default Re: Commodore 8026 printer / typewriter

Is it possible that one or more of the stepper motor drivers is short circuit? This would lock the motor and possible explain the juddering as the motor is pulled back to the step that is locked.
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Old 22nd Apr 2024, 11:27 pm   #246
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

I take it the resistor which gets hot is the same one as before, the 10R associated with Q3. I'm wondering if one of the carriage motor sink transistors (Q7, Q8, Q9, Q10) has gone short-circuit collector-emitter and is keeping one part of the motor energised whenever Q3 is turned on.

We had Colin measure Q7 through to Q10 from base-emitter and base-collector but we neglected to ask for a check between collector-emitter for each of those transistors as well.

Edit: Cross posted with Mark.
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Old 23rd Apr 2024, 4:41 am   #247
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

I think that the fact that the motor does something means the processor is running which is a good sign.

I would expect the motor to be locked. The stepper motor common (all the pink wires on J1) is fed from +9V via a diode and +36V via Q3, etc. The latter is used when the motor is moving, the former just to hold the motor when the carriage is not to move. At all times I would expect 2 of Q7-Q10 to be turned on to energise the windings.

With the unit turned off, but with J1 (at least) connected, measure the resistance between the collector of Q3 and the collectors of Q7-Q10 in turn. Do they all give about the same resistance? (about 8 or 9 ohms?)
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Old 23rd Apr 2024, 8:07 am   #248
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

In addition to the above checks requested by Tony, could you (Colin) please also disconnect the carriage motor from the PCB and check Q7, Q8, Q9, Q10 for possible short-circuits or low resistance between collector and emitter: Also check the diodes which are connected to the collectors of those transistors.
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Old 23rd Apr 2024, 5:21 pm   #249
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

No shorts on any collector-emitter pairing from any of Q7 through Q10.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
I take it the resistor which gets hot is the same one as before, the 10R associated with Q3. I'm wondering if one of the carriage motor sink transistors (Q7, Q8, Q9, Q10) has gone short-circuit collector-emitter and is keeping one part of the motor energised whenever Q3 is turned on.

We had Colin measure Q7 through to Q10 from base-emitter and base-collector but we neglected to ask for a check between collector-emitter for each of those transistors as well.

Edit: Cross posted with Mark.
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Old 23rd Apr 2024, 5:26 pm   #250
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

Q3 Collector -> Q7 collector - 9.4 ohms
Q3 Collector -> Q8 collector - 9.5 ohms
Q3 Collector -> Q9 collector - 9.4 ohms
Q3 Collector -> Q10 collector - 9.5 ohms

Colin.

Quote:
Originally Posted by TonyDuell View Post
I think that the fact that the motor does something means the processor is running which is a good sign.

I would expect the motor to be locked. The stepper motor common (all the pink wires on J1) is fed from +9V via a diode and +36V via Q3, etc. The latter is used when the motor is moving, the former just to hold the motor when the carriage is not to move. At all times I would expect 2 of Q7-Q10 to be turned on to energise the windings.

With the unit turned off, but with J1 (at least) connected, measure the resistance between the collector of Q3 and the collectors of Q7-Q10 in turn. Do they all give about the same resistance? (about 8 or 9 ohms?)
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Old 23rd Apr 2024, 5:35 pm   #251
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

No shorts.

Resistance values below:

Q7 C->E - 2.6M ohms
Q8 C->E - 2.7M ohms
Q9 C->E - 2.7M ohms
Q10 C->E - 2.4M ohms

Sorry - here's my inexperience again; what checks need to be carried out on the diodes please?

Colin.


Quote:
Originally Posted by SiriusHardware View Post
In addition to the above checks requested by Tony, could you (Colin) please also disconnect the carriage motor from the PCB and check Q7, Q8, Q9, Q10 for possible short-circuits or low resistance between collector and emitter: Also check the diodes which are connected to the collectors of those transistors.
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Old 23rd Apr 2024, 6:15 pm   #252
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

The transistors certainly seem OK or at least not s/c.

Quote:
what checks need to be carried out on the diodes please?
There are four identical ones, so the main task is to see that they all read identically, more or less. As well as measuring their resistance both ways - a low resistance in single figures or double figures would be cause for alarm - use the 'Diode Test' feature of the meter to check the forward voltage drop, which would usually be in the region of 0.6-0.7V, although that may vary if you are measuring them in-circuit.
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Old 23rd Apr 2024, 6:23 pm   #253
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

So (This is mainly a question for Tony) - if the carriage is not already at the 'home' position, as indicated by the home sensor not detecting whatever it detects, then at switch-on I would expect the motor to rotate smoothly in a single direction until the carriage activates the 'home' position sensor. Then I would expect the motor to stop, held only by the 'holding' voltage which passes through those diodes from the 9V rail.

Is that the behaviour we are expecting, and in that case should we expect to see a continuous train of drive pulses from pins 33, 32, 31, 30 and passing through the four corresponding gates of U1 through to Q7, Q8, Q9, Q10 as U9 attempts to step the motor steadily in one direction until it sees the 'home' sensor activate?

Colin, you could have a look at that if you like - what activity or states do you see (with your scope) on those four U9 pins (33, 32, 31, 30) immediately after power-on, and if you do see activity is it continuous?

Last edited by SiriusHardware; 23rd Apr 2024 at 6:37 pm.
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Old 23rd Apr 2024, 6:29 pm   #254
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

That is what I'd expect to happen.

The home sensor is a slotted optoswitch on the underside of the carriage which detects a bit of metal on the chassis frame when the carriage is at the left side.

The microprocessor firmware mght time out and stop the motor if it doesn't detect 'home' after a certain number of steps. But you certainly should see it run for a time.
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Old 23rd Apr 2024, 6:33 pm   #255
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Maybe we should also check that the 'home' sensor only produces an output when the carriage is actually in the 'home' position, as well. If it is (incorrectly) outputting a continuous 'homed' signal regardless of the position of the carriage, then of course U9 will not try to move the carriage, thinking it is already 'homed'.
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Old 23rd Apr 2024, 6:38 pm   #256
TonyDuell
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Default Re: Commodore 8026 printer / typewriter

Yes, what do you see on the Interrupt (int) pin of the microcontroller when the carriage is at the left side and when it is moved away from there?
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Old 23rd Apr 2024, 7:28 pm   #257
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

OK - all measurements in circuit with nothing connected to the bard.

Three of the diodes read 0.64V in one direction and 0V in the other.

The diode that is physically between Q7 and Q8 reads 0.64V in one direction and 1.24V in the other,

Is that an issue?

Colin.

Quote:
Originally Posted by SiriusHardware View Post
The transistors certainly seem OK or at least not s/c.

Quote:
what checks need to be carried out on the diodes please?
There are four identical ones, so the main task is to see that they all read identically, more or less. As well as measuring their resistance both ways - a low resistance in single figures or double figures would be cause for alarm - use the 'Diode Test' feature of the meter to check the forward voltage drop, which would usually be in the region of 0.6-0.7V, although that may vary if you are measuring them in-circuit.
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Old 23rd Apr 2024, 7:32 pm   #258
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Quote:
Three of the diodes read 0.64V in one direction and 0V in the other.
I'm thinking the '0V' in your figures really means 'OVerload' or 'out of range', in other words, higher than the meter can measure. That one different reading is interesting though, it could lead us to something.

Which 'Q's collector is that odd-man-out diode connected to? Can you disconnect one end of that diode and measure it again using 'diode test'?

(Carry on and do the other checks while we are considering this).
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Old 24th Apr 2024, 8:27 am   #259
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

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.

Last edited by SiriusHardware; 24th Apr 2024 at 8:39 am.
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Old 24th Apr 2024, 8:52 am   #260
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Question for Tony: The 'Home' sensor circuit is slightly more complicated than I anticipated - Q3 / Q4 looks like a transistor multivibrator circuit influenced by the 'home' sensor, so it looks as though, rather than generating a static 'high' or 'low' signal to feed to the INT input, it will generate a pulse train. Can you walk us through the operation of that part?
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