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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 12th Apr 2024, 12:14 pm   #181
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

I think this is the first time in my time here that I've been measuring amps so apologies and I've learnt something.

On the A setting, I get a reading of 0.72A.

Colin.


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Originally Posted by SiriusHardware View Post
I did say, a few posts back...

Quote:
Note that your meter will usually have a separate 'Amps' input - sometimes one for Amps and one for mA - which you will need to move the red lead over to in order to measure current, and it will need to be set to the corresponding DC Amps / DC milliamps range.
Try again - first with the red probe in the 'A' socket and with the range switch set to DC Amps - if the measured current is half an amp or less, move the red lead to the 'mA' input and set the range switch to DC mA for a more exact readout of the current.

Afterwards, don't forget to move the red lead back to the 'normal' socket input before attempting to make any other type of measurements, especially voltage.
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Old 12th Apr 2024, 2:09 pm   #182
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Wow, that's high. No wonder that 327 is getting a bit hot under the collar. Can you find the capacitor Tony mentioned, lift one end of it and see if that reduces the current?
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Old 12th Apr 2024, 4:55 pm   #183
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Default Re: Commodore 8026 printer / typewriter

Capacitor leg lifted, but still reading 0.72A.

Colin.
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Old 12th Apr 2024, 4:58 pm   #184
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Can you trace the 5VSW line leading away from the 327 collector lead and identify / note down every single component or component socket and / or connector that it goes to? That current has to be going somewhere, through something.
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Old 12th Apr 2024, 5:13 pm   #185
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Default Re: Commodore 8026 printer / typewriter

With my meter, I get a positive continuity test to the top leg of the capacitor marked with a red arrow on the attached picture.

Should I lift leg that and re-test?

Colin.
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Old 12th Apr 2024, 5:25 pm   #186
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Default Re: Commodore 8026 printer / typewriter

These are at least some of the things connected to +5VSW

Vcc of U1 (7404, used to drive the carriage motor driver transistors)
Vcc of U3 (ULN2068, Ribbon control solenoid driver)
Vcc of U4 (74LS259)
Reset pin of the processor (U9) -- but that is currently removed
270 ohm base pull-up resistor on Q22 (Hammer driver)
390 ohm base pull-up resistor on Q21 (Ribbon motor driver)
Clr/ pin of U6 (8212 latch, used to drive the feed and daisywheel motor drivers)

MR/ pin of U1 _on the GPIB PCB_
2k2 pull-up RP1f on the GPIB PCB
22uF capacitor to ground on the GPIB PCB

You could try desoldering the purple wire in the ribbon cable between the 2 boards to isolate the last 3 parts
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Old 12th Apr 2024, 5:28 pm   #187
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Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by ScottishColin View Post
With my meter, I get a positive continuity test to the top leg of the capacitor marked with a red arrow on the attached picture.

Should I lift leg that and re-test?

Colin.
Yes, it can't do any harm....
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Old 12th Apr 2024, 6:47 pm   #188
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

If that doesn't reduce the current, may I ask for a breakdown of which components and other circuit elements listed in #186 are are NOT present at this time?
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Old 12th Apr 2024, 6:58 pm   #189
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Default Re: Commodore 8026 printer / typewriter

Purple wire removed as per post 186 with no change.

Capacitor removed as per the photo in post 185 with no change.

No connectors to any wiring in the print carriage are connected.

So by my reckoning that leaves U1, U3 and U4 from that list.

Colin.
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Old 12th Apr 2024, 7:05 pm   #190
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Default Re: Commodore 8026 printer / typewriter

If you are OK with removing ICs in that PCB, as I am sure you are, I would start with the smallest one (fewest pins to desolder, easiest to remove) and work up through the bigger ones. Is U6 not also fitted, or is that already removed?
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Old 12th Apr 2024, 10:46 pm   #191
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Default Re: Commodore 8026 printer / typewriter

You're right - U6 is still there.

I'll get onto it tomorrow. I'll draw lots out of a hat to see which one to start with. U2 might be a start as it has green legs from the battery...

Colin.


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Originally Posted by SiriusHardware View Post
If you are OK with removing ICs in that PCB, as I am sure you are, I would start with the smallest one (fewest pins to desolder, easiest to remove) and work up through the bigger ones. Is U6 not also fitted, or is that already removed?
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Old 13th Apr 2024, 3:32 am   #192
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Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by ScottishColin View Post
I think this is the first time in my time here that I've been measuring amps so apologies and I've learnt something.

On the A setting, I get a reading of 0.72A.

Colin.
>>
>>

Quote:
Originally Posted by SiriusHardware View Post
Wow, that's high. No wonder that 327 is getting a bit hot under the collar. Can you find the capacitor Tony mentioned, lift one end of it and see if that reduces the current?
Well it is a little-high, however BC327's are rated for 500mA / 800mA (depending which make's datasheet you read) collector-current.
So in theory, they should still be able to pass that without getting very-hot.
- Providing they have enough base-current to ensure they are operating fully-saturated with very little voltage drop between emitter & collector
(The datasheet spec's Vce to be < 0.7V @ Ic=500mA and Ib=50mA - as hFE often reduces dramatically at higher collector currents)

Although nearly 1V across the transistor would still keep power @ 0.7A to 0.625W max power rating
- at +25degC (So would maybe require a clip-on heatsink to handle that continuously)

But maybe base-current isn't high-enough in this case for it to handle that collector current
- Which most-likely is rather higher than it should be, otherwise the BC327 transistor would be very-marginal.

That 0.72A measured may actually be higher without the Ammeter in-series with BC327 'Output'
- which (along with leads) will drop a bit of voltage that could reduce current drawn by 'linear' loads)

But you could just temporarily short out (collector to emitter) the BC327, so that it doesn't get hot. And then use thermal camera to see what is getting hot so is probably drawing an excess current.
- Although disconnecting as much as possible from the +5Vsw supply first isn't a bad idea - if it can be done by just removing IC's etc.in order to rule those out.

Last edited by ortek_service; 13th Apr 2024 at 3:40 am.
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Old 13th Apr 2024, 4:34 pm   #193
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Default Re: Commodore 8026 printer / typewriter

OK - U2 removed and no change in the Amp reading.

U1 removed and the Amps have dropped to 0.08A on the 10A reading.

Does this mean a replacement of U1 (7404) is the correct next step?

Colin.
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Old 13th Apr 2024, 4:40 pm   #194
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Default Re: Commodore 8026 printer / typewriter

Probably. My only concern would be if something driven by U1 was shorting, but I think that's unlikely. You want to use a 7404 here, not any other version. And fit a DIL socket
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Old 13th Apr 2024, 5:01 pm   #195
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Default Re: Commodore 8026 printer / typewriter

Ordered.

And if we could all ignore the fact that I took U2 out unnecessarily instead of U3, I'll be very grateful. Never happened.

Colin.
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Old 13th Apr 2024, 5:56 pm   #196
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Default Re: Commodore 8026 printer / typewriter

Well as a double-check, you could try just powering the original 7404 IC off a 5V supply (Pin 14=+5V, Pin7 = 0V), with all the other pins isolated (the inputs are effectively internally pulled-up on standard 74 / 74LS TTL, so don't need to worry too much about floating).
- You could put it on a bit of breadboard, and feed 5V to it from the unit via an Ammeter.

But even a standard non-LS 7404 should not be able to source 100's of mA's from its outputs, so I'd also say it's faulty.
- It will also probably be getting quite-warm with that amount of current going into it.
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Old 13th Apr 2024, 6:21 pm   #197
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Default Re: Commodore 8026 printer / typewriter

Part of me is thinking: Hooray! We've found the fault. The other part of me is remembering that there is never, ever, just one fault on one of Colin's Commodore gizmos. There is one mildly disturbing aspect of of this, which is that, with the power off, there wasn't a seriously low resistance from the 5VSW rail to 0V. This does lend some weight to the idea that maybe the 7404 was being forced to drive an excessive load, rather than being faulty in itself.

Is it just me, or is it weird that the outputs of the 7404 appear to go straight to the bases of Q7 through Q10 without resistors in series, as that will surely clamp the maximum voltage out from the 7404 gates to 0.6-0.7V or maybe 1.2V if they happen to be Darlingtons?

I've just been trying to see if a fault on U1 could account for that odd behaviour from the CMen / CMcomm driver Q3 and its associated resistor getting too hot - looks like two elements are not used and four are involved in controlling the transistors which sink current through the four different sections of the carriage motor.

I could only see this causing a problem if the carriage motor actually was connected (a faulty U1 could cause current to be drawn through all sections of the carriage motor at the same time by turning all of the sink transistors on at the same time regardless of the states of the control signals at the 7404 inputs).

I love that we are now able to muse and speculate over what the problem (and any potential problems) might be. This is only possible because of the work put in by Tony and Colin to reconstruct the circuit diagrams.
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Old 13th Apr 2024, 9:38 pm   #198
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Default Re: Commodore 8026 printer / typewriter

Colin pointed me (at least) to an Olympia ES100 schematic. Although it is not the same as the CBM8026 it also uses 7404 inverters directly driving the bases of the transistors. So guess it's OK.

I wonder if one of the transistors in a totem-pole output stage of said chip has shorted. If so, then it could easily pass excessive current, either through the other transistor when said other transistor was turned on or through the base-emitter junction of the load.

Yes, 4 of the 6 inverters are used as base drivers for the carriage motor driver transistors. The other 2 are not used, one has the input tied high, the other tied low (just because that was easiest on the PCB I guess). There are no sections used on other sheets of the schematic.
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Old 13th Apr 2024, 10:13 pm   #199
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Default Re: Commodore 8026 printer / typewriter

I'm very glad we've found a fault now, but I'm fairly sure given my track record this will be the first of several.

Colin.
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Old 13th Apr 2024, 10:30 pm   #200
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Well, while we await the arrival of the new 7404 you (Colin) could always put the original one in an experimenter breadboard - purely to avoid having to chase the chip around the bench with the meter probes - and measure the following resistances:

With the red probe on pin 14 of the chip, measure the resistances from there to:

Pins 1, 3, 5, 9, 11, 13 - all six should show about the same resistance.
Pins 2, 4, 6, 8, 10, 12 - all six should show about the same resistance.

With the black probe on pin 7 of the chip, measure the resistances from there to:

Pins 1, 3, 5, 9, 11, 13 - all six should show about the same resistance.
Pins 2, 4, 6, 8, 10, 12 - all six should show about the same resistance.

The reasoning behind this, applicable to most logic gate ICs, is that the chip is a 'box' containing more than one identical part (in this case, six identical parts). If any of the inputs measures significantly differently to the rest, or any of the outputs measures significantly differently to the rest, then either that element of the chip is dud and the rest are OK - or - less likely - that one element is OK and the rest are dud. The point is that they should all read the same on a healthy device.

If they do all read the same as expected that is still not an absolute guarantee that the chip is OK because the fault could be so far 'inside' the chip that it makes no measurable difference to the resistances at the pins.

But, if you do see one element which reads differently from the rest, then the chip is almost certainly dud.
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