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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 13th Apr 2024, 11:39 pm   #201
ortek_service
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Default Re: Commodore 8026 printer / typewriter

Well if you have a look at the (TI at least) datasheet for the 7404: https://www.ti.com/lit/ds/symlink/sn74ls04.pdf

Then it doesn't seem possible for it to draw the amount of current it was drawing, even with all 6 outputs shorted - Which is not recommended.

Max recommended current output current is also only 400uA (sourcing) / 16mA (sinking). So it would seem to be rather bad practice to connect the outputs directly to the base of transistors (even if they have some emitter resistance, and high-level output voltage is typically 3.5V, rather than 5V even with no load, plus 7404's internal resistance will limit output current - especially when sourcing which does have resistors in-series with supply to output-high transistors)

If any of the transistors it is driving have gone short to base (especially from a higher than 5V supply voltage), then it's possibly they could damage the 7404 driving them.
So it may be worth checking these, before fitting another 7404 (although they are rather common / cheap, so hopefully will have ordered a few spares - maybe (un?)lucky not to have had to change one in the other Commodores so far, to already have some spare).
And it does seem like these Commodores do seem to like having many faulty parts (but not often directly-connected to each other)
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Old 14th Apr 2024, 11:48 am   #202
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Default Re: Commodore 8026 printer / typewriter

So here's the current situation and my plan while I wait for the (second-class) delivery of the 7404s (I bought two just in case).

I currently have the following not in place on the boards:

The purple wire to the IEEE board
A capacitor on the IEEE board
A capacitor on the main board close to the 327
The collector leg of the 327
U1 (7404) and U9 on the main board
U1 on the IEEE board
One leg of a diode that was removed by Tony earlier in the process

I'll put back the purple wire and the two capacitors one by one, measuring the effects of each one as I go.

Then I'll carry out the test from post 200 later on today/tonight.

I'm getting the vibes that the 7404 may not actually be the problem so are there any other components I can test while I wait for the postie?

Colin.
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Old 14th Apr 2024, 1:17 pm   #203
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Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by ScottishColin View Post
>>
>>
I'm getting the vibes that the 7404 may not actually be the problem so are there any other components I can test while I wait for the postie?

Colin.
I think the 7404 is definitely faulty, from the amount of current that it was drawing
- as suggested, you could verify this by putting it on a breadboard and taking some measurements (particular its supply current).

But there was a felling it may not be the only problem (as seem typical with these old Commodores!), and something may have damaged it.
- rather than most of us thinking it was not actually faulty itself.

It's probably not a bad idea to refit / reconnect everything else, and check that it now runs OK - with nothing now getting hot - without the 7404 fitted. You may then be able to test some other things / see if mechanism can be driven (other than by the transistors driven by the 7404)
- although it does depend on what the other 7404 gates do, that didn't drive the output transistors and how vital they are for the processor-side etc to run.

It may also be worth checking the transistors that were driven by the 7404, to ensure none have failed - particularly short-circuit.
With the 7404 removed, and mechanism disconnected, then it may be possible to check these in-circuit with your new multi-function tester.
But if they don't measure OK, then isolate all 3 pins from the circuit on one, and re-test it, just to be sure.
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Old 14th Apr 2024, 3:35 pm   #204
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Default Re: Commodore 8026 printer / typewriter

Two of the six gates in the 7404 are not used, the other four directly drive the bases of Q7 through Q10, unusually without series resistors between the 7404 outputs and the transistor bases, although Tony has confirmed that the diagram for the standard version of this Typewriter shares that arrangement. Certainly Colin could check, while U1 is out, that none of Q7, 8,9,10 are short-circuit or low resistance from base to emitter.
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Old 14th Apr 2024, 4:26 pm   #205
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Default Re: Commodore 8026 printer / typewriter

The TIP112s used for Q7 through Q10 appear to be Darlingtons as I thought they might be - the datasheet doesn't show any integral base series resistor, so it does seem as though the upper output voltage from each 7404 gate will be clamped to about 1.2V maximum. Presumably, the 7404 can normally tolerate that.
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Old 14th Apr 2024, 4:43 pm   #206
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Default Re: Commodore 8026 printer / typewriter

This reminds me of the MK14 use of 7408 to direct drive the segments of the led display, relying only on the internal resistance of the 7408 to limit the current. Forward voltage of the darlington transistors would be similar, slightly lower than the forward voltage of older leds.
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Old 14th Apr 2024, 4:49 pm   #207
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Default Re: Commodore 8026 printer / typewriter

I never really liked that either, but in that particular case it is mitigated by the fact that the LED display is multiplexed so each display cell is only energised for a relatively small amount of time. I don't know if the same can be said of the carriage motor in this, is it only energised for short periods of time?
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Old 14th Apr 2024, 5:00 pm   #208
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Default Re: Commodore 8026 printer / typewriter

None of Q7, Q8, Q9 or Q10 are short or low resistance from base to emitter.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
Two of the six gates in the 7404 are not used, the other four directly drive the bases of Q7 through Q10, unusually without series resistors between the 7404 outputs and the transistor bases, although Tony has confirmed that the diagram for the standard version of this Typewriter shares that arrangement. Certainly Colin could check, while U1 is out, that none of Q7, 8,9,10 are short-circuit or low resistance from base to emitter.
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Old 14th Apr 2024, 5:07 pm   #209
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Default Re: Commodore 8026 printer / typewriter

OK, if you want to (possibly) reassure yourself that U1 really is faulty and worth having sourced a replacement for, try the resistance measurements outlined in #200. No need to post detailed results but if you find that any input or any output measures differently from the other, identical input or output pins of the device, that would be of interest.
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Old 14th Apr 2024, 5:26 pm   #210
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Default Re: Commodore 8026 printer / typewriter

All of the following reconnected/reinserted one by one and the Amps measurement stays at 0.088A (including putting back in the IEEE IC and the main board processor).

I have removed the IEEE IC and the main processor again just in case.

Testing as per post 200 next.

Colin.
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Old 14th Apr 2024, 5:45 pm   #211
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Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by ScottishColin View Post
None of Q7, Q8, Q9 or Q10 are short or low resistance from base to emitter.

Colin.
Can you also check than none are shorted base-collector.

The TIP122 is indeed a darlington. I thought it was and confirmed it in Tower's when I drew out the schematics. The double emitter arrow is the way I learned to draw such a component.
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Old 14th Apr 2024, 5:56 pm   #212
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Default Re: Commodore 8026 printer / typewriter

I've honestly never come across that before but will remember it in future.

I normally draw out the Darlington pair in full, with a circle around them to indicate that they are in one package (as per attached, found online).
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Old 14th Apr 2024, 5:59 pm   #213
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Default Re: Commodore 8026 printer / typewriter

No shorts B->C on any of those components (Q7->Q18 tested).

Colin.

Quote:
Originally Posted by TonyDuell View Post
Quote:
Originally Posted by ScottishColin View Post
None of Q7, Q8, Q9 or Q10 are short or low resistance from base to emitter.

Colin.
Can you also check than none are shorted base-collector.

The TIP122 is indeed a darlington. I thought it was and confirmed it in Tower's when I drew out the schematics. The double emitter arrow is the way I learned to draw such a component.
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Old 14th Apr 2024, 6:03 pm   #214
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Default Re: Commodore 8026 printer / typewriter

OK, if you could do those monotonous resistance tests on the 7404 out of circuit?
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Old 15th Apr 2024, 12:09 pm   #215
ScottishColin
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Default Re: Commodore 8026 printer / typewriter

I'm finding it very difficult to get consistent readings, especially for the first set of pin 14 tests suggested.

Here's the best I can get after several attempts and lots of cursing. They're not good imho. All readings are M ohms

With the red probe on pin 14 of the chip, measure the resistances from there to:
Code:
Pins 1,   3,   5,   9,   11,  13
     7.5, 5.3, 5.5, 3.0, 2.7, 1.2

Pins 2,   4,   6,   8,   10,  12 
     1.6, 1.6, 1.5, 1.5, 1.4, 1.2

With the black probe on pin 7 of the chip, measure the resistances from there to:
Code:
Pins 1,    3,    5,   9,    11,   13
     18.8, 12.7, 7.3, 12.7, 11.1, 4.7

Pins 2,    4,   6,   8,   10,  12
     15.8, 9.8, 4.4, 3.2, 3.6, 3.0
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Old 15th Apr 2024, 12:23 pm   #216
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Default Re: Commodore 8026 printer / typewriter

Well, if those readings in the above post are consistent then that's good(?) because, as stated earlier, all the readings in line 1, should be virtually the same, all the readings in line 2 should be virtually the same, same for lines 3 and 4.

Because they aren't, I think it is reasonable to think that some elements of the 7404 are faulty.

Only one caution: When making these measurements did you take reasonable care to make sure that you were not inadvertently measuring yourself? If your fingers were touching the metal parts of the probes when you were trying to keep the probe tips on the pins, you would be part of the result.

If you are sufficiently interested, try the same sequence of measurements on one of your known good replacement 7404s when they arrive to see if what I suggested (that all inputs should read similarly and all outputs should read similarly with respect to the +ve and 0V supply pins) does hold true.
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Old 15th Apr 2024, 12:42 pm   #217
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Default Re: Commodore 8026 printer / typewriter

No fingers involved.

I guess I'll wait for the postie now.

Colin.
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Old 15th Apr 2024, 11:59 pm   #218
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Default Re: Commodore 8026 printer / typewriter

Well, whilst waiting, you could always try applying 5V to the removed 7404 (Pin 14 = +5V, Pin 7 = 0V) to see if it does still draw lots of current with nothing connected to its outputs. I reckon it will
- Possibly drawing even more current than in-circuit when the BC327 was probably limiting the current to it due to lack of enough base-current
(hence probably why that was getting hot, as there was a significant voltage drop when the BC327 is not fully-driven into saturation)
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Old 16th Apr 2024, 12:26 am   #219
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Default Re: Commodore 8026 printer / typewriter

I'm not sure there is an independent fixed 5V or variable voltage supply available for this purpose. The most likely 5V regulated supply that anyone would have lying about would be a a 5V USB charger / supply but that would only be usable if broken out onto screw terminals or bare wire ends.

With the arrival of the replacement 7404s surely now imminent, this may all be almost academic as we'll soon find out whether the original 7404 was responsible for the excess current load.
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Old 16th Apr 2024, 10:20 am   #220
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Default Re: Commodore 8026 printer / typewriter

Well I was thinking that the Commodore 8026's 5V supply - before the BC327 so on permanent +5V - could always be tapped out for this, if no other option.

Its (78S05?) regulator should be overload protected, although I think the overload current limit on these is typically rather more than the normal rated max. current.

Doing this would prove in advance that the 7404 itself had gone permanently faulty (rather than anything on the outputs affecting its supply current) as I have a feeling is the case. And hopefully a new 7404 should resolve this fault at least.
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