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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 11th Apr 2024, 10:27 am   #161
SiriusHardware
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Default Re: Commodore 8026 printer / typewriter

Despite the smoke the resistor may have survived - it's a 10R, so measure it in-circuit first and if you see 10R or less it is probably OK. If you see significantly higher than 10R then it may be damaged.

'Concrete' 10R resistors like that are not hard to come by, although they are less commonly found with that built-in thermal fuse action, as Graham described.

We do have a problem whereby the 10R resistor and its associated circuitry appear to have nothing to do with the BC327 and its associated circuitry. We need to have some idea of where all the current passing through Q3 is actually going, if there is currently no carriage motor load connected to it.
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Old 11th Apr 2024, 10:46 am   #162
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Default Re: Commodore 8026 printer / typewriter

In circuit, it's 9.9 ohms.

Colin.
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Old 11th Apr 2024, 10:47 am   #163
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Default Re: Commodore 8026 printer / typewriter

Thanks - so if the solder blob is still there then I'm in with a fighting chance of that component being OK.

As per my other message, it measures 9.9 ohms in circuit right now.

Colin.

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Originally Posted by Station X View Post
The idea is that if the resistor overheats the solder blob melts and the wire springs away breaking the circuit.
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Old 11th Apr 2024, 12:05 pm   #164
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Default Re: Commodore 8026 printer / typewriter

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Originally Posted by ScottishColin View Post
While I work on this later on today, the resistor that gave me magic smoke is marked as

212-4 10R 10% Z

Photo attached.

I've never seen one like this before - can anyone give me any details of this component please?

The component next to it is marked the same with a Y at the end; does that make any difference?

Colin.

I'd somehow missed that these had this built-in 'fuse'.
(I have previously encountered similar ones sometimes used in Philips etc TV's back in the 70's, where 'special' Low Melting Point (LMP) solder was used, to ensure it melted and the connection sprung-open).

But these 'VTM' make ones are being quite widely sold on eBay, if you do have one that fails completely, and not able to solder back together.

And it looks like these 'Resistor Circuit-Breaker' types are still being made, plus that they had a KF or KT prefix before the 212-4 series number, depending whether they were axial or vertical mounting: https://www.vtm.co.uk/PDF/Vitrohm/Vitrohm%20KF-KT.pdf

Although they may now have a KFF-prefix, unless these are different: https://www.vitrohm.com/content/file...-datasheet.pdf

(Also attached both here)

It seems 'VTM' have been acquired by Yageo, but like many resistor / passive company acquisitions/mergers, they may still be using their original branding and part numbers.

If these are getting very hot with no mechanism connected, then it seems like that's another thing to aim your IR thermal camera at , when (briefly) powering-up again to see if BC327 getting very-hot issue has now been resolved (BTW, even if the old one still works, it may have been rather stressed by the overheating. So - if you do have a few more new ones - putting another new one in may be advisable for best future reliability).
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File Type: pdf vitrohm-kff-wirewound-fusible-resistor-datasheet.pdf (755.0 KB, 30 views)
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Old 11th Apr 2024, 12:15 pm   #165
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Default Re: Commodore 8026 printer / typewriter

New 327 soldered in. Checked for any stray solder splashes and can find none.

Plugged back into the earth wire and the PSU connector. No other connectors are attached to the board.

Sadly, after about 20 seconds, the new 327 again goes up to 200 degrees on my thermal camera.

No obvious heat on the resistors, and the ICs are warming up but not horribly. I can get temperatures of them if required any time.

Colin.
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Old 11th Apr 2024, 2:27 pm   #166
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Default Re: Commodore 8026 printer / typewriter

Try this: Remove the 327 and refit it with the B and E legs in place, but the C pin cranked out sideways. Use whatever means you have to connect your meter, set to Amps, between the transistor C lead and the pad the C lead normally goes to. 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.

Switch on, note the current being indicated on the meter, then switch off.
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Old 11th Apr 2024, 3:26 pm   #167
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Default Re: Commodore 8026 printer / typewriter

Forgot to say, if at all possible, make these meter connections to the 327 C lead and its pad 'hands free' using crop clip leads or whatever - don't try to depend on your ability to keep the meter probes on points A and B, or one of the probes is bound to slip off and hit something it shouldn't.

I would do the same if it was me trying to measure the current.
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Old 11th Apr 2024, 4:18 pm   #168
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Default Re: Commodore 8026 printer / typewriter

Some more thoughts.

The +5VSW line (+5 volts switched) from the collector of this transistor goes to the IEEE-488 board too. It's used to reset the control IC. But on that board there's also a 22uF capacitor from +5VSW to ground. I think it might be a tantalum thing and could well be shorted.

The 5V supply rail comes from a 78S05 regulator in the power supply. So you're not going to be able to get much more than 2A from it. Makes things a little easier.

When I had the boards here, I desoldered one end of a diode in the battery circuit. If the charging circuit is complete with no batteries fitted then one port pin on the processor will get 9V applied to it. I hope you've not refitted this!

Anyway, I'd remove the processor and the GPIB chip -- both are socketed -- mainly because they would be hard to replace. Then try to find out what is drawing excessive current.
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Old 11th Apr 2024, 8:11 pm   #169
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Default Re: Commodore 8026 printer / typewriter

All unobtanium chips removed as suggested.

I'm not sure this makes any sense - I got a reading of 0.29mA with the test in #166. I took a quick look at voltages while I was there and it was 4.7V.

I suppose it's worth noting that the two chips are removed and the Collector line of the 327 seems to have continuity to pin 17 of the processor socket.

Colin.
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Old 11th Apr 2024, 8:23 pm   #170
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Default Re: Commodore 8026 printer / typewriter

I'm not sure this has gone according to plan.

Even with all known loads apart from the 120R resistor removed from the collector circuit I would expect the collector current to be something like 40mA because of the presence of the 120R resistor.

The current was definitely 0.29mA, and not 0.29A?
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Old 11th Apr 2024, 8:32 pm   #171
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Default Re: Commodore 8026 printer / typewriter

I've left that end of the diode disconnected.

Colin.

Quote:
Originally Posted by TonyDuell View Post
Some more thoughts.

The +5VSW line (+5 volts switched) from the collector of this transistor goes to the IEEE-488 board too. It's used to reset the control IC. But on that board there's also a 22uF capacitor from +5VSW to ground. I think it might be a tantalum thing and could well be shorted.

The 5V supply rail comes from a 78S05 regulator in the power supply. So you're not going to be able to get much more than 2A from it. Makes things a little easier.

When I had the boards here, I desoldered one end of a diode in the battery circuit. If the charging circuit is complete with no batteries fitted then one port pin on the processor will get 9V applied to it. I hope you've not refitted this!

Anyway, I'd remove the processor and the GPIB chip -- both are socketed -- mainly because they would be hard to replace. Then try to find out what is drawing excessive current.
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Old 11th Apr 2024, 8:33 pm   #172
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Default Re: Commodore 8026 printer / typewriter

No - nor I.

I've re-done it - 0.1 A, 0.51mA.

This doesn't make sense does it and it was a nice plan too.

Colin.


Quote:
Originally Posted by SiriusHardware View Post
I'm not sure this has gone according to plan.

Even with all known loads apart from the 120R resistor removed from the collector circuit I would expect the collector current to be something like 40mA because of the presence of the 120R resistor.

The current was definitely 0.29mA, and not 0.29A?
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Old 11th Apr 2024, 8:40 pm   #173
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Default Re: Commodore 8026 printer / typewriter

Quote:
I've re-done it - 0.1 A, 0.51mA.
Those are very different values, one is one-tenth of an Amp, which I could believe, the other is just over half a milliamp. What is changing, between you making those two measurements?
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Old 11th Apr 2024, 8:44 pm   #174
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Default Re: Commodore 8026 printer / typewriter

Here's the photos of my meter and the dial settings. All I'm doing is going from one dial setting to another. Am I mis-using my meter?

Colin.
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Old 11th Apr 2024, 9:00 pm   #175
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Default Re: Commodore 8026 printer / typewriter

I think you've got the red meter lead plugged into the wrong socket. It needs to go into the 10A or mA socket depending on the expected current.

I can't read the writing obscured by the red lead though.
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Old 11th Apr 2024, 10:05 pm   #176
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Default Re: Commodore 8026 printer / typewriter

Got it thanks.

It's almost like I don't know what I'm doing sometimes....

Colin.
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Old 12th Apr 2024, 12:09 am   #177
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Default Re: Commodore 8026 printer / typewriter

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, 1:41 am   #178
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Default Re: Commodore 8026 printer / typewriter

Yes, there won't be much of a current flow between the (10M resistance to Common) 'Volts' input socket and the 'Common' one.
- Although at least there will be less things getting hot!
(I'm surprised that << 120R to ground hasn't been measured from BC327 collector to ground, if there is a fault. But maybe it isn't a permanent short, and just an IC drawing far too much current when enough voltage is applied. A capacitor - like the previously mentioned Tantalum - could also do this, if it only breaks-down above a certain voltage)

A look round with the IR thermal camera, may spot where current is going, but the more IC's you can remove / cables to other boards disconnected the better in terms of isolating where fault lies.


You do also have to be careful when using the mA's input, as often that input is fused at quite a low value - maybe only a few 100mA. And more-expensive meters like Fluke often use large v.expensive (>£10 each!) fuses in these (often 1A?).
In many cheaper DMM's, the (10)Amps input socket is unfused, but again Fluke's etc tend to have a 10A etc. v.expensive fuse on that as well.
Although at least Fluke's often detect and remind you that you're using the wrong socket, by beeping at you - but usually just a bit annoying when changing between Amps and Volts, knowing you need to change the red lead position. I seem to recall another make doing something mechanical with shutters across the sockets , controlled by the function rotary control.
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Old 12th Apr 2024, 10:14 am   #179
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Default Re: Commodore 8026 printer / typewriter

Quote:
(I'm surprised that << 120R to ground hasn't been measured from BC327 collector to ground, if there is a fault.
The in-circuit resistance of the 120R resistor, in parallel with the other loads which were still present at the time, has been measured at 109R (Post #148).
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Old 12th Apr 2024, 11:20 am   #180
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Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by SiriusHardware View Post
Quote:
(I'm surprised that << 120R to ground hasn't been measured from BC327 collector to ground, if there is a fault.
The in-circuit resistance of the 120R resistor, in parallel with the other loads which were still present at the time, has been measured at 109R (Post #148).
Well that isn't really much less - nearly within tolerances of the resistor.
So it doesn't seem like there is a permanent near-short and that there is more likely an active component that is producing an excess-current (possibly with an overload on on output from that, so not directly on the +5Vsw rail)

But maybe once the current is correctly measured (briefly, to prevent further damage) / some of the loads on +5Vsw isolated from it, then it should be easier to track-down. And it maybe worth temporarily removing that Tantalum capacitor, knowing how prone these are to breaking-down / if it is on a separate board, then disconnecting its supply.
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