Commodore 8026 printer / typewriter

[ScottishColin]

Yes - I did that while I was there. I await them having the time to get it out and look.

Also I have contacted what remains of Olympia and they have pout me in touch with someone who they recommend for their older typewriters.

Also I have asked someone on the forum64.de forum who has a contact who used to repair these printers.

The feelers are out.

Colin.

Originally Posted by SiriusHardware

If it's the same maybe you can also ask him to read the values of the capacitors you've replaced as well.

[ScottishColin]

All my leads have gone cold so I've bought one of these in the hope that it may help me identify that transistor so I can get a new one in and try to move forwards.

https://www.amazon.co.uk/dp/B0B7X4TYL2

Colin.

[ortek_service]

Originally Posted by ScottishColin

All my leads have gone cold so I've bought one of these in the hope that it may help me identify that transistor so I can get a new one in and try to move forwards.

https://www.amazon.co.uk/dp/B0B7X4TYL2

Colin.

Yes, I bought one of those (via eBay? from China) a few years ago - the successor to previous ones I'd got and often used to measure ESR of (usually faulty) Electrolytic capacitors.
Unfortunately the seller hadn't packaged mine very-well, and although outer-packing wasn't damaged when it arrived, the fragile display was cracked (despite being under the undamaged clear-plastic of the top cover).
I did eventually get a full refund from them (and didn't have to return it as that would have cost much more back to China), but it seems buying a new display for it would cost nearly as much as I'd paid for it.

However, with transistors, it probably won't really tell you much-more than a DMM with built-in transistor-tester - essentially the hFE 'DC cyrrent gain' for a Bipolar Junction Transistor.
- Although it does automatically work out the pinout for you.

IIRC, schematics etc. weren't available for this board. But it may be able to work out what's required from some voltage measurements / tracing-out some of the surrounding-circuitry.

[SiriusHardware]

It should also identify whether the transistor is NPN or PNP, but unfortunately it will still require you to add extensions to the leg stumps so you can put it in the tester - and if you can do that you can just re-fit the original transistor without even needing to know what it is (assuming that all that's wrong with it is that the legs have been eaten off).

[SiriusHardware]

What it won't do - in case you wondered - is to identify the device by its actual BCxxx part number. It will tell you whether it is NPN or PNP and will most likely tell you which lead is which and what the gain of the device is.

[ScottishColin]

Well, yes - you're a mind reader. I had rather hoped it would at least give me some clues towards the device number.

Have I wasted 20 quid here or will it take me at least a little way down the path of identifying the component?

When I get a bit more time, I'll try to draw the components around it and look for the voltages from those components to the transistor.

Will that help?

Colin.

Originally Posted by SiriusHardware

What it won't do - in case you wondered - is to identify the device by its actual BCxxx part number. It will tell you whether it is NPN or PNP and will most likely tell you which lead is which and what the gain of the device is.

[SiriusHardware]

You haven't wasted your money, this is potentially a very useful general purpose tester for not much money. It's just that usually, you know what the component is (you know what it's number is, and you use the tester to check that the component is behaving as that type of transistor should.

The information it does give you, if you can somehow get the original device connected to the tester for long enough to test it, will go some way towards identifying a possible replacement.

You can use it to test other components (like capacitors) as well, but if you do, always be careful to discharge the capacitor under test before connecting it to the tester - as per the cautionary note shown on the front panel.

[ScottishColin]

OK - the results are in from the Scottish jury (see attached).

Does this help?

I have some other PNP transistors - is there merit in trying them to see if I can get a match?

Colin.

[TonyDuell]

I'd be inclined to shove in a 2N3906 and hope

[SiriusHardware]

Bipolar transistors 'magnify' current, so if you pass a current (I) through a transistor's base-emitter junction, that causes a current tens or hundreds of times (I) to flow between the collector and emitter. Small variations in base-emitter current cause big variations in collector-emitter current. It is this behaviour which allows transistors to act as amplifiers.

Your tester is really just looking for this typical bipolar transistor behaviour - it 'tries' the wiring to the three leads of the transistor in every possible combination and with both polarities until it finds 'typical' bipolar transistor behaviour and then reports the gain figure, pinout and polarity (NPN or PNP) for the transistor - and that is about as much as it can do.

There are other properties, such as the maximum frequency of operation, which can only be determined with a more specific test, and still others, such as the maximum voltage that the transistor can withstand across its collector-emitter junction and the maximum current which the transistor can pass continuously between collector and emitter which can only be found by testing the transistor to the point of destruction. So these properties remain unknown.

You can test a selection of your spares box PNP transistors until you find one which has the same pinout and roughly the same hFE figure, but without knowing what voltage and current the original device was expected to handle this is always going to be somewhat experimental.

Can you list the PNP types you do have, and we will look them up and see which, out of the ones available, is likely to be the sturdiest?

[TonyDuell]

As I said many posts ago, if it were mine I'd trace out ('reverse-engineer') the complete circuit diagram of the printer. The PCB doesn't look that complicated to me, it would take me a fewd days at most to do it.

Then, armed with said diagram, you can work out the maximum voltage and current that the transistor has to stand.

[ScottishColin]

In my mixed box of transistors, I have the following PNP:

BC327
BC557
BC558

Colin.

Originally Posted by SiriusHardware

Bipolar transistors 'magnify' current, so if you pass a current (I) through a transistor's base-emitter junction, that causes a current tens or hundreds of times (I) to flow between the collector and emitter. Small variations in base-emitter current cause big variations in collector-emitter current. It is this behaviour which allows transistors to act as amplifiers.

Your tester is really just looking for this typical bipolar transistor behaviour - it 'tries' the wiring to the three leads of the transistor in every possible combination and with both polarities until it finds 'typical' bipolar transistor behaviour and then reports the gain figure, pinout and polarity (NPN or PNP) for the transistor - and that is about as much as it can do.

There are other properties, such as the maximum frequency of operation, which can only be determined with a more specific test, and still others, such as the maximum voltage that the transistor can withstand across its collector-emitter junction and the maximum current which the transistor can pass continuously between collector and emitter which can only be found by testing the transistor to the point of destruction. So these properties remain unknown.

You can test a selection of your spares box PNP transistors until you find one which has the same pinout and roughly the same hFE figure, but without knowing what voltage and current the original device was expected to handle this is always going to be somewhat experimental.

Can you list the PNP types you do have, and we will look them up and see which, out of the ones available, is likely to be the sturdiest?

[ScottishColin]

I will try to do this for the components near the transistor, but it's quite a daunting task for me.

Plus I need to find the time.....

Colin.

Originally Posted by TonyDuell

As I said many posts ago, if it were mine I'd trace out ('reverse-engineer') the complete circuit diagram of the printer. The PCB doesn't look that complicated to me, it would take me a fewd days at most to do it.

Then, armed with said diagram, you can work out the maximum voltage and current that the transistor has to stand.

[ScottishColin]

Based on the attached photo when the transistor was in place, and the fact that it's a PNP, I believe that the legs go CBE from left to right.

If that is the case, I have measured the following voltages:

C - -3.95V
B - +3.5V
E - +0.75V

The collector pin is connected to a resistor marked Brown, Red, Brown Gold which I believe is 120ohms

The base pin is connected to a resistor marked Red, Violet, Brown, Gold which I believe is 270ohms

The emitter pin is connected to a track which (amongst others) connects to the Vcc pin of a 74LS259PC and a 74LS533PC IC.

Does this help or am I better off asking someone nicely to draw the schematic?

Colin.

[ScottishColin]

Last piece of measuring for now; maximum voltage on any of the pins from the Power Supply is 33.9V

Colin.

[SiriusHardware]

In my mixed box of transistors, I have the following PNP:

So first of all, try all of those in your tester and see if any or all of them have the same pinout as the original device.

Any which don't, put aside for now.

There is some confusing info in your earlier findings - you say the emitter pad can be traced to the Vcc pins of two LS ICs, suggesting that the emitter is almost certainly connected to hard +5V. This would be OK for a PNP transistor.

However, when you have measured the voltages you only have +0.75V on the emitter pad and a larger minus voltage on the collector pad. One possible reason for this is that the point you were using as your 0V return for the meter wasn't really 0V, so try those measurements again - I am assuming with the transistor out?

Of course your +5V supply may actually be missing in action - with power applied and everything connected, what voltage do you have on the VCC pins of those ICs?

Finally, could you please take well-lit, well focused overhead and underside views of that PCB - ideally frame them so that the PCB fills the whole image from edge to edge with no dead space beyond the edges of the PCB, and post the images as jpegs in .zip files (as you have been doing). This will give us the best chance of reconstructing the circuit 'remotely', if possible.

[ortek_service]

Originally Posted by ScottishColin

In my mixed box of transistors, I have the following PNP:

BC327
BC557
BC558

Colin.
>>
>>
[

Well out of these, the BC327 would be your best-bet, as the max. highest ratings (I used to use these and their BC337 NPN complement a lot in the past as my 'universal' TO92 replacements in DC / Low-frequency circuitry).

BC327 45Vceo 0.8A 625mW hFE=100-600 (@100mA pulsed!)
https://www.farnell.com/datasheets/314706.pdf
- Although I recalled these as being rated at 0.5A
m (which one version of NPN BC337 is as low as on this) so may vary across makes / suffixes.

BC557 45Vceo 0.1A 625mW hFE=120-800 (@2mA)
BC558 30Vceo 0.1A 625mW hFE=120-800 (@2mA)
https://www.onsemi.com/pdf/datasheet/bc556b-d.pdf

I doubt the circuit is placing much of a demand on a TO92 transistor, as max. current is often rather limited by quite-low package's max. power dissipation.
So you won't find many transistors rated at over an amp in a standard plastic TO92 package, as you need to have a very-low collection-emitter saturation-voltage to keep the power-dissipation at max current within 625mW.
And it's unlikely that you'd need much over 30V max collector-emitter voltage rating, unless the transistor is driving an inductive load like a relay (that usually has a back-emf 'catch'-diode) or a transformer as part of a DC-DC converter oscillator-driver (or it is being used as a driver via a transformer to drive a more-powerful output-stage transistor)

All 3 of these have the same very-standard for BC... series pin-out. From left to right, when looking at the front and legs pointing down: Collector, Base, Emitter
I couldn't tell from your photo of the original being tested, exactly where the leads '1' '2' and '3' of the tester socket went to, as it seems the transistor was out of shot.
- But (especially with long-enough leads) it's usually possible to for them to match any of the various TO92 pinouts.

[ortek_service]

Originally Posted by ScottishColin

I will try to do this for the components near the transistor, but it's quite a daunting task for me.

Plus I need to find the time.....

Colin.

It should mainly only need the connections to the emitter and collector (maybe base as well, for good measure) tracing as to what they connect to - so only a partial 'reverse-engineer' (as I'd also previously suggested would be useful to do).

A full Rev-Eng is probably only best when you've got faults in other places as well / you want to make a clone.
Or if you just like having a full schematic of things for future reference / repairs / to modify it
- I often did that for many things back in the 1980's, before the Internet, when it was generally much harder to obtain circuits (especially if you couldn't buy a service-manual or service-sheet (mainly only produced for Radio, TV's, VCR's + some computers) for these.

[ScottishColin]

Photos attached. I'll remeasure later - point to note is there is no earth pin on the power connector of the printer so I'm testing to the chassis of the printer itself which is where it seems that the earth connectors go to.

Did I need to find a better way of finding earth?

Colin.

Originally Posted by SiriusHardware

So first of all, try all of those in your tester and see if any or all of them have the same pinout as the original device.

Any which don't, put aside for now.

There is some confusing info in your earlier findings - you say the emitter pad can be traced to the Vcc pins of two LS ICs, suggesting that the emitter is almost certainly connected to hard +5V. This would be OK for a PNP transistor.

However, when you have measured the voltages you only have +0.75V on the emitter pad and a larger minus voltage on the collector pad. One possible reason for this is that the point you were using as your 0V return for the meter wasn't really 0V, so try those measurements again - I am assuming with the transistor out?

Of course your +5V supply may actually be missing in action - with power applied and everything connected, what voltage do you have on the VCC pins of those ICs?

Finally, could you please take well-lit, well focused overhead and underside views of that PCB - ideally frame them so that the PCB fills the whole image from edge to edge with no dead space beyond the edges of the PCB, and post the images as jpegs in .zip files (as you have been doing). This will give us the best chance of reconstructing the circuit 'remotely', if possible.

[ortek_service]

Originally Posted by ScottishColin

Photos attached. I'll remeasure later - point to note is there is no earth pin on the power connector of the printer so I'm testing to the chassis of the printer itself which is where it seems that the earth connectors go to.

Did I need to find a better way of finding earth?

Colin.

Well the chassis is usually a fairly-good ground. Although it's best to find a point closest to where you are measuring, to minimise ground noise / voltage-drops due to resistance etc.
It seems that in the picture of the board on the first three attachments, the ground is the thicker track that runs around the edge of the PCB underside ( With +5V? supply running around the edge of the top layer).
And can also use the bottom left pin of logic IC's, if you have a suitable hook-clip to connect onto one of those.

Whereas, on the board shown on your 4th attachment, they have nicely labelled the Gnd and +5V thicker tracks.

BTW, did you have a better-photo of your transistor-testing, that shows which pin of the transistor went to which pin of the tester? - We can then work-out if your BC327 etc would likely be a direct drop-in, or whether some re-arrangement of the legs is required.