UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Computers

Notices

Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

Reply
 
Thread Tools
Old 10th Apr 2024, 5:46 pm   #141
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

So do I understand correctly that the BC327 is the transistor which originally had its legs rotted off?
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 5:58 pm   #142
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

If so, power off and measure the resistance, in-circuit, of the 120R resistor connected to the collector of the BC327.

If you connected a 120R resistor directly between 5V and 0V the resulting current would be a little bit over 40mA, which is rather a lot for a small transistor like that.

The issue of the resistor elsewhere also getting hot may or may not be related, let me look at the diagram for a bit longer.

If the transistor really ran that hot I am afraid it is probably toast.
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 6:24 pm   #143
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

What you refer to as the bottom left 212-6 (high power resistor) appears to be connected to the emitter of Q3 - maybe you can confirm that? If so I think we are looking at this circuit section (attached), with the 10R resistor likely to be the one which got hot. I'm assuming that the annotations CMen and CMCOM probably mean 'carriage motor enable' and 'carriage motor common' respectively.

Can you also confirm that thus far, you have only had the PSU connection plugged into this PCB while power has been applied, with none of the other connectors plugged into it?

Also - everyone help me out here - it looks as though the 5V supply switched by the BC327 goes to the reset input of the processor as has been mentioned before, but it also seems to go to the +ve supply pin of the ULN2068 - does it go to anywhere else on the logic PCB that I might have missed?
Attached Thumbnails
Click image for larger version

Name:	Q3.jpg
Views:	48
Size:	17.3 KB
ID:	296214  
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 6:30 pm   #144
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,281
Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by SiriusHardware View Post
Also - everyone help me out here - it looks as though the 5V supply switched by the BC327 goes to the reset input of the processor as has been mentioned before, but it also seems to go to the +ve supply pin of the ULN2068 - does it go to anywhere else on the logic PCB that I might have missed?
It's also the Vcc power line to U1 (7404 NOT gates to drive the carriage motor transistors), U3 (ULN2068 drivers for the ribbon control) and U4 (74259 addressable latch).

I wonder if one of those is drawing too much current.
TonyDuell is offline   Reply With Quote
Old 10th Apr 2024, 6:34 pm   #145
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

Agreed - Colin, could you please take an in-circuit resistance measurement of the 120R resistor which is connected to the emitter of the BC327 (Q20).
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 6:39 pm   #146
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,281
Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by SiriusHardware View Post
What you refer to as the bottom left 212-6 (high power resistor) appears to be connected to the emitter of Q3 - maybe you can confirm that? If so I think we are looking at this circuit section (attached), with the 10R resistor likely to be the one which got hot. I'm assuming that the annotations CMen and CMCOM probably mean 'carriage motor enable' and 'carriage motor common' respectively.
Yes.

Basically there are 3 off 4-phase stepper motors (carriage, paper feed, daisywheel) in this thing, the driver circuitry is similar (but not identical) in each case.

If we consider the carriage motor, the ends of the 4 windings (CM(0)..CM(3)) are switched to ground by Q7-Q10 under processor control. Sequencing them is what gets the motor to turn.

The common connections are CMCOM. This is switched to the 36V line by Q3 when the motor is to move. CMEn is what enables this. When the motor is stationary, then Q3 is turned off, but CMCOM gets 9V via a fat diode. This provides enough current through the windings to hold the rotor in place.
TonyDuell is offline   Reply With Quote
Old 10th Apr 2024, 7:11 pm   #147
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

Assuming the carriage motor is not currently connected, why would Q3's associated 10R resistor be getting so hot? That 'fat' diode between CMCOMM and +9V, could that be short-circuit?
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 7:19 pm   #148
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,849
Default Re: Commodore 8026 printer / typewriter

Confirmed nothing else plugged into the board except the earth wire to the typewriter chassis and the PSU connector.

Resistance in circuit between the BC327 collector and the 120? resistor - 0.1?

Resistance in circuit of the 120? resistor - 109.6?

Colin.
ScottishColin is online now   Reply With Quote
Old 10th Apr 2024, 7:25 pm   #149
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

327 to 120R, one end, should be directly connected so that is OK.

Quote:
Resistance in circuit of the 120? resistor - 109.6?
Strange.

How confident are we that the resistor really is / was 120R originally? Is that the one which got stripped by the battery acid, or was that a different one? Tony, can you think why that resistor (which is from 5VSW (switched 5V) to GND needs to be such a low value? I would have thought the components whose supplies are switched by it would present enough of a load for it not to need a 'dummy' load as well.
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 7:39 pm   #150
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,849
Default Re: Commodore 8026 printer / typewriter

I've had a look under the board and found two legs in the section that was damaged that were too close. I've gently pushed them apart. They are marked in the red box on the attached photo.

However, when I test continuity after moving them apart, I start with a continuity reading of 0 (ie there is a connection) that slowly rises to over 300? when I stopped measuring.

Is that right between these two components?

Colin.
Attached Thumbnails
Click image for larger version

Name:	potential short.jpg
Views:	61
Size:	94.6 KB
ID:	296219  
Attached Files
File Type: zip potential short.zip (932.2 KB, 51 views)
ScottishColin is online now   Reply With Quote
Old 10th Apr 2024, 7:41 pm   #151
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,849
Default Re: Commodore 8026 printer / typewriter

btw that 120 ohm resistor is original and has not been replaced.

Colin.

Quote:
Originally Posted by SiriusHardware View Post
327 to 120R, one end, should be directly connected so that is OK.

Quote:
Resistance in circuit of the 120? resistor - 109.6?
Strange.

How confident are we that the resistor really is / was 120R originally? Is that the one which got stripped by the battery acid, or was that a different one? Tony, can you think why that resistor (which is from 5VSW (switched 5V) to GND needs to be such a low value? I would have thought the components whose supplies are switched by it would present enough of a load for it not to need a 'dummy' load as well.

Last edited by ScottishColin; 10th Apr 2024 at 8:00 pm.
ScottishColin is online now   Reply With Quote
Old 10th Apr 2024, 8:45 pm   #152
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

Whenever you (a) suspect there might be a short and (b) see a possible cause for it, verify the presence or absence of a short before you do anything to clear it. (Before you move anything, before you scrape anything). As it stands, you think there might have been a short but you can't know for sure now because you didn't check before moving the leads apart.

Quote:
I start with a continuity reading of 0 (ie there is a connection) that slowly rises to over 300?
When making measurements, please always remember to state the units (Ohms? K?). A low resistance rising to high over time usually means you are measuring a capacitor, or there is a capacitor closely connected to the two circuit nodes you are making the measurement from - the capacitor isn't charged at first but the voltage being produced by the meter probes slowly charges it up. The larger the capacitor value, the slower the change.
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 8:55 pm   #153
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

Can you measure the resistances between the BC327 pins in-circuit

C to E
B to E
B to C

Alternatively, remove it and get out your new(ish) component tester. We need to know whether that transistor is fried, so connect it to your tester and see what it makes of it. If it correctly identifies it as a PNP transistor, it may have survived.
SiriusHardware is online now   Reply With Quote
Old 10th Apr 2024, 10:11 pm   #154
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,849
Default Re: Commodore 8026 printer / typewriter

Apologies - I should have said it better. I found the short with my meter so can confirm the legs were indeed touching.

In circuit measurements:
C to E: 355 ohms
B to E: 559 ohms
B to C: 0.913k ohms

The tester states it is a PNP - see attached photo.

Colin.
Attached Files
File Type: zip PXL_20240410_210943937.zip (1.91 MB, 30 views)

Last edited by ScottishColin; 10th Apr 2024 at 10:27 pm.
ScottishColin is online now   Reply With Quote
Old 10th Apr 2024, 11:47 pm   #155
ortek_service
Octode
 
ortek_service's Avatar
 
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,506
Default Re: Commodore 8026 printer / typewriter

Well it looks-like the transistor did at least survive after all, from getting very hot.

It's not just an excess of their Ic(Max) (Continuous) collector-current max. rating (of 500mA / 800mA for BC327's depending on the Manufacturer) that can cause these to get hot.
But also whether they are switched fully-on into saturation, so that the voltage across the Collector and emitter is causing a power-dissipation in excess of their max. rating of only 625mW @+25degC.
So as well as an overload on the 'output' side (collector for a PNP 'power-switch'), lack of base-current to fully-switch it on can also be an issue (or a too-low hFE (current-gain) transistor).

Now that you've cleared the short you found (which may have been on the base or the collector side? - would have to study the RevEng circuits to see exactly where the short was), then there's probably no further harm in trying powering it up again for a brief period with this transistor refitted
- But taking care to ensure this doesn't still get hot (or maybe even rather warm), so only apply power briefly, checking the transistor temperature all the time (a quick touch with a finger may be sufficient, although if your IR camera responds quickly then that might save a possibly burn! A bit of freezer spray / a drop of water on the transistor would also indicate if it is getting hot quickly, as would soon start producing steam)

Last edited by ortek_service; 10th Apr 2024 at 11:56 pm.
ortek_service is offline   Reply With Quote
Old 11th Apr 2024, 12:19 am   #156
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,827
Default Re: Commodore 8026 printer / typewriter

I agree that if you found and then cleared a bona-fide short close to the 327 then this may have accounted for it getting tremendously hot, and you can maybe try again while monitoring the 327 and the 10R resistor, perhaps through your thermal camera, ready to switch off quickly.

I would still like to gather opinions as to why the 327 is lumbered with a relatively low resistance 120R load in addition to the various other inputs and chips it supplies.
SiriusHardware is online now   Reply With Quote
Old 11th Apr 2024, 2:52 am   #157
ortek_service
Octode
 
ortek_service's Avatar
 
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,506
Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by ScottishColin View Post
>>

Then I noticed that worrying electrical smell, so I took a photo with my thermal camera which shows that the BC327 was at near enough 200 degrees.

Followed by magic smoke from the bottom left 212-4.

>>

Colin.
Well '212-4' appears to be a type number on the 3off 10R 3W/5W? Wire-wound resistors. And it appears these might be the 10R ones on the rev-eng'd circuits that feed the +36V supply to the power-driver transistors fed from that (But not too easy to be sure, with no power ratings / reference designators etc)

And if that's the case, then this doesn't seem to be connected with the BC327 getting hot - as that's all around 5V(sw) supply.

It appears that the short at one end of the 6k8 resistor was the ground end.
And I presume the resistor with no markings left on it is the 220R between the BC327 base and BC237 collector.
So if the short to 'ground' (end of 6k8) was at BC327 collector then there wouldn't be a problem as the BC237 is designed to ground that normally.
However, if short to 'ground' (end of 6k8) was at BC327 base, then that would be putting 5V across emitter-base of the BC327, and I would have expected it to have gone bang very quickly - unless the 5V supply has a low / foldback current limit.
ortek_service is offline   Reply With Quote
Old 11th Apr 2024, 3:05 am   #158
ortek_service
Octode
 
ortek_service's Avatar
 
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,506
Default Re: Commodore 8026 printer / typewriter

Quote:
Originally Posted by SiriusHardware View Post
>>
>>
I would still like to gather opinions as to why the 327 is lumbered with a relatively low resistance 120R load in addition to the various other inputs and chips it supplies.
I can only think that this is to ensure that any capacitance on the +5Vsw rail is discharged quickly at power-off, ready for next power-up.
And it seems to be about the lowest they could have used, whilst keeping within it's 1/4W rating (as 100R would have been exactly 1/4W, at 5V).

I recently had to do something similar (but at 12V so I used 820R 0.4W) with a few 1000uF, as the power-control IC didn't have that low an internal output-discharge resistance.
With it setting its 'fault' status output, whenever a voltage was still present at the output and its enable input wasn't active
- which could be for a few seconds with no load to discharge the output capacitor quickly.
ortek_service is offline   Reply With Quote
Old 11th Apr 2024, 9:05 am   #159
ScottishColin
Octode
 
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,849
Default Re: Commodore 8026 printer / typewriter

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.
Attached Thumbnails
Click image for larger version

Name:	PXL_20240410_162330874.jpg
Views:	54
Size:	97.7 KB
ID:	296242  
ScottishColin is online now   Reply With Quote
Old 11th Apr 2024, 9:37 am   #160
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,512
Default Re: Commodore 8026 printer / typewriter

The idea is that if the resistor overheats the solder blob melts and the wire springs away breaking the circuit.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 9:46 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.