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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 10th Apr 2022, 6:02 pm   #1
ScottishColin
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Default Commodore CBM 4040.

I've bought one from eBay which has arrived. I've opened it up and it doesn't look like the disaster area that my PET did, but there's no power at all getting through.

I'll check the obvious power issues and come back to you when I can see some voltage somewhere inside the unit.

Colin.
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Old 10th Apr 2022, 8:05 pm   #2
SiriusHardware
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Default Re: Cbm 4040

Do I take it this thing has its own mains power input / cable? I know you know, but a reminder, don't even think of connecting or separating the drive and the PET unless the power to BOTH items is off. Just remember how expensive those exotic MC3446 buffers are.
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Old 10th Apr 2022, 8:16 pm   #3
ScottishColin
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Default Re: Cbm 4040

It does - I do not actually have an HPIB cable yet so no danger of doing anything bad (yet).

Colin.

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Originally Posted by SiriusHardware View Post
Do I take it this thing has its own mains power input / cable? I know you know, but a reminder, don't even think of connecting or separating the drive and the PET unless the power to BOTH items is off. Just remember how expensive those exotic MC3446 buffers are.
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Old 10th Apr 2022, 8:19 pm   #4
SiriusHardware
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Default Re: Cbm 4040

With power applied to the drive, what voltage do you see (with respect to 0V) on the bar / striped end of diodes CR7, CR8 and CR9? They are the diodes which are right on the edge of the PCB next to the regulator heatsinks.

Diagram etc for the 3040/4040 is here:-

http://www.zimmers.net/anonftp/pub/c...schematics.pdf
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Old 11th Apr 2022, 1:35 pm   #5
ScottishColin
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Default Re: Cbm 4040

OK - no power to CR7/8/9. So back to basics. I've checked the fuses and they're OK I've unscrewed the PSU (transformer?) out of the case but left it wired in and carefully tested that 240V is getting into it.

However, there's a 'spare' wire from the middle of the on/off switch (see photo). Anyone any good ideas where that is meant to go?

Colin.
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Old 11th Apr 2022, 2:13 pm   #6
ScottishColin
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Default Re: Cbm 4040

OK - there is a place on the underside of the transformer that used to have a wire attached - pin 6 in the photo. Does that look like a good place to start?

Colin.
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Old 11th Apr 2022, 3:23 pm   #7
TonyDuell
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Default Re: Cbm 4040

I've never worked on a 4040, but I've done the 8050 and 8250LP and the PSU is likely to be much the same. CBM power supplies of this vintage are quite simple electrically, mains goes into the transformer which supplies low-ish voltage AC to the main PCB. Here is it rectified, smoothed and regulated to provide at least +5V (logic supply) and +12V (for the motors). Maybe a -ve supply rail too for the analogue circuitry.

In any case, get the transformer working first. The mains cable earth wire (obviously) goes to the chassis. The live wire goes to the fuse and switch in series, then to one end of the mains transformer primary. The mains neutral wire goes to the other end of the transformer primary, possibly via another pole of the switch. The transformer secondaries go to the connector that plugs into the logic board.

It appears your unit has a single-pole switch, meaning it should switch the live side of the mains only. It is likely the wire hanging in mid-air is the live mains feed to the transformer (!). So start tracing things. The live wire (brown) of the mains cable, where does that go? (I would expect to the fuse holder). Where does the neutral (blue) wire go? (A tag on the transformer?). If the latter does connect to the transformer, then that loose wire should go to the transformer too, to the tag which is the other end of that winding. With an ohmmeter, check that the tag you think it should go to (a) has some resistance to the tag the neutral wire is connected to and (b) has infinite resistance (open circuit) to the tags that are linked to the connector to the logic board.
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Old 11th Apr 2022, 3:39 pm   #8
SiriusHardware
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Default Re: Commodore CBM 4040.

Can we have views of ALL of the terminals on that upper edge of the transformer and also a separate clear view of the label which is underneath the terminals? One possible pitfall here is that the transformer primary may have connections for both 110V in and 230V in.

That thin black wire dangling off the (mains?) switch is a death trap while it is like that so don't connect it to the mains again until you have made some sense of what goes where. With the half-view that we have of the terminals it seems like terminal 4 is a tap with (logically) terminal 2, out of sight in the photo, being the other end of the 230V winding. The tap could be a centre tap (ie, 110V) or it could be a tap for 220V with the whole winding being 240V.

I have to admit I'm worried - why has the mains side wiring been disturbed by the previous owner?

Last edited by SiriusHardware; 11th Apr 2022 at 3:47 pm.
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Old 11th Apr 2022, 4:04 pm   #9
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Default Re: Commodore CBM 4040.

OK - photos attached of the top/bottom/label of the transformer, another showing the brown/blue wiring to the fuse and one showing the blue/black (from the transformer) wiring going to another component that I'm not clear of.

The next post will have a close-up of that to aid identification.

Colin.
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Old 11th Apr 2022, 4:05 pm   #10
ScottishColin
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Default Re: Commodore CBM 4040.

Photo of component.
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Old 11th Apr 2022, 4:51 pm   #11
SiriusHardware
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Default Re: Commodore CBM 4040.

[Throwing my hands up in horror]. Wow, that is rough.

Conventionally speaking the incoming brown live wire should go to one side of the fuse holder (which it does). The other side of the fuse should go to one side of the switch, and then there should be a wire going from the other side of the switch to terminal '6' on the transformer.

The Neutral (blue) meanwhile should go to terminal 2 on the transformer, but actually, there is a black wire going to terminal 2.

All of the above is not taking the mysterious component into account. That looks like a mains filter which is probably meant to have one of its wires going to Live, another of its wires going to Neutral and its body going to chassis / earth.

Can you draw a hand sketch of what the wiring looks like now, from the incoming mains cable to the filter and to the transformer. Indicate the colours of the wires either by drawing them in the appropriate colour or by labelling them with what colour they are, take a photo of the sketch and post it?

How many wires does that filter have going into it? Two? Or four? What colours?
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Old 11th Apr 2022, 5:25 pm   #12
ScottishColin
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Default Re: Commodore CBM 4040.

Hopefully this shows everything.

The filter has two wires in (black and blue) and two out (also black and blue). It also has a tab which is then screwed to the chassis which I am assuming is earth?

Colin.
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Old 11th Apr 2022, 5:46 pm   #13
SiriusHardware
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Default Re: Commodore CBM 4040.

Pretty horrible choice of wire colouring (not yours, whoever came up with it).

From what you have so far it seems like the two blue wires are incoming AC mains into the filter, and the two black wires are mains outgoing from the filter. One black wire goes to terminal 2 of the mains transformer wiring and the other one should therefore go through the fuse, through the switch and then onwards to terminal 6 of the transformer.

Let's check this - take the mains plug apart and start at the neutral wire in the mains plug and measure for continuity from there to either the transformer terminal 2, or the wire going to the mains switch. Which of those two places does Neutral go to?

Same for the Live wire in the plug, check continuity, it should be connected either to the transformer terminal 2 or to the mains switch. Which is it? Your drawing shows it well enough, I just want to establish that the blue wires really are the input wires to the filter and the black wires really are the output wires from the filter.
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Old 11th Apr 2022, 6:13 pm   #14
ScottishColin
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Default Re: Commodore CBM 4040.

Neutral in plug has continuity to Terminal 2 of the transformer.

Live in the plug has continuity to the switch.

While I was there, earth in the plug has continuity to an area of the chassis with paint scraped off.

Colin.
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Old 11th Apr 2022, 6:23 pm   #15
SiriusHardware
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Default Re: Commodore CBM 4040.

Yep, that's OK then. Couple more checks, what is the resistance of the transformer measured between terminal 2 and terminal 6?

What is the resistance between live and neutral at the mains plug - plug NOT plugged into the mains, obviously.

What is the resistance between Live and Earth in the plug?

What is the resistance between Neutral and Earth in the plug?

The first measurement is likely to be quite low, as you are measuring the resistance of a very long piece of wire.

The three measurements afterwards should be very high or infinite.

Last edited by SiriusHardware; 11th Apr 2022 at 6:29 pm.
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Old 11th Apr 2022, 6:57 pm   #16
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Default Re: Commodore CBM 4040.

If the measurements in #15 are as stated, then remove that spindly bit of black wire from the mains switch and reinstate the missing connection shown by the brown wire, upper right, in the attached Fisher-Price style diagram. Find a bit of scrap 3 core mains cable, strip the overall cover back and harvest a length of brown wire long enough to go from the centre pin of the switch to terminal 6 on the transformer, strip the ends and solder it in place. Once this works, you are going to have to tidy it up, with heatshrink over the connections on the fuse holder, switch and transformer. It's just too dangerous the way it is.

I'm assuming the output wires on the other side of the transformer are connected to the PCB by some kind of unpluggable plug. If so, disconnect that for now.

Still with the mains plug unplugged, move the on/off switch to the 'on' position and then plug in at the wall and switch on the mains. I won't lie to you, this could go bang, if there is a fault either in the mains filter or the transformer primary. There is still a question over why the mains wiring was ever messed with in the first place.

If it doesn't go bang, switch off, reconnect the low voltage output to the PCB, switch on, and look for life signs, but be very careful to keep away from those exposed mains connections.
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Old 11th Apr 2022, 7:05 pm   #17
ScottishColin
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Default Re: Commodore CBM 4040.

2->6 - 32.4 ohms

L->N - 0

L->E - 0

N->E - 0

Is that OK for me to do your suggested re-wiring in the next message? I have solid core mains cable or stranded cable - which is better?

Colin.


Quote:
Originally Posted by SiriusHardware View Post
Yep, that's OK then. Couple more checks, what is the resistance of the transformer measured between terminal 2 and terminal 6?

What is the resistance between live and neutral at the mains plug - plug NOT plugged into the mains, obviously.

What is the resistance between Live and Earth in the plug?

What is the resistance between Neutral and Earth in the plug?

The first measurement is likely to be quite low, as you are measuring the resistance of a very long piece of wire.

The three measurements afterwards should be very high or infinite.
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Old 11th Apr 2022, 7:11 pm   #18
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Default Re: Commodore CBM 4040.

Maybe check that the mains plug has a reasonable fuse, not 13 amp, it might reduce the size of a bang. I wonder if that filter had let out some smoke in the past so someone bypassed it, did you check it for shorts and opens?
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Old 11th Apr 2022, 7:12 pm   #19
SiriusHardware
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Default Re: Commodore CBM 4040.

I hope the 'O' on your meter means 'Out Of Range' and not 'Zero'. Make the L-N, E-N, E-L measurements again but with the meter on the highest ohms range it has, if you were not already doing that. If it still indicates very very high or infinite resistance, that's fine.

32 ohms is quite a low resistance but the transformer primary winding wire looks quite thick, so I am going to say yes, try it.
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Old 11th Apr 2022, 7:14 pm   #20
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Default Re: Commodore CBM 4040.

Quote:
Did you check it for shorts and opens
It's not open as Colin has already measured continuity through both arms. The checks between L-N, L-E and N-E before reinstating the lost connection to the transformer's pin 6 are aimed at seeing if any element of the filter has failed low resistance or short circuit.
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