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Old 27th Mar 2021, 9:40 pm   #1
Leon_Night
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Default 2 dead CBM 8032-B 'PETs'

Hi all,

Well, here's another PET thread..

I have three CBMs now (I'll refer to them as PETs now because I am sure to forget what they are called) and none of them work. My first was a 3016 and last week I acquired two with the model 8032-B.

PET 1 looks immaculate and uses a IEC cable while PET 2 has a hardwired power cable and is badly fire damaged and because the later ones had plastic cases the monitor housing is a bit melty and bubbled.

Inside PET 1 is pretty clean and I couldn't see anything obviously out of place while PET 2 has a lot of black residue all over everything though having said that, the mainboard looks to be in fairly good condition.

When I switch either of them on there is a faint hum of the transformer and that is pretty much it. There is no sign of the screen heater glowing and if left on for a while there is some warming of the components on the mainboard so I guess some power is getting there.

I am hoping this will be a good learning experience because so far my entire electronics experience has been replacing some exploded RIFA caps in my Osbourne 1. Which now works so yay me!

I have a soldering iron and a new multimeter but that is it.

So...

I found a picture online which detailed the expected voltages on the pins of J8 which is where the power connects to the mainboard. So I figured that armed with that I would learn how to test voltages one PET 1 with my new multimeter.

Here's what I got compared to what the internet said I should expect.

1 - 9.2v (Should be 8v AC)
2 - 0.002v (Should be +9v)
3 - GND
4 - 0v (Should be 8v AC)
5 - 9.3v (Should be 8v AC)
6 - GND
7 - KEY
8 - 8.1v (Should be 14v AC)
9 - 5.3v (Should be 14v AC)

So those seem to be fairly different from what I expected though it should be noted I am currently fairly clueless as to what is right and what isn't..

On top of that (see piccy) I noticed that pin 7 (the missing pin) has a wire going to it on the plug and then pin 6 which should be an earth doesn't have anything connected to it at all.

I welcome any help you can offer!
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Old 27th Mar 2021, 11:05 pm   #2
PJL
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Default Re: 2 dead CBM 8032-B 'PETs'

The wire colours on the connector don't correspond either, I would say you have a mix&match there.
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Old 27th Mar 2021, 11:12 pm   #3
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

Hello Leon (May we call you that?)

Some of your difficulties are due to the fact that you are using GND as the reference from which to measure both the DC and AC voltages. Generally speaking you do measure DC voltages relative to GND, but AC voltages coming out of transformers often do not have a direct reference to ground.

To begin with, can we check that the correct voltages are coming out of the transformer.

With the J8 connector NOT plugged into the board and the mains powered on can you set your meter to AC volts and measure:-

-The AC voltage between one blue wire (connector pin 8) and the other blue wire (connector pin 9)

-The AC voltage between one brown wire (connector pin 1) and the other brown wire (connector pin 5)

-The AC voltage between the brown wire in connector pin 1 and the black wire in connector pin 3

-The AC voltage between the brown wire in connector pin 5 and the black wire in connector pin 3

Can you also give us a rough idea of your current level in electronics, can you read circuit diagrams, can you read the colour codes on resistors, do you know how the pins on ICs are numbered? This is just to give us an idea of how much or how little information to include in whatever steps or measurements we ask you to carry out.

For anyone wanting to get involved I think the offboard mains transformer assembly is this one:-

http://www.zimmers.net/anonftp/pub/c.../8032051-3.gif

And the regulator circuit (which is on the main board) is this one.

http://www.zimmers.net/anonftp/pub/c...8032029-11.gif
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Old 27th Mar 2021, 11:14 pm   #4
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

Quote:
I would say you have a mix&match there
Not necessarily, let's see what is coming out of the transformer.
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Old 27th Mar 2021, 11:32 pm   #5
PJL
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Default Re: 2 dead CBM 8032-B 'PETs'

Diagram attached but not great. It certainly won't work without pin 6 connected as that should be the reservoir capacitor -ve.

If the black on pin 7 goes to the big electrolytic can then prize the wire out of pin 7 with a screwdriver and put it on pin 6.
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Old 27th Mar 2021, 11:40 pm   #6
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

Notice the offboard electrolytic -Ve also goes to a GND point on the chassis? The connection between capacitor -ve and main board may be by some other route, assuming that the mainboard 0V / GND is also connected to chassis GND.

I would be interested to know where you think the unregulated +9V which should be on J8 pin 2 comes from (see link to regulator diagram in #3).
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Old 27th Mar 2021, 11:49 pm   #7
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Default Re: 2 dead CBM 8032-B 'PETs'

Leon, as well as the AC voltage checks in #3, can you also, with power OFF and J8 NOT plugged in, measure the resistance between the connector pin 2 (red wire) and connector pin 4 (the other red wire).
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Old 28th Mar 2021, 2:34 pm   #8
Leon_Night
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Default Re: 2 dead CBM 8032-B 'PETs'

Hi all,

That's my mind completely blown then! Thank you for so much help!

I'll try to go through as much as I can in here:

PJL, thank you, I thought the same actually and it would have given me a really good excuse to give up but I have two of these machines and they both have exactly the same plug and connector so somehow it looks like it is meant to be that way..

Also, I'm not sure about moving wires around in the plug as it does look like they are supposed to be there and it has been working that way. I have had a few things go bang recently (including PET 2 last night) so I might hang on until I feel more intrepid!

Sirius, thank you, yeah, Leon is fine (is Sirius fine?)

My electronics background stretches back several months! I can read bits of circuit diagrams but nothing complicated yet. I am aware of colour codes on resistors but haven't memorised the colours. I know about the pins on ICs as I have one on another PET that has snapped off and I have been looking into ways to repair, replace or ignore it. I will try anything, I am happy to show my ignorance if I don't understand something.

I did the tests and got the following results (they all wavered a tiny bit but I averaged it out)

Pins 8 & 9 (blue and blue) - 15.9v
Pins 1 & 5 (brown and brown) - 18.5v
Pins 1 & 3 (brown and black) - 9.27v
Pins 5 & 3 (brown and black) - 9.23v

Thanks for posting the links to the transformer and motherboard diagrams, I can even understand some of what they show!

Both red wires (pins 2 and 4) connect to the big gold capacitor (?) and then from the other end side of the same there are four black wires, two of them going to pins 3 & 6 of J8 and the remaining 2 going to the transformer (terminal marked 5 on the transformer) and to the earth screw on the metal base.

What is ve- please?

And..

I'm not sure how to measure resistance with my multimeter but I would be delighted to learn, I have added a piccy of the big cap and my meter!

Thank you!
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Old 28th Mar 2021, 3:43 pm   #9
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Default Re: 2 dead CBM 8032-B 'PETs'

Sirius is all fine here.

-Ve = shorthand for 'Negative'. In the context in which I used it, I meant the negative pin of the capacitor - if you're not already aware, electrolytic capacitors like that have a positive (+Ve) terminal and a negative (-Ve) terminal.

I agree that you should not move any of the wires from their 'historic' positions.

The voltages from the power transformer all look good.

By observing that the red wires on connector 2 + 4 both go to the same place on the capacitor you have already found out what I was trying to establish, whether those two red wires were connected together. So this explains how the +9V regulated voltage (going out on J8 pin 4) arrives back on J8 pin 2.

To answer your question about measuring resistance anyway, it looks like the resistance measurement setting on your meter is the third click clockwise from the 'off' position, labelled with a little circle of three different sub-functions.

If you turn the dial to that setting it will probably be in resistance measurement mode initially, and a brief press of the function / hold button will most likely make it step through the other modes, one of which is 'continuity', and the other of which is 'diode test'. We'll explain those modes as and when you need to make those sorts of measurements. The golden rule regarding resistance measurements is that you always make them with the power to the circuit under test turned OFF.

The next thing you can do is to plug the connector back onto J8, turn the power on and measure the DC voltage between pin 3 (with black wire going to it) and pin 2 (with red wire going to it). What voltage do you see there?

I don't have one of those PET PCBs in front of me but there are test points (TPx) indicated on the circuit diagram where the power supply voltages can be measured. Can you see any of those test points, especially TP1, TP2, TP3, TP4, marked on the PCB?
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Old 28th Mar 2021, 4:27 pm   #10
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Default Re: 2 dead CBM 8032-B 'PETs'

Quote:
So this explains how the +9V regulated voltage...
Sorry, that should have read ..."+9V unregulated voltage..."
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Old 28th Mar 2021, 4:51 pm   #11
Leon_Night
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Default Re: 2 dead CBM 8032-B 'PETs'

Great thank you!

Ok, I popped the board back in and connected everything and checked pin 2 to pin 3 and there isn't anything.. The reading was 0.002 with the probe off and 0.006 with it on.

I have looked at the boards from both PETs and there don't seem to be any TP markings anywhere.
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Old 28th Mar 2021, 7:35 pm   #12
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Default Re: 2 dead CBM 8032-B 'PETs'

You should have about +9VDC on pin 2, so we need to find out why you haven't.

Please carry out the following checks:-

Power off, unplug the connector from J8, select resistance measurement on your meter. Touch the meter probes together and the meter should indicate zero ohms resistance. (This is just to confirm you have the resistance mode correctly selected).

Then, measure the resistance between the following points:-

-Pin 2 of the plug in connector to pin 4 of the plug-in connector. It doesn't matter which meter probe you put on which pin.

-Pin 1 of the mainboard J8 connector to pin 5 of the mainboard J8 connector.

-Pin 2 of the plug-in connector to pin 3 of the plug-in connector, with the meter red lead on pin 2 and the meter black lead on pin 3.

-Pin 2 of the mainboard connector J8 to pin 3 of the mainboard connector J8, with the meter red lead on pin 2 and the meter black lead on pin 3.

The readings you get in some cases above may change quite substantially as you watch, rising from a low resistance to a higher resistance - if so, just report that, rather than trying to read the exact resistance.

When we are happy that the above resistance checks are OK we'll go back to checking voltages.
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Old 28th Mar 2021, 11:50 pm   #13
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Default Re: 2 dead CBM 8032-B 'PETs'

Will you get sensible resistance measurements of the main board through the bridge diodes?
Also, the meter shows no obvious way of selecting AC/DC measurements?
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Old 29th Mar 2021, 12:42 am   #14
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Default Re: 2 dead CBM 8032-B 'PETs'

I think what can be leading to some confusion, is that a copy of the images that Leon posted do not appear to be correct:

The wiring diagram should show:
a. That J8 pins 3 & 6 are connected together (as Leon said the 2 black wires from these both go to capacitor -ve and the centre-tap on the 8V-0-8V transformer winding -as well as chassis ground)
b. That J8 pins 2 & 4 are connected together (as Leon said the 2 red wires from these both go to capacitor +ve)

On the J8 Power Connector pin-out (changes) list for the 8032:

a. Pin 4 is NOT 8V AC, but is actually +9V UNREG

b. The connections on this list are from PCB point of view. So It's also useful to be aware that:

Pin 3 (marked GND), is also the centre tap of 8V-0-8V winding and

Pin 6 (marked GND) doesn't actually go anywhere off the board, as the 2nd black Gnd wire actually goes to pin 7 (Key). So not quite sure why Commodore did that, and didn't put the 2nd black wire on Pin 6 (As someone else had suggested moving this to, although I don't think it would actually stop it working, just that there is twice as much ground wire resistance than there could be and having 2 ground connections could increase connector reliability a bit more)


So anyone trying to follow this for the first time, could get rather confused, and probably a good idea to pencil some corrections to printed copies!

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Old 29th Mar 2021, 8:03 am   #15
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Default Re: 2 dead CBM 8032-B 'PETs'

Quote:
Originally Posted by PJL View Post
Will you get sensible resistance measurements of the main board through the bridge diodes?
I'm looking to see if the BRIDGE is short-circuit, among other things. One of several possible reasons why we may not have +9V out on pin 4 / In on pin 2.

Owen, after briefly reading around a bit I understand that this same transformer assembly was used in different models, in some of which some of the wires may be redundant.
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Old 29th Mar 2021, 9:34 am   #16
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Default Re: 2 dead CBM 8032-B 'PETs'

The circuit is also confusing as the bridge is used as two full wave rectifiers giving +/-9V unregulated.

Leon, it is possible the 9V DC is there and the meter is measuring AC volts.
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Old 29th Mar 2021, 9:42 am   #17
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Default Re: 2 dead CBM 8032-B 'PETs'

If the meter does not autodetect AC/DC then most likely you get from AC to DC mode and back with a single press of the Function / Hold button.

The Ohms / Continuity / Diode position of the meter clearly shows these functions on a circular track suggesting that these functions do have to be cycled through.

The 'Volts' position does not have a similar circular diagram with 'DC' and 'AC' on it, so my default assumption is that the meter decides for itself, but you may be right, it may have to be manually selected.
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Old 29th Mar 2021, 1:00 pm   #18
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Default Re: 2 dead CBM 8032-B 'PETs'

Quote:
Originally Posted by SiriusHardware View Post
Quote:
Originally Posted by PJL View Post
Will you get sensible resistance measurements of the main board through the bridge diodes?
I'm looking to see if the BRIDGE is short-circuit, among other things. One of several possible reasons why we may not have +9V out on pin 4 / In on pin 2.

Owen, after briefly reading around a bit I understand that this same transformer assembly was used in different models, in some of which some of the wires may be redundant.
Yes, It may be that Commodore also used a selection of different transformer assemblies (like with the main boards) in PET models.
So that wiring diagram could be for a slightly-different one.

Although the incorrect function-label on 8032's main board J8 Pin 4, did seem to be an error - unless you had to use a single red-wire on Pin 2 transformer assembly with that particular board (so not quite always interchangeable between all the various main boards (like with 8096 one) and transformer assemblies).
But at least all the measured AC output voltages from the original one did appear correct, so a bridge-rectifier diode fault does look like mostly likely
(assuming there isn't very bad contacts on connector)
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Old 29th Mar 2021, 2:54 pm   #19
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Default Re: 2 dead CBM 8032-B 'PETs'

There's also a diode, CR3, (reverse polarity protection?) across the +9V return input (from the offboard capacitor) on pin 2-pin 3 - also potentially low resistance or short circuit.

The resistance measurements I asked Leon to try are aiming to check most of these potential issues before we go looking for voltages again.

When I first looked at the regulator circuit I couldn't make out where the +9V regulated voltage on the uppermost rail was coming from, it was only when Leon noticed that the two red wires on the connector (pin 4, pin 2) both go to the +ve terminal on the capacitor that it became clear - it comes from the + terminal of the CR2 bridge, out of pin 4, along one red wire to the capacitor + terminal, and then back along the other red wire to pin 2, from where it continues as '+9V unregulated' and goes to various destinations including the inputs of the +5V regulators VR3 and VR4.

The path between pin 4 and pin 2 is not illustrated on the transformer assembly diagram I posted earlier - possibly not the exact right one for this machine, but with the additional information Leon gave us, close enough.

Last edited by SiriusHardware; 29th Mar 2021 at 3:03 pm.
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Old 29th Mar 2021, 4:12 pm   #20
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Default Re: 2 dead CBM 8032-B 'PETs'

Yes, I presume they though it was better to have the +9V supply fed-through the reservoir capacitor, rather than the capacitor 'T'-ed off from the supply. Although the extra inductance / resistance of the wiring to a spur-connected capacitor is probably not much of an issue at 100Hz full-wave rectified frequency and ESR of capacitor.
And it does seem that on some boards there is only one pin on the J8 connector that has +9V DC on it. (If Pin 4 is not always this / 8096 doesn't have it on Pin 2 - Maybe that one has an on-board capacitor).

Hopefully DMM was one DC-Volts when originally checking the 9V DC, as not sure if there are ones that auto-detect. I remember analogue meters giving a different reading on DC- presumably as they were designed to read rms value.
Whereas Digital ones are rather variable: Measuring a 9V battery, a Fluke 23 reads 0V, but a cheap £2 CPC-Duratool one reads 18V!

To check the diodes properly, will no doubt have to discover the joys of the diode-test function, as DMM's typically have < 0.4V across probes on resistance so not enough to forward-bias most diodes.
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