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Old 12th Oct 2021, 10:10 am   #41
SiriusHardware
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Default Re: Commodore PET 2001 voltage regulators

We could come to that point if we ever get the power back up on this one - it's kind of you to offer and we may take you up on that if that is your wish but as the owner of a PET yourself you might want to think about keeping them handy.

You know how it is, any tool you give away will be the one you need the very next day.

I still have a good handful of suitable EPROMs here surplus to my requirements but it would be undeniably easier, and a way for you to contribute to this case, even if you only loaned them out.

Of course if you do pass them on and then eventually find you need them again, there would be no problem knocking up another set at that point.
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Old 12th Oct 2021, 10:21 am   #42
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Default Re: Commodore PET 2001 voltage regulators

As Sirius says it looks like the five suspect diodes are ok although I'm still nervous about CR3 (Y). Also we do seem to be looking for a poor connection/dry joint ahead of VR3 and VR4 so I'd like to suggest the following actions:

1. Lift (unsolder) one leg of CR3 and perform a diode test in both directions. This will at least conclusively eliminate CR3 from the equation if nothing else.

2. Reflow the solder joints of all five diodes (CR1, CR2, CR4, CR5 & CR3).

3. Whilst the board is out slacken the screw terminals of the large electrolytic and dose with IPA or contact cleaner before retightening.

Once the board is back in place perform the measurements suggested by Sirius. A small general point but do make sure that continuity tests are carried out with the machine switched off.

For reference and so that we all know the meter's capabilities I'm attaching a copy of the user manual.

Alan
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Old 12th Oct 2021, 12:22 pm   #43
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Default Re: Commodore PET 2001 voltage regulators

Hi all. Thanks again for all the replies. I'll try to get round to the new measurements after work today. I was thinking (and please correct me if this isn't a good idea) of performing the DC voltage tests first (post 37), then performing the work Alan suggested, then checking the DC work again.

A question that was asked previously that I didn't see is that I do have an EPROM programmer - but it's also a kind offer of the EPROM should it be required.

Thanks again,
Tim.
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Old 12th Oct 2021, 12:31 pm   #44
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Default Re: Commodore PET 2001 voltage regulators

Perfectly fine to check voltages first. Hopefully you won't need an EPROM programmer. Colin's PET was a pretty extreme example of a delapidated machine and I don't think your's will be too difficult to fix in due course.

Alan
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Old 12th Oct 2021, 12:35 pm   #45
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Default Re: Commodore PET 2001 voltage regulators

Here's my next thought. I have the same motherboard, so if anyone wants to know any readings from my working PET for comparison, I'm quite happy to do that.

The only downside is I am away on holiday for the next ten days, but if this thread is still running Iit may be of use then.
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Old 12th Oct 2021, 12:59 pm   #46
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Default Re: Commodore PET 2001 voltage regulators

Could turn out to be very useful Colin although I'm sure Tim will have his PET up and running by the time you return from holiday. Ever the optimist!

Just in case Tim hasn't seen it this is a link to Colin's thread:

https://www.vintage-radio.net/forum/...d.php?t=174829

I've hesitated to post the link before for fear of putting Tim off completely. It was truly an epic saga and must hold some kind of forum record.

Alan
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Old 12th Oct 2021, 5:15 pm   #47
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Default Re: Commodore PET 2001 voltage regulators

Quote:
performing the DC voltage tests first (post 37)
Provisionally yes, but note I made an error in post #37, corrected in #39.

The corrected version:

-Check voltage on Cathodes (tapered ends) of CR1 / CR2, (previously marked as 'Z')

-Check voltage on JP8 connection 4 (Should be exactly the same as the above voltage).

And the answer to my question - where are you putting the black probe when measuring DC voltages?

Last edited by SiriusHardware; 12th Oct 2021 at 5:30 pm.
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Old 12th Oct 2021, 5:50 pm   #48
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Default Re: Commodore PET 2001 voltage regulators

The random character display on the screen is most usually caused my the processor not starting up, which can be because of a PROM fault, the low RAM chip or a fault on the 555 reset circuit, or in the case of the #320251 version and later the DRAM refresh.

The PROMs on early PETS with 6540 ROMS got very hot (almost too hot to touch) not sure about the 3516/3532 ones though as the only PET I have with those doesn't work (on my todo list with its 1000 friends).

The fact you get a garbled display is in many ways a good sign, as it shows the monitor and graphic hardware are at least working, which makes diagnosis easier!
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Old 12th Oct 2021, 6:43 pm   #49
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Default Re: Commodore PET 2001 voltage regulators

Let's get a better view of the area being worked on, circuit diagram wise and layout wise.

I have crudely drawn in the offboard wiring, including the centre-tapped transformer secondary which feeds the area we are interested in. I've left the other transformer secondary and its connections to the CR6 / CR7 / CR8 / CR9 bridge out for now.
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Old 12th Oct 2021, 7:41 pm   #50
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Default Re: Commodore PET 2001 voltage regulators

Hi. I've attached a picture as to where I'm connecting the black probe when checking voltages - the negative lead of the capacitor near JP8.

I've just checked the DC voltages, and have the following:

- CR2 = 6.86v (red probe on tapered end)
- CR1 = 6.89v (red probe on tapered end)
- JP8 pin 4 = 1.68v

One thing I did notice with JP8 and pin 4. If you keep the red probe on, then the voltage starts to drop. I tested this a few times, and if the voltage was around 1.68v, then keeping the probe on meant a drop. However, when I tested again at different intervals, the initial reading was around 1.44, which then went up and stabalised at 1.44v.

I also checked the DC of pin 2 whilst checking pin 4, and it was also around 1.68v when tested.

Is it potentially worth trying the IPA/contact cleaner on the screw terminals of the large (off board) capacitor, then looking over the voltages again?
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Old 12th Oct 2021, 7:55 pm   #51
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Default Re: Commodore PET 2001 voltage regulators

C62 -Ve terminal is an ideal place to use as the 0V return for all DC voltage measurements in this area so for consistency, keep using that as your 0V.

Unless the information we are working from does not match your machine, your measurements would appear to show a problem between the meeting point of CR1/CR2 and JP8 pin 4, which should be directly connected together but can not be because the voltages on those two points are so different.

With the power off and JP8 connector plugged onto the PCB, please:-

-Measure the resistance between CR1 / CR2 tapered ends ('Z') and the contact on the JP8 plug-on connector pin 4, resistance =?

-Unplug the JP8 connector and measure the resistance between CR1 / CR2 tapered ends ('Z') and the PCB connector pin 4, resistance = ?
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Old 12th Oct 2021, 8:22 pm   #52
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by SiriusHardware View Post
C62 -Ve terminal is an ideal place to use as the 0V return for all DC voltage measurements in this area so for consistency, keep using that as your 0V.

Unless the information we are working from does not match your machine, your measurements would appear to show a problem between the meeting point of CR1/CR2 and JP8 pin 4, which should be directly connected together but can not be because the voltages on those two points are so different.

With the power off and JP8 connector plugged onto the PCB, please:-

-Measure the resistance between CR1 / CR2 tapered ends ('Z') and the contact on the JP8 plug-on connector pin 4, resistance =?

-Unplug the JP8 connector and measure the resistance between CR1 / CR2 tapered ends ('Z') and the PCB connector pin 4, resistance = ?

Hi. That's good to know about the 0V connection. I've been using that throughout, so will continue to do so.

I've just run the resistance tests as well (with the power off). Both were run in the 200 ohms range on the multimeter.

With JP8 connected, black on pin 4 (JP8) and red on tapered CR2 = 00.2. For CR1 = 00.2

With JP8 unplugged, black on pin 4 (JP8) and red on tapered CR2 = 00.2. For CR1 = 00.2


I've also had another look over the top of the board and can only see a reference for ABBY No. 320351 (which I think I reference before). I can certainly check the rear of the board to see if any further details are available (if needed).
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Old 12th Oct 2021, 8:37 pm   #53
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Default Re: Commodore PET 2001 voltage regulators

Do you have some crocodile clip leads with shrouded clips on each end? If so, make a temporary connection between either CR1 or CR2 tapered end and the +ve terminal on the large offboard capacitor - the one with the red wires from JP8/2 and JP8/4 going to it.

Leave all wires which are already connected to the offboard capacitor in place.

If you don't have croc leads, bare the ends of a suitable length of insulated wire and connect the ends to the two points mentioned. However you make the connections, ensure that they are completely secure at each end and can not slip off.

Once that is in place, power on the machine. If it doesn't come on within the usual time that you would expect it to take, power off and remove the bypass wire.
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Old 12th Oct 2021, 8:53 pm   #54
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Default Re: Commodore PET 2001 voltage regulators

I have some crocodile clip leads so will give this a look over now. I'm hoping they're large enough for the +ve side of the capacitor as it looks pretty congested around there!

I'll report back with what I find.

Thanks.
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Old 12th Oct 2021, 9:00 pm   #55
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Default Re: Commodore PET 2001 voltage regulators

Be careful not to short the crocodile clip against the regulator heatsink even if the clip is insulated. There's not much room.

Alan
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Old 12th Oct 2021, 9:00 pm   #56
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Default Re: Commodore PET 2001 voltage regulators

Please do be careful not to cross-connect with anything else in that area. Take your time. If you need to daisy chain more than one croc lead to get the length, wrap tape around the clips in the 'join' in the middle.

Edit: Cross posted with AJ.
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Old 12th Oct 2021, 9:03 pm   #57
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Default Re: Commodore PET 2001 voltage regulators

Hi. We have some success! I connected the clip to CR2 for a start, then to one of the lugs for the red wires on the large capacitor - it seemed the easiest place to attach it. I've also included some pictures so you can see the set up.

On powering up the PET, we get a garbled screen!

I then powered down and tried the same using CR1; again, success.

So is it potentially looking like an issue with pin 2 and pin 4 (or their route to CR1/2) with not supplying voltage?
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Old 12th Oct 2021, 9:05 pm   #58
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Default Re: Commodore PET 2001 voltage regulators

Quote:
Originally Posted by SiriusHardware View Post
Please do be careful not to cross-connect with anything else in that area. Take your time. If you need to daisy chain more than one croc lead to get the length, wrap tape around the clips in the 'join' in the middle.

Edit: Cross posted with AJ.
Thanks, both, for the advice. I was nervous as there's not too much room, but hope you can see fro the pictures that it looked okay. I didn't leave the PET on for long for each test and have now removed the clips.
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Old 12th Oct 2021, 9:22 pm   #59
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Default Re: Commodore PET 2001 voltage regulators

<Blowing out cheeks>.

OK, so in spite of one measurement which seems to confirm it and another which seems to contradict it, it looks as though there is a problem with the connection between the plug-on JP8 connector's socket 4 and the PCB connector's pin 4, even though your resistance measurements argue against that - it's possible that the connection is making when you press on it to make the measurements and then opening up when you remove the probe. To resolve this apparently impossible combination of measurements, I asked you - as you no doubt realised - to bypass JP8 pin 4 by way of a casting vote.

The right thing to do is to try to fix the apparent JP8 / pin 4 problem before proceeding. If you prefer, you can install the bypass wire as detailed earlier in a semi-permanent fashion and have a look to see what other problems the machine has.

Unfortunately, the randomized screen seen when the power was getting through shows that although the display RAM and hardware are almost certainly working, the processor subsystem is not working well enough now to manage to clear the screen, so getting to the bottom of that will be the next job once the power is restored properly.

It may be that powering up the machine with only half of the supply output voltages present - which you could not avoid because that supply died without warning - has killed some of the ICs, unfortunately.
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Old 12th Oct 2021, 9:30 pm   #60
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Default Re: Commodore PET 2001 voltage regulators

Thank you so much for the help so far. I really do appreciate it.

I think I'd be more comfortable in trying to fix the power issue with pin 4, without using the bypass wire. It would be nice to know that the power is all okay, then move onto other checks.

I'm just happy that I've the screen back again!

Your comment about some of the ICs being damaged. Yes, I'm afraid of that too. I'm hoping things are maybe not too bad, but we'll see how it goes for the next steps.
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