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retromit 8th Oct 2021 1:26 pm

Commodore PET 2001 voltage regulators

Iím wondering if you can help me. I have a Commodore PET 2001 (assembly 320351) and Iím in need of some advice for the voltage regulators VR4 and VR3.

To give some background, it was bought locally and had been stored in a loft. I bought it non-working, but it did boot (had a garbled screen). Looking over some other posts, it seemed like it could be a video RAM problem Ė so I replaced the two MPS2114 with two MM2114N-3 (from a Commodore 64). This solved the garbled screen as it showed the Commodore BASIC text, as well as the memory available Ė good news. After switching it off then trying again later, nothing was displayed on the screen.

I checked the voltages for VR6 (-5.06v) and then CR12 (+12.16v). On checking CR11 and CR10, they both showed voltages of around 0.48v. So, Iím thinking the voltage regulators for the 5v line may have gone (hence nothing appearing on the scree - although I could be wrong here). The PET I have has LM323K STEEL regulators for the +5v.

What Iím asking is do you know of any modern replacements for the LM323K STEEL which you could recommend, and would it also be prudent to change the +12v at the same time?

Thanks in advance, and please let me know if you need any further details or have any further ideas/suggestions on what Iíve detailed.

SiriusHardware 8th Oct 2021 1:55 pm

Re: Commodore PET 2001 voltage regulators
Assuming this is similar to ones we have worked on in the past there are actually two +5V regulators with the outputs not tied together, so for both to have no output implies failure of something common to them both, possibly the input to the 5V regulators. What voltage do you see on the cathodes (ringed ends) of CR10 and CR11?

If you had the board out and the connectors disconnected to fix the original fault, look especially carefully at the power input connector J8, make sure none of the solder joints on that connector are cracked.

Well done fixing the original screen fault, by the way.

SiriusHardware 8th Oct 2021 2:32 pm

Re: Commodore PET 2001 voltage regulators
Although not the reference you quoted, all the part numbers in this circuit diagram match those you have quoted.

What's not clear from this diagram is that the AC power for the +9V and -9V supplies comes in from the transformer on J8 pins 1 and 5 (so check across those two point for AC Volts) and then +9V comes out from the bridge rectifier on JP8 pin 4, goes to the +Ve of the large capacitor on the offboard power supply 'lump' and then comes back onto the board via JP8 pin 2. From there it goes to the input pins of the two +5V regulators, so if you have no output from both of the +5V regulators the most likely reason is a missing +9V supply to the regulator inputs.

So try looking for:

...AC volts across JP8 pins 1 and 5

...Around +9VDC (with respect to 0V) on the following points in turn:-

...JP8 pin 4

...+ve terminal of the large offboard capacitor

...JP8 pin 2

...CR10 cathode / CR11 cathode (the ringed ends of the diodes).

retromit 8th Oct 2021 3:10 pm

Re: Commodore PET 2001 voltage regulators
Hi. Many thanks for the reply.

I may have been lucky when replacing the video RAM as they were already socketed (white sockets), so the board hasn't been taken out. I've only a basic multimeter, but have recorded the following:

- AC across JP8 - pin 1 = 07.6, pin 5 = 07.6
- JP8 pin 4 = 1.84v (DC)
- JP8 pin 2 = 1.84v (DC)
- CR10 ringed side (left leg) = 1.83v (DC) and right leg = 0.58v
- CR11 ringed side (left leg) = 1.83v (DC) and right leg = 0.55v

As for the large capacitor, I'm unsure on how to test this. There looks to be two screw terminals on the right hand side (looking at it).

Please forgive any naivety on my part as I'm still a novice, but trying to learn!

merlinmaxwell 8th Oct 2021 4:40 pm

Re: Commodore PET 2001 voltage regulators

Please forgive any naivety on my part as I'm still a novice, but trying to learn!
Learning is the best tool in the box, no forgiveness needed.

If you take my first "electrical" thing of wiring up a LEGO lighting up brick as the start I have been doing electronics for 56 years, I still learn at least one thing a day, bloomin' brilliant.

SiriusHardware 8th Oct 2021 5:02 pm

Re: Commodore PET 2001 voltage regulators

- AC across JP8 - pin 1 = 07.6, pin 5 = 07.6
Sorry Retromit, have to ask you to clarify: Normally when asked to make voltage measurements you would make them with respect to GND / 0V, so, the black probe on 0V and the red probe on the point being measured.

However, when measuring AC voltages they will often not have a direct reference to GND so you have to put your meter directly on the two sides of the AC supply, in this case JP8 pins 1 and 5. Can you look at that again, meter on ACV, red probe on JP8 pin 1, black probe on JP8 pin 5, and just confirm your measurement again?

Could you also please post a clear photo of the JP8 connector and the surrounding area just in case we are barking up the wrong tree, circuit diagram wise. Your power supply might be a bit different.

retromit 8th Oct 2021 7:01 pm

Re: Commodore PET 2001 voltage regulators
4 Attachment(s)
Sorry about that. I'm used to measuring DC, but not AC. I've now measured as you suggested between pin 1 and pin 5; this comes out as 16.1v.

I've also taken (and attached) four pictures which hopefully show the area requested. Please let me know if any other information/higher resolution is needed.

Thanks again for your help (and patience!).

SiriusHardware 9th Oct 2021 8:43 am

Re: Commodore PET 2001 voltage regulators
Having looked around a little bit more I expect that when you measure the AC voltage between JP8 pin 1 and JP8 pin 5 it will be about 15V AC. If you find that to be the case, measure the DC voltage on the screw terminals of the big offboard capacitor (Red meter lead on capacitor +, black meter lead on capacitor -). That should be approximately 9V DC.

Let us know what you find.

retromit 9th Oct 2021 9:23 am

Re: Commodore PET 2001 voltage regulators
I've measured the AC between pin 1 and pin 5 this morning and it comes out at as 15.9v AC. I checked it a few times and it seems pretty constant. I then checked the DC of the big capacitor; it read 1.83v. I tried the meter leads at various points around the screw terminals, and it was 1.83v at those points. I lef the PET on for a few minutes to check the voltages again, and they came in around the same values for both AC and DC.

It looks like my post from last night has been authorised, so it contains some of the pictures of the board/JP8 that were requested. Hope these help, and thanks again.

SiriusHardware 9th Oct 2021 9:47 am

Re: Commodore PET 2001 voltage regulators
Retromit, thanks for the detailed images. I should point out that as a new forum member your posts are held for moderation, after which they appear in their correct chronological position in the thread. When I posted #8 above your post #7 wasn't yet visible.

OK, so - As explained earlier, what should happen is that your 16V AC goes to a group of four diodes on the mainboard and is turned into +9V and -9V DC.

The +9VDC then goes back offboard out of JP8 pin 4 on a red wire to the + terminal of the large metallic blue capacitor mounted offboard, then comes back from there down another red wire and back in on JP8 pin 2. From there, it goes to the cathodes of CR10 / CR11 and the input terminals of the two 5V regulators.

From your measurements so far you have the AC voltage (about 16V) going into the board on JP8 but you don't seem to have the DC voltage which should be coming out of the rectifiers, so the supply is going missing somewhere in that area.

I'm about to go out for the day, have a look in the general area of JP8 - lift the board and look at it for problems like cracked solder joints underneath, especially on JP8 itself. If you need more specific point to point help with tracing the problem I'll sit down with the diagrams later and try to give you an exact list of points to measure between. (Anyone else who can do this earlier, please pitch in).

retromit 9th Oct 2021 8:24 pm

Re: Commodore PET 2001 voltage regulators
1 Attachment(s)
Hi. Thanks again for the info. I haven't taken the board out yet as I didn't have a lot of time today. However, I did try the continuity testing with the pins/cathodes as suggested. Here's what I found:

- JP8 pin 4 to +ve terminal on capacitor was fine; had continuity.
- JP8 pin 2 to +ve terminal on capacitor was fine; had continuity.
- pin 2 on JP8 to left leg of CR10 and CR11 had continuity.

I measured the voltages again on all these, and they reported the low values as previously posted. I then checked continuity again, but then noticed that the screen had come on (I've attached a picture). I had been very careful when checking voltages/continuity, but was surprised to see the screen. I powered off/on the PET a couple of times and it seemed to keep bringing the screen on. So some good news. I left it for a while, then tried again; it came on but with the garbled details. I did take the voltages at this point, and they all recorded what you guggested; the big capacitor around 9v DC, CR10 and CR11 (right leg) around 5v DC.

I didn't do anything else for a while, then went back to it. Turned it on and no screen. Checked the continuity; all okay. Checked the voltages, all low.

One thing I did notice when the screen was working for a while was that UD6 and UD7 were getting quite hot to touch. I don't know if this is relevant.

So it's be a step forward (but I don't know why it sprung into life), but back to no screen working again and low voltages.

I'm hoping none of the chips have been damaged by the drop in voltage (or whatever caused it). I'm wondering if the two 2114 RAM chips I changed for the video RAM could be fried (or any other chips for that matter).

I'll have another run through of everthing tomorrow to see if anything is different, but thanks again for your help with this.

SiriusHardware 9th Oct 2021 8:37 pm

Re: Commodore PET 2001 voltage regulators
1 Attachment(s)
I see that your post #9 was pending, not yet visible, when I posted #10, sorry about that. I'm going to have to allow more time for your replies until you get clear of new user moderation.

Given your previous observations can you now make measurements - with the power on - between the following points marked up on one of your PCB images?

AC volts, one probe on 'X', the other probe on 'Y', voltage =?
DC volts, Black probe on 'W', Red probe on 'Z', voltage =?

You might have to scrape a clean area on each of those diode legs (with the power off) in order to get a good connection, as they look rather dull.

retromit 9th Oct 2021 8:50 pm

Re: Commodore PET 2001 voltage regulators
Yes, it will be good when the new user moderation is cleared as I don't want you to think I'm ignoring the posts!

I'll certainly try the new measurements from the picture/details you've given in post 12 tomorrow morning, then reply to the post with the findings.

Thanks again.

SiriusHardware 9th Oct 2021 9:16 pm

Re: Commodore PET 2001 voltage regulators
Yes, you are going to have to get to the bottom of that power problem before you can seriously tackle anything else.

Random stuff on the screen is (counter intuitively) a sign that your screen RAM and display hardware are working normally, but the main system is not managing to clear the screen, one of the first jobs it usually does.

Do you have the means to program EPROMs? There is some test code which could be very useful to run at this point but it needs to be programmed into an EPROM.

UD6 and UD7 are PROMs and PROMs CAN run quite hot in normal operation although I can't say whether yours should. One of the jobs the test code I mentioned does is to checksum the PROMs, which gives a useful indication as to whether they are OK.

ajgriff 10th Oct 2021 9:15 am

Re: Commodore PET 2001 voltage regulators
The missing voltages and intermittent operation suggest a possible problem with molex connector J8. I had exactly this issue when tackling my PET 2001 (earlier model but similar enough). The problem was cured by removing the board and reflowing the solder joints to the molex connector. This would be a worthwhile first step along with cleaning the connector pins/contacts. Once you've got consistent line voltages and a screen display we can worry about the next steps as indicated by Sirius.

By the way the PROMs normally get disturbingly warm so don't get too concerned about that at the moment.


SiriusHardware 10th Oct 2021 11:52 am

Re: Commodore PET 2001 voltage regulators
Thanks to AJ for confirmation about the PROMS being likely to run warm normally.

I'm afraid a physical inspection of the solder connections on JP8 on the underside of the PCB is becoming increasingly necessary, if only to rule it out, and if you get that far you may as well resolder / reflow the joints on the JP8 connector, assuming you have the means to do so.

In addition to these continuity checks already made:-


- JP8 pin 4 to +ve terminal on capacitor was fine; had continuity.
- JP8 pin 2 to +ve terminal on capacitor was fine; had continuity.
- pin 2 on JP8 to left leg of CR10 and CR11 had continuity.
...could you also unplug the JP8 plug and check for continuity between the JP8 PCB connector pin 1 and 'Y' indicated on the image posted earlier, JP8 connector pin 5 and 'X' indicated on the image, and JP8 connector pin 3 and 'W' indicated on the image?

retromit 10th Oct 2021 4:36 pm

Re: Commodore PET 2001 voltage regulators
3 Attachment(s)
Hi both. Many thanks for the replies. Okay, the following has been done today:

First off, the voltages in relation to the image details:
AC volts, one probe on 'X', the other probe on 'Y', voltage =16.2
DC volts, Black probe on 'W', Red probe on 'Z', voltage =7.03

After that, I decided to remove the board to see if anything was amiss underneath. Having a look around JP8, it looked okay to me, but I decided to reflow all the connector points as advised, then use contact cleaner in the molex socket (and pins). Looking at the 12v and 5v regulators, they looked a bit crusty, so I also reflowed them (I've attached pictures). Underneath the board, it also seems as if there are a few bodge wires and it also looks like the main RAM has been socketed at some point (a lot of flux around the pins - picture also attached). I haven't done anything about this for the moment.

After going through that, I put the board back in, but still the same voltages when checking AC and DC - no screen and low voltages. I measured the voltages mentioned at the beginning of this post, and they were almost identical.

I've just checked the continuity between the following:

JP8 pin 1 and Y - no continuity (but continuity with X)
JP8 pin 5 and X - no continuity (but continuity with Y)
JP8 pin 3 and W - continuity.

Thanks again for your patience and advise with this.

SiriusHardware 10th Oct 2021 4:49 pm

Re: Commodore PET 2001 voltage regulators

JP8 pin 1 and Y - no continuity (but continuity with X)
JP8 pin 5 and X - no continuity (but continuity with Y)
I confess I took an educated guess which would go to which based on physical position, trust me to get it wrong on a 50-50.

OK - with power off and JP8 plug disconnected from JP8 pins, can you look for very low resistances on the following components in-circuit:-

-The main group of four diodes at lower left (where my X, Y, and Z marks are)

-The diode by itself up beside the big dark blue capacitor (where my 'W' mark is).

You're not looking for any particular resistance value, the reading may very well change markedly as you are looking at it due to the presence of capacitors nearby, and it will be different depending on which way around you put the probes on the diode. What you are looking for is any of those diodes showing a constant low resistance (in the single figure to low treble figure ohms range).

What have you got?

SiriusHardware 10th Oct 2021 5:02 pm

Re: Commodore PET 2001 voltage regulators
1 Attachment(s)
Another thing, those metal regulators have three connections, one of which is the metal case. Look at the point I have ringed in orange on one of your photos, that rivet or whatever it is is supposed to carry the connection from that thick green track through to the metal body of the regulator. Can you check that the connection from the track, through the board to the body of the regulator actually is continuous? Also check any other similar through-hole connections as well.

retromit 10th Oct 2021 7:12 pm

Re: Commodore PET 2001 voltage regulators
I've just checked the resistance of the diodes, and all three are showing three figures when testing; around 620 for bottom four, and around 580 for one near capacitor.

I checked the rivet you circled, and it does indeed have continuity with the thicj track. I also tested the continuity from pin 2 (JP8) on the back of the board to the two +5v regulators (and tracks from them) and it all seemed okay.

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