Commodore PET 2001 voltage regulators
 

Hi,

I’m wondering if you can help me. I have a Commodore PET 2001 (assembly 320351) and I’m 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 – 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 – 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, I’m 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 I’m 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 I’ve detailed.

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.

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.

http://www.zimmers.net/anonftp/pub/c...01N/320349.pdf

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).

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!

Re: Commodore PET 2001 voltage regulators
 

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.

Re: Commodore PET 2001 voltage regulators
 

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.

Re: Commodore PET 2001 voltage regulators
 

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!).

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.

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.

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).

Re: Commodore PET 2001 voltage regulators
 

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.

Re: Commodore PET 2001 voltage regulators
 

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.

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.

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.

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.

Alan

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:-

...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?

Re: Commodore PET 2001 voltage regulators
 

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.

Re: Commodore PET 2001 voltage regulators
 

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?

Re: Commodore PET 2001 voltage regulators
 

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.

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.

Re: Commodore PET 2001 voltage regulators
 

Thanks for checking those connections - when stating the results of resistance measurements could you also please state the units of measurement as well as the figures (so Ohms, K-Ohms M-Ohms, etcetera). I guess with your stated figures of 620 and 580 you mean Ohms, but maybe not.

If your meter has a 'diode test' range on it could you please do them again but this time measure all five with the probes placed red to tapered end, black to flat end, and then measure them all again but this time with black probe to tapered end, red probe to flat end. In this mode your meter indicates the voltage drop across the diode rather than its resistance.

Incidentally these measurements need to be made with JP8 plug unplugged, otherwise you may be measuring the mains transformer as well as the diodes.

Re: Commodore PET 2001 voltage regulators
 

For ease of reference I'm attaching a copy of the 320351 board layout. This might help in identifying test points in relation to the schematic as component designations are clearly marked.

Alan

Re: Commodore PET 2001 voltage regulators
 

I can't help feeling that it would be a good idea to lift one end of each of the suspect rectifier diodes (CR1, CR2, CR4, CR5 & CR3) in order to carry out proper open circuit diode tests. If nothing else this would at least eliminate them from our enquiries. In circuit testing can produce such confusing results. The solder pads are quite large and with a little deft application of the soldering iron it should be possible to avoid troublesome board damage.

It does concern me a little that the board has been worked on before which can make things tricky in diagnostic terms especially if the previous repairer gave up and consigned the PET to the loft in frustration.

I've just switched on my PET 2001 for the first time in a while and am relieved to say that it's still working well.

Alan

Re: Commodore PET 2001 voltage regulators
 

I'm not sure (because we don't know Retromit well yet) how confident he (or possibly she) is at desoldering components cleanly especially in antique double sided PCBs, so I was hoping to see an obviously bad diode without desoldering any, but if the results are inconclusive then you are right, it may be necessary to measure them with one end disconnected from the circuit. If we do identify a bad one in-circuit then it would be easier and less stressful to the PCB to snip out the diode body and remove the legs separately.

Going all the way back to #1 we can see that the machine has worked for a period, however brief, so we can say that the various mods are harmless and probably original, except for the socketing of the RAM which was obviously done when someone had a RAM problem at some point in the past. I have some concerns that the machine running without +5V but with the other supplies still present may have caused some device damage that was not there originally.

Re: Commodore PET 2001 voltage regulators
 

Let's see how the OP feels about a bit of soldering work. With regard to possible damage due to having no +5V lines the fact that the PET briefly showed signs of life again @ post #11 is a hopeful sign. However this again indicates a possible poor connection/joint rather than complete component failure. I must admit that I'd be doing a bit of wiggling and prodding (plastic chopstick being the instrument of choice!).

Alan

Re: Commodore PET 2001 voltage regulators
 

The case history as I understand it is that when acquired it had an apparent display fault, was fixed by replacement of the screen RAM, was turned off apparently working but on next start, would not turn on and, on investigation, had lost its 5V supplies.

The machine has not been seen working since, so we'll just have to see what happens once the supplies are up again.

Re: Commodore PET 2001 voltage regulators
 

My understanding of post #11 is that the PET did briefly produce a random character screen after the OP had carried out some of the suggested checks before dying again.

Alan

Re: Commodore PET 2001 voltage regulators
 

Indeed so, I should pay more attention.

Re: Commodore PET 2001 voltage regulators
 

Hi,

Thanks for all the replies/thoughts, and apologies for my late update on these.

Just from some of the replies, I've included the original screen shot of when I first aquired the PET. It was from this that I replaced the video RAM which (briefly) showed the Commodore BASIC screen. After switching the PET off, then back on, this is where I am now.

It did briefly kick into life (with the garbled screen picture I posted), but since them the screen has been blank.

I'll start to go through the other replies, and some more tests that have been carried out.

Regarding soldering, I'm happy to do this. I have worked previously on other 8-bit machines, and have soldered/desoldered. This is the first time I've ever had the chance to look over a PET.

Right, I'll update with the rest of the findings!

Thanks,
Tim.

Re: Commodore PET 2001 voltage regulators
 

Yes, that pattern is more reminiscent of a problem with the display RAM which you obviously did fix when you replaced that. Just have to get to the bottom of this ******* power problem and then we'll see where we really are.

Good to know that you are confident with soldering, we never really know where to pitch advice initially - if we aim too low we run the risk of patronising someone who has been working with electronics for years but if we aim high we could end up frightening off a newcomer who doesn't understand a word of what we are saying. We usually find the right level in the end.

Re: Commodore PET 2001 voltage regulators
 

For the diode test, I carried out the following. I believe the output is in ohms, but with this being a basic multimeter, there is no value for the unit.

Multimeter on diode test and red end tapered, blank flat.

All the bottom four diodes (X and Y, plus the two others near the silver capacitor) registered a value of '1'; the diode (W) near the capacitor registered 1689.

When black was on the tapered end, and red on flat:

The bottom four diodes (left to right) measured:

1 = 570
2 = 591
3 = 578
4 = 583
W (near the capacitor) = 532

Re: Commodore PET 2001 voltage regulators
 

I looked over the resistance readings again, and apologies for missing the units that should have been applied. When setting the multimeter to 2000 ohms range, the following was carried out:

Black on flat end, red tapered:

All bottom four diodes registerd '1'. The diode near the capacitor (W) measured a 4 figure value.

Red on flat end, black tapered:

All showed a four figure value in the 2000 ohms range.

Re: Commodore PET 2001 voltage regulators
 

If your meter was on 'Diode test' (was it?) then those readings in #31 look broadly OK. If you try doing one of the later group again you may see a decimal point ahead of the three figures, ie, '.570'. If so that means a forward voltage drop of 0.570 - generally speaking a forward voltage reading of around 0.5V to 0.6V is healthy for a Silicon diode, which these are.

Your resistance readings also look OK to me.

So it doesn't look like that is it either. :(

Re: Commodore PET 2001 voltage regulators
 

Please don't worry about patronising me! I am a beginner who enjoys working on these old machines. I'm just self-taught, but happy to keep learning (and am learning!) each day. I just like to work on these machines and try and get them working again. Again, thanks for all the help you've already provided.

Re: Commodore PET 2001 voltage regulators
 

Yes, the meter was on diode test (it looks to be the same function selection as the continuity test). It's only a very basic multimeter (Astroai am33d).

Re: Commodore PET 2001 voltage regulators
 

I am just wondering why there was a difference in the diode test for the diode near the capacitor (marked W on the picture you gave). When the multimeter was on diode test with the red end on the tapered side and the black on the flat side, then all the other diodes registered a value of '1', whereas the 'W' diode registered 1689?

I thought they would all register '1'?

Re: Commodore PET 2001 voltage regulators
 

Going back to some measurements you made earlier -

That 7.03 volts on the output of the positive voltage rectifier is not actually far short of what it should be but it's low, so something may be loading it. We'll get to that in a bit. Let's just first find out how things stand now. Sorry to take you round in circles but when things keep changing we have little choice but to go back and re-check. Can we ask you to make the following measurements again:-

Referring to the layout diagram kindly attached by AJ in post #22, please measure the DC voltage (with respect to 0V) on

-CR1 / CR2 tapered ends (which I previously marked as 'Z')

-JP1 pin 4

Electrically speaking, these are the same place / same circuit node so if they are not at exactly the same voltage there must be an open circuit or high resistance between them.

If the voltage on 'Z' has changed again and is the same as the voltage on JP1 pin 4, then it looks as though we have an intermittent fault to contend with. I suppose this is to be anticipated since we have already seen the supply present, then absent, then briefly present, then absent again.

By the way - what / where are you using for your 0V point to put the black
meter lead on when measuring voltages?

Re: Commodore PET 2001 voltage regulators
 

You are measuring the diodes in-circuit, so the reading you see is the reading from not only the diode but other components around it. In the case of the 'W' diode, that one has a large electrolytic capacitor and the inputs of the two 5V regulators, to name just a few things, connected across it. That is why AJ suggested desoldering one end of each diode so that when you made your measurements you would ONLY be measuring the diodes.

If one of the diodes had been short-circuit or very low resistance (which is the most usual way for them to fail) then that would have been obvious even when measuring them in-circuit. There are times when it's reasonable to try to measure things in-circuit and times when you definitely should separate the suspect component from everything else it is connected to before trying to measure it.

At the moment my feeling is the diodes are OK, but we may come back to them.

Re: Commodore PET 2001 voltage regulators
 

Grr, I really shouldn't try to do this late at night. I meant JP8 pin 4, sorry.

Re: Commodore PET 2001 voltage regulators
 

If it's ever required, I still have the two test EPROMs with the daver2 and slothir's test code on them if that'll help. Happy to post them on to the OP of this thread.

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.

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

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.

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

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.

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

Re: Commodore PET 2001 voltage regulators
 

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?

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!

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.

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?