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CBM 8032, working but not

BrianG

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Hello all!

Been lurking awhile, finally posting to ask a question. New to vintage computers in the last year, new to electronics. Enthusiastic about both!

I have a Commodore CBM 8032 that I purchased several months ago. I know nothing about this unit's history. From my first day with it, it would power on, make the initial beeps, and the screen would come on going directly to all forward-slash characters. No garbage-screen before that. Ten minutes pass, nothing changes. Keyboard entry does nothing. Spent the weekend, tearing the whole thing apart and cleaned it up nicely, hoping that maybe that would be enough to do the trick. It was very dirty so why not. Got it all back together, and it shined up nicely. There are 3 chips that are removable, I removed them and reseated them. Fired it back up an no change. Powers on, initial beeps, screen goes straight to all forward-slashes. Actually, I think they are forward-slashes separated by spaces. Top to bottom, left to right, screen filled with slash, space, slash, space, etc.

Looking at the mother-board, all the chip pins seem to be in really good condition, except for one chip at the UE12 location, which has a bit of corrosion but not seemingly so bad as to make it inoperable. The board is a Universal Dynamic Pet 8032089, though on the bottom side of the board it says 8032090.

So I am wondering if anyone here has any theories on what may be the problem. From poking around online, it seems that maybe one of the ROM chips could be bad? Bad RAM? Is there a way to test the chips without desoldering each one, one at a time? Any way to narrow down the possible problems? Could it be something else altogether? And besides that, does anybody have a link of a schematic for this particular motherboard? I can't seem to find one, that is legible, for this board. One that displays what each chip is, each capacitor, etc.

If anybody takes a moment to respond to this thread, thank you in advance, and please dumb it down for me a bit. I am new to this, but eager to learn. I've got a soldering iron, a voltmeter, and a buddy with an oscilloscope.

Happy New Year!!!
 

daver2

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Welcome BrianG.

Where are you located? US, UK somewhere else?

The fact that it beeps on start-up is good - it is trying to do something at least...

The interesting thing is that the screen is filled with a regular pattern of characters. That implies that the firmware set the video RAM up like that - or there is something amiss with the video circuitry. any chance of posting a picture of the screen on start-up?

Yep - get over to http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/ and try and identify the specific schematics for your particular machine (probably univ or univ2).

Let me think on that video problem for a while...

Dave

EDIT1: Just out of interest (after you boot the machine to the 'slash space' screen) - just try typing "PRINT CHR$(7)" and hit the <RETURN> key. Out of interest - does it beep? Just thinking that the machine may be working but something is wrong with the video - if we get a BEEP we know that is potentially the case. A bit of a long shot though!
 
Last edited:

BrianG

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Las Vegas, NV
Thank you all for such a fast reply!

I found a legible schematic using the suggested links. There are a few differences with the board that I actually have, but everything else looks spot on.

daver2, I tried typing in what you had suggested, and no beep in response. I tried a few times just to make sure but nothing happened.
BTW, I'm in the US, in Nevada.

Hopefully, the attached photo is included in this post. This is the screen after powering on. It goes straight to this, no garbage screen.


2356.jpg


Cheers!
 

dave_m

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This is the screen after powering on. It goes straight to this, no garbage screen.


View attachment 35132
After it's been on for a while, and you quickly cycle power, do you see a brief garbage screen before it goes to even 'space' and odd '/'?

It seems to me the odd lower nibble Video RAM (UC6) is showing F hex instead of 0 hex, so a cleared screen is reading $2F (/) in the odd positions (address $8001, 8003...) where the even addresses are reading correctly $20 (space). This also assumes the PET firmware is running out of program just after it clears the video RAM or you would see the sign on message. Can you piggy back a 2114 RAM in UC6?
 

daver2

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No problem with the 'fast replies'. We like doing this and getting a few more PETS working again! It is also quite useful working in multiple time zones - I get up 5 to 8 hours ahead of you guys - but get to bed earlier...

+1 for Dave's suggestion regarding the odd video ram. There is nothing wrong vertically. The problem is horizontal with alternate characters (from the odd and even video RAM) seemingly bad.

Let's concentrate on getting the video hardware correct to start with, and then working on the CPU.

Dave
 

BrianG

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So, after a 10 minute warm up period, I did a quick power off/on and I saw the garbage screen for maybe a half-second. Then, back to the forward slashes. Tried it twice more and same results. Thank you dave_m!!!

As a side note, kept the computer on for another ten minutes, then felt all of the DIP chips on the main board and none of them felt particularly hot.

So I don't have any 2114 chips laying about for the piggyback. Before I go shopping for a few, are there any other specs that they need to meet besides being a 2114 DIP static RAM chip with 18 pins? I see specs for voltage and speed as I am looking at some online. Is there a particular parameter that they need to meet to be used safely in the Commodore?
 

MikeS

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...
+1 for Dave's suggestion regarding the odd video ram. There is nothing wrong vertically. The problem is horizontal with alternate characters (from the odd and even video RAM) seemingly bad.

Let's concentrate on getting the video hardware correct to start with, and then working on the CPU.

Dave
Yup; definitely looks like a problem with UC6 (to begin with) as Dave suspects. Make sure you're using the correct schematics; for your board I believe it should be the 8032087-xx and 8032090-xx print sets which should correspond more or less exactly to your board; if you are looking at these and they don't match your board exactly, what are the discrepancies?
 

dave_m

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Before I go shopping for a few, are there any other specs that they need to meet besides being a 2114 DIP static RAM chip with 18 pins? I see specs for voltage and speed as I am looking at some online. Is there a particular parameter that they need to meet to be used safely in the Commodore?

The PET is an old design running at 1 MHz and with +5V power and works with even the oldest 2114 RAM chips or equivalent. So 400 nS or faster access time will be fine.
 

BrianG

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Yup; definitely looks like a problem with UC6 (to begin with) as Dave suspects. Make sure you're using the correct schematics; for your board I believe it should be the 8032087-xx and 8032090-xx print sets which should correspond more or less exactly to your board; if you are looking at these and they don't match your board exactly, what are the discrepancies?

Looking at the board at http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/univ2/8032090-7.gif
- It seems the only difference that I see is a missing C89 and FB26 on the schematic. On my board these are just below UD2 and UD3.
 

BrianG

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Received a few of the 2114 chips today. Just got home from work and opened up the package. I piggybacked a chip onto the UC6 and the result is now all forward slashes, without any spaces in between. See photo. So...progress!!! It's actually very exciting to me, good call guys! But, now what?

IMG_2984.jpg
 

MikeS

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Hard to tell from the original screen shot, but are we sure it's the odd RAM that's bad and not the even? What happens when you piggyback UC4 instead?

m
 

dave_m

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Hard to tell from the original screen shot, but are we sure it's the odd RAM that's bad and not the even? What happens when you piggyback UC4 instead?

m
That's quite possible, but then I'd hope for no change rather than going to all slashes, but let's see what happens when UC4 is tested that's the right next step.
Brian, why is image rotated 90 degrees? Did that happen when you transferred the file to the computer?
 

BrianG

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Just piggybacked UC4 and the resulting image looks the same as the first image that I posted, with alternate slash-space-slash-space along the horizontal.
 

BrianG

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That's quite possible, but then I'd hope for no change rather than going to all slashes, but let's see what happens when UC4 is tested that's the right next step.
Brian, why is image rotated 90 degrees? Did that happen when you transferred the file to the computer?

Sorry about that! I did not see the image when I posted to the forum, there is a delay on all my posts.
The image rotation happened during upload to the forum. Perhaps I shouldn't take vertical iPhone photos with the intention to upload?
 

MikeS

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That's quite possible, but then I'd hope for no change rather than going to all slashes, but let's see what happens when UC4 is tested that's the right next step.
Brian, why is image rotated 90 degrees? Did that happen when you transferred the file to the computer?
I was kinda hoping it'd go to all spaces but I'm probably missing something...
 

MikeS

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I was too. Maybe the problem is the Video RAM latch, but that would mean only half the latch is stuck high??

Well, assuming that the screen should be blank then piggybacking UC6 has actually incorrectly added $0F to the correct $20, whereas what we want is to subtract $0F from the slash $2F turning it into $20, no? Or have I got something wrong?

@Brian: When the screen alternates / and space, can you tell whether the space is at the beginning or end of the line?

m
 
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