PET 4016 Repair...Again!

[KevinO]

Hello all. I could use some guidance from the PET gurus here. Apparently the PETs don't store well, especially in non-temperature controlled environments! My collection had to be stored in a barn for the last 10 months, and they didn't fare well. My good working 8032 had a mouse nest in it!

Anyway, now of my 5 PETs, I now have zero working ones. I brought in my 4016 that the fine folks here helped me fix several years ago, and it's no longer working. I get the expected garbage screen that clears after a second, the startup chirp, then nothing on the screen at all.

Where to start troubleshooting?

 

[retro-pc_user]

Check the capacitors and solder joints on the board that deals with the video portion (character ROM, video board, etc).

 

[VERAULT]

Reseat everything, corrosion, mouse pee, dust, etc. add up quickly.

I buy these special art erasers from Japan, they have sand in them and they are AMAZING at removing corrosion from pins or even board traces without damaging the trace! (might remove some solder mask no biggee!)
here:
https://www.ebay.com/itm/Tombow-MON...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[KevinO]

Fortunately, this one didn't seem to have any signs of rodents in it, like my 8032 did. I got that one fixed! See my other thread on that.

The other thing on the PETs is the chip sockets are horrible. I don't have it in front of me today, but I *think* I have replaced all those white sockets that always make such poor connections.

Anyway, I should have written more about what I've already done. Reseated all socketed chips and cleaned contacts. Checked power supply voltages and they look ok. Since I get initialize, chirp, and screen clear, it almost has to be in the video section. But that's where I start to flounder, because I don't recall which chips are involved with the video. Also, does anyone know, did the 4016 have the end of line chirp like the 8032? I can't remember. That was always a good test on the 8032 to see if the computer was working but you just weren't getting video. You just hold down the space bar until you got to the end of the line and it would beep. If so, I'm not getting that....

 

[VERAULT]

Hey just curious, where did you get all these PET's and how much did it set you back? I wanted an original 2001-8 so I paid out the nose for it.

 

[KevinO]

Hey just curious, where did you get all these PET's and how much did it set you back? I wanted an original 2001-8 so I paid out the nose for it.

Gosh, if only I could remember. My first 8032 was given to me almost 20 years ago by a guy who used to work on them. It's never worked, and has been sitting waiting for my attention all that time. Then somewhere along the way, I picked up my second 8032 and the 4016. I don't recall paying very much for either of those, but neither was working. Then my last 8032 was given to me by a friend just a few years ago. Also non-working. There used to be a couple great guys here on this forum that knew PETs like nobody else, and they helped me fix the two I had working. MikeS and Dave_M.

Oh, and my weird hybrid....the 2001N-16, which is in an 8032 case. I got that one, also non-working for $50 back in 2012, and between those guys and many hours of troubleshooting, we got that one working too. Unfortunately, it's dead again! Look at these things the wrong way and they quit working.

 

[AndyG]

Hi

I had a motherboard that behaved in a similar way. For me it turned out to be a 6520 chip (UB16 in particular) and a few ram IC’s.

Do you have a PET vet style ROM/RAM board to bypass the onboard memory and roms to see if it is memory or Rom based.

The fact it chirps and gives a brief garbage screen is a good sign as it is initialising the CRTC video chip but it cannot boot from the rom for some reason. This could be one of the big IC’s, basic roms or memory ic

I have taken the easy route and acquired a rom/ram board, that has been invaluable in helping me fix a couple of PETs I own.

Am assuming this is a fat 40 universal motherboard configuration .... as you said it chirped.
Andy

 

[KevinO]

Hi

I had a motherboard that behaved in a similar way. For me it turned out to be a 6520 chip (UB16 in particular) and a few ram IC’s.

Do you have a PET vet style ROM/RAM board to bypass the onboard memory and roms to see if it is memory or Rom based.

The fact it chirps and gives a brief garbage screen is a good sign as it is initialising the CRTC video chip but it cannot boot from the rom for some reason. This could be one of the big IC’s, basic roms or memory ic

I have taken the easy route and acquired a rom/ram board, that has been invaluable in helping me fix a couple of PETs I own.

Am assuming this is a fat 40 universal motherboard configuration .... as you said it chirped.
Andy

I don't own a PETvet...but I have considered it a time or two over the years. It would be helpful.
That 6520 is a PIA....does it have anything to do with display? I thought that was only for keyboard and external peripherals.
I think you're right, this is what they call a "fat 40". And yes, it has the speaker.
I finally got my 8032 working again, so I can use the "sumcheck" program I was given years ago to double-check the ROMs and make sure they are good.
It might be the weekend before I get back to it.

 

[AndyG]

Yes the 6520 chips are PIA but they fail regularly. They can hold the IRQ or Reset lines down preventing the PET from booting. Similarly for failed ROMS.....

In my case, I fixed the RAM and still could not get it past the Chirp until I removed UB16.... then all was well. I think you can remove both 6520 chips and it will still boot to some display.

I do not think it is the Video circuit at this point.

From a cold boot, does the PET chirp without showing the garbage screen (ie you just get a blank screen) ... then if you reset it shows one briefly before clearing it (the CRTC PETS behave differently to the 9inch versions with regards to displaying garbage screens because on boot they initialize the video first, clearing the video memory .... so from a cold start you shouldn't see it but from a quick, hot, reboot you can see it sometimes before it clears the video memory )?

I would check the IRQ and Reset lines for the 6520's, 6522's and also check if the ROMS and RAM are OK.

Cheers

 

[David_M]

I doubt it is the video circuit.
You get garbage on the screen for a short time, that tells you the sync, character generator and video shift register are all working.
It clears the screen, that tells you the video RAM, the data path and address bus for the video are all working as they should.
The screen clear also tells you that ROM (or at least part of it) is working and so is the stack page in system RAM.

Its possible it is waiting for an IO device to signal it is ready or perhaps one of the ROMs has failing bit or two. I'm not familiar with the PET startup process so that just an educated guess.

 

[KC9UDX]

I don't think IO comes into play until you get the READY prompt. At least it doesn't seem to on the non-CRTC variants.

I can have a stuck-asserted IRQ on my 2001 and I get a READY without a blinking cursor.

Could the issue be BASIC ROM?

 

[AndyG]

With the universal motherboards, if the 6520 or 6522 hold the reset line (or IRQ) line due to a fault it will stop the rest of the board booting. You can get the chirp and nothing else.

I desoldered UB16 when I found that was what mine was doing and all booted fine. Has happened on two boards now.

Worth checking both roms and the IO chips - my two penneth.....

I don't think IO comes into play until you get the READY prompt. At least it doesn't seem to on the non-CRTC variants.

I can have a stuck-asserted IRQ on my 2001 and I get a READY without a blinking cursor.

Could the issue be BASIC ROM?

 

[KevinO]

I haven't had any real time to troubleshoot, but I did have a quick minute to test what AndyG suggested. I removed UB16- no change. Also removed UB12- no change. These were soldered already. I might have put in sockets when I worked on it the last time...I can't remember.

But something else is going on that wasn't happening on Friday. I'm not able to get a quick reset anymore. Flipping the power off and back on gets me no chirp now. I have to turn it off and leave it off for a full minute before I can get a chirp again. Something wrong with the reset circuit maybe?

EDIT: I snuck down to my lab for a quick check before going to work. Pin 3 output of the 555 goes high for about 2 seconds, and then goes low. This is inverted to a low. Checking Pin 40 of the CPU, it stays low on power-up for about 2 seconds, and then goes high. So reset looks to be ok. Thought I was onto something there for a minute...

 

[AndyG]

Ok, I would also check the roms and ram, including the databus ..... hopefully it is something not too complex.

Did this change you now see happen before or after the 6520ics were removed ?

 

[KevinO]

Ok, I would also check the roms and ram, including the databus ..... hopefully it is something not too complex.

Did this change you now see happen before or after the 6520ics were removed ?

The change was before I removed the 6520s. Removing them didn't change anything either.
Since I've fixed my 8032, I should be able to use the sumcheck program to do a cursory check of the ROMs. I'm hoping to get to that tonight.

 

[AndyG]

Diagnosing the ram will be more of a challenge I guess.... advice I was given a while ago maybe of interest... “ faulty RAM could pull the databus high or low, so you could try and check for any pins on the databus which are not wiggling appropriately, i.e. stuck high or low or floating in the middle at indeterminate logic levels. “

You can use your 8032 as a benchmark ..... however I thought I read somewhere (hope my mind is correct) you can create an eprom to replace the kernal chip ( I think) that tests the ram ... as the pet chirps it may work if your roms check out ok. I cannot recall without doing a search again where to find the .bin file.

 

[KevinO]

FIXED! I actually went back through my original post back in 2012 to retrace my steps, so to speak. Had I done that earlier, I may have solved it sooner. Back then I replaced 2 RAM chips. Today I replaced the other 6. I had upgraded from 16K to 32K since then, so to test I just took the low ram out, and moved the high ram to the low ram positions. All had been socketed previously, so it was pretty easy.

The symptom just seemed different than what I had last time, so I was on a wild goose chase for a while...



EDIT: I keep forgetting that this forum only does pics in landscape!

I forgot to mention that I also found the documentation on how to run the "sumcheck" program to do a quick check on the ROMs, and I used the 8032 to check the kernal and BASIC ROMs before I got to the RAM issue.

 

[David_M]

Am I alone in feeling betrayed? 40 years ago I was taught that semiconductors don't wear out. They would only fail due to heat or the wrong voltages being applied.

These days I see more and more DRAMs and Mask ROM's failing due to what appears to be nothing more than old age.

 

[AndyG]

Glad you got it working...... and your ram ic’s are socketed!! With these old machines it’s always a process of elimination ....
Sometimes replacing one or two ram chips is fine and they work great for ages, whilst at other times (my experience with the 4032 fat-40 setup) for some reason after replacing with “newer” chips there is a gradual cascade of failures in the remaining older ones.....

My other fat-40 motherboard had failures in 2 6520 ic’s, 2 of the ieee 3446’s, most of the ram chips and for good measure one basic 4 rom... could have been a power spike ... or had been worked hard as it was used as an external data logging system

FIXED! I actually went back through my original post back in 2012 to retrace my steps, so to speak. Had I done that earlier, I may have solved it sooner. Back then I replaced 2 RAM chips. Today I replaced the other 6. I had upgraded from 16K to 32K since then, so to test I just took the low ram out, and moved the high ram to the low ram positions. All had been socketed previously, so it was pretty easy.

The symptom just seemed different than what I had last time, so I was on a wild goose chase for a while...

View attachment 44036

EDIT: I keep forgetting that this forum only does pics in landscape!

I forgot to mention that I also found the documentation on how to run the "sumcheck" program to do a quick check on the ROMs, and I used the 8032 to check the kernal and BASIC ROMs before I got to the RAM issue.

 

[KevinO]

Am I alone in feeling betrayed? 40 years ago I was taught that semiconductors don't wear out. They would only fail due to heat or the wrong voltages being applied.

These days I see more and more DRAMs and Mask ROM's failing due to what appears to be nothing more than old age.

I agree. We were totally sold on the idea that semiconductors are forever. SOLID STATE! It never occurred to me they could fail, or corrode INSIDE the chip. Bil Herd talked about the "purple creeping crud", which you can see under a microscope.

I sure do appreciate you guys talking me through stuff. It's always more fun to have a collaborative effort and share the victory!