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Commodore P500

Pet Rescue

Experienced Member
Joined
Dec 15, 2010
Messages
484
Location
Staffordshire, England
It's taken a long time but I've finally bagged a P500! :happy2:

The good news is that she has her original box and even clear plastic bag with commodore printed on it and poly's

There's always a but! and it's she has the black screen of death!!

After checking voltages there is +12V,-12v,+5v and +2.5v from the power supply.

I decide to check the 6509 first and there was a clock and reset but the address lines were high apart from A11,A12 & A13 so were the data registers D0-d7.

I took the plunge and used my 710 to test the 6509 cpu in it and low and behold it was u/s.

I then took the working 6509 cpu from the 710 and put it in the P500 and up came the boot screen. Happy days!!

Well not exacty, upon typing the keys were intermittent and upon investigation some of the pins had slipped up inside the keyboard connector.

I must point out at this point if you pressed enter you got the syntax error message when returning after typing rubbish as expected.

After pushing the pins back into the connector properly and reseating it, all of the keys worked. Happy Days!!

No again it was too good to be true. After typing the mandatory ' 10 ?"Hello World" ' test program after pressing enter the computer locks up!

In fact just pressing enter alone locks the computer up, press reset and the ready prompt comes back.

I'm a bit worried about the 6509 cpu getting damaged especially as they are getting few and far between.

It's stumping me as to what could cause this kind of fault (Kernal? PLA? Memory?)

Have you had a similar problem


A couple of questions for P500 owners:

1. Does the pi symbol work on your keyboard or does a checkered pattern appear instead of the pi symbol
2. Do you get the odd artifacts flicker on the screen a bit like the 64 sparkle bug


P.S Just noticed that typing garbage sometimes returns a syntax error but not always.
When it locks of some typed characters sparkle. (jiggle about!)
 
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Congrats and best of luck. According to: http://www.insectria.org/b128/dusk.html
it's likely the 6509 or 6525 chip that causes the lockup. Unfortunately both chips are almost impossible to find.

1. This is normal. The P500 boots into lowercase. The Pi symbol is only available in the "uppercase/graphics" set. It's one of the few characters that are different between sets. Press SHIFT-NORM/GRAPH to change sets.
2. Very early VIC-II chips are known to have this problem just like in the C64. You can replace the VIC-II with a newer revision to see if that fixes it. Luckily VIC-II chips are a little more common ;-)

Steve
 
Congrats and best of luck. According to: http://www.insectria.org/b128/dusk.html
it's likely the 6509 or 6525 chip that causes the lockup. Unfortunately both chips are almost impossible to find.

1. This is normal. The P500 boots into lowercase. The Pi symbol is only available in the "uppercase/graphics" set. It's one of the few characters that are different between sets. Press SHIFT-NORM/GRAPH to change sets.
2. Very early VIC-II chips are known to have this problem just like in the C64. You can replace the VIC-II with a newer revision to see if that fixes it. Luckily VIC-II chips are a little more common ;-)

Steve

Thanks Steve It took me a while and never honestly thought I'd get one. I won't ever get a C65 but the P500 I had set my heart on having.
Well I'm halfway there now but a bit gutted I have to rob one machine to fix another especially how long it took to repair the 710 but hey needs must.

That's great info on insectria as I've never seen that before thanks, I did google but nothing came up.
The 6509 works fine in the 710 so that couldn't work in one but not the other could it?
Regarding the mos 6525a, am I right in saying there are two of them in the P500?
You guessed the next question which one would be responsible for the hang?
Too top it off sod's law they are the only big chips not socketed.:sad3:

One more question what is the main configuration for the power supply mounting, mine is top case mounted but a lot I've seen fit on the lower section.
 
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The P500 might be my favorite "released" Commodore machine although I admit I haven't done much with mine. And of course having a c65 would be quite nice, but I won't be buying one any time soon...

Ed's site has some good info. I hope my own site is helpful as well ;-)

Yes, the chip situation is not good. Hopefully in time there will be FPGA replacements so we can restore our old machines.

If a chip is 100% good it should work in either machine. If it works in one but not the other then it might actually be faulty, or there is a fault in the machine. Yes, there are two 6525A's in the machine... one for keyboard the other for IEEE. You can try swapping them to see if it makes a difference. Likely it's the IEEE one (U2) as it deals with IRQ handling as well.

Very early machines had bottom mounted supplies and later ones had top mounted. Of course with Commodore anything is possible.
What's your P500's serial#?

Steve
 
I can see if I have either chip in my inventory... I have been quietly collecting lots of Commodore chips over the last couple of years I might be wrong but I thought I might have seen 6525 in one of them.
 
The 6525 was also used in the Commodore Magic Voice...
I didn't know that (since I've never opened up the MV). We have two of those in our club, and we will now be extra careful when handling them.
...and Commodore's IEEE interface for C64...
I've never come across one of those.

FCUG celebrating 33 years,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm
 
I can see if I have either chip in my inventory... I have been quietly collecting lots of Commodore chips over the last couple of years I might be wrong but I thought I might have seen 6525 in one of them.

That would be very good of you thanks falter.

It's strange that I've used my smd desolder heat gun on quite a few boards without worrying about it but with a machine so rare I would normally cut each leg and desolder destroying the chip but with the chip being rare as well and not being sure of it's functionality.

The only downside of the heat gun is that it can bubble the fibreglass board slightly, which does go back flat but you see the hatched pattern underneath.
 
I have just had another look at the P500 today and used my willem programmer to read the Kernal, Basic Lo and Basic Hi ROM's.

All three ROM's have checked out ok, I would like to read the FPLA chips but what can you use to read them. Can they be read as an eprom?
 
I have just had another look at the P500 today and used my willem programmer to read the Kernal, Basic Lo and Basic Hi ROM's.

All three ROM's have checked out ok, I would like to read the FPLA chips but what can you use to read them. Can they be read as an eprom?
Yes they can. Jim Brain sells EPROM/PLA dapter boards for a couple of dollars. You can fit the socket and headers either way round to allow you to read the PLA as an EPROM.
 
Yes they can. Jim Brain sells EPROM/PLA dapter boards for a couple of dollars. You can fit the socket and headers either way round to allow you to read the PLA as an EPROM.

Thanks crock, I realised I had read my C64 PLA a few years back by using Ray Carlsen's 82s100 to 27c512 eprom read adapter.
I have tested both FPLA's on the P500 and they have tested O.K.

Looks like the 6525 are the next to be checked.
 
I had a look at the two 6525's today but from past experience I feared I could not remove them without damaging the board as the tracks and pads are so thin.
So I decided that I had to cut the chip legs thus destroying the chips but saving the board from damage, really gutted but it had to be done! I then desoldered
the legs and fitted sockets what a pain in the @$$! The replacement 6525A's from my working 710 were fitted but no go it still locks up when you press return.

One exception is that if you press return with the shift lock in, the computer does return to a new line without locking but if you type in garbage and press return
it does return to a new line but does not give a syntax error! Release the shift lock and press return and it locks up again.

To say I'm stumped is an understatement!
 
This is meant to be constructive criticism:

I think you need to work on your desoldering. I haven't yet met a chip I couldn't desolder without damaging anything.
 
This is meant to be constructive criticism:

I think you need to work on your desoldering. I haven't yet met a chip I couldn't desolder without damaging anything.

They certainly do need a bit of work, I don't have any fancy equipment just a soldering iron and trusty solder sucker oh and an smd heat gun but it can cause damage to the fibreglass board. Knowing my limitations I didn't want to take the risk on a computer I've wanted for a few years. I've tried solder braid but I just can't get good result without lifting traces.
 
They certainly do need a bit of work, I don't have any fancy equipment just a soldering iron and trusty solder sucker oh and an smd heat gun but it can cause damage to the fibreglass board. Knowing my limitations I didn't want to take the risk on a computer I've wanted for a few years. I've tried solder braid but I just can't get good result without lifting traces.

Again, I'm trying to be helpful though this probably doesn't read that way:

I recommend sticking to a soldering iron and a good solder sucker. I've never had good results with the bulb type. I always use the cheap piston type. I'm sure a heat gun is necessary for some surface mount work, and braid does have its applications, but good technique with the iron and sucker really does the trick.

Get some junk boards, especially ones with fragile traces and components and practise.

1) Prime the sucker.
2) Apply the iron to one pin until the solder melts
3) Put the sucker squarely over the hole, and release it.

Do opposite corners of a chip consecutively. If you think the chip is getting too hot, pause for a while.

When you've done that to all the pins, you may have to do some of them again (this is normal). Also, a 'solder tool' or screwdriver is handy: If you have a stubborn pin, get most of the solder off with the sucker; and then heat it up and pry it around with the solder tool or screwdriver so that when the solder cools, the solder doesn't adhere.

With a little practise, you can be very proficient at this, I think. At least, I am pretty proficient, and I don't have any magical talent. I routinely desolder chips of all types and after socketing them, place the old desoldered chips in the sockets. I can't think of a time that I had a chip that didn't survive desoldering.

A fancy temperature controlled iron is nice, but I survived decades with cheap Radio Shack irons.
 
I had a look at the two 6525's today but from past experience I feared I could not remove them without damaging the board as the tracks and pads are so thin.
So I decided that I had to cut the chip legs thus destroying the chips but saving the board from damage, really gutted but it had to be done! I then desoldered
the legs and fitted sockets what a pain in the @$$! The replacement 6525A's from my working 710 were fitted but no go it still locks up when you press return.

One exception is that if you press return with the shift lock in, the computer does return to a new line without locking but if you type in garbage and press return
it does return to a new line but does not give a syntax error! Release the shift lock and press return and it locks up again.

To say I'm stumped is an understatement!

If I have an 8-bit that basically boots but hangs, I would be going down the faulty RAM route now. On the insectria site there was a nice suggestion about removing the BASIC ROM. This means the computer boots into the monitor. You can then start selectively filling memory with various bit patterns to try and determine the issue. It's possible that if it is memory, and it affects the zero or stack pages, that the monitor will probably hang as well. At that point I would resort to making a bespoke kernal that does some very basic RAM checks, thought I don't know whether is is possible for you.

Regarding replacing your 6525's - do not despair, they are much easier to find than 6509's as they were also used in 1551 disk drives and in Amiga CD-ROM drives. I see them maybe once every few months on ebay, though you might want to include worldwide in your search criteria.

+1 for what KC9UDX said. Just practise on junk boards. If you are not certain that the chip is dead or not easily replaceable, never cut the pins as most will survive a de-soldering. The odd broken trace or via (the holes in the pcb, which are usually plated through) can be easily spotted and fixed once the chip is removed. My technique is to use a Weller 35W iron with a chisel tip, which is not temperature controlled, and a pump action solder sucker. I heat each pin until the solder visually melts, plus about 1 second, then suck. After doing all 40 pins, flip the board and using quick applications of the iron, push the legs inwards along both sides of the chip and you usually feel a soft click as the leg frees itself from the outside wall of the via. Most new chips have the legs spread outwards so even after sucking, the legs will often still be attached to the outside with the small amount of residual solder. I then work along each pin checking it is free, using a small screwdriver. You can then usually insert a screwdriver under one end of the chip and it will come free with a gentle twist at both ends. A *small* amount of force is OK here but it should not need much. If it isn't coming free, recheck each pin and repeat as required.

You will very occasionally find an obstinate bugger that needs a bit more persuasion, this is when you can find that traces get lifted or a via gets pulled out on the leg of the chip. You should perform a careful visual check of the legs of the pin to see if any via's have been pulled out and the board to check for lifted, broken traces. I usually clean up the board with braid and then PCB cleaner after removing the chip and this makes it much easier to spot faults. If you have, just make a note of where the traces run and fix with fine cored wire on the back side of the board once the socket is fitted.

Hope this helps, Rob
 
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Again, I'm trying to be helpful though this probably doesn't read that way:

I recommend sticking to a soldering iron and a good solder sucker. I've never had good results with the bulb type. I always use the cheap piston type. I'm sure a heat gun is necessary for some surface mount work, and braid does have its applications, but good technique with the iron and sucker really does the trick.

Get some junk boards, especially ones with fragile traces and components and practise.

1) Prime the sucker.
2) Apply the iron to one pin until the solder melts
3) Put the sucker squarely over the hole, and release it.

Do opposite corners of a chip consecutively. If you think the chip is getting too hot, pause for a while.

When you've done that to all the pins, you may have to do some of them again (this is normal). Also, a 'solder tool' or screwdriver is handy: If you have a stubborn pin, get most of the solder off with the sucker; and then heat it up and pry it around with the solder tool or screwdriver so that when the solder cools, the solder doesn't adhere.

With a little practise, you can be very proficient at this, I think. At least, I am pretty proficient, and I don't have any magical talent. I routinely desolder chips of all types and after socketing them, place the old desoldered chips in the sockets. I can't think of a time that I had a chip that didn't survive desoldering.

A fancy temperature controlled iron is nice, but I survived decades with cheap Radio Shack irons.

KC9UDX I hope my reply didn't read as me taking offence as I didn't take it as a criticism.
It's always a good thing when someone can offer ways of repairing equipment or the methods they can use which will help others with their repairs.
I think my fear of lifting traces goes back to 1988 when I was repairing my most precious C64 I'd had her since 1983 and she cost my mum and dad £399
back then when my dad was on around £100 a week. I was removing the faulty memory and replacing with sockets due to that horrible brick supply going bad.
I was using a normal fine tip iron and piston type solder sucker, all the solder looked clear and the pins seemed to be free but when lifting the chips up came
the traces too, to say I was devastated at the time was un understatement but a few hours later and a bit of transformer wire she was fixed, not pretty but
at least she worked again.

When I was doing the repair yesterday I had a proper sweat on! maybe it's old age or just the fact it had taken me ages to get a P500 and the fear of ruining
this lovely piece of commodore history was playing on my mind and that was with cutting the chip legs, I'd have been nervous wreck with chips in place.
I've had great results in the past with the heat gun but on smaller memory chips but not on the larger 40pin dips as the heat tends to damage the board.
I've had plenty of experience desoldering IC's but never been great on the larger chips. It was a case of knowing my limitations I guess so maybe it's time
to put that right and get the scrap boards out!
 
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