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Old 4th Mar 2021, 8:51 pm   #1
Slothie
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Default PET 6540 PROM adapter

Well I got the PCBs today for the adapter and tried out my skills at SMT soldering, something I haven't done in 15 years. Considering this, my terrible eyesight and that I was soldering with a potato, I managed to solder 3 of 5 of them up OK, one got a damage PCB because a MOSFET exploded during soldering (still not sure how that happened) and one the IC went on skew-whiff. I might be able to get that off with a heat gun and redo.
When I get my arduino-based universal chip tester working I'll be able to test the boards to see if they actually work!
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Old 4th Mar 2021, 9:00 pm   #2
SiriusHardware
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Default Re: PET 6540 PROM adapter

Well done Slothie, another piece in retro parts jigsaw. I don't know if you have a flux pen, a bit of that run across the legs followed by a soldering iron tip (ideally not a potato) applied to each leg in turn will often make the solder joints reform into nice neat shapes. When it works really well you can hardly tell that the soldering was not done by a machine.

Was that using lead-free or lead/tin solder?
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Old 4th Mar 2021, 9:02 pm   #3
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Default Re: PET 6540 PROM adapter

I suppose all you need now is your PET to try them out in? It's an early one, like AJ's?
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Old 4th Mar 2021, 9:08 pm   #4
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Default Re: PET 6540 PROM adapter

Impressive and I look forward to reading the next instalment. I think Slothie's PET is indeed a 2001-8 which originally had 6550s and 6540s.

Alan
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Old 4th Mar 2021, 9:10 pm   #5
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Default Re: PET 6540 PROM adapter

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Originally Posted by SiriusHardware View Post
Well done Slothie, another piece in retro parts jigsaw. I don't know if you have a flux pen, a bit of that run across the legs followed by a soldering iron tip (ideally not a potato) applied to each leg in turn will often make the solder joints reform into nice neat shapes. When it works really well you can hardly tell that the soldering was not done by a machine.

Was that using lead-free or lead/tin solder?
I was using leaded solder. I did try the flux pen and run a soldering iron over it and it improved a little , I guess I need a better soldering iron.
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Old 4th Mar 2021, 9:16 pm   #6
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Default Re: PET 6540 PROM adapter

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I suppose all you need now is your PET to try them out in? It's an early one, like AJ's?
Well I have a PET in storage, and I'm hoping that in the coming months I will be being rehoused and get access to it. If AJ thinks he can test one for me and PMs me his address I will post him one I'm hoping I will only need 1 or 2, but they're not expensive to produce and if they do work I might even make an etsy account and sell them....
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Old 4th Mar 2021, 9:28 pm   #7
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Default Re: PET 6540 PROM adapter

I assume the adaptors allow the use of a common EPROM or perhaps EEPROM in place of the 6540.

A reverse version of that adaptor, to convert the pinout of the 6540 to a common EPROM layout, would also be useful as that would let original PROMs be read by one of the relatively cheap USB EPROM programmers as though they were 2716s or whatever.

Then again, I suppose all you have to do is programme your 'substitute 6540' with the code for any PROM suspected of being faulty - if the machine works with that in but not with the real PROM in you can probably say that the original PROM is duff even if you can't read it directly.
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Old 4th Mar 2021, 9:49 pm   #8
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Default Re: PET 6540 PROM adapter

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Originally Posted by SiriusHardware View Post
I assume the adaptors allow the use of a common EPROM or perhaps EEPROM in place of the 6540.

A reverse version of that adaptor, to convert the pinout of the 6540 to a common EPROM layout, would also be useful as that would let original PROMs be read by one of the relatively cheap USB EPROM programmers as though they were 2716s or whatever.

Then again, I suppose all you have to do is programme your 'substitute 6540' with the code for any PROM suspected of being faulty - if the machine works with that in but not with the real PROM in you can probably say that the original PROM is duff even if you can't read it directly.
Its designed to take 5v 2716 EPROM chips (of which I have hundreds) or 28C16 EEPROM chips, both in DIP-24 packages, Each adapter replaces a single 2k chip.

Unfortunately, due to the "active electronics" the PCB can't be used in reverse. One other possible use of this adapter PCB will be for me to create a "diagnostics" ROM to replace the F800 PROM with so I can do hardware tests. The PETs built in diagnostics are rudimentary to say the least. It does have a fancy diagnostic LED which no-one uses though!
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Old 4th Mar 2021, 10:02 pm   #9
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Default Re: PET 6540 PROM adapter

Exploding mosfet could possibly be due to popcorn effect if it has absorbed some moisture from the atmosphere and you used a big enough soldering iron to heat the body really fast. Usually only occurs in smt reflow machines. If smt parts have been out of the drypack for a day or more they are often baked at 90C for a few hours before use in the reflow process.

I just soldered an epm7128s pqfp-100, to a board last week with a soldering iron, unfortunately damaged an 8mil trace using a solder sucker to remove excess solder, though i think I’ve been able to link the broken track. Waiting for delivery of new toys, usb microscope and a temp controlled hot air gun, so I might actually be able to inspect the soldering instead of just buzzing it out with the multimeter.
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Old 4th Mar 2021, 11:16 pm   #10
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Default Re: PET 6540 PROM adapter

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Originally Posted by Slothie View Post
If AJ thinks he can test one for me and PMs me his address I will post him one ....
Happy to test a ready programmed adapter and will make contact by PM to arrange. The emulator's diagnostic tools might be of use if things don't go according to plan for some reason.

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Old 5th Mar 2021, 12:01 am   #11
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Default Re: PET 6540 PROM adapter

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Quote:
Originally Posted by Slothie View Post
If AJ thinks he can test one for me and PMs me his address I will post him one ....
Happy to test a ready programmed adapter and will make contact by PM to arrange. The emulator's diagnostic tools might be of use if things don't go according to plan for some reason.

Alan
Ah, I dont have a programmer at the moment, its kind of locked away with the PET. Unless you can get a programmed eprom from somewhere. I will be experimenting with programming an EEPROM with an arduino, but thats still on my todo list.
The diagnostic program is vapourware at the moment

If I get anything together with the diagnostics I you might be able to beta test that!
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Old 5th Mar 2021, 1:00 am   #12
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Default Re: PET 6540 PROM adapter

That's a skill which might still come in handy in the other PET thread, I know a bit of assembly language generally but have almost no experience of 6502 apart from a brief few weeks on a friend's BBC B when that machine was still contemporary. I'm thinking that in the other PET thread we need something we can plug into the Kernel socket which will generate a binary count on the user port or a square wave on just one port pin just to prove whether the CPU is or is not able to fetch and execute code. (The apparently dud 74164 will have to be replaced first in any case).

I have a few 2716 EPROMs here if you can point me at appropriate code to programme into them for adaptor test purposes, perhaps some PROM images in the first instance. Or maybe this is the incentive you need to get your Arduino -> 28C16 programmer up and running...
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Old 6th Mar 2021, 1:25 pm   #13
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Default Re: PET 6540 PROM adapter

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Originally Posted by Mark1960 View Post
Exploding mosfet could possibly be due to popcorn effect if it has absorbed some moisture from the atmosphere and you used a big enough soldering iron to heat the body really fast. Usually only occurs in smt reflow machines. If smt parts have been out of the drypack for a day or more they are often baked at 90C for a few hours before use in the reflow process.

I just soldered an epm7128s pqfp-100, to a board last week with a soldering iron, unfortunately damaged an 8mil trace using a solder sucker to remove excess solder, though i think I’ve been able to link the broken track. Waiting for delivery of new toys, usb microscope and a temp controlled hot air gun, so I might actually be able to inspect the soldering instead of just buzzing it out with the multimeter.


Yes, although isn't usually MSL-3 rated IC 's in PQFP etc packages that have issues with moisture affecting solderability / maybe getting inside package via legs as not hermetically-sealed. I've never seen an issue with SOT23 (especially hand soldering). Hopefully these weren't fake ones (can't see that being worthwhile for small signal FET's)


I too was recently hand-soldering a large pin-count FPGA on an ZX-Uno board, after getting the IC cheap from China. But it wasn't too well packaged, so had to carefully straighten some crush corner pins again with a scapel etc. I think I may have tried moving one with soldering iron tip, to get it a bit more central on the pad, and that started to lift.

Even though I was using quite rare now 34swg solder, and a micro-tip iron, they were still quite large relative to IC pin size, and as I like to have a decent amount of solder over pins, to ensure no dry-joints, do get occasionally a bit too much across to adjacent pin even soldering under a microscope etc.

Luckily, I had some good micro-size (0?) desoldering braid, too remove excess solder and then re-apply a bit to get a nice looking joint.
You can also just put lots of solder across all the pins, then remove it with the braid, and should be left with just enough solder in place to have connected them all OK (But I still like to cover the top of the bottom leg of the pin with solder). You do have to be a bit careful with some desolder braid, as can stick to several pads if too large / isn't still quite new with good flux in it. (The Red RS-branded stuff was notorious for seemingly having no flux in it, whereas 'Soder wick' etc. come in vacuum sealed packaging.
Although I usually add some extra flux from flu-pens , a bottle with a needle nozzle, but it seems you have to use different types of flux for lead-free and leaded solder (at least they sell different flux pens for each)

The recoil from desolder pumps often damages pads. With even the best 'Green colour RS-components branded' ones I've found, still damaging delicate fine tracks. The first one I got from Tany's (Rado-Shack) in the 80's had so much recoil, it would jump out of your hand as well as damage most tracks on veroboard etc.
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Old 6th Mar 2021, 1:32 pm   #14
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Default Re: PET 6540 PROM adapter

The standard home-constructor method of soldering flat-pack SM ICs has always been to do what you would never normally do - intentionally flood all of the pins along each side together with solder, and then use solder wick to take off 95% of it to leave all of the pins soldered, but separated.

There is a minor risk with that technique, that some of the solder may possibly run in behind the pins and refuse to come out, in which case you have little option but to remove, clean up, reposition and resolder the device all over again.

At work I have a Metcal iron with an exceptionally fine point so I usually prefer to solder the pins individually.
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Old 6th Mar 2021, 2:04 pm   #15
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Default Re: PET 6540 PROM adapter

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Originally Posted by SiriusHardware View Post
The standard home-constructor method of soldering flat-pack SM ICs has always been to do what you would never normally do - intentionally flood all of the pins along each side together with solder, and then use solder wick to take off 95% of it to leave all of the pins soldered, but separated.

There is a minor risk with that technique, that some of the solder may possibly run in behind the pins and refuse to come out, in which case you have little option but to remove, clean up, reposition and resolder the device all over again.

At work I have a Metcal iron with an exceptionally fine point so I usually prefer to solder the pins individually.
Yes, there's also a video of a backstreet in China method of applying solder to lots of pins and bringing the PCB sharply down to flick off all the excess solder!

Many years ago at Nepcon exhibition, I saw a demo from Pace on their concave tips, that used surface tension to draw solder away from pins and allowed you to just run the tip down the ends of the pins and solder them all in one go without shorts occurring. But I've not heard much of others doing this more recently.
We seem to have mostly moved away from Pace systems at work, with Metcals mostly favoured by production as fixed temperature. Have some of the lab, but mostly larger ones, and can't get many small tips for higher-temperature lead-free use.

So I would go for JBC if buying more systems, as they have the widest range of tips, and full temperature etc. control. But it wasn't that long ago I bought-in the lab some Weller micro-iron etc. systems, with cartridge element-tips, so I currently mostly use when accessible.

Although all a bit steep new for home use, and still have quite a few old 24V Weller TCP Magnastat's and EC-series temperature-controlled ones I've had for ages - Buying a TCP new back in the 80's, after getting fed up of flimsy Antex ones with fragile tips and handles got so hot at end they melted!
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Old 6th Mar 2021, 2:20 pm   #16
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Default Re: PET 6540 PROM adapter

I've had access to those concave tips, sometimes referred to as 'Gull-Wing' tips, but never had much luck with them probably because I didn't believe they worked, but I've known them be used on production lines where exerienced operators could solder a four sided fine pitch device in mere seconds. I just never got the hang of them.

I think we've now thoroughly explored Slothie's options for the next time he chooses to solder one of his PROM adaptor PCBs.

Slothie, if you need a programmed 2716 or two to pass on to AJ let me know what you want put into them.
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Old 6th Mar 2021, 2:26 pm   #17
Slothie
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Default Re: PET 6540 PROM adapter

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Originally Posted by SiriusHardware View Post
I've had access to those concave tips, sometimes referred to as 'Gull-Wing' tips, but never had much luck with them probably because I didn't believe they worked, but I've known them be used on production lines where exerienced operators could solder a four sided fine pitch device in mere seconds. I just never got the hang of them.

I think we've now thoroughly explored Slothie's options for the next time he chooses to solder one of his PROM adaptor PCBs.

Slothie, if you need a programmed 2716 or two to pass on to AJ let me know what you want put into them.
Its looking like if I come up with a semi-useful diagnostic tool then I will stick it on a ROM somehow and send it to AJ to test (presuming he hasn't passed on his PET to some other lucky soul by then!). I've been enjoying writing a bit of 6502 assembler, got some code that checks the zero page and stack areas of memory, fills and then clears the screen and initialises all the PETs PIAs and the VIA, so thats a start.....
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Old 6th Mar 2021, 6:50 pm   #18
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Default Re: PET 6540 PROM adapter

I think hot air is the best way to solder smt components, with possible exception of connectors with plastic construction, uSD card connectors etc.

Just got an 858D temp controlled hot air kit. Looks to be a few different variations, I picked one that can control both the temperature and the air flow. It seemed to be intermittent at first, but I think I might have been holding it wrong, it has a tilt switch so it only operates when pointing downwards, point it above the horizontal and the heater turns off then after cooling it stops blowing.

So far I’ve only reflowed the joints on the PQFP after applying more flux. The soldering now looks a lot better.

Also added 0603 decoupling caps, apply blob of solder to each pad using soldering iron then place capacitor with curved point tweezers and reflow with hot air. Best appearance is after applying a bit more flux and reflow a second time.

Another tip when designing pads for smt components for hand soldering is to extend the pad a little bit further beyond the lead of the component, this gives a bit more landing space for a soldering iron to the pad. I still messed up the PQFP soldering even with this extra space by allowing the soldering iron to touch a couple of leads and dragging them out of line with the pads. Its quite tricky to bend them back to the correct position.

The hot air is also much better for heat shrink, no need to worry about my wife catching me with her hair dryer
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