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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment. |
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10th May 2023, 9:21 pm | #1 |
Octode
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,803
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Quiet Commodore PET Speaker
I managed to find a little time tonight and have resolved this issue, but I'm not really sure why (I know what I did but I don't know why it fixed it).
I've bought a Stupid PET Tricks device that plugs into the User Port mainly so I can connect to BBSes over the internet which is fun. It also has a speaker jack on it. I plugged in an external speaker and got the same problems as I did before - sound but very very quiet. I then started to measure with my scope as Sirius suggested a while back and couldn't find anything sensible on pin M of the User Port so I went backwards to pin 19 of the 6522 (UC5) and got the same. I then remembered that the 4040 disk drive I have has a 6522 in it so I swapped that in and the problem is resolved. So I had a fully working PET in all other respects except for quiet sound, and a replacement 6522 has fixed the sound. I'm happy that it's fixed, but I'm curious why it might have happened. Colin. |
10th May 2023, 11:29 pm | #2 |
Octode
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,803
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Re: Quiet Commodore PET Speaker
As an addition, I put the 'broken' 6522 into the 4040 disk drive and that seems to work fine.
Colin. |
10th May 2023, 11:54 pm | #3 |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,294
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Re: Quiet Commodore PET Speaker
Maybe there is faulty pin on the 6522 that is not used in the 4040.
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11th May 2023, 8:02 am | #4 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,553
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Re: Quiet Commodore PET Speaker
I remember this issue, every diverse sound output solution you tried was much quieter than expected so it was beginning to seem like the source, i.e. the 6522 pin, must be the problem. This probably came about from someone connecting an unsuitable load to it (like an 8 ohm speaker) at some time in the past. The 6522 pin is not 'powerful' enough to drive such a low resistance load directly, hence the need for either a high impedance piezo sounder, or, if a low impedance speaker is to be used, an amplifier in between the 6522 pin and the speaker.
It's fortunate that the 6522 with the dud pin seems to work in the drive unit but you might want to look at the drive diagrams to see what that 6522 pin is meant to do in the drive, as it could cause a subtle problem on the drive which you won't notice straight away. For example, does the second drive still work? Do both drives still write as well as read? I would also put a label on the underside of the 'faulty' 6522 because you may, at some point in the future, have a need to 'borrow' a 6522 to diagnose a problem on something else, by which time you might have forgotten about the suspect pin. Ultimately, I would be looking for another 6522 to replace the 'faulty' one which I would then quarantine, well labelled. |
11th May 2023, 8:14 am | #5 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,437
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Re: Quiet Commodore PET Speaker
Quote:
Pin N is Ground, so if PA2 was set to a high-level output, there's a possibility of forcing it low - Although I'd expect the 6522 should be tolerant to this. Also possible for a brief-short to PB7 on Pin L, and if they are set to outputs in opposite states, then an excess-current could flow. I'm not familiar with that Stupid PET tricks interface, but if it has its own power-source, then it could also be trying to 'parasitically' power the PET via PA2 if the PET was off and the interface had pulled PA2 high with not much current-limiting on this line. Or as trying to power itself from PA2, if its power supply was missing - Best to have some series-resistance in line with PA2 to prevent this / possible / latch-up. Finally, there's always a possibility of an Electrostatic Discharge event occurring, and that pin getting zapped, whilst plugging it in. Although the original 6522 wasn't CMOS, so shouldn't be particularly sensitive.to this (although I do recall an Atari ST Midi interface opto-coupler dying, when the other end of the midi-lead had touched the screen of a CRT monitor). |
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11th May 2023, 8:54 am | #6 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,553
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Re: Quiet Commodore PET Speaker
Briefly aside, I wondered why Colin kept sounding so negative about an add-on he obviously quite likes, so I looked into it and the device actually is called the 'Stupid PET Tricks' module.
https://www.bit-zeal.com/product/stu...ricks-module/3 |
11th May 2023, 10:31 am | #7 |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,437
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Re: Quiet Commodore PET Speaker
I had assumed that was the actual name of it, so I'd repeated the name without quotes on it. But I'll take a look at the link to it, to see what it actually does 'Stupid Tricks' wise.
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11th May 2023, 5:15 pm | #8 |
Octode
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,803
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Re: Quiet Commodore PET Speaker
Thanks for the above - I have ordered a replacement 6522 as suggested. The original one was marked 3879 as were the 6520s next to it so it looks original and I'm always sad to lose an original part, but if it doesn't work, then it doesn't work.
I'll do some further testing on the disk drive with it in but on first glance it works OK. I'll let you know. Now I need to find a 9-pin joystick. It never ends does it. I'm also ordering parts to fit this diagnostic clip and I'll let you know how it goes. It needs a small amount of SMD soldering so that's new for me. https://github.com/svenpetersen1965/PET-Diagnostic-Clip Colin. |
11th May 2023, 10:38 pm | #9 |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,437
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Re: Quiet Commodore PET Speaker
Yes, I always like to use IC's with the right period date codes, when restoring original computer boards (Although not as bothered when building replicas - especially ones that aren't a close-copy, but an updated design).
But finding New Old Stock or good used 'pulls' 6522's quite that old will be tricky, even on the 'Bay etc. Especially as major users of these like the BBC computers, weren't around until a couple of years later (Although they may well have still been making that model of PET by then). - I don't think I've got any that old amongst the surplus IC's or old PCB's, I've picked-up at radio-rallies. Well 9-pin Joysticks used to be very easy to come buy, but not sure if anywhere still makes new ones so might have to get a used one. I presume it uses the same pin-out as standard 'Atari-VCS compatible' / VIC-20 & CBM-64 pin-out ons (Although an original Commodore-branded one would be nice). Thanks for the info on that diagnostic clip, which I hadn't seen before. I thought it looked like something that might be useful in getting one of my old PET's going, but after looking at it more-closely I've found it's really just an (EP)ROM adaptor, and requires most of the computer to be already working - especially the CPU and the RAM (plus address-decoding. So I'm not sure if it really gives you much over pulling out the original ROM's and directly inserting a non-Kernal reliant diagnostic test ROM into the Kernal ROM socket (even if an adaptor is required, if non-standard pin-out ROM's were used). If the 6502 CPU is socketed, then I'd expect the ROM's probably would be. A 6502 uP bus tester (that also plugs into the 6502 CPU socket, but doesn't require a working 6502 to be fitted a well) may be the most useful with a dead 'PET' etc. As that should be able to locate shorted data / address buses and work-out the memory map, testing RAM plus check-summing the ROM without needing the CPU. Unfortunately, whilst several of these used to be sold back then, there doesn't seem to be any ones produced commercially any more. But there is some interest in re-creating these - possibly based around a fairly-cheap Arduino (Mega) / Raspberry-Pi etc modules (I'm trying to get a thread going to discuss ones we have / ideas for creating a one using more-modern hardware) Soldering that 74LS08 AND-gate TTL SO-14 1.27mm pitch SMD IC shouldn't be too difficult. The (fairly-standard) way I hand fit these is to: 1. Apply a small amount of solder to a corner pad, to be able to tack it down onto this first. 2. Place IC flat on the board, the correct way round, just to the side of the pads for it, then re-heat that solder blob whilst slowly sliding the IC into the exact correct position so that the legs are squarely on all 14 pads. 3. Apply solder to the leg on the opposite-corner, slightly moving the IC as necessary to ensure the legs are exactly in the middle of all their pads. 4. Apply solder to the remaining 12 pins / also re-solder the initial 'tacked' one, if necessary. I used to do these by eye, but working under a magnifier with a good light-source or a microscope (including the fairly-cheap digital ones, designed for this work, generally work quite well) helps these days - even at this 'largest' 1.27mm SMD IC pitch. It's best to use the finest-size solder you can get - Might be just about do-able with 22SWG, but easier with 26SWG (Or even much harder to find 34SWG). And you'll probably also find some very-fine size (eg size '0' or size '00') desoldering-braid will come in handy, to remove excess if 2 adjacent pins accidentally get soldered to each other. There is also another approach, where you deliberately apply excess solder, so that they are all soldered to their pads, but shorted together. Then remove all the excess, by wicking it up the desoldering-braid, until all the shorts are removed, and there's just enough left between the pins on their pads (a more subtle approach than heating them all, then sharply-hitting the board onto a hard surface to flick off the excess solder, that I once saw demonstrated on a Youtube video - in a backstreet facility in China?) This can be a very useful approach on finer-pitch SMD IC's, when you haven't got a micro-point soldering tip and the finest-size solder. |
11th May 2023, 11:17 pm | #10 |
Octode
Join Date: May 2012
Location: Perth, Scotland
Posts: 1,803
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Re: Quiet Commodore PET Speaker
The diagnostic clip seems to test more than say DaveR2's EPROM does - it looks like the cassette ports and video RAMs get tested for example.
I'm just building it for fun really - the creator is very helpful and responsive to questions; he also got some PCBs printed and sent them to me (although all the files are there on github) so I reckon it's time I tested them on my 3016. Here's his web page with a bit more detail: http://tech.guitarsite.de/petcbm_diag_clip.html Colin. |
12th May 2023, 12:18 am | #11 |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,437
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Re: Quiet Commodore PET Speaker
Yes, it does look like it's based on an original official Commodore service aid. And the (EP)ROM is designed to fully-test the ports - with the aid of loopback connectors etc, as required.
It should in theory be possible to just run it's / the official Commodore one's EPROM in the existing ROM sockets (Although if it's larger than original ROM-sizes, then might need more than one / an adapter to wire to the right pins). But if you've already got the PCB's, then probably shouldn't cost too much extra to build-up this version. It was a bit easier to do this on Commodore's later computer, as they had a memory-cartridge slot. I've got an original C64 Diagnostics cartridge. Plus a friend gave me a Memory-cartridge PCB he'd done for the VIC-20 (that also had RAM-expansion cartridges) which allows you to select one of several ROM-images in a larger ROM. So you could have various versions of diagnostic ROM available, to try. I wonder if anyone ever did a Memory-cartridge interface for the PET? - But might have to fit cartridges inside, by opening the 'bonnet'! Could also probably adapt this tester, to have a plug-in cartridge containing the EPROM. Although it does map-out the Kernal ROM, so can't use ROM's in the cartridge that expect that, without duplicating the Kernal in that (I think C64 & VIC-20 ones still retained the Kernal which checked for presence of a ROM-cartridge at certain addresses and could jump to running from that). Last edited by ortek_service; 12th May 2023 at 12:27 am. |