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Old 16th May 2017, 5:47 PM   #16
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The VIC 6561's Teutonic dispatch is now confirmed Once it's arrived and I have the mainboard nominally working I'll restore it a bit - replace dodgy caps, clean it, tidy up some of the repair solder from a previous replacement of a 6522VIA etc. etc.


So, with the chip some weeks away my attention now turned to the keyboard.
Aside from being ABSOLUTELY filthy there was also a missing MINUS (-) key and the spline for the key snapped off.

So I split the keyboard, providing access to the switch contacts, allow removal of the keyboard movement and replacement of the broken spline.



I tied the keyboard cable to the backboard, as unsupported it's individual wires are prone to fracture.



Then I turned my attention to replacing the broken spline.

It was at THIS point that I said.... "Bother". For like the donor C64 keyboard this Vic20 keyboard is made by Mitsumi, but the movement is significantly DIFFERENT from the donor keyswitch
The shaft is shorter and fatter, the spline dimensions different, the distance between the base of the key and the end of throw slightly greater - and finally the spline recess is less deep into the 2-shot cast key. They will not fit.





So now I need to precisely measure up both keys and attach a sawnoff portion of the REPLACEMENT spline and shaft onto the EXISTING shaft. Attaching it won't be difficult: I'll just drill the existing and replacement shaft , run a steel pin through both and infill with a bit of 2-part polystyrene casting goop and it'll be quite strong. Determining the dimensions of said replacement shaft, however will be quite 'interesting' - looks like I'll be digging up my vernier gauge and doing some maths tomorrow.

At least the key colours match up - gotta be happy about SOMETHING!

Callan
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Old 16th May 2017, 5:54 PM   #17
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it's great to see such a detailed and technical worklog ticking along.
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Old 16th May 2017, 6:23 PM   #18
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It is tempting just to replace it with a C64 keyboard outright (they're a dropin replacement), but I wouldn't be able to get the function key colour correct.
This keyboard has actually seen a lot of action - the splines have significant scoring (which I guess is a good thing as it shows it was loved and used). The whole thing will need to be disassembled, cleaned and serviced. The painted on graphics key images are still in good nick, however.

Neat tip: Mr Sheen furniture polish is simply silicone oil in solution. Sparingly and strategically applied it lubricates plastics brilliantly - the carrier liquid evaporates off leaving a powdery silicone film which doesn't wick, spread or attract dust, lubricates well without attacking the plastic.

I'll probably do the keyboard restoration before repairing this one key, as I don't want to disturb the repaired keyswitch anymore than is necessary.

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Old 17th May 2017, 1:26 PM   #19
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Keyboard Day!!



Blergh!..


That dust is really gritty and abrasive, too - it's scored the keyboard shafts and bound up the shift-lock mechanism. Nasty stuff. Could be worse I suppose: at least the former owner wasn't a smoker.


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Old 19th May 2017, 9:21 AM   #20
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Late last night I ransacked my scrap pile and came across a very dead Rev1 Vic20 (from Girton College, Bendigo) that I'd forgotten I had. Bit of a tragedy as it's a VERY early example (16th week 1982). It's a PET keyboard with bizarre reworking in wirewrap under the backboard, but quite unsalvagable. The case is in good condition however, but more importantly it has a seemingly working VIC6561 So for now I've dropped it into the project VIC, and in the next few days I'll work on the board. Having a tooth implant and some minor face surgery tomorrow, so I might be out of commission for a bit.

When the German VICs arrive I might still have a go at repairing the Girton machine, but it seems pretty far gone. Schools are not kind to computers.

I had mixed success with the keyboard repair: still a work in progress.

Pics later.

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Old 19th May 2017, 3:58 PM   #21
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Sounds like you're making progress with the VIC but nothing good comes easy. Thanks for the updates.

All the best with the dental work, doesn't sound like a good time. Praise be to modern drugs.
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Old 19th May 2017, 7:51 PM   #22
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Callan, what's the make/model of your scope?
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Old 19th May 2017, 10:23 PM   #23
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It's a Sainsmart DS202.
The current incarnation (which is probably a lot better) is the DSO note II. Here's an Ebay link (I don't vouch for the seller: it's just someone who sells it.)

I've never quite worked out how to drive this one - the manual is an appalling chinglish translation: it can do storage and basic signal generation, apparently. But it's a portable oscilloscope, lithium battery powered, and handles the frequencies my retro gear generates and gives me the info I need.

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Old 20th May 2017, 12:14 AM   #24
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Quote:
Originally Posted by @rt View Post
Where are you getting the signal you’re showing?
The messy one doesn’t look like the output of a clock buffered with Schmidt triggers.
How does it look on the way in to the custom chip? Straight after the buffers.

If the clocks on pins 38 & 39 are clean, can’t you then assume you’re screwed?
Found the reason that the 4.4Mhz signal is so dodgy: it's the dodgy oscilloscope I'm using: It only has a 7.5Mhz sampling rate. 4.4Mhz is above the nyquest limit (3.25Mhz), so sampling artifacts are generating the sub-harmonics/foldback you're seeing.

It told me what I needed to know, though so that's enough

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Old 20th May 2017, 12:39 AM   #25
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Hey this is awesome stuff man.

I have a bunch of vic-20 game carts in their respective boxes, but i'm realising that I'll probably never get a vic20 to try them out them on

http://i.imgur.com/2uAedYA.png
cactus quality, but theyre in front of the game cubes
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Old 21st May 2017, 7:31 PM   #26
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I love it! It looks like they still have their boxes.. 👍👍
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Old 22nd May 2017, 7:48 AM   #27
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Quote:
Originally Posted by callan View Post
Found the reason that the 4.4Mhz signal is so dodgy: it's the dodgy oscilloscope I'm using: It only has a 7.5Mhz sampling rate. 4.4Mhz is above the nyquest limit (3.25Mhz), so sampling artifacts are generating the sub-harmonics/foldback you're seeing.

It told me what I needed to know, though so that's enough

Callan
I was gonna ask about the sampling rate, I've had issues with portable affordable scopes in the past regarding sampling rates - Without even asking you just answered my question, sadly.

Good job on the Vic my friend!
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