Re: R-DOS (REX DOS??) on an oceanic 1541 clone drive

From: Gerrit Heitsch <gerrit_at_laosinh.s.bawue.de>
Date: Mon, 06 Oct 2014 17:56:54 +0200
Message-ID: <5432BBC6.7030504@laosinh.s.bawue.de>
On 10/06/2014 05:37 PM, Ville Laustela wrote:
>
> Gerrit Heitsch <gerrit@laosinh.s.bawue.de> kirjoitti 6.10.2014 kello 18.11:
>
>> On 10/06/2014 01:28 PM, Julian Perry wrote:
>>>
>>> In an unrelated query - I've been reviving a 1982  KU-14194HB
>>> silver-label machine. It's now working fine (RF modulator was dead) but I'd love a schematic
>>> for the thing, and I've been unable to locate one.  Has anyone seen
>>> one?
>>
>> If it's really a KU-14194HB board (the 2114 color RAM on U6 is located between CPU and Char-ROM), you need to check C107 and C108. Theses are 2 small electrolytic capacitors located left and right from the 74LS629 (which is right next to VIC)
>>
>> If those are black, very small and sitting directly on the board, you need to replace them immediatly (10F/25V will do). They don't contain much, but they will leak and it's enough for C107 to destroy the trace between Pin 15 if the 74LS629 and R26. Once that connection is gone, the VCO inside the 74LS629 will no longer do its job and no clock signal will be generated.
>>
>> You can't go by looks, the leak might be limited to directly below the capacitor.
>>
>> I had to fix this trace on more than one KU board.
>
> Is that the colour clock signal? I am currently working on a KU-14194HB board that is working otherwise perfectly well, except that it loses colours after couple of seconds. And while warm, it doesn't give colours at all (only b/w picture), only after cold start it works and only for a couple of seconds. The trimmer next to the VIC has no effect (I have even replaced the trimmer).

No, it's the master clock, without it, there will be NO other clock 
signal in the system since all are derived from the one 17.7xx MHz 
crystal (PAL).

If you have color when you turn it on, then it means that the color 
subcarrier is there, just shifted too far outside the range your 
TV/monitor can decode once the circuit warms up.

It could be the crystal itself that aged a bit too much, see here for an 
example:

  http://www.pagetable.com/?p=672

Another possibility is that C70 is no longer what it's supposed to be 
(15pF as far as I know).


> My other bet is on the modulator: the top of it is rusty and there are little signs of oxidation inside the modulator, but I haven't bothered to unsolder it (yet).

The modulator is unlikely if you have the same symptoms when using a 
monitor.

Anyway, check C107 and C108 on your board before it's too late.

  Gerrit



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Received on 2014-10-06 16:01:30

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