Apple II Europlus ; 14Mhz on clock ?

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Apple II Europlus ; 14Mhz on clock ?

Hey everyone,

 

I'm repairing my first II europlus, altough i've repaired quite a lot of more modern pcs.

 

The problem i'm facing is that the Mac (II europlus) does not boot, has no video, no light on the keyboard either.Voltages are fine. Powersupply is actually a new one bought by the previous owner.I tried removing all cards, same story. There is a ram expansion card that becomes very hot to touch after a few minutes. (RAM chips very hot, logic stays cold)No chip on the board is hot to the touch. ALl chips have proper +5VOscilloscope shows a RESET on the reset pin at boot and when I do CTRL-RESETCLock on 6502 chip is strangely enough measured at 14Mhz, not 1Mhz (?!) which probably means something is very wrong at the clock divider side.has anyone encountered such a problem ?

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Check the clock generator

One or more of the components in the region A1, A2, B1, B2, C1, C2 is faulty, so check those.

The oscillator is probably ok, it runs at about 14 MHz and is divided down to 1 Mhz by the other components.

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The 74S175 is probably the

The 74S175 is probably the culprit, as if I read the schematic right, it seems to be used to divide frequency and output "Theta 0" which is the CPU clock

Altough this goes thru a chip at B11 beforer going into the CPU

 

 

What would be a modern replacement of a 74S175 , or S195 ?

 

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74LS175 might be easier to

74LS175 might be easier to find, or 75HCT175.  But if Apple used a 74S175 instead of an LS, there may have been a reason for that.  You can still find old stock 74S175 if you look around, so I'd recommend getting one of you can.  Otherwise try the LS or HCT.

 

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ThanksI ordered a couple of

ThanksI ordered a couple of NOS spares from ebay in a nearby country,I'm a bit worried about the RAM card extension that has all it's chip hot to the touch, probably related to the massive overclocking.It's likely a lot of things are damaged... Maybe even the CPU...

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For anyone having the same

For anyone having the same problem in the future (that's why we post on forums right)

Replacing the 74S175 with a 74LS175 does not work. As far as I can tell it may be due to the LS beeing twice slower than the S, and the clock rate beeing 14Mhz this might be an issue. The datasheets are hard to come by ( 90% of spam websites ) so it's hard for me to verify. Take this with a grain of salt.

At least in my case ; replacing S with a LS (I found on a Language card) leaves clock ( Theta 0) at level high all the time.

I'm waiting NOS 74S175 to see if this problem is solved.

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Also interestingly, there is

Also interestingly, there is an error in the APple II plus/europlus schematic (at least the one if found) where they mixed up Q2 & Q3 in their 74S175

The pins are correct ( Theta0 get indeed out of pin 7 , which is Q2 according the S175 datasheet) , but the name of the FlipFlops are wrong in the Apple docs.

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CedsRepairs wrote:...Theta0 .
CedsRepairs wrote:
...Theta0 ...

 FYI -

Not to be picky, but you keep referring to the Phase0 signal as Theta0. If anything, it would be Phi0, but we usually just call it Phase 0.

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No thanks for letting me know

No thanks for letting me know, the schematics i've got are a bit blurry and it look like a theta to me :)

Phase 0 makes for sense, so does Phi 0

 

I'm here to learnn this is my first AppleII 

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In case someone in the future

In case someone in the future ever comes in this topic

 

The problem was with the 74S195 in C2

 

Replacing it made my clock back at 1Mhz and not 14Mhz  (!!!) anymore. (clock measured on the 6502)

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Thanks

for letting us know.

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