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21-12-2018, 14:17   #1
deadl0ck
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C64 not loading from tape

Hi all - My C64C won't load from tape at all. I have a thread about this issue on lemon64 but no solution yet.

Anybody here seen anything like this before ?
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21-12-2018, 17:53   #2
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if it can write but read seems to fail when reading I'd say it's most likely one of the CIA chip .

looking at the schematics ...
There's a shared data line (pin 5 of the serial bus) and the read signal ( at the serial bus pin 1) is triggered by pin 24 of the CIA chip U1 . It looks like it's active low, so I assume it's high all the time and should turn low when loading. If it's stuck high or doesn't go low enough this could well be the issue and the chip needs to be changed
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21-12-2018, 17:56   #3
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What happens when you try to load from tape?
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21-12-2018, 18:02   #4
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Is there anyone you can borrow a datassette from?Looking at the lemon thread,I reckon that's the problem.Could be your psu too.I once had a dodgy psu that caused loading issues.When the psu was swapped with a known good one the c64 and datassette worked fine.
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21-12-2018, 20:22   #5
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Thanks for the info @BGOllie - hopefully it's not that....

@magentis - I'll have to see if I can get another tape deck to try - or try mine somewhere. There is a guy in Mullingar that might test it for me - by chance just came across him the other day: https://byteandabit.ie/

The PSU is brand new from here
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21-12-2018, 22:42   #6
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Quote:
Originally Posted by magentis View Post
Could be your psu too.I once had a dodgy psu that caused loading issues.When the psu was swapped with a known good one the c64 and datassette worked fine.
I was wondering about that so I went back to the schematics and interestingly it looks like the PSU sends two 9v AC lines (pins 7 and 6) to (after going to various coils and cap) 7812 and 7805 that regulate the +12 and +5v DC power. It also derives the unregulated 9v DC from this too.
But back on the psu there's another pin, going to a separate conversion that generates the 5v to keep the "Pull high" threshold for some of the chips, like the RAMS and the CIA chips. It does this via transformer and caps. So if the value is wrong coming out of the PSU some of these chips might be thrown off and not work properly.
Wouldn't hurt to check the voltages right at the PSU:

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21-12-2018, 23:23   #7
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Quote:
Originally Posted by BGOllie View Post
I was wondering about that so I went back to the schematics and interestingly it looks like the PSU sends two 9v AC lines (pins 7 and 6) to (after going to various coils and cap) 7812 and 7805 that regulate the +12 and +5v DC power. It also derives the unregulated 9v DC from this too.
But back on the psu there's another pin, going to a separate conversion that generates the 5v to keep the "Pull high" threshold for some of the chips, like the RAMS and the CIA chips. It does this via transformer and caps. So if the value is wrong coming out of the PSU some of these chips might be thrown off and not work properly.
Wouldn't hurt to check the voltages right at the PSU:

Thanks - I'll check that first thing tomorrow when I get a chance !
I'm assuming it's fine as it's a new PSU but you're right - I should check it !
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22-12-2018, 10:57   #8
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Quote:
Originally Posted by BGOllie View Post
I was wondering about that so I went back to the schematics and interestingly it looks like the PSU sends two 9v AC lines (pins 7 and 6) to (after going to various coils and cap) 7812 and 7805 that regulate the +12 and +5v DC power. It also derives the unregulated 9v DC from this too.
But back on the psu there's another pin, going to a separate conversion that generates the 5v to keep the "Pull high" threshold for some of the chips, like the RAMS and the CIA chips. It does this via transformer and caps. So if the value is wrong coming out of the PSU some of these chips might be thrown off and not work properly.
Wouldn't hurt to check the voltages right at the PSU:

OK - I got measurements as follows:
  • Pins 2 and 6 -> 9.3VAC
  • Pins 2 and 7 -> 12.3VAC
  • Pins 2 and 5 -> 5.02 V DC
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23-12-2018, 01:10   #9
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mm pin 7 should be around 9v ideally. but since it goes to a 7805 regulator then it's converted to 5v regardless. If this part failed you'd have no 5v to supply all ICs and the computer wouldn't even start. so I'd say your ok with the power supply. Pin 5 is only the one that could affect the CIA chip in the way you're experiencing and it's fine at 5.02.

One thing you could try is test the voltage across pin 1 (ground) and pin 24. It should be high (around +5v) when the tape player is off and pulled low (around 0v) when the tape is loading. This pin is an input pin, meaning the signal comes from somewhere else , in this case the tape port. So check it's high when nothing's going on, load a tape and check again :
If it is low but the tape isn't loading you most likely have an issue with the CIA chip at U1 and it needs to be replaced.
If it is high and the tape doesn't load it could mean that the tape player itself isn't sending the read signal and need to be fixed (or replaced)
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23-12-2018, 12:10   #10
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Quote:
Originally Posted by BGOllie View Post
mm pin 7 should be around 9v ideally. but since it goes to a 7805 regulator then it's converted to 5v regardless. If this part failed you'd have no 5v to supply all ICs and the computer wouldn't even start. so I'd say your ok with the power supply. Pin 5 is only the one that could affect the CIA chip in the way you're experiencing and it's fine at 5.02.

One thing you could try is test the voltage across pin 1 (ground) and pin 24. It should be high (around +5v) when the tape player is off and pulled low (around 0v) when the tape is loading. This pin is an input pin, meaning the signal comes from somewhere else , in this case the tape port. So check it's high when nothing's going on, load a tape and check again :
If it is low but the tape isn't loading you most likely have an issue with the CIA chip at U1 and it needs to be replaced.
If it is high and the tape doesn't load it could mean that the tape player itself isn't sending the read signal and need to be fixed (or replaced)

Thanks for the advice Ollie - I really need to study electronics and get up to speed!!

I'll check that today hopefully - I'll double check my pin readings again too while I'm at it.

Fingers crossed it's a deck issue as opposed to a mainboard issue.

That being said I suppose I could get a new CIA chip if I really needed and put it in
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23-12-2018, 14:33   #11
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I think I have a spare tape deck for a C64 if you are badly stuck
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23-12-2018, 16:29   #12
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Quote:
Originally Posted by Stoner View Post
I think I have a spare tape deck for a C64 if you are badly stuck
Thanks - I really appreciate that !! I may take you up on it, but I still need to test the stuff @BGOllie suggested first.
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23-12-2018, 20:22   #13
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Quote:
Originally Posted by BGOllie View Post
mm pin 7 should be around 9v ideally. but since it goes to a 7805 regulator then it's converted to 5v regardless. If this part failed you'd have no 5v to supply all ICs and the computer wouldn't even start. so I'd say your ok with the power supply. Pin 5 is only the one that could affect the CIA chip in the way you're experiencing and it's fine at 5.02.

One thing you could try is test the voltage across pin 1 (ground) and pin 24. It should be high (around +5v) when the tape player is off and pulled low (around 0v) when the tape is loading. This pin is an input pin, meaning the signal comes from somewhere else , in this case the tape port. So check it's high when nothing's going on, load a tape and check again :
If it is low but the tape isn't loading you most likely have an issue with the CIA chip at U1 and it needs to be replaced.
If it is high and the tape doesn't load it could mean that the tape player itself isn't sending the read signal and need to be fixed (or replaced)
OK - So taking the reading as follows:


I get:
  • Normal : 4.8V
  • Tape Loading: 100.5 mV

So it looks like it might be U1 that's the issue ?
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23-12-2018, 20:29   #14
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Actually - looking at this is seems like U2 is the one that deals with user ports ?

Code:
U1 	906108-02 (6526/ A or 8521) CIA 
     Startup screen normal but no cursor, or blank screen if chip is shorted 
(remove to test). No keyboard or joyport access. Partial failure: some keys or 
joystick positions don't work, "stuck" line may print a character at startup. 
Cartridge works.

U2 	906108-02 (6526/ A or 8521) CIA 
     Lines or blocks instead of startup screen, but blank screen if chip is 
shorted (remove to test). Partial failure: marginal or no serial / user port 
access, keyboard and cartridges still work.
Me confusted

Edit: BTW - I'm on a C64C with the "short" board - PCB ASSY No. 250469
Another Edit: Actually, I see it's pin 24 on U1 on that diagram that gets the data...

Last edited by deadl0ck; 23-12-2018 at 20:44. Reason: Extra info
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23-12-2018, 21:34   #15
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you can do a continuity test just to be sure which chip is which

but the signal shifts as expected to low and the game isn't loading. Which would point to U1 being faulty indeed.
It could also be sth wrong further down the line on the Data bus, but these CIA chips are known to fail easily . I would consider changing that first.

I might still be worth checking there's activity on the other lines (ie, the data line might be having issues and not be receiving the data from the tape ) on the cassette port but you'll really need a scope or at least a logic analyser for this. Might be worth double checking with a known working tape deck first just to completely rule that part out
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