C-64 Weirdness.

[shakalnokturn]

Inserting cassette and pressing "Play" or "Play+Rec" pulls "CASS SENS P4" low.
This is the first condition for the MPU to drive "P5" low and enable cassette motor once a LOAD or SAVE command is used.

A shorted motor could bring the 9V line down, but you'd probably notice that due to Q2 overheating.

When you say "if i remove U7 VIC the circuit all the sudden works perfectly" does that mean you actually get the cassette to spin? With something in the 6 to 7V range on Q1-Emitter? measuring a voltage there unloaded is bad diagnosis.

A wrong model VIC could bring the 9V line down but you'd probably notice that due to VIC overheating.

This could be a case of starting at the beginning.


Are you sure of your Power supply? Original model or substitute with sufficient 9V AC current? Have you been able to use your PS on another C64 with a 12V VIC?
Next check either voltage drops when loaded as you go along, or contact resistances unpowered.

Power socket contacts.
Weak solders on power socket, rectifiers, L1 choke, fuse+holder resistance.
Check C63 is still good enough.
Check power switch contact resistance/voltage drop. This would be the perfect culprit IMHO.

[Bashstreet]

Inserting cassette and pressing "Play" or "Play+Rec" pulls "CASS SENS P4" low.
This is the first condition for the MPU to drive "P5" low and enable cassette motor once a LOAD or SAVE command is used.

A shorted motor could bring the 9V line down, but you'd probably notice that due to Q2 overheating.

Ok i think i understand the basic function of the circuit now thank you.
I have tried with two other cassette drives they do not function ether.

When you say "if i remove U7 VIC the circuit all the sudden works perfectly" does that mean you actually get the cassette to spin? With something in the 6 to 7V range on Q1-Emitter? measuring a voltage there unloaded is bad diagnosis.

If U7 VIC is removed pin 3 in cassette connector gets 6.8 volts and cassette drive functions of course cannot boot the computer without video processor.

A wrong model VIC could bring the 9V line down but you'd probably notice that due to VIC overheating.

Both VIC's are correct version for the board 8565R2

Are you sure of your Power supply? Original model or substitute with sufficient 9V AC current? Have you been able to use your PS on another C64 with a 12V VIC?
Next check either voltage drops when loaded as you go along, or contact resistances unpowered.

Yes i have tested to board with two power units one original one build the board and other boards work with these units (rebuild one has 3 amp 5V supply and 9V has 10 VA)
The VIC (VIC2) uses purely 5 volt line for power. SID of this board uses 9V both lines are fine on on U6 MPU pin 6 and U9 SID pin 28.

Power socket contacts.
Weak solders on power socket, rectifiers, L1 choke, fuse+holder resistance.
Check C63 is still good enough.
Check power switch contact resistance/voltage drop. This would be the perfect culprit IMHO.

C63 Should be fine after all the cassette player works fine (i get 6.8v (constant) on pin 3 of cassette connector and the cassette drive plays rewinds etc just fine after removal of Q2.
But the device is not found.

Could it be that somehow the MPU do not detect the drive and that is why it do not activate the circuit ? (tested with two drives)
What is peculiar as MPU detects my SD reader (it used purely 5 volt supply from the cassette port)

[shakalnokturn]

Could it be that somehow the MPU do not detect the drive and that is why it do not activate the circuit ? (tested with two drives)
What is peculiar as MPU detects my SD reader (it used purely 5 volt supply from the cassette port)

Sure that could be a cause... Even more if your SD reader works.
Engaging cassette and pressing "play", can you confirm that the P4 line goes from high to low?
You can compare what happens on P4 line with your SD reader. Note that while you " LOAD" or "SAVE" to your SD the 6.8V should appear at Q1-e.

Better check the schematic for the tape drive before trying but as P4 line has a pull-up resistor, you can probably force this line low to check.

Other than messy power I still don't get why removing VIC would enable the tape motor... I'm no C64 guru though.

[Bashstreet]

Could it be that somehow the MPU do not detect the drive and that is why it do not activate the circuit ? (tested with two drives)
What is peculiar as MPU detects my SD reader (it used purely 5 volt supply from the cassette port)

Sure that could be a cause... Even more if your SD reader works.
Engaging cassette and pressing "play", can you confirm that the P4 line goes from high to low?
You can compare what happens on P4 line with your SD reader. Note that while you " LOAD" or "SAVE" to your SD the 6.8V should appear at Q1-e.

Better check the schematic for the tape drive before trying but as P4 line has a pull-up resistor, you can probably force this line low to check.

Other than messy power I still don't get why removing VIC would enable the tape motor... I'm no C64 guru though.

The SD reader far as i know only takes 5 volt from the cassette port (I might be wrong) and communicates through serial bus.

I tried pressing play play button but there was no effect what so ever on the lines.
But there is difference if i just plug in the cassette drive. Q1's Emitter starts to produce 0.8 volts.. Q2's base still gets the 0.8 volts from R3 Resistor.
Any case pressing rewind and forward should work all the time..
When i boot if left on rewind button runs for few hundred milliseconds as it gets the power but soon as something happens screen appears it stops and voltage is not provided. (what usually happens if you leave play button on and reboot it moves tiny bit but rewind forward should keep running)

Removing VIC is puzzling to me also but it might be that without VIC the MPU do not boot and as result it not to interfere with the Q2 (line stays low and Q1 activates)

P0
    LORAM. If set to 1 (default), the BASIC ROM is mapped into $a000-bfff, if set to 0, accesses to this area address the RAM.
P1
    HIRAM. If set to 1 (default), the KERNAL ROM is mapped into $e000-ffff, if set to 0, accesses to this area address the RAM. Alas, this bit also controls the mapping of the BASIC ROM, i.e. if the KERNAL ROM is deactivated, the BASIC ROM is deactivated, too. This means if you want to modify the KERNAL, you have to not only copy the KERNAL from ROM to RAM, but the BASIC ROM, too, since you cannot switch off the KERNAL ROM individually.
P2
    CHAREN. Setting this bit to 1 maps the I/O registers into $d000-dfff, otherwise the character ROM is selected. Note that if you want to access the I/O registers, both P0 and P1 must not be 0 simultaneously.
P3
    Cassette data.
P4
    Cassette sense. 0 = PLAY button pressed.
P5
    Cassette motor. 0 = motor spins (only if $0192 (cassette motor control flag) is not 0).

If you set both P0 and P1 to 0, the I/O ports will be deactivated.

[Bashstreet]

Contact resistances seems fine.

[Bashstreet]

 

[fzabkar]

ISTM that removing the VIC is a red herring. If the MPU doesn't see the VIC, then I suspect that iatt may halt or hang during the POST, in which case the cassette port may not be initialised.

IIUC, the 5V pin supplies the logic in the cassette while the 6.8V (6.2V ?) pin supplies the motor (and analogue circuitry?). Therefore you should see P4 go low when PLAY is pressed. I would think that this pin would be an open collector output from the cassette, in which case you should be able to short it to ground without risk of damage (as already suggested by others). If P4 does not go to ground (as you appear to be saying), then the fault must be in the cassette or the cable. But you have tested other cassettes with the same result. :-?

[shakalnokturn]

All the datasette schematics checked show that 6.8V is "motor on" (motor supply only).
The P4 line is driven low by switch that closes on pressing any key that requires the motor to spin.

[Bashstreet]

ISTM that removing the VIC is a red herring. If the MPU doesn't see the VIC, then I suspect that iatt may halt or hang during the POST, in which case the cassette port may not be initialised.

IIUC, the 5V pin supplies the logic in the cassette while the 6.8V (6.2V ?) pin supplies the motor (and analogue circuitry?). Therefore you should see P4 go low when PLAY is pressed. I would think that this pin would be an open collector output from the cassette, in which case you should be able to short it to ground without risk of damage (as already suggested by others). If P4 does not go to ground (as you appear to be saying), then the fault must be in the cassette or the cable. But you have tested other cassettes with the same result. :-?

I agree, i think removing VIC was red herring and it only stopped MPU initializing.
Still this lead to situation where C3 pin got voltage cassette drive needs to operate the motor this was "simulated" with removal of Q2.
Results of this was cassette player functioned (motor) and play rew etc worked but the cassette player was not detected by computer and could not be used.
P4 do not seem to go low with press of play (I think reason is that the MPU?" do not detect there being a cassette player)
It might be the core of the problem is MPU do not detect the player  (I have tried with 3 players)

All the datasette schematics checked show that 6.8V is "motor on" (motor supply only).
The P4 line is driven low by switch that closes on pressing any key that requires the motor to spin.

Yes this is confusing me , There should be no need to press play to active Q2 as rewind and forward should work all the time.

[fzabkar]

If you examine all the schematics URLs on the following page, you will find that the PLAY switch simply connects the P4 pin to ground.

http://www.zimmers.net/anonftp/pub/cbm/schematics/datassette/

[shakalnokturn]

All the datasette schematics checked show that 6.8V is "motor on" (motor supply only).
The P4 line is driven low by switch that closes on pressing any key that requires the motor to spin.

Yes this is confusing me , There should be no need to press play to active Q2 as rewind and forward should work all the time.

Confusing me too actually! I don't know enough on the C64, sure some people out here do though.
As the only way to supply motor is through Q1, the P4 line must be pulled low also for REW/FF.
Then I'm not sure how this is handled once you have cued the tape and LOAD or SAVE a program or do you just have to: cue, pause, LOAD, un-pause, wait... ?

Your problem could very well come from the MPU not detecting the Datasette (therefore no motor power switching), however it is rather unlikely that the CPU itself will stop the P4 line from changing state as all you have there to oppose to that is a pull-up resistor (RP5) and a logic input (if that was destroyed you'd have heat or other problems driving P4 low).
If you can get P4 to go low (you can simply force it low for experimentation using a ground strap) and you have checked that the motor power switching works, if you still have no normal operation that leaves you with the possibility of a defective MPU.

I suggest you open a Datasette and connect it to the C64 check it's cable continuity including ground and shielding.
When Play is pressed P4 must go low, first problem to solve.
Very often the supple contacts on tape mechanisms develop a black oxidation giving high contact resistance. (Does P4 decrease at all?)
A contact spray and strip of paper as an abrasive is usually enough to do the trick.

[Bashstreet]

Well the mystery is finally solved.

There was nothing wrong with any of the circuitry components IC's or power rails...
It turns out Jiffy DOS i installed do not support cassette drive.

"It is not possible to use a tape drive (Commodore DATASETTE, etc) while the JiffyDOS
Kernal is selected. If tape access is attempted, an ILLEGAL DEVICE NUMBER ERROR
will occur.

It is kind of good mystery is solved i just wish i did not have to waste hours upon hours troubleshooting and desolering and resoldering and lifting one pad..

Any case Cheers for everyone for your help. 

[jeffheath]

Well the mystery is finally solved.

There was nothing wrong with any of the circuitry components IC's or power rails...
It turns out Jiffy DOS i installed do not support cassette drive.

"It is not possible to use a tape drive (Commodore DATASETTE, etc) while the JiffyDOS
Kernal is selected. If tape access is attempted, an ILLEGAL DEVICE NUMBER ERROR
will occur.

It is kind of good mystery is solved i just wish i did not have to waste hours upon hours troubleshooting and desolering and resoldering and lifting one pad..

Any case Cheers for everyone for your help.