FIXED UP! Now it can read, write and format Double Density disks perfectly. It can also read High Density disks but, as a result of the fix, it cannot write or format them. I am going to relate the repair process. May it help people repair their drives.
First I searched the Internet for the circuit board photo and compared to mine.
The attachment dsc_0143.jpeg is no longer available
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I found three differences in my board:
1) One added R33 resistor in parallel with the existing one in the other side of the board.
2) One added resistor in parallel with the last resistor in the RM1 resistor network (termination resistors).
3) An accidental tin solder drop over the R8 resistor which was shorting it.
Then I searched for the schematic but I could not find it. Instead, I found the schematic of the Chinon F-502II or F-502LII which seems to be quite similar for the most part to that of the FZ-502.
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The two main ICs (Sony CX20185 and Toshiba TC8600F) are the same but there are some differences between this schematic and the FZ-502 circuit board:
- R33 in my board is R34 in this schematic.
- R15 in my board is R32 in this schematic.
- The terminal resistors (RM1 resistor network) are before the diodes, the other way around in this schematic.
- R11, R12, R13 and R14 in my board are the RM2 corresponding to DIR, STEP, WGATE and SIDE signals in this schematic.
- R7 in my board is R12 in this schematic.
- R9 in my board is R11 in this schematic.
- R8 in my board is R9 in this schematic.
- Maybe IC4 and IC5 are of the same model in this schematic but they are of different models in my board (different pin numbers).
- ...
There are more differences between the schematics and much more important than these without a doubt, but I did not want to deepen the subject (I had already found what I was looking for...).
The original R33 resistor in my board (R34 in this schematic) is 2,7 Kohm and the added one in parallel is 1,6 Kohm, so the resulting total resistance is 1 Kohm.
The resistor network includes seven 1 Kohm resistors but the last resistor in the network must be defective because the total resistance with the added resistor in parallel is 1 Kohm.
Maybe the drive did not pass the quality control and was repaired. I suppose that both resistors where added in the repair process and that the tin solder drop fell on R8 (R9 in this schematic) during this process. Then the write function of the drive was not properly verified. Because the one to blame is the shorted R8.
What I have learnt after my research on this malfunction is that DD disks and HD disks are made with different materials featuring different coercivity (300 oersted for 5.25" DD disks and 600 oersted for 5.25" HD disks) meaning that HD disks require a stronger magnetic field than DD disks to be written. The current passing through the heads of the drive generates the magnetic field so it is needed a higher current to write a HD disk than to write a DD disk.
R8 (R9 in this schematic) is the resistor which value determines the write current passing through the heads. With a short in R8 the current is too high to write a DD disk properly but it is fine to write a HD disk. After removing the short in R8, the current generated with the 3.3 Kohm resistor is fine to write a DD disk but it is insufficient to write a HD disk.
You cannot have it all.