Jun 012018
 

Received from Germany this faulty DonPachi PCB for repair:

Board played almost ‘blind’, most of graphics was missing, only some corrupted text was visible:

According to MAME source there are three layers of graphics:

ROM_REGION( 0x100000, “layer0”, 0 ) /* Layer 0 */
ROM_LOAD( “atdp.u54”, 0x000000, 0x100000, CRC(6bda6b66) SHA1(6472e6706505bac17484fb8bf4e8922ced4adf63) )

ROM_REGION( 0x100000, “layer1”, 0 ) /* Layer 1 */
ROM_LOAD( “atdp.u57”, 0x000000, 0x100000, CRC(0a0e72b9) SHA1(997e8253777e7acca5a1c0c4026e78eecc122d5d) )

ROM_REGION( 0x040000, “layer2”, 0 ) /* Text / Character Layer */
ROM_LOAD( “text.u58”, 0x000000, 0x040000, CRC(5dba06e7) SHA1(f9dab7f6c732a683fddb4cae090a875b3962332b) )

I translated this info on hardware and figured out the relevant circuit of each layer:

Each circuit consists in a QFP custom ASIC (maked ’38’) which addresses an 8Mbit MASK ROM (or a 2Mbit EPROM for the text layer) reading back data that then are written to two 8K x 8-bit static RAMs.After succesfully  checked connection between ASIC, ROM and RAMs my suspicions fell on the 8k x 8-bit SRAMs, they were all manufactured by Sanyo so in my experience not a great guarantee of reliability.Probing the ones @U50 and U51 (which lie in the ‘layer 0’ circuit) revealed weak or stuck signals on many data lines:

I removed both:

Actually only the one @U51 failed the out-of-circuit testing:

Now all graphics were visibile.Backgrounds and sprites were fine but text was corrupted throwing garbage over the screen :

The ‘layer 2’ identified in MAME source as ‘ Text / Character Layer’ was obvioulsy the involved one.Checking the two Sanyo 8K x 8-bit SRAMs @U55 and U56 revealed again weak signals on data lines:

I removed them and installed machine sockets:

Actually both RAM chips successfully passed the out-of-circuit testing of my different programmers.Anyway, replacing them fixed completely the graphics:

But sound was horribly scratchy and corrupted as you can hear in the above video.Here it comes again to help my audio probe for checking the relevant circutit:

“Listening” to various points revealed the sound was clear on the analog output of the two OKI MSM6295 and still good on outputs of the LM324 OP-AMP and input of the LA4460 amplifer.It was fine too on one output (pin 7) of the LA4460 amplifier:

 

But corrupted on the other output (pin 9)

I ruled out all electrolytic capacitors checking them in-circuit with my ESR meter (bad ones can affect sound in this way) so I decided to remove the LA4460 amplifer:

Put back a good one and some thermal compound for a better heat dissipation:

This restored a crystal-clear sound.Mission DonPachi accomplished!

 Posted by at 9:10 pm

  One Response to “DonPachi repair log #2”

  1. Very good intuition on detecting faulty RAM even passed on testing them on two different programmers.
    This proves you are really good on repairing arcade boards.
    Excellente log repair.
    Loved it.