20 Mar 2019

Raiden - Seibu 1990 (repair log)

Got this board in a lot of faulty boards in a trade. It came in an antistatic bag with a note written on it "will not boot".
Upon first inspection I noticed board was very filthy so probably hadn't been stored in the bag all the time. Also custom chip UEC-52 was completely missing (ripped from the board). Fortunately Caius from jammarcade.net sells reproductions of it, only I placed an order for one few days ago for an other board which had the chip scratched to the point only legs of the SMD ICs mounted on it where left. At least I'll have the ICs reference from the reproduction.

I usually find very few information about what a board should do with missing elements, which is a pity since this can be extremely useful for repairs.
Here are few things I can tell:
- without the UEC-52 custom chip (DAC used for colours in SIL22 package) the game still boots and plays blind
- the board requires -5V for sound, this probably explains with the board was noted "will no boot" when it fact it did but played blind, only to notice it you need -5V
- sound part is completely independent so even with a completely dead main CPU or sub CPU you are still able to hear the coin jingle if the sound part is alive since it's an interrupt (jingle is played whenever you insert a coin, wherever you are in the game)
- without the OBJ1 daughterboard game still boots and plays blind (black screen)

So as said game wasn't completely dead but played blind due to the missing custom chip. Fortunately I had received an other Raiden board few weeks ago which was perfectly working (and turned out to be an undumped set).
Could it be as simple as installing back the custom chip to get a fully working game?

Not really, I did so and then found valid signals on the RGB outputs, which was progress, but sync, despite having activity, wasn't valid. I inspected the board again and found a severed trace on the part side. I patched it and that did the trick:

Game fixed.

13 Mar 2019

Strikers 1999 (Strikers 1945 III) - Psikyo 1999 (repair log)

The second board I got from ekorz, or should I call him my friend after the gift he made to me.

Again board had colour issues:

Green was completely absent:

Going back from the JAMMA connector I quickly ended up on a cracked resistor on the final stage of the green colour amplification (33Ω SMD). I replaced it and colours were fixed:

I continued my tests and found left direction for player 1 wasn't registered:

Again starting at the JAMMA edge I found one of the custom resistor array (possibly containing capacitors too) stamped MLR8C4 IAM (or is it 1AM?) measured resistance between pin 2 and pin 3 was ~4kΩ when it should be 100kΩ. I pulled it but the lower resistance between pad 2 and pad 3 was still present. I was left with a single chip in line: a HCT244 (U42), I confirmed the ~4kΩ resistance between its pin 11 (input) and ground. Once replaced all controls were then working.

Game fixed.

20 Feb 2019

System 16B motherboard - Sega (repair log)

The third and last board I got from ekorz. It was sent as non going but I found game logic was actually working but:
- sound was totally absent
- colours were wrong (mostly yellow screen)
- text and background layers were missing

It's hard to see but sprites are actually drawn in white.
By looking at the board something caught my eyes: UMC (United Microelectronics Corporation) branded RAM chips (known to be unreliable).
I probed them:
- 1 * 6116 type for sound RAM: was bad
- 2 * 6116 type for palette RAM: one was bad
- 2 * 6116 type for text RAM: both were bad

Once replaced:

Motherboard fixed.

13 Feb 2019

Hammer Away - Sega+Santos 1991 (repair log)

I got this game in a lot of faulty boards from ekorz. I'd like to thank him a lot, the boards were in great condition. According to him romboard was fine but motherboard had colour issue and a repair had been previously attempted. I could confirm both, game had a saturated blue colour, and both palette RAMs and a couple of nearby TTL chips had been put on sockets.

I started by probing the palette RAMs and found signal /WE of IC42 was floating. I traced it back to a LS244 (IC40) but piggybacking it made no difference. Then I found the two enable signals (1G/2G) of this LS244 were tied together and held high when not writting to the RAMs. This causes the output signals to go in high impedance state, however probing /WE signal of the second palette RAM revealed it was high so there clearly was a pull-up resistor involved. I measured 1kΩ resistance with Vcc whereas the floating output measure few MΩ. Looking at the board closely I discovered a 1kΩ resistor between the two palette RAMs. On was tied to Vcc as expected the the other, altough aligned with the /WE pin of the RAM IC42 was connected to nothing. I could see the trace was going straight to the rivet (only few millimetres away) of /WE pin so to me had to be connected there. I restored connection and colours were fixed:

I then tested inputs, sound, etc. but found nothing else wrong.
Game (very easily) fixed.

6 Feb 2019

Master System II - Sega (uneconomical repair)

Some repairs aren't worth the effort an time they would need. I'm looking at you faulty Pit Fighter PCB! But giving up is the easy way and I like challenges.

Here we have a faulty Master System II. According to previous owner left direction for player 1 is stuck. I powered the console (with Alex Kidd built-in) and sure enough Alex went straight to the left. I opened the unit and found the custom chip handling (amongst other things) inputs was a 315-5237. SMSPower website has tons of info about the MS and is an extremely valuable source, thanks to all of you guys contributing to it. So I quickly found pinout and probed pins used for controls:
- P1 down was floating
- P1 left was stuck low (= seen as button depressed)
- P1 right was stuck low (= seen as button depressed)
Other controls seemed fine (P1 up/buttons, P2 controls). This was confirmed by testing the player 2 port with different games, however behaviour for player 1 changed from game to game:
- Alex Kidd = keeps going left and impossible to crouch/duck
- Double Dragon = keeps going down/right
- etc.

Not that way Alex!

Ugh... I'm cornered.

I suppose this all depends on how each game was coded and what priority was given between up/down and left/right directions.

Anyway pull-up resistors for those inputs are embedded in the custom chip and previous owner already installed an external resistor but it didn't do the trick. I measured resistance for all 3 faulty inputs and found:
- P1 down wasn't connected to anything internally (no pull-up resistor measured, in fact resistance was infinite with any other pin)
- P1 left and P1 right had only few dozens of ohms resistance with ground so wasn't surprising the external 4.7kohm resistor didn't work

I could have stopped there, custom chip being faulty I would have needed a replacement from an other Master System II with the same motherboard. Sega hardware is generally rock solid and only faults I've repaired before on Master System II were broken solders on the AV port and severed traces, in other words even if I had found a compatible donor it's likely repairing it would have been simple. At least simpler than pulling the DIP48 custom chip and transplanting it in the other console.

Having repaired older Sega hardware similar to the Master System (e.g. Mark III) I knew controls could be read and data sent on the bus with simple TTL chips. My idea was to reverse engineer the part of the 315-5237 custom chip handling inputs.
I started to build a small test circuit with the custom chip on a socket and a GAL chip to recreate the faulty part of the custom chip. I lifted pins of the custom chip corresponding to P1 down/left/right and data bits D1/D2/D3 in order to isolate them.

The Master System reads control trough two mapped ports $DC and $DD, inputs are latched to the data bus:
- $DC.d0 = P1 UP
- $DC.d1 = P1 DOWN
- $DC.d2 = P1 LEFT
- $DC.d3 = P1 RIGHT
- $DC.d4 = P1 BUTTON  1
- $DC.d5 = P1 BUTTON 2
- $DC.d6 = P2 UP
- $DC.d7 = P2 DOWN

- $DD.d0 = P2 LEFT
- $DD.d1 = P2 RIGHT
- $DD.d2 = P2 BUTTON 1
- $DD.d3 = P2 BUTTON 2
- $DD.d4 = RESET BUTTON (not present on MS2 anyway)
- $DD.d5 = Cartridge slot CONT signal

Given all other functions of the custom chip were working fine I went for the minimal replacement solution, that is to say reading of P1 DOWN/LEFT/RIGHT inputs and latching them to D1/D2/D3 on the data bus. It was quite easy to figure out equations. Here is the PLD file I wrote for a GAL16V8 chip in case someone else has a similar issue and wants to take inspiration (for some reason WinCUPL didn't allow me to use more than one term for the OE equations so I used a spare output, it might be a GAL16V8 limitation):

Name      315-5237_repair;
Partno    ;
Date      ;
Revision  ;
Designer  Apocalypse;
Company   ;
Assembly  None;
Location  None;
Device    G16V8;
/**  Inputs  **/
PIN 1 = JyDs;
PIN 2 = A0;
PIN 3 = A6;
PIN 4 = A7;
PIN 5 = RD;
PIN 11 = IN_OE;
/**  Outputs  **/
PIN 18 = D1_BUS;
PIN 17 = D2_BUS;
PIN 16 = D3_BUS;
PIN 15 = D1_5237;
PIN 14 = D2_5237;
PIN 13 = D3_5237;
PIN 12 = OUT_OE;
/** Logic Equations **/
OUT_OE = !RD & !JyDs & !IOREQ & A6 & A7;
D1_BUS = !A0 & DOWN # A0 & D1_5237;
D2_BUS = !A0 & LEFT # A0 & D2_5237;
D3_BUS = !A0 & RIGHT # A0 & D3_5237;
D1_5237.OE = !IN_OE;
D2_5237.OE = !IN_OE;
D3_5237.OE = !IN_OE;
D1_5237 = D1_BUS;
D2_5237 = D2_BUS;
D3_5237 = D3_BUS;
And then was successfully tested:

Console (uneconomically) fixed.

30 Jan 2019

Slot PGM - IGS (repair log #2)

I received this faulty PGM slot as part of my Christmas gift from my friend Lorenzo2mars.
Problem was red colour being saturated in-game:

However some screens appeared fine:

Colours bars in the hardware test menu also appeared correct:

So did the RAM check (from my experience colour RAMs U23/U24 correspond to RAM 5 in the test):

To me problem was downstream in the late colour generation stages (DAC). I followed data lines from the colours RAMs leading me to the IGS custom chip U6 (stamped IGS026) then on the other side of this custom were custom resistor arrays (stamped RSA1) and few other components (transistors for amplification and few resistors to drive them). I quickly found each analog colour signal could be mixed with one coming from the expansion connector (same one used for 3p/4p inputs). Was this functionality ever used (photo booth or similar?). I probed the transistors used for the red colour (Q3/Q4) but they seemed ok. I went back to the custom resistor array noted RN1 on the silkscreen and found connectivity was lost between its legs in the middle. Having a spare IGS motherboard in my part bin I was able to replace it. While pulling the faulty one this happened:

It was probably cracked before but I couldn't notice it. Once replaced red colour was back to normal:

An extremely easy repair this time!
Board fixed.

23 Jan 2019

Sega System E to JAMMA

While working on a side project (multi System E) I decided for once to make a nice JAMMA adapter instead of a handmade one with dozens of sagging wires.

Nothing fancy here, it's easy to install and does the job: