PCB Repair Logs – Page 43

Vimana repair log

May 062017

Got this pcb from a friend for a repair.

At first it showed a OBJECT RAM ERROR which was fixed by reflowing the custom BCU-2 which had some lifted pins.

At this point the game showed all the sprites with black pixels

After some trials, I could locate the circuit which draws the sprites.

There are 4x srams 6116, by piggybacking sram @12C ( marked in white on the below pic) the sprites were shown perfectly.

After exchanging it, I got another issue

From the above picture it is difficult to see, but on the upper half of the screen, the alternate lines of the sprites were displaced on the bottom side.

I was really puzzled and I was sure that something else got broken in the meanwhile.

After some days of blindly checking other parts of the circuits, I decided to probe the address pins of the very same sram I exhanged.

A10 and A9 were tied to ground on all 4 srams of the sprite circuit, while only A8 was floating on the sram @12C

The address pins were connected to some 74LS157 and all were working correctly.

That meant the trace A8 was somewhat broken underneath.

Fortunately I had another Vimana from which I noted to which 74ls157 output A8 was connected.

After fixing the connection, the games was 100% fixed

SEGA System 16 repair log #2

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May 052017

I received this system 16 motherboard from an arcadeitalia member called Consoleman.

The motherboard was missing completely the sound and didn’t sync with the monitor.

After checking with my sound probe that the sound was really missing by probing the pre amps, I started to check the Z80 which was confirmed good and RAM which was faulty.

Changing the ram restored the sound completely.

The Sync problem was not easy to check because the TTL which outputs the signal to the finger board is under the romboard.

In anycase I managed to test it from the solder side and I noticed the composite signal was about 14khz, that is why was not syncing.

I proceeded to desolder it

and tested out of circuit with my programmer as good….

Instead of soldering the same part, I decided to find another 74Ls125 among my junk boards.

It was not common but in the end I found it on another board and soldered it.

Motherboard was 100% fixed.

Probably my programmer hadn’t enough sensibility to declare the part as bad. First time it happens.

Black Tiger repair log #1

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May 052017

A friend of mine sent this pcb to check why the background was all messed up

I immediately noticed in demo mode that all the characters fell down in an endless loop because they couldn’t walk on the platforms

After searching for the fault in the video board for some days, I decided to test all roms starting from the program ones.

Program rom 04 didn’t match with anything in Mame and after checking the pattern I saw it was full of garbage.

After reprogramming with a good dump, the game was restored perfectly.

It seems rom 04 has only the layout of all levels because even without its presence the game is booting

Wild West C.O.W. Boys of Moo Mesa repair log #3

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May 032017

Today I received this Wild West C.O.W. Boys of Moo Mesa PCB for repair:

PCB was faulty, initial RAM/ROM check failed showing a ‘SOUND SYSTEM BAD’ error and then resetted :

Sound circuitry was already reworked: two RAMs, the Z80, the YM2151 and the ‘054539’ ASIC were replaced, the ‘054986A’ module was socketed too.But this was not enough to locate the fault :

The sound system reported as bad is ruled by a Z80 CPU, probing it revealed its pin 16 (/INT) was asserted, this meant the CPU was undergone an interrupt request stopping execution of code.It was time to use my Fluke 9010A.When I performed a BUS test, it reported address bit 15 (pin 5 of Z80) tied LOW:

This doesn’t necessarily implies the address line is shorted to GROUND but also that something is forcing it low.But in my case it was really shorted (only few Ohms of resistance to GROUND measured)

Doing some continuity test on a good board I could figured out that pin 5 of the Z80 is connected only to pin 5 of the PAL ‘054744’ @E7 and pin 1 of the 74ALS157 @C7 so in my faulty board one of these IC was bad :

For first I intentionally cut the trace going to pin 1 of the 74ALS157 @C7:

This actually cleared the short to GROUND of pin 5 of the Z80, hence the TTL was internally shorted.Tested out-of-circuit it obviously failed:

Now board could pass the RAM/ROM check and enter in game:

But sound was very faint, barely audible.I was forced to set volume to maximum to hear something:

The ‘054986A’ module was already recapped with thru-hole electrolytic capacitors:

A faint sound meant the main amplifier was working but something was wrong in pre-amplification performed by the 4558 OP-AMP on the underneath of the module.So I went straight and replaced it with a compatible LM358:

Sound was back loud and clear!Job done.

Truxton repair log #3

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Apr 262017

Another faulty Truxton PCB on the bench:

These were the issues: sprites were totally absent, sounds was loud and screen was disturbed by some wavy lines

Interferences on video are a common issue on this board, usually they are caused by electrolytic capacitors with increased ESR (especially the filter ones connected to +12V line).On my board the capacitor @C1 connected to +12V was previoulsy replaced:

But who replaced it managed to break the internal connection to GND of its negative terminal, you can see in the below picture that pads (and internal rivets) were ripped off:

The capacitor lost its GROUND reference hence it was not operational.This also explained why sound was loud: the amplifier was oscillating due the missing function of this capacitor (the role of these capacitors in audio circuit is indeed to avoid dangerous oscillations of components) and this affected the video too.I restored the connections, this cured both loud sound and video interferences so I moved on to troubleshoot the lack of sprites.Object data are stored in four 1Mbit 28 pin MASK ROMs :

Probing the address lines revealed they were all stuck low and hence data lines too.Address lines come from outputs of some 74LSALS163 but they resulted good when tested in circuit by my logic comparator.Probing around revealed nothing abnormal until I came across this custom chip (DIP 42 package) marked ‘TOAPLAN-02 M70H005’:

All the pins from from 1 to 21 ( not counting pin 21 which is GND) were toggling but all the ones from 22 to 42 on the other side (except pin 29 GND, and 23-42 VCC) were stuck, these were most likely the outputs.I compared the behaviour on a good board and had confirm that the stuck pins had to be active hence the custom was dead.I desoldered it and I found a ‘GXL-02’ silkscreening under the chip:

I remembered I had a couple of other Toaplan boards I harvested (Hellfire and Out Zone) with same silkscreening on PCB but different marking (”FDA MN53007T0A’) of the custom chip: