PCB Repair Logs – Page 54

NebulasRay repair log #2

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Dec 152016

Got this genuine NebulasRay for a repair:

Game played fine but suffered from a color issue, screen was all yellowish, self-test on boot reported a problem on palette RAMs:

A yellowish screen means that problem is in the BLUE color generation so I started to study the hardware and figured out the RGB circuit:

As you can see from the above picture, there are three 8K x 8 bit static RAMs, each one for a each color.These SRAMs are addressed by the custom ‘156’, their data go to the custom ‘116’ which processes them and generates the different color shades.These digital signals are converted to analog and formed into a single color by three 1K Ohm resistor arrays.Lastly each is color is amplified by a PNP transistor and routed on JAMMA edge pins.

With this knowledge for first I checked the 6264 SRAM @5X (which does the BLUE color), it showed normal activity on data/address bus until I probed its pin 27 (/WE) , it was not toggling like in other two RAMs but it was stuck HIGH so RAM was never written:

/WE signal of this RAM (like the other two ones) are generated by the custom ‘116’:

But with my multimeter I found no continuity, at a closer look the pad of pin 27 of this RAM lost contact with the trace.I promptly restored it and this gave me a fully working board again.End of job.

Super Spacefortress Macross and DoDonPachi repair log

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Dec 142016

Although issues were different in these two repairs (well, quick fix more than repair), the cause was the same.

Here’s the first board, a Super Spacefortress Macross :

Board had issue on some backgrounds which showed colored blocks :

A visual inspection revealed a lift pin on a QFP custom ASIC:

Reflowing the pin fixed the game:

Second PCB was a DoDonPachi :

with wrong music and sound FXs:

Also here the culprit was a lilfed pin (an address line, specifically) of the Yamaha YMZ280B-F (PCM/ADMPCM decoder)

Promptly fixed and sound 100% restored :

So, the bottom line is that you must always keep attention when you troubleshoot PCBs that use SMT devices, always check these for first…

Juno First repair log #4

PCB Repair Logs 2 Responses »

Nov 272016

Got this original Juno First (by Konami) for a repair :

100_9351

Board played fine but some sounds were missing, the most noticeable was the laser shot:

Here is a MAME record for a comparison:

All the sounds are generated by the AY-3-8910 chip.Probing it revealed that three I/O (pin 19-20-21) were stuck high, these should exchange data with same number of I/0 lines of the Intel 8039 MCU like shown in schematics:

80393r

The 8039 was dead, clock was missing on pin 2 and 3.I replaced the 8MHz quartz and the two 10pF capacitors with no change.This MCU has no internal ROM so it needs an external one :

100_9353

Using a dummy ROM file in MAME reproduced exactly the issue, some sounds were missing like in my board.This lead me to think the MCU was bad so I removed it:

100_9355

The 8039 is not widely used on arcade system but I was luck and found one on a Gyruss bootleg.Installed a socket and chip:

100_9479

and all mssing sounds were back.End of job.

Truxton repair log #1

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Nov 062016

I got this Truxton PCB (export version of Tatsujin) in a recent operator raid:

100_9253

Once powered it up board showed GFX issue, the sprites were blocky :

Besides, there was some kind of video interference, this is a common issue on this board and usually it’s due capacitors in the audio section, indeed I found a 470uF 16V one with bad capacitance :

100_9275 Back to the sprites issue, relevant data are stored in four 1Mbit 28 pin MASK ROMs :

100_9257

I went to dump them and found a faulty one:

b65-04_bad

Replacing it with one from a donor board didn’t fix the issue therefore I went on in my troubleshooting.Each data bit of the sprites MASK ROMs is fed into inputs of four 8-bit 74LS166 register.I probed them with my logic comparator and led on output pin 13 lit for two of them:

100_9259

I desoldered the ICs:

100_9260

They both failed the out-out-circuit testing (obviously they were from Fujitsu) :

74ls16623a-24a

Sprites were fully restored :

100_9304

The last issue I had to fix was that Player 1 Start input didn’t work.The inputs circuitry was previously reworked by someone (note the replaced 74LS240 and some patched traces)

input_circuitry_reworked

I traced Player 1 Start back from pin 17 parts side of JAMMA connector to an end of a 220 Ohm resistor @R17 whose other end was tied to pin 13 of the 74SL240 @21K :

p1_start_tracing

Signal on pin 17 of the JAMMA connector and on first end of the resistor was high and correctly toggling when pressing the button while was totally missing on pin 13 of the 74LS240.Therefore the only culprit had to be the resistor.Indeed I got no reading measuring it on circuit:

100_9265

I removed and tested the resistor out-of-circuit having confirm it was opened:

100_9269

100_9268

Job done.

WWF Wrestlefest repair log #2

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Oct 312016

When switching ON the game, sometimes it had very fast tempo music.

It happened 1 one out of 10 but it was very annoying.

So I used my frequency counter when it happened and I noticed that the 3.58mhz quartz ran at about 10.58mhz!

I changed it with a know good one but again it happened very often.

I was sure it was something connected to the quartz oscillator circuit and given that the resistances are difficult to be faulty I decided to change the cap C47 (33pF) with known good one.

After testing about 30 cycles of switching on and off the game didn’t develop again the problem

Game fixed.

wwf

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