Caius

Vapor Trail – Hyper Offence Formation repair log #2

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Oct 292018
 

Received this Vapor Trail PCB for a repair :

Board had two noticeable issues, the sound samples were corrupted and sprites glitched like if part of them were floating on screen

I decided to troubleshoot for first the sound issue.A quick visual inspeaction revealed a 470uF electrolytic capacitor was missing in audio section:

But it was just the one which filters the +12V so reinstalling it didn’t cure the issue.

PCM samples are played by two OKI MSM6295 on main PCB while data are stored in two 27C010 located on ROM board:

I dumped the devices and they matched the MAME ROM files:

ROM_REGION( 0x40000, “oki1”, 0 ) /* ADPCM samples */ ROM_LOAD( “fj06”, 0x00000, 0x20000, CRC(6e98a235) SHA1(374564b4e494d03cd1330c06e321b9452c22a075) ) ROM_REGION( 0x40000, “oki2”, 0 ) /* ADPCM samples */ ROM_LOAD( “fj05”, 0x00000, 0x20000, CRC(39cda2b5) SHA1(f5c5a305025d451ab48f84cd63e36a3bbdefda96) )

But then I remembered I repaired another same PCB which had only one EPROM for audio samples whereas the other socket was empty.Other pictures found on the net confirmed this too like this one taken from ‘system11’ blog:

So I removed the one @1F and sound was fully restored.

Now the sprite issue.Hardware used the custom ASIC (in QFP160 package) marked ‘MXC 06’ which generate objects  :

We can see its pinout in the Oscar schematics:

Since the ROM board covers entirely the ‘MXC 06’ custom ASIC denying me any access to probe its pins I decided to replace it given also the fact there is almost nothing else involved in sprite generation  :

I took the spare from a dead Sly Spy/Secret Agent PCB and soldered it pin by pin with the iron tip:

When I powered the board up again, the glitches on sprites were gone so I could declare the board as 100% working.End of job.

 Posted by at 7:50 pm

Bucky O’ Hare repair log #4

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Oct 212018
 

Received from Greece this Bucky O’ Hare PCB :

Owner told me the PCB developed some sound issue so he wanted the ‘054986A’ audio module replaced with a repro of mine.This is what I did but it didn’t cure the problem, the sound samples were corrupted and some of them missing too :

The MASK ROMs test reported two bad devices @A6 and B6:

They are the 16Mbit MASK ROMs that stores PCM samples:

This wass unlikely but possible so I removed and read them resulting in good dumps.At this point there was nothing else to check, the PCM samples data are processed by the near custom ASIC marked ‘054539’ which was most likely faulty.I prepared the board and desoldered the chip with hot air :

Then cleaned the area and soldered the spare taken from a donor board:

Sound was back clear and complete again.End of repair.

 Posted by at 7:21 pm
Oct 142018
 

Some time ago I bought an untested Sega Dreamcast console (with no video cable or other accessories included) with the intention someday to replace the optical drive with a GDEMU :

I was lucky because it was a japanese HKT-3000 model with VA1 motherboard and 3.3V GD-ROM assembly so perfectly compatible with Deunan’s GDEMU :

 

Some days ago I finally got an A/V cable so it was time to power up the console for the first time.With my disappoint all I got was a disturbed video signal, nothing came up on the screen:

I opened the console and did an inspection :

First of all I looked at PSU and immediately my attention was caught by the big 100uF 200V electrolytic capacitor @C3 which was clearly ‘bulging’ (you can see how top of the metal can is dilated)

This capacitor has a very important function as it filters (suppressing the ripple) the AC source that then get transformed by the rest of the PSU circuit.It measured little more than 7000 pF (0,007 µF) when tested out-of-circuit showing no ESR value at all :

I replaced it with a low-ESR one and checked in circuit the ESR of the other electrolytic capacitors, they were all good:

I powered up the console and I was delighted by the startup intro animation (sorry for B/W picture but my Philips CM8833-II monitor doesn’t accept NTSC signals)

Now waiting for a joypad and then time to play with wonderful Deunan’s GDEMU ODE!

 Posted by at 10:08 am

Silent Dragon repair log #2

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Oct 122018
 

Received today this Silent Dragon PCB (a not so well known beat ’em up manufactured by Taito) that I bought as faulty :

According to the seller board played fine but sound was absent.He was right:

Some one tried to replace invain the audio amplifier:

A quick check with my audio probe revealed the issue had a digital and not analog cause.So I went to probe the Z80 audio CPU and found its /INT line always asserted:

This means a maskable interrupt to the CPU was triggered by an external I/O device.This causes execution to jump to a specific interrupt vector (which is some code at a fixed location).Execution  can then continue after the interrupt vector routine has finished but in my case the /INT line was LOW all the time so the CPU was not properly running.

A further check  revealed pin 21 /RD and pin 22 /WR were shorted each other:

I turned the board over and did a visual inspection on solder side, my attention was caught by this particular:

Under a microscope:

A solder bridge was shorting two vias connected not only to pin 21 and 22 of the Z80 but also to two pins of the ‘TC0140SYT’ ASIC (the sound and interrupt controller)

I removed the bridge with my iron tip and sound was back.Job done

 Posted by at 5:24 pm

Hellfire repair log #3

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Oct 102018
 

Another shoot’em up ‘under surgery’.Here we have an Hellfire PCB I received from Portugal:

Board booted into game but sprites were all blocky:

Looking at hardware I could figured out the the parallel data from the four 1Mbit MASK ROMs of sprites are converted into serial by four 74LS166 8-BIT shift registers (one for each ROM)

Probing the 74LS166 @5F revealed the output (pin 13) was stuck high :

While the inputs were active:

I removed the IC:

It failed the out-of-circuit testing :

This lead to an improvement  but still not perfect as sprites were missing lines :

 

Looking again at hardware I could figure out the sprite line buffer is made by eight 2149 SRAMs (1K x 4-bit devices, pin to pin compatible with 2114/2148)

The one @6K was hot to the touch, probing it with a scope revealed weak signals on data lines :

Ther RAM chip failed when tested out-of-circuit :

 This fixed sprites completely:

The last issue concerned the sound, it was faint and corrupted:

Looking at sound circuit I noticed a Nec uPC1181H amplifer was fitted in place of the Fujitsu MB3730 that silkscreening suggests :

Although the two amplifiers have pretty same pinout, different is the typical application circuit and power output (14W for the MB3730 and 9.2W for the uPC1181H) so I installed a proper one.Looking at sound section on another same board I noticed three mylar 100nF capacitors were used by factory @C61-C62-C63:

Whereas on my PCB an electrolytic capacitor was installed @C61 and the one @C62 was missing :

I restored the factory condition gaining a loud and crisp sound again.Repair accomplished.

 Posted by at 10:06 pm