Porchy

Current blog status

Oct 062020

Over the last few months I’ve had tons of messages from readers of this blog asking about the lack of updates and all sorts of nice words so I think its time to address this officially here.
My interest in arcade repairs has pretty much dwindled down to nothing over the last 12 months or so and I have found myself finding a love for other hobbies which I am throwing myself into and loving them more and more.
That doesn’t really leave me with much free time for repairs and it certainly doesn’t leave me with any time to write up a detailed blog post about it either.
Right now, my official status on this is I wont be doing anymore repairs logs.
With that said I do put a lot of time and effort into dealing with PAL’s and will still be maintaining the WIKI and the blog will still stay with that too.

If the time comes that I wont be continuing with the site then it will either all be handed over to someone else that is willing to keep the information going or it will be saved and made available via other methods. I’m not going to let the last 10 years of work from myself and other contributors disappear.

Any of the other contributors can still post here if they wish and if anyone else wants to contribute then I will create an account for them too

I hope that puts a few minds at ease

Pacman Diagnostic ROM

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May 292020

Philmur has sent me his Pacman diagnostic ROM.

Pacman test ROM – philmurr 05/20

Fits at location 6E

Tests programme, colour, video and sprite RAM 1 (note sprite RAM 2 is write-only)
Displays sprites dropping from screen to show sprite select and addressing is working (tries to show all 8 but most boards only have 6)
Shows real-time status of inputs and DIPs

Colour RAM – 4P, 4L
Prog RAM – 4R, 4M
Video RAM – 4N, 4K

Sprite RAM 1 – part of 4R,4M but may make it clearer to people when testing

File can be downloaded in the “Downloads/ROM files/Diagnostic” menu

Thanks to Philmur for his hard work and for sharing this

Operation Wolf repair log #5

Apr 132020

Yet another Operation Wolf repair and this one gave me trouble.
First test we had watchdogging. All ROM’s checked out fine in the programmer and seeing nothing else obvious I removed the work RAM both of which failed.

Now on boot up I got a screen seemingly running game but all the screen was garbage.

Probing the data lines of the screen RAM show pretty much all of them across two RAM chips were dead. I desoldered and replaced them both.

Now I got a running game but the colours were messed up

There are two 74LS245’s attached to the data lines of the colour RAM at location IC78 & IC90. Probing these revealed stuck bits on IC78. Replacing this gave me perfect results on the colour test screen in test mode but dull colours in game. Other issues in-game showed me all the sprites were missing and the screen doesn’t scroll when entering a game.

Stupidly I chose to ignore the colour issue and concentrate on the other two (I will get back to this later).
The sprite and scrolling issue worried me because these areas are related to the PC080SN and PC0900J custom chips.

Paulcan69 sent me a scrapper PCB so I could rob the customs off it if needed so I started out by replacing the PC0900J.

Replacing this brought my sprites back but the scrolling issue was still present.
I didn’t enjoy replacing that IC so I started checking around to see if the scrolling issue could lie somewhere else.
I found nothing wrong so removed the PC080SN custom and replaced it with the spare but it made absolutely no difference. A lot of work time and effort for nothing.
I spent time over the course of the next few days looking into this but found nothing.
Eventually I got sick of looking at the washed out colours on screen and replaced IC90 to fix the last of the colour issues.
I couldn’t really believe it when this fixed my scrolling issue as well.
I still don’t understand why this chip caused the scrolling issue but I’m really glad it did.
All issue are now fixed on this board.

Onto the sound PCB.
The original sound PCB that came with this board was a complete write off. Every chip I pulled from this board was broken and both amps were also burning hot.
Among all the other boards that Paulcan69 had sent me he also kindly sent me a spare sound PCB for Muddymusic to have.
There was a sticker on this board saying “No Sound”.
On first test I found that the music played but the volume was really low. Turning the volume up to the maximum made the sound audible but not nearly enough.
I checked the resistance of the volume pot and found that if you move the pot off maximum setting then it read megaohms. I tried cleaning it but it didn’t work so I replaced it.
This made the sounds much better but like with most of the others I’ve repaired some of the samples were scrambled and didn’t stop playing when they should.

I quickly found another 74LS688 that was dead. Replacing this made the sounds stop when they should but the samples on channel A were scrambled.
Looking at the schematics shows where the sample data goes.

Using an audio probe I checked the output at pin 10 of the 5205. The sound was the same. This ruled out issues caused by bad caps and things like that.
Next I checked the 574 at IC33. Both the input and output looked good so moved onto the 74LS157 at IC 44.
Probing all the outputs with a logic probe showed pin 9 was stuck HIGH

Replacing this fixed all the samples.
That’s the last one fixed
Big thanks to Paulcan69 for the spares board. Without it this board would be scrap too.

Operation Wolf repair log #4

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Apr 112020

Another Operation Wolf repair!
This one belongs to Frothmeister on UKVAC.

Again, we have a nice little fault label

So to start with there was no sound. I did a couple of signal checks to see if commands were being written to various IC’s but saw none of them so I just removed the RAM which of course failed.

Fitting a new RAM brought the sounds back but the samples were garbage

Samples are all stored in the 40 pin MASKROM.

I attempted to read the MASKROM as a 27C400 EPROM in my programmer but it gave me errors

As I had a scrap sound board here I swapped this ROM out and retested.
Samples were now restored but I had the same issue as in a previous repair where the sounds started playing and never stopped.

Time to check out the comparators.
I quickly found the comparator with a stuck output at location IC18

Replacing this fixed my faulty sample playback for this one but I found another by playing sample 2E.
Using the same technique I found another dead 688 at IC39.

To determine which channel the game uses for various samples I use the wonderful MAME debugger.
If you look at the memory map for the sound CPU in the MAME source you can see the addresses that get written to to set the START and STOP values.

Address ranges $B000 – $B006, $C000 – $C006, $D000 – $D006 & $E000 – $E006 are what we are concerned with so set some breakpoints in the debugger and fire up the Operation Wolf test menu (dont forget to set the debugger focus onto the sound CPU).
Play the sounds that cause issues and see which address gets written to.

Now look at the schematics

From here we need to work out with signal is address $C000.
A15 & A14 give us an address of $C000. the 74LS138 at location IC14 gives us the enable line for the channel. A15 enables the 138 itself and A14 selects the relevant output, in this case its input C which enables (active LOW) pin 11. This in turn enables IC54 which as you can see from the signal names is channel B.
Following the schematics further we find the 74LS688 comparators responsible for this

Probing the only output on those comparators (pin 19) and playing the sample shows me which one isn’t toggling. In this case it was IC39.

All fixed

Operation Wolf repair log #3

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Apr 102020

Another few Operation Wolf boards in at the moment. This one is from Unit504.

There is a nice little tag on this telling me the faults which really helps me keep all these boards together and where to start looking.

First job was to look into the gun shot register issue.
I did a quick test and could see the screen flash when the trigger was pulled so I knew that wasnt the fault
Following the circuit I come to the 74F74 IC which is used to latch the gun co-ordinates.

I normally wouldn’t start looking at this part but it was already socketed so decided to pull the chip and test it. It failed an out of circuit test.

I replaced this with a 74ALS74 and it seems to be be fine.
Now all the gun shots register as expected.

Next onto the sound.
None of the sounds were working at all and I didnt believe that all the seperate circuits for making sounds would be dead so looked a bit closer at the CPU side of things.
First off I checked the ROM and it dumped out fine.
Next the RAM. An inspection of the RAM showed signs of corrosion

I removed and tested it and thankfully it failed

Looks like that corroded pin has broken contact somewhere. Anyway, replacing the RAM brought all the sound back to life and completed this repair.

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