Porchy

I have no background in electronics or programming. Everything I have learnt over the years has been through self teaching and the help from other kind people willing to lend a hand.

Nov 042017
 

The second of Alex’s board for repair is Donkey Kong Junior.
This board booted to static screen that wouldn’t sync. This ended up being a broken 30k pot on the H-Sync.

As I don’t currently have and 30k or 50k pots I opted to temporarily fit an 18k resistor to the video PCB to let me move on.

The game now booted to a static screen of garbage but when touching the Z80 CPU it booted. The socket looked old and a bit crusty so I replaced it.
The game now played but there was a lot of garbage still on the screen.

I hooked up the Fluke 9010 and did some basic reads and writes to the video RAM that sits between address 0x7400 – 0x77ff.

Clearly bits 0 and 1 were stuck low.
Using the schematics I started checking at a 74LS245 at location 6A which buffers the databus.

Straight away I found although the signals were going to the chip the outputs on DAT0 and DAT1 were floating.
As the outputs were floating I tested by piggybacking a good 245 chip and everything came up good.

Replace the 245 and fitted a new one.

Tested the game and all sound and controls work too.

Another one fixed.

 Posted by at 2:52 pm
Nov 042017
 

Got a Donkey Kong 3 to repair from my good friend Alex @ Nintendo Arcade
On boot up I got a static green of garbage

I checked the voltages at the far end of the PCB and saw they were quite low compared to what my PSU was set at. Adjusting the voltage to a bit higher brought the PCB voltage back up to a good point but made no difference to the fault.
Next job was to check the program ROM’s

Found 7C, 7D & 7E had suffered from bit rot. I erased these and reprogrammed them with the correct data from the MAME set.
It didn’t make any difference at all so I started looking at the Z80 CPU signals and this is where I got a little confused.

At first glance and without really engaging brain too much I automatically assumed that there were two Z80 CPU’s side by side but checking the voltages and the signals left me scratching my head as they were not what I had thought. On closer inspection I realised that the one on the left is actually a Z80 DMA chip. I’ve not seen or even heard of these before so it something new to me.
I took to the internet and downloaded the datasheet for the DMA chip and also the schematics for this PCB too.
The CPU and the DMA chip are tied to the same busses and the DMA chip didn’t appear to be releasing the data bus to allow the CPU to do its thing.
I checked all the incoming signals to the DMA chip but found nothing odd so at this point I figured I needed to buy a new DMA chip so ordered one from eBay.
To help confirm my diagnosis I pulled the DMA chip and powered up the game. Sure enough it booted fine and ran but was missing the sprites which is to be expected really.

A couple of days later I received the new chip and now we get this

All sound and controls working too so that’s this one sorted.
There appears to be some skewing at points in the pictures above. This is due to my supergun setup and not a PCB fault.

 Posted by at 12:41 pm

‘For Sale’ Section

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Sep 242017
 

As you may now notice there is a new tab on the menu bar called ‘For Sale’.
At present this is just a simple gallery showing items that we may be selling at any particular time.

If your interested in any items then please contact the seller directly via the contact form

 Posted by at 6:03 pm

ROMCMP Frontend

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Aug 272017
 

In an attempt to help more people start using MAME’s fantastic ROMCMP utility I have made a simple GUI frontend for it.

Program can be found in the Downloads section under “PC Software”

 Posted by at 10:47 am
Aug 262017
 

This fantastic monitor has been on my test bench for as long as i’ve been in this hobby and its been a great little work horse.
I powered it up the other day to test something and noticed the upper half of the screen had what I would describe as thick scanlines whereas to lower half of the screen looked a little compressed.
I hooked up my NES to it and the problem was a lot more apparent

Keep you eye on Luigi when he jumps to the top of the screen. As he moves up he gets longer

Asking a couple of people quickly, they all recommended changing the capacitors. The ‘shotgun’ approach isn’t really something I like doing as I like to understand what would cause issues before I randomly poke around but monitors are not my strong point so I started ESR testing in-circuit.
All the capacitors I checked were perfect with really low ESR values and all the capacitance readings were within spec. All except for one little capacitor, C306. This is a 47uF 25v capacitor and is connected to pin 12 of the vertical deflection circuit (TDA1670A)

I tested the capacitor out of circuit and confirmed for sure it was actually bad.

With the monitor taken apart I hooked up the Muter BMR95 to the tube to check the condition of the guns
A tube can usually be identified by the label that is on the back of the tube

E2940B22 is the one we want. Next we cross reference this to the supplied manual to find which adapter we need to use.

Adapter 808 is the one we need for this tube.
All hooked up

The guns aren’t too bad and certainly no need to attempt any cleaning or rejuvenating. Sometimes this can cause more harm than good. I have a spreadsheet prepared too as outlined in the BMR 95 manual to give a better indication of the life expectancy of a tube but i’ve not included it here.

With all that done it was time to test the monitor again

All seems to be fixed again and im happy with that.
You can see the stretching better with a picture comparisson

Before

After

Never underestimate the importance of a single capacitor!

 Posted by at 2:34 pm