Wei-Ya switching PSU repair

Quick repair from the weekend just gone... I needed you use my Jamma test rig for a new board I'm repairing, and being the cautious type I always like to check my PSU voltages before plugging a board in. Well it was a good thing I did, my Wei-Ya PSU, a clone of an original by all accounts was out putting 10Vdc on the 5V line! Yikes... The voltage adjuster did nothing, I was stuck with 10Vdc on the 5v line. I checked the other voltages; +12Vdc & -5Vdc and to my surprise they were sitting at +24v and -10v respectively. I very nearly decided to bin the thing, but my tech head got the better of me and I decided to crack the case open to see if I could spot anything obvious.

Well as you can see below, there was a hairline crack (highlighted) in the PCB, when I used my meter to test continuity the outside and middle track of the three were broken and contact could be made by flexing the PCB.

I scraped away the solder mask around the affected area.

I then made some links using some old wire and soldered them across the traces. 

Once reassembled, the PSU was working to the correct spec and the adjuster controlled the 5v line perfectly. I could have finished this off by applying some lacquer (Or the wife nail varnish!) over the bare traces, but really its not required for how I'm using it, if it was in a cab I may have done so but not for my test rig, the solder side of the PCB does have a sheet of insulating material between the traces and the metal case and for me that will do for now.

I can only imagine this was a manufacturing defect that was worsened by me moving my test rig around...

Cheers, Mart.

Atari Video Pinball PCB repair

Time for another game to break, this time Video Pinball went faulty mid game all of a sudden there was no video image on the display, upon re-powering the game the was a high pitched ping from the speaker and nothing else visible on the display, all the play field LED's were also not illuminated.

First thing first was to check the game voltages, The un-reglulated 10Vdc was present at the edge connector, the LM323 regulator was still doing a fine job of providing 5Vdc to all the game logic. The 18Vdc required for sound was also still present. Time to get the logic probe out and find out what was going on...

I first checked the Reset line on the CPU, this was low, you would usually see it held high in working condition and pulsing high/low if the game was resetting or more commonly mentioned watchdogging.

Next up I needed to check the clock circuit and see if all the horizontal and vertical timing signals were present. If these aren't all correct and present the game wont even get as far as attempting to run.

The image below shows the Horizontal timing part of the circuit:

The main 12MHz clock signal was good and all the horizontal timing signals from the outputs of LS161's at N6, M5 and L5 were all correct and present. It was time to scroll down page 4-5 and look at the horizontal timing signals.

Image below showing that part of the circuit:

As I noted on my printed schematic, all of the vertical output signal were low, these should be seem to be pulsing on a logic probe, I noted that the *VRESET signal was low and incorrectly thought the LS161 at N5 would have been faulty. I removed this from the PCB and tested it and was proven wrong, it was OK. I should have studied the data sheet further, the way these LS161's are used in the circuit requires a state change on pin 9.

The circuit creates a loop where the output signal *VRESET from pin 6 of F4 is needed to make the vertical timings active which in turn are used via a PROM to generate the *VRESET signal... confusing or what!

The above image shows the signal flow of what I tried to explain. Moving on I manually forced the *VRESET signal high whilst monitoring the vertical outputs on my logic probe, it would get high outputs on the vertical clock signals and would then hear the game reset.

Next I decided to remove the LS175 from F4 and test it. I was running out of IC's which could be at fault! :o) This turned out to be fine which should have left me with the 'SYNC PROM' at H4 being the problem, I had no easy way to test this so I removed a similar PROM from my Fire Truck cab and fitted it to newly installed IC socket and boom the game was running again! I have sourced the correct PROM and one of my arcade buddies has programmed it, so once I collect it i will be able to consider the game fully working again.

For future reference if anyone finds this post while looking for Video Pinball or Fire Truck repair logs, the SYNC PROM in my Video Pinball cab is labelled 9402-01 and the SYNC PROM in Fire Truck is labelled 9114-01. Using these files from MAME they contain exactly the same data and should be fully interchangeable between these two game and possibly any others of the era that use the a PROM to generate the Sync timings.

Thanks for reading, Mart.

Sega System 24 - Crack Down

Atari Fire Truck PCB Repair

So after having a good few years of reliability of my Fire Truck game, it decided to develop a small fault after a 100 mile journey to the Revival Retro gaming show last weekend... The lower part of the screen where the score text is displayed, showed corruption for the top half of the letters. There was nothing I could do over the weekends event and it didn't seem to disrupt the game plays over that weekend. Any way back to having the cab home I broke out my small test monitor and worked on the game from the back of the cab.

Image of the fault at the show, as you can just about make out, the bottom section of the text is correct with the top half showing incorrectly:

The part of circuitry that deals with the text generation on screen is shown below, eight data lines are fed into the 74LS174 at N3 from shared RAM N2 & P2 (Not shown), they are clocked through the the 74LS174 by the *HRDWR signal. When the signal is low, the CPU is stopped by the STPCLK signal, and data will pass through into the Prom at R3 where its parallel output is converted to serial and is then summed with the other video signals.

I was hoping that the Prom faulty as that would have been more time consuming to fix! As it turned out, it was OK, I knew from documenting these schematics previously that when in test mode, no text above or below the play field should be visible, so lets take a look at what my test mode screen showed.

Clearly some sort of text was still visible, The outputs of the 74LS174 should all be held high in test mode essentially making all the bits of the Prom output no data. Checking with my logic probe output pins 5, 7, 10 & 12 were held high but pin 2 was pulsing away with data and triggering the Prom to display garbage data on the screen all the time. I temporarily linked pin 2, to the 5v supply (forcing the signal high) and sure enough the garbled text went away

I removed the device, fitted a socket and replaced with a new 74LS174. Upon re-powering the game everything was working as it should be. Replaced IC shown below.

Thanks for reading, Mart :)

Super Breakout PCB repair (Bonanza Enterprises Clone)

So part of my Cocktail restoration was to fix the Super Breakout game board that came with it. I knew it was faulty after my initial testing of the cab a few months back.

The board itself has now manufacturing marking or anything to say who made it. From studying photos of Atari boards this appears to be a direct 1:1 copy of a Rev-03 board. The only difference I noted was changing of the audio amplifier which would be a TD1004, this PCB has an LM380 on a small daughter board that plugs into the TDA1004 socket location. Image below showing the PCB in its original faulty state.  

When plugged in with a basic adaptor providing 5V DC for the game logic and video output to my monitor. The game was displaying garbage on the screen that was almost static but some characters did move and change here and there. The game looked the same in test mode as well. 

So checking the reset line on the CPU (pin 40) it was constantly changing state when it should have been held high after the initial board power up, this means the game code is not running correctly for some reason and the CPU is being restarted to try and get the game code from the ROM's running and initialised correctly.

After a few more quick checks of the clock signals and confirming the address & data bus were active, I took a look at the signals on the main ROMs the run the game program and found 3/4 of the ROM select lines we not active. From memory one had no signal and the other two were stuck high. THE IC location was F2 and is a 3901 chip which is obsolete and unobtainable, With the help of someone on the DLF forums and PhilMurry I had some converter PCB's made up which worked with a 7442 IC. 

I had previously made a very rough adaptor shown below.

These new adaptor boards are a vast improvement!

Changing this IC got the ROM select lines active but the game was still resetting and the screen was full of garbage still.  Checking around the reset circuit I found another 3901 IC that I suspected was faulty. This one at location E8 should have been triggering a Timer Reset signal which if working correctly would clear a timing circuit that stops the CPU from being reset assuming everything else was working OK.

Next up I decided to check the RAM with my logic probe, visibly using the probe all the signals were active, but I had little other way to confirm if they were working as the game was not running. I decided to remove all eight RAM's and socket them. I then fitted some new 2102 ram, using this method once the game was running I could put back the original RAM and confirm which one did or didn't work.

So with all eight RAM's changed we had what look like part of the Playfield being drawn but not a lot else. The CPU was still resetting so the code still wasn't running correctly.

Flicking the game into test mode the display was now stable and showing a ROM error, a good sign. This ROM was at PCB location L1. I removed it and cleaned the chip legs and fitted it back in the socket.

Boom! the game was running, but the balls we just faint 'dots' on the screen and not moving correctly. More diagnosing found another two 3901 IC's that were faulty, each of these IC's should have been sending ROM and position data to the Motion generator circuit where the balls and their movement is calculated. These two IC's were at locations K8 and P7, the upside of fixing this fault was that it also fixed an audio fault i hadn't yet discovered! The IC at P7 and output signal (*TONES) on pin 11 was stuck high. This is an enable line for the audio latch that combines data signals and turns them into the game sound. I check with an audio probe and found I had a constant garbage sound playing when in game and test mode.

So with those two IC's replaced the game looked to be playing, test mode showed no problems, but the balls were being doubled up, each of the three balls was being generated twice, they were tracking each other and moving in an identical pattern, they just shouldn't have been there. I thought this would be an easy fix... 

Fast forward Seven or so hours of faultfinding I found the problem. Now this was something a Logic probe and an Oscilloscope couldn't help find.

Eventually the only IC's I was left with that could have something to do with the problem were a pair of 7483's they are used as comparators to compare a RAM value with the vertical line that is being displayed, if they match three load pulses are triggered which are used by the video shift registers.

I didn't have replacements for these but decided to remove them and fit sockets. I intentionally put them back in different positions, and the fault appeared to be gone. Fitting them back into their original positions the fault came back. Looking at the schematics we can conclude that the IC at M4 had a faulty input on pin 13. I will temporarily use them but have ordered some new replacements.

A final look at the board wired to my test rig. The total IC's replaced is as follows:

F2 - 3901 - ROM Selects
F,H,J,K4  - Ram 2102
F,H,J,5     - Ram 2102
E8 - 3901 - Reset circuit
K8 - 3901 - Ball motion circuit
P7  - 3901 - Ball motion circuit & Sound
M4 - 7483 - Ball motion circuit

The game has been play tested in the cabinet and all controls, sound, Input and Output all work correctly.

If you made it this far well done!

Thanks for reading, Mart. :)