| Arcade Collecting > Restorations & repair |
| Arcade PCB Repairs - New to the game... |
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| bperkins01:
My approach to fixing PCB's - thoughts of a first timer Digging into the restoration of my Centipede machine lead me toward fixing the PCB. Once I got going, it was quite interesting and I like that kind of trouble shooting. I picked up a pair of dead Centipede boards from a KLOV'r purely to learn how to fix them. Having a working Centipede as a reference is helpful of course. Long story short - I managed to get one of the two boards working enough that the game fired up and displayed within about 20 minutes. Well now I'm hooked. I'll create a repair log for each individual board I fix (probably) - this post is going to cover my overall approach and what I've bumped into as I learn to fix PCBs. People that have been doing things for years forget the "duh.." moments that they had when they started.. my "duh" moments are fresh in my mind. My dead PCB repair process: My first thought is fix what you know - my Centipede is fully functional and the PCB is running 100% properly. I have a known reference point. If you want to dig into the electronics and fixing of PCBs, nothing better then having an operational PCB for comparison. With that comes the assumption that your full cabinet is running properly - the monitor, sound and power are all good, etc.. First timers PCB work bench: People who fix many boards have some pretty nice test benches - Jamma adapters, bench power supplies, bench monitors, control boxes with coin, start, joysticks, etc. You can spend a lot of time just setting it up and debugging it - to fix a single board. My thought is - fix a few boards - make sure you like it. Then set up the ultimate bench. Once you have a workflow you will decide what you need. Schematics for the PCB are required. You need to know the voltages and test points that are expected on the board. KLOV will likely have the ones you need. I was able to find the full size PDF's and printed them at Staples. Certainly this isn't a requirement. Have it nearby on a laptop and have at it. Large print drawings makes it easier for me and I can write on it. The schematic will tell you all of the voltages the board expects. On this one - it has a power input section to provide the specific details. STEP 1: Visual inspection Take an inventory of the board and look at all of the chips, look for obvious missing parts. My dead PCB was missing the Pokey chip (I was told this was the case). Other parts could be worn or broken off too. Being able to compare to a working board is helpful. Some boards have empty sockets by design.. The other thing I did was remove chips that were socketed, cleaned the pins, sprayed a little Deoxit and reinserted them. The fiberglass pen works great - it really cleans up the contact areas of the card edge connector and the legs of any chips. Shine up the edges and pins like using a pencil eraser. The puller is an upholstery staple puller. It has enough shape and strength to just get under the edge of a chip and pry it up just a little. Pry a little at a time from each side and be careful not to bend the legs - no need to rush on any chips here. You want to chip to come straight up as much as possible, not tip out and bend the legs. Once the chip is out, I use the pen to buff up the legs inside and out. Finally I squirt a little Deoxit on the pins and the socket and reseat the chip. Plenty of videos exist on extraction and reseating. If you've never done it.. Don't practice on expensive boards.. break old crappy ones until you get a good feel for it. I cleaned and reseated all of the removable chips on my dead PCB. STEP 2: Plug the PCB into your good cabinet! Validate you have all of the correct voltages on the PCB. The 5v+ should be very accurate (i.e. within .1v or so +/-) on the PCB. There will be other voltages all over the board. My original Centipede had these: PCB Test Points Test Point Actual Voltage +5v +5.063v -5v 0.569v +15v 0.001v -15v -15.12v +22v +24.72v (unregulated) -22v -24.93v (unregulated) -30v -14.00v I documented the repairs in my restoration thread: PCB Repair The main point is to check and repair any of the voltage regulators first. Each test point is going to lead back to a regulator or something on the board creating that voltage. My dead PCB had all of the correct voltages (I was surprised). After plugging it in and seeing nothing on the screen, I happened to touch one of the EPROM chips and it was very hot. I'm going to bring the laser thermometer to the shop and check the boards with it moving forward.. My bet is they all should be pretty close temperature wise - but none should be HOT! Bad EPROM chip identified. STEP 3: Verify that the EPROM chips are good. The EPROMS are the software that is the game. This part requires some investment. I purchased the GQ-4X4 EPROM Programmer Centipede has 6 EPROMs on the board. I'd already found one bad one. But without verifying each EPROM individually - you will never really know if you have a software issue. The EPROM programmer and reader is a bit glitchy, it locked up a couple times, but for the most part - if its acting weird - just exit the program - unplug the reader and start over. It worked quite well overall. Do get a 9V power supply for burning replacement EPROMs. USB *MAY* have enough power to do it, a power supply will absolutely do it. There are a few sites out there to test the EPROMS - This one worked great for me. Arcade Rom IDentification Tool Read the EPROM, save it to your desktop and drag/drop it onto the webpage. It will either verify the EPROM or tell you it has no idea what it is (meaning bad program). Perfect! I found 2 additional bad EPROMS. Now to burn new ones... I picked up this inexpensive EPROM eraser and matching EPROMS off EBAY. The Centipede EPROMS erase with UV. This little $20 eraser is pretty crappy from a quality point of view. The knob is supposed to be a timer - its crap. I opened the box spliced it out of the circuit. There is a small UV bulb inside - the on/off switch now just turns it on/off. Notice the position in the drawer of the EPROMS. My eBay EPROMS were not all erased. I cleaned the windows with acetone and filled the drawer. 45 minutes under the light and only some of them erased.. But I figured it out. You really need to line them up like this so that the windows sit DIRECTLY BELOW the UV light inside the drawer. Once I did this I got a good erase on all of them every time. On the EPROM programmer screen in Windows - put a blank EPROM in, test it for being blank. I would test 2-3 times to be sure.. Like I said - it can be a little glitchy. Once verified blank - get the correct EPROM bin files. I used my MAME zip files and extracted the ones I needed. Each MAME file will have the ROM number in its file name, etc.. Write it to the blank EPROM and verify it 2-3 times. Once you verify and check all of the EPROMS - you know with certainty the PCB has the correct software. Cover the little UV windows with a label and the EPROM number. So Far.... * We have visually inspected the board - replace obvious broken or missing chips * Look for bad solder joints, disconnected jumper wires, etc. * Cleaned, Deoxit'd and reseated all of the socketed chips * Cleaned, Deoxit'd the card edge connectors * Plugged the board into a working machine * Checked all of the chips for excessive heat (finger, laser thermometer) * Checked all of the voltage test points per the schematic * Replaced voltage regulators, etc. * Get all of the voltages correct now. * Verified all ROM chips programming and replaced any failed chips. These steps alone fixed one of my two dead Centipede PCB's. My other dead Centipede PCB will require additional diagnostics.. I'll update the next steps once I actually DO them.. But I think the above outline is a good starting point for board repairs. |
| Mike A:
Very nice write up. |
| Arroyo:
Always two steps ahead Bobby. That’s a great write up and I will be “borrowing” all that homework you’ve done for the future. Very cool. |
| bperkins01:
Log of Centipede boards repaired to date: My first attempt at repairing a PCB is how I determined my approach to triage and setting a baseline for repair. After watching hours of videos with different people and different techniques - I've seen three types in general: * Guys who just start replacing parts without much diagnosing * Guys who logic probe through the board and fix them with reasonable success rates * Guys who break out a Fluke and Oscilloscope as their first step and map everything along the way. There are merits to each of these - For the moment - using a logic probe is going to be the best way to figure out how boards work in general and once I've strained it to the point where its not enough - I'll think oscilloscope.. Board Zero - The Origin Story: Board Zero was the board that came in the Centipede machine I've been restoring. Triage: Board played * Sound issue where it cut in/out at weird times * No high score retention * +15v missing * -30v missingThis board repair was covered here: Working on the electronics.. * Replaced 7815 voltage regulator for +15v * Replaced 555 timer which fixed the -30v * Replaced 7905 voltage regulator on the ARII boardBoard works! Board #1 Purchased off KLOV as a spare and training board Triage: * Missing Pokey chip * All voltages on board good (+5v, -5v,+15v, -15v, +22v, -22v, -30v) Board came up into test pattern once. Work done: Removed and cleaned legs of all chips that were socketed. Found a VERY hot ROM chip. Swapped it with a ROM from another board and the game started. EPROM reader/burner was on order at this point. Once the burner showed up: read/validated all of the ROMS. Found two more bad ones. Replaced ROMS. Game played but with all the mushrooms coming down on the left or right side only. Replacement Pokey chip showed up. Board works! Note: The pokey chip generates the sound, but is also the random number generator. It obviously has a part in mushroom distribution! Board #2 Purchased off KLOV as a spare and training board Board dead - no picture or test mode * Missing Pokey chip * All voltages on board good (+5v, -5v,+15v, -15v, +22v, -22v, -30v) Work done: Removed and cleaned legs of all chips that were socketed. Verified/replaced missing and bad ROMS (3) Checked for any hot chips (none) Installed replacement Pokey chip Board started running Machine would coin and play blind, but the sound was double speed and double playing No video however.. but there was life! I started checking the video output circuit looking for voltages on the RGB wires and worked my way backwards. I borrowed an oscilloscope (its from the 70's) an was able to see the sync signals, but I had a suspicion that vertical was wrong.. it looked funky based on a gut feeling.. I had it hooked to the LCD because it works, but finally I thought - its showing voltage on the pins, connect it to the K4600 (even though its convergence sucks at the moment and see what happens) - smart move! Here is what was happening! Twice the centipede, twice the fun. It took me a while to think through it, but from this: * Video memory is working * Program is running * Horizontal sync working * Vertical sync messed up - explains why the LCD could not lock onto the signal * Game played- in double bezerk more.. but it played. The screen was doubled - but each side was sliced into 4 sections. So whatever part of the circuit that scanned through the video ram was getting bad signals and was just displaying stuff out of order. The issue is vertical in nature since the screen is sliced horizontally 8 times Working my way backward in the schematic, it took me to the sync circuit. Just using a logic probe, I hit the 1v, 2v, 4v,....128v pins in this area. All of them were pulsing EXCEPT 32v! It was pinned high. Hitting all the pins with a logic probe tends to reveal patterns, this one was not following the pattern and it is right in the middle of the vertical sync circuit. Ordered the replacement chip. I really wanted to try that thing where you piggyback a chip on a known bad chip.. The screen went all fuzzy, so that didn't work. Must be for other chips, but not this one. So I socketed and installed the chip and this happened! Parts of the playfield missing, rain drops. But it was an easy fix.. When I did the piggyback trick - it clearly fried the chip.. Note to self - don't do that. I had ordered spares.. New chip installed Board works! During testing I determined high score retention was not working. Since the board had +15v and -30v in the right spots - I knew the only chip it could be was the ER2055. I had a spare, replaced it and high score retention is fixed. Testing also revealed that the trackball response is off.. My other two boards work perfectly. This board if you spin the trackball quickly, it kinda skids and goes the wrong way for a bit - then catches up and does what it is supposed too.. I'd suspect the trackball itself (and it could ultimately be that) - but my other 2 boards work great. There are only 2 chips on the board that can really be effecting this. One being 74LS157. I'm going to try swapping it out first since its the most likely suspect. Last item to fix is the trackball response (once the part shows up) |
| jennifer:
I realize this is board repair 101...But it should be noted diagnostics can be a really expensive pastime, A necessary evil, Like my workbench for example, I thought basic repairs back in the day but over the years it got nasty, even with thrifty shopping and used equipment Jenn has dumped approximately 25k into the project...Probes, scopes, power supplies, audio testers, you name it...Just saying careful friend, that is the gateway to to the Matrix. |
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