How to fix PCBs - A Beginners Guide

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PCB power questions.

3.) So I have a two board set that is not working, at the very least, because of a 5V voltage drop. How does one tell if the voltage drop is due to components on the boards or due to a power supply issue? In other words, is it possible for the power supply to be putting out proper +5V, +12V, etc... to the edge connector, but fail to deliver that voltage once connected? What if the voltage drop is due to crummy edge connectors themselves? What is the correct process to diagnose the source of the problem?

Depends on the degree of voltage drop, if it drops to zero or close to zero you have a dead short on the board due to a faulty component somewhere, or a PSU that cannot drive any load (rare).

Voltage drop that doesn't go to zero is usually a result of wiring that is too thin, or as mentioned above, a single wire used when the board has 2, 4 or 6 pins dedicated to a single power rail. The amount of copper in the conductor determines how much current a wire can deliver, the result of a board trying to pull too much current through wiring that is too thin to support it is heat and voltage sag. Its why you often find boards with burnt up edge connectors, the contact got dirty and oxidised, the contact failed and the board pulled current through a smaller surface contact area. This causes runaway heating.

In general older boards and physically larger boards take more current than more modern smaller boards. A modern 15-in-1 board will draw a fraction of the current that an original 1980s/90s would draw, even though both need 5 volts. As such modern harnesses you buy on ebay are designed for modern boards. Their power wires are puny, even if they look fatter than the signal wires they are usually mostly insulation. Put a thirsty board on one end of one of those, and an arcade PSU at the other and you will find 5V at the PSU leads to 4.x volts at the PCB.

Replace the 5volt and the ground wires with thicker wire, or more parallel runs of thing wire and the voltage drop should vanish. Don't skimp on the ground wires either, that's the return path.
 
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PCB power questions.
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2.) Say I want to test the board on my workbench with a switching power supply. Can I run an alligator clip from the +5V to one of the edge connector pins (assuming parts/solder side are both +5V), so do I need to supply feeds to ALL of the edge connector pins with that value? (Same question for ground).

If the board has test points for +5v and GND, you can clip power leads onto them. That's what I do if I don't have a harness for the game.

3.) So I have a two board set that is not working, at the very least, because of a 5V voltage drop. How does one tell if the voltage drop is due to components on the boards or due to a power supply issue? In other words, is it possible for the power supply to be putting out proper +5V, +12V, etc... to the edge connector, but fail to deliver that voltage once connected? What if the voltage drop is due to crummy edge connectors themselves? What is the correct process to diagnose the source of the problem?

You can use your meter to measure the resistance between +5v and GND and compare it to a working board of the same game. If the board is question is a lot lower, then it could be too big a load for the PS. Clean the edge connector and re-pin (all of) the +5v and GND pins, and if there is an electrolytic cap connected between +5v and GND, replace it. If that doesn't help, then it may be a bad chip, and finding which one can be tough. You may get lucky and have the chip ID itself because it gets warmer than the others, or you may need something like the "LeakSeeker" to try to track it down.
 
Thank you so much for your awesome posts! I am new to arcade PCB's but not to hobbyist electronics, and this is a huge help! Most of the guys on Facebook want to tell you how they can fix it but not share to help us fix our own stuff. I love trying to get these old boards working for my own collection. This is a huge help!

Gavin
 
Thank you so much for your awesome posts! I am new to arcade PCB's but not to hobbyist electronics, and this is a huge help! Most of the guys on Facebook want to tell you how they can fix it but not share to help us fix our own stuff. I love trying to get these old boards working for my own collection. This is a huge help!

Gavin

Second that with the Facebook crap, it's more of a repair guys advertising page than much of any real help. Just a bunch of noobs on KLOV they say, well I've gotten 10 times the amount of help here...
 
Strange; I have quite the opposite experience. People contact me to repair their crap. But I don't repair for money. Only my own crap and for friends, that often gives me something (maybe other PCBs) in return.
I don't mind giving advise, but I expect people to put an effort into it themselves. I don't hand out cook books };-P
 
I want to learn. I like this hobby a lot. Klov and posts like these have helped me and others significantly. I hope to contribute more myself when i can help others!
 
What if you get nothing on the clock crystal with using the probe, then replace the crystal, and still get nothing?

You usually can't probe crystals directly. The signal is too weak, as it isn't TTL. There's usually a buffer that comes after them, which is where you want to probe.

Also, many clocks are pretty fast, and some probes will report them differently (e.g., on some probes with audio, you won't get audio for anything above a certain frequency, though the LED should still work.)

You can also try checking any sub-rate clocks that are derived from the master clock, to tell if the main clock is working.
 
Great write ups. Thanks
Are there any guides to using a scope? I have a good one that works well and I have used it successfully to find faults. But I feel like I'm fumbling a bit in the dark getting it setup. A beginners guide to setting and hookup of a scope for ttl and monitor troubleshooting would really be helpful and help me build confidence.
 
Great write ups. Thanks
Are there any guides to using a scope? I have a good one that works well and I have used it successfully to find faults. But I feel like I'm fumbling a bit in the dark getting it setup. A beginners guide to setting and hookup of a scope for ttl and monitor troubleshooting would really be helpful and help me build confidence.

Youtube has a million good scope tutorials.
 
@fortytwo definitely check youtube, there are videos directly showing you how to use a scope and tons of PCB repair videos showing how the tech isolated a specific problem. Two that come to mind are Classic Game Repairs and OneCircuit, both even walkthrough the problem end to end.
 
Which scope do you have?

Depending on the type or model you have I can recommend some great reading...short training books in PDF format.
 
Seek out Tektronix XYZ's of oscilloscopes. It should give a good overview of oscilloscopes in general.

And read your owners manual. READ IT. Don't skip around. Sit with the scope and go cover-to-cover at least once before getting serious about troubleshooting stuff. Make sure you understand each section in the manual. It won't teach you how to use the scope for troubleshooting....that comes through practice and troubleshooting experience. But it will show you what the controls do and give you a basic understanding of the tool's abilities and how to make measurements.

You can practice by measuring the calibration signal that is generated by the scope. Do it manually, using the cursors. Measure the top and bottom voltages relative to ground (ground = 0 volts). Measure the difference between the top and bottom (amplitude). Measure the period (time) and frequency (Hz). Measure the positive pulse width ("high time") vs. the negative pulse width ("low time"). Measure the duty cycle. Duty cycle = (high time) / period. Adjust the time/division to be about 1/10th the period. Then count the pulses on screen. Measure everything you can think of about that signal.

(hint: when measuring a signal make sure it takes up the whole screen. You'll get more accurate results!)

Then use the scope's automatic measurement capabilities to measure the same things. Are the results the same? They'd better be. If not...practice some more.

Now grab a random working game PCB. Find a 74161 counter on the board. (or 74ls161, 74hc161, or 74ls163, etc.)
Measure the clock on pin 2and Q[2:0] on pins 12-14 and try to get a stable display. Make sure they make sense to you. This will test your basic triggering skills. And your binary counting skills too.

(hint: when looking at signals that are related in time....trigger on the slowest changing signal.)

Find the fastest signal on your PCB. This is usually the output of a digital inverter connected to the crystal oscillator and it usually goes straight to some counters among other things. Try to measure that high speed clock. It might be 12-15 MHz as an example. While still viewing the clock onscreen enable the 20MHz filter function of your scope. It's usually found in the vertical setup controls for each channel. How did it change the clock signal onscreen?

Play around and get comfortable with the scope. Make up things to measure and check if the results match what you expected.

Then when you're really debugging and you want to measure something (time, voltage, delay between 2 events, etc.) you'll at least have a vague memory of having seen that type of measurement or something similar in the manual and in your play sessions.
 
Do you need different hardware to read/write EPROM vs EEPROM. Any product suggestions?
 
Do you need different hardware to read/write EPROM vs EEPROM. Any product suggestions?

No.

An eprom programmer can do both.

The only difference is that you'll need a UV light to erase the Eproms, but not the EEProms.

The first "E" in EEprom is for "Electrically"-Erasable , which is done by the programmer prior to writing.

The only caution I can think would be to have a programmer that's "new" enough to do EEPROMs.

The really old programmers like the data I/O 29b pre-dates the EEproms, so they don't support them.

Any programmer from the late 80's on-ward should be fine.

Good luck,
Steph
 
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