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Entry level, ultra-budget oscilloscope for troubleshooting old Arcade PCBs
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donluca:
Hi everyone!

Just registered, so here's a little info about me:

My name is Luca, I'm 32, I live in Italy and I'm a huge videogame fan, all the way back to old consoles such as NES and PC memorable titles like arkanoid and pacman.

As of late, I've finally got one of my childhood dream come true: I've acquired an authentic SEGA arcade cab, a SEGA Astro City and have been spending a fortune since then on repairs and acquiring new PCBs of the games I love (Shinobi, Golden Axe, Asterix, Neo-Geo titles such as Metal Slug and so on).

Up until now, I've been able to diagnose and solve all kind of problems just by using the boards' schematics (when available), an EEPROM programmer, a good multimeter and common sense, but now I have come to a halt due to a Capcom CPS2 board which has graphical issues.

By using the multimeter to check if voltages are alright, continuity between points and shorts I couldn't find anything wrong, so I've asked around and people have suggested me two new instruments: a logic probe and an oscilloscope.

Since it's super cheap, I've already bought a logic probe (it should be here by the end of the next week) but while a probe will help me troubleshooting new issues such address lines on the EPROMs stuck high or low, it won't help me understanding if a digital IC (for example a RAM chip) is spouting garbage or good data and that's when an oscilloscope comes into play.

Now, the first question is: before dumping money into a 'scope, is there any other tool I should be getting first?

If the answer to that question is "nope", then we can jump straight into the main topic: what oscilloscope should I get?

There are two *huge* restraints at play:

1 - I'm really strapped for money, I can go up to 150€ max. I don't mind waiting or buying used, though.

2 - The space I have is limited (even more now that I have an arcade cab with a 29" CRT monitor  :palm: ), so those old, sweet tektronix which I've seen going for 100€ (and even less!) are out of the question.

People have suggested that I should go for a 'scope with an input frequency no less than 50Mhz which means I'm looking at 'scopes like Rigol 1052e (350€) or, cheaper, Hantek 5072P (~200€).

I've seen the Owon SDS1102 going for ~160€ on eBay, but I've read lots of people discouraging buying Owon products, otherwise that could hit the mark.

Is there a way out?  :-BROKE
rhb:
The scope won't tell you if you're getting garbage from a memory chip unless it has a logic analyzer function or it's really blown badly.  And an MSO is about 10x your budget.

As you are cash strapped, get one of the Chinese Saelae knockoffs.  That should let you dump bus traffic to disk for analysis.  At the same time, look for a cheap analog scope in good working order.

If the CPU is socketed, make a "free run harness".  This is a socket which has a NOP hardwired on the CPU data bus.  So with the free run harness plugged into the socket and the CPU plugged into that it will sit and cycle through memory.  Run the board side of the harness to the logic analyzer.  This will dump the entire address space of the board to your PC.  With a memory map of the system, you can then identify bad ROMs, RAM, etc.

If the CPU is not socketed but is obtainable, I recommend installing a socket.  An alternative approach is to cut the data bus board traces and then bridge them when you've solved the problem.

The newer the gear, the harder it gets to do this.  It's a very 80's and earlier technique.  It only works if you can access the data lines.  I used it on a Vic20 over 30 years ago.

Watch out for low voltage logic.  I built a 1.5 V buzzer continuity tester a few days ago so I could trace out 1.8 V logic lines without blowing junctions.  I did this after discovering my HP 34401A supplied over 7 V for the continuity test function.

You're playing in a niche field.  It is very likely that you can design and build test gear which is both cheaper and more useful than anything you can buy.

I'd like to suggest you consider getting  an ARM based RPi or BB  and building your own test gear around that. What you would need to add is level shifting at a minimum and preferably  comparators with an adjustable threshold.  You'll need to check clock rates, etc.  Nothing could interest me less than a video game, so I have no idea about the HW, clock rates, etc.

There's a good chance that some time spent developing a custom, fit for purpose test bench would make repairing video games a profitable hobby.
spec:
Hi donluca

What an excellent opening post: informative, well structured, to the point. I enjoyed reading that.  :-+
FrankBuss:
A scope is very useful for looking at problems with analog signals, or if there are glitches in digital signals and for a first test if there is some digital input and output for ICs. But I think a logic analyzer is more useful, because you can capture the signals of buses in the videogame and then analyze it to see if there is something wrong. The more channels the better, but I think with 8 channels you could already do something, e.g. if you monitor a few address bits and a few data bits, then a few other address bits etc.

It gets much easier, if you can just sample all of the system. For my C64 MIDI cartridge development, I built a 64 channel logic analyzer with 100 MHz samplerate:

http://www.frank-buss.de/parallella/sampler/

I could see all the odd behavior of the expansion port signals of the C64 and C128, and design and test the CPLD program for my cartridge until it worked. It was not as polished as a commercial 64 bit logic analyzer, but it was a lot of fun to program this and much cheaper.

Recently I've started to build a much better system:

https://www.eevblog.com/forum/projects/4-channel-adc-10-mhz-8-bit-design/

I can now already sample 128 bits with 25 MHz samplerate, but unlike the Parallella project, it uses the DDR3 RAM on the DE10 Nano board, so I can store up to 1 GB continuous data. With the Parallella project I had only a few kB of the internal FPGA RAM and had to set triggers and there were always gaps in the recording. With the new board I can just sample everything and then analyze all the data on a PC. I posted some dumps and another one could analyze the data with PulseView to reverse engineer most parts of the programming protocol for a microcontroller:

https://www.eevblog.com/forum/blog/eevblog-1144-padauk-programmer-reverse-engineering/msg2098303/#msg2098303

Instead of sampling analog data, I can sample digital signals as well. Might be very useful for my retro-gaming projects later as well. I can even combine it, like 36 channels digital input and 4 ADC channels, like the big DSOs. Maybe I can get it to work at 100 MHz as well.

So the conclusion is, if you do this as a hobby and don't want or can't spend a lot of money on test equipment, it can be fun to build this yourself. A Beagle Bone can be used as well for a multi channel logic analyzer:

https://hackaday.com/2015/02/19/turn-your-beagleboneblack-in-to-a-14-channel-100msps-logic-analyzer/

It is easier to program than a FPGA and you can write custom realtime code for triggers, and videogame specific analyzer functions with the PRU and in C on Linux, and even generate test signals, or emulate cartridges with the PRU etc. But might need a Beagle Board instead of a Beagle Bone, which has more IO pins.
KL27x:
The major problem with the Owon PDS 1102 I own is the LCD screen. When new, you had to fiddle with the dial to get it in just the right place to view the screen, and even then it looked washed out. After time, I had to jam some shims between the housing and the screen in just the right spot to see anything. It still works, last I checked. The only upside of this scope is it runs on batteries. I'd sell it to someone in need for a pittance, but I'm too lazy to ship it.

I upgraded to a Hantek. Can't remember the model, but it's a bit out of your price range, anyway.
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