Isolation Transformers

Today lets discus about Isolation Transformers. They’re a must have for troubleshooting unknown or problematic circuits.

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Duinomite and BSD

A while back I bought a Duinomite Mega. It’s a small single board computer that uses a PIC micro controller and runs BASIC. After playing with it and got very annoyed with it since the ROM it was shipped with was faulty and had to flash it so I could use the SD card port and still wanting to toss the keyboard into the screen due to it being so buggy I just tossed it in a box and tried to forget about it.

The other day I came across RetroBSD and sure enough the Duinomite is compatible. So I gave it a shot and I am very pleased. It took some head scratching on getting it to flash and ended up using a Windows port of mphidflash. I tried doing it on my laptop running Linux but no mater how I tried it I got a permission denied or it wouldn’t see the board. I copied the mphidflash executable to the directory I decompressed RetroBSD to and ran a command prompt windows with Administrative permissions and ran the flash, same command as the Linux/MacOS syntax that is stated in the Duinomite manual for updating the firmware.

mphidflash -w unix.hex -v 15ba -p 0032 -r

Be sure to put the board into the “debug” mode by holding down the BUT and hit RESET so the yellow and green LEDs are flashing.

I wrote the SD card file to the SD card with my laptop but Win32DiskImagier should work just fine for Windows.

To test the system I used Minicom on the Laptop and connected it via USB/Serial and hit the Reset to reboot the board and it came to life.

Now there are some major downsides. Since the RetroBSD team is small there isn’t much working on the Duinomite. No UART other then the USB, no video and no keyboard. If I had more time I would attempt to port some stuff but I don’t know, we shall see.

CRT TV as a computer display

Way back a lot of computers had a option to use a composite video source to a TV to be used as a monitor. Over time this became uncommon then came back again in the 90s and up until HDMI was becoming standard.

So what is the deal? Well VGA is a analog video signal unlike CGA and EGA that are digital. Old TVs are analog so I thought maybe I could build a adapter instead of dishing out some cash for a proper adapter box. Sure enough there are plenty of circuits floating around on doing this. The most simplest circuit I found uses a 680ohm resistor and the red gun signal of the VGA producing a monochrome (B&W) picture. Fine for me since I wanted to use it as a command line interface terminal. I forced my test machine to use the most lowest possible resolution and tried it out. It some what works and had to swap out the fixed resistor to a 1K pot to get a better picture however I learned real quick that NTSC uses a different clock signal then VGA. I think NTSC uses around 15KHz and VGA uses 31KHz for 640×400. The picture was mirroring as in showing three pictures, left and right were cut off and the center was crushed in vertically. After digging around I found a super old website where some guy tossed in a B&W CRT into his desktop computer. He modified the video card’s ROM to use a slower clock. Later on I found a webpage that covered to force a video card in software via Linux and Xorg to use a slower speed but it only listed some old ATI and Matrox cards that I don’t have anymore and I doubt the cards are compatible in modern distros of Linux since over the years they have cut out thousands of compatible hardware from the Kernel.

I kinda knew I wasn’t going to get far but it was fun to try. Looks like I’ll have to ether buy a converter adapter or maybe reprogram my MaxiMite clone to be used as a serial terminal.

Expand a scope to four channels

I was going through a book that has some vintage circuit schematics and came across a nifty one in the CRT section. This circuit will expand a single or dual channel scope to four channels.

SCOPE

The circuit provides a four channel display with a HA-2405 four channel programmable Opamp. It provides a unity gain per channel, the bandwidth of DC to 5MHz. The maximum input voltage of plus or minus 10 Volts and a slew rate of 15 Volts per micro second.

The top right side chip is a 7473 Flip-Flop. Also the voltage divider for the “From Scope Gate Out” is a 5K and a 1K. The 1K is going to ground.

I haven’t tried this schematic but let me know if you are willing to try.

Retro / Vintage computers

I remember back in the late 90s people were tossing out their old computers from the 80s. One time my mother knew a guy that worked for some small computer repair center and she of course mom being mom she would “gloat” about my hobby with computers because at that time I had built myself a computer. Next thing I know she comes home with four box loads of of computer parts that the computer guy she knew was tossing out old parts that were pulled from old systems. I had Seagate ESDI/MFM drives, piles of floppy drives, memory and motherboards. I think there was two IBM XT cases as well. Granted I could really do much with them since I didn’t have enough cases, power supplies and no networking hardware to speak of I was able to slap together a couple of systems for tinkering around with. I had most of these parts laying around up until I moved out from home.

Today I was surfing around YouTube and came across retro computer builds and such and to my amazement people are paying about $500.00 and upwards to build one of these systems. I went on eBay for kicks and sure enough the same hardware I had that was at one point trash is going for top dollar. I kinda want to kick myself because now this stuff is considered collectable. Not to mention a lot of gold refiners attend to buy this stuff to strip out the gold.

Looking back I had tons of stuff that some sap would pay big bucks for.

ATX Power Supply modification

The simple ATX power supply is a PC standard. However some OEMs will use different pinouts for the voltage/signal rails and once in a blue moon use a different rear panel layout like putting the exhaust fan in the middle or putting the IEC power socket on the left or right side. Typically it’s doesn’t matter but rarely it does.
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Free Server

Well I got the server yesterday and man I was wrong. It’s a HP Proliant ML310 Gen 4. It’s got 4GB of ram and a Intel Pentium D CPU.

The Power supply is bad so I tossed in a 270watt supply. The server had a 410watt supply but it’s running lowend stuff so it’s fine. I tossed on Debian and if it wasn’t for the HP SAS RAID array card it would be a run of the mill desktop.

I’m thinking to toss in a couple of bigger drives and toss on FreeNAS and use it for backing up the computers.

News 8/7/2016

Nothing new really. Still waiting on some parts for my Analog scope terminal. I found on eBay some stuff I would like to get such as a Fluke 27FM and a vector display but I’m low on funds so that stuff. I’m currently saving up to get a business license to start a computer repair company. So until then I have been doing odd jobs here and there. Last job I painted a couple of rooms for a family friend.

On other things the main desktop computer for the wife and I share is starting to get on my nerves. When I built it I used a AOpen desktop case. Since it lacks front USB and front audio jacks I am planning to transplant the system into a different case. Nothing special really, just a run of the mill HP case.

s-l225.jpg

I used the AOpen case because I was planning to have it on the desktop and to give it that retro feel. On the replacement case I might give it a paint job. The top of the case is all scratched up. I might try some Plasti Dip and just paint the top. The old case could be used for something else, I should measure it and see if it fits my entertainment center and I could use it for my retro gaming rig so it’s not sitting out on the floor.

The retro gaming rig is a hit with the younger kids. I figured the older two wouldn’t care and sure enough they don’t care. Gives me ammo to point out their not truly Gamers like they claim to be since they don’t play the classics every now and then. They are currently playing some game called “Overwatch”. To me it looks like Blizzard puked all over Valve’s Team Fortress and took a few elements for other games like Counter Strike and Enemy Territory. I don’t care for it myself.

A guy I know is giving me some sort of a HP server and a couple of monitors. From what I have gathered it uses non-standard hardware. He said it’s a Xeon CPU but I’m thinking it might not be, It could be one of Itanium systems. We shall see when it gets here. If anything it will have a happy home here and will become into great use when I can get it running.

Clean electronics after spill mishaps

Before I go into details here lets get one thing straight. If you have a device that is on and a liquid comes into contact and the device dies then chances are very slim that cleaning it alone will fix it. The device could be repaired but will involve replacing parts and might cost more to fix then replace.

Here is a example, Your little brother trips and spills Redbull on your PS4. The PS4 was off at the time. With the right cleaning products such as Rubbing Alcohol (highest proof you can find at the drug store, 91% usually), compressed air, cotton swabs and a soft brush you can take the unit apart and squirt on some Rubbing Alcohol and carefully scrub it clean. Sit the parts in a warm area and let it drip dry for about a hour. After a hour then spray it down with compressed air so it can release possible trapped Rubbing Alcohol from under BGAs and such. Let it sit for another 30 minutes and look it over. If it is completely dry then reassemble the device.

Take note, the longer the liquid such as water, soda and what not is in contact with a electronic device will cause issues such as rust/corrosion to traces and parts. The Rubbing Alcohol will displace the most of the liquid. Some people have had success on using distilled water and some glass cleaner to clean a circuit board but you should always use something to displace and evaporates the water afterwards.

Items such as cell phones and tablets are subject to once wet = broken due to the fact when the device is “off” it’s still powered. Even newer electronics non-portable devices that use a soft power solution can make things difficult and lessens the chances to getting a device clean and still working afterwards. Reason being is they’re still powered on to a degree. A lot of times there is a small fuse that blows when something is shorted out but since we live in the world of SMD electronics you would need a very steady hand and a strong magnifier since the fuses look like SMD resistors.

Heard of a bag or bowl of rice to “soak up” water or soda from a electronic device? It doesn’t always work and makes me laugh a bit when people do this method.
Now here is why the “rice method” doesn’t work. Granted rice can draw out moister but not entirely let alone what is left behind from the liquid. The moister needs a way out so leaving a device in it’s casing would just end up being a coffin. Liquid can get into places that rice can’t reach. Rice can not soak up contaminants such as sugar and most chemicals that are in soda and energy drinks. Let alone what is left over from water such as minerals and other things. 90% of that stuff left is conductive so it will have no effect with rice alone.