News 6/10/17

It’s been a while. Been busy at work and this is the first weekend in a while that I have some free time.

Last month I finally got my junk together by buying a bookshelf from a thrift store so I could properly store everything instead of piling it up in the corner.

I also bought early last month a Eico 950 Resistance, Capacitance, Comparator Bridge. It’s so old it uses Vacuum Tubes. I bought it from Ebay for around $40.00. The Ebay seller said he restored it but when I opened it up the unit still has a lot of paper caps that are probably ticking time bombs. Some time I plan on recapping it and replacing the AC cord with a polarized one.

To make some extra room on the work bench I put my Analog scope in storage. Before doing so I wanted to checkout inside the case since I never opened it up. Besides some dust bunnies I found something that explaned why the triggering function was a PITA to use. I fixed it and it triggers much better.



I was visiting my mother and found an old V-Tech learning computer that my brother and I use to play with when we were kids. I took it home and took it apart to clean it up since it had a good layer of nicotine stains on it and to my surprise it has a z80 and  static RAM inside of it.


I’m thinking to hack it and toss on a Zilog SIO or PIO chip and a EEPROM and see if I can do something with it. Although I’m already building a z80 from scratch. Speaking of that I got some needed parts on order but will take a while to show up. I ordered some z80 20MHz CPUs from China. They claim they’re new old stock but we shall see.

Last week I got some ATMega328P-PU ICs for two bucks that I ordered months ago and forgot about. I tested them out and they didn’t want to work. After looking in my email for the invoice it said the chips were preprogrammed with the Arduino bootloader. After spending about two hours trying some stuff I finally got them working. I had to reset the “fuses”. I plan to use a couple of them for a Serial Terminal project for my z80 build. I would like to finish my TerminalScope clone but Grant Searle’s is cheaper and more customizable. I might go find myself one of those portable B&W TVs and make a mini old school looking dumb terminal.


My HP TouchSmart TX2 laptop bit the dust. When you power it on the Cap Lock and Number Lock lights blink and no picture comes up on the monitor. I found out the GPU had loosen up on the mainboard. My Acer Laptop has a few issues as well and I think it is starting to fall victim to GPU failure aswell so I pulled out the old Family desktop my wife has had for eight years. It’s a Gatyeway DX4200-09 with 4GB of RAM and a 500GB Hard drive. I tossed in a nVidia GeForce 220 GT and a USB 2.0 PCI card for more USB ports. It’s not a bad system, I installed Windows 7 on it for now and works quite well. I might toss in a second hard drive and dual boot Debian Linux on it.

Elenco XP-720 Modifications

A while back I bought a Elenco XP-720 off of CraigsList. It was $40.00 and seemed like a deal at the time.

I brought it home and started to play around with it and realised it is missing some major features such as Meters and an Output ON/OFF switch. So I quickly bought the meters and added an Output ON/OFF switch. The switch has a lot of bounce but im not a NASA engineer so it works for my needs.

When the meters arrived and I got around to putting them in I had to back trace the wiring to what went where. I noticed a few things such as the constant 5V+ section had a 16v rated cap when it outputs around 20v from the transformer and rectifier. So I switched it out to a 50v 105*C rated cap. I think I even used double the capacitance as well since after all it’s only a filter cap. The meters I bought were not what I expected. The voltage sense and power are tied together so after I get around 2.6V the meters kick on. I need to figure out how to add a voltage sense lead or get better meters.

I currently have the constant 5V+ disconnected. I was thinking of a way to get a 3amp variable voltage output but after looking at the schematic the input of the regulator is in a voltage divider path. I would have to rebuild the whole circuit for that to work right.

20170610_193011The left Meter is for the positive voltage and the right Meter is for the Negative voltage. Between the pots is the Output ON/OFF switch. The smaller switch between the meter and positive pot was going to be a current limit LED but I think I’ll use it for the constant 5V+ Output ON/OFF.

I goofed on cutting the holes for the meters. I used a Drill for the four corners for each hole and a Coping saw with a metal-cutting blade. Kinda made myself mad since I measured everything five times and still made them a bit big.

I’m planning to repaint the case black and use white lettering for the front panel. This way I can use some black silicone to fill the gaps of the meters. Unless I have to change out the meters for the proper voltage sense and power.

I also wanted to add some 10 turn pots but the pair I ordered from Ebay were not the correct value. The Ebay seller sent me 100K instead of 2K that was advertised.

The Adventures of a home brew 8Bit Computer

Got a few parts in such as the MC6800 and the 74s189 chips. For some odd reason I thought the 74s189 was a Static RAM chip. It’s not, it’s a Dynamic RAM. So now I need to find some proper Static RAM.

Also the 6821 chips I had in my junk box don’t work. I got these chips from a box of electronic parts from a friend when he went to ITT Tech and I have come across a few 74 series logic chips that are dead so it wasn’t a surprise the 6821 ICs were dead as well.

However this is great news so now I can go ahead and go with a z80 CPU. After all I bought ten z80420A PIO chips when I bought the 74s189 chips.

The overall form factor of the system is going to be a back plan with a CPU card, Memory/ROM card, video card and a general IO board. Maybe a switch console or parallel LCD. The video card will be the last item to build so I’ll mostly be using a serial terminal. I guess I should also find some z80 SIO chips to toss in as well. If I do that I will have the PIO, SIO and RTC on their own board.

I plan to wire wrap the computer boards but for the back plan I might etch a PCB or strip board it. Since the cards need to be inserted and removable wire wrapping the sockets on the back plan would be a back idea. I would love to use the old ISA style edge card design but the card edge sockets cost too much so I might just use double row or right angle single headers.

In all I don’t plan to try to do the smallest chip count or making it small as possible. I also don’t plan to build this overnight and expect it to work. This is going to be long-term project and once in a while chime in on here about the progress of it. The overall goal is to getting a standalone system up and running CP/M or MP/M.

DIY 8bit computer

I’ve had a couple of 680x PIA chips laying around. Every now and then I come across a DIY retro 8bit computer project and makes me want to build one. So I went ahead and bought a 6800 CPU and a couple of 74s189 SRAM chips.

A while back I had sent off for some TerminalScope PCBs and they have been sitting collecting dust in a box somewhere. I guess I can toss one together for the 8bit computer.

While I was shopping for the SRAM chips I came across some z80 compatible PIA chips. It was 15 bucks but it comes with ten ICs. So maybe down the road I can build a Z80 based system with CP/M

In all I got a lot of info from Grant Searle’s website. He has a few small chip count mini 8bit computers that can be built on bread boards.

Right now im awaiting parts so I can start. In the meantime I am researching and thinking of a few things such as power supply and storage. I am thinking of a simple linear supply and making sure I have filter caps at every chip’s V+ and ground pins. Also im leaning towards Compact Flash for storage since it is a Parallel device. Doubt I could get Ethernet with a TCP/IP stack on it so simple direct connection via Serial should be plenty. I think I have a MAX232 chip in my junk box. If not I could use a 7404 Hex Inverter for it or for fun use some switching transistors and resistors. It would be fun to have an array of switches to control functions kinda like the Altair 8080. I’ll just add some LEDs to the inputs and outputs for some blinken lights.

If anything on the final build I can add the TerminalScope circuit to the mainboard. It’s a dirty way but the easiest without using too much system resources.

Keithley 179

I acquired a Keithley 179 Ture-RMS bench top multimeter. The case is a smokers beige and has a few scratches here and there. The date code on the IC chips are dated from 1978 so it’s an early revision.

When I received it I took the top off to take a look around and sure enough the filter capacitors were leaking electrolyte everywhere. I went ahead and powered it on and it some what worked. I don’t have any voltage references or resistance references to see how far out it is but comparing it with a cheap none True-RMS meter it keeps up.

Today I recapped the whole unit. Since RadioShack was having another store sale off they had the electrolytic caps I needed, not to mention the axial type as well. They didn’t have any tantalum caps so I had to use electrolytic caps instead but the two caps I changed are only for filtering so it works just fine. The other two tantalum caps still had fine ESR and checked out fine so those were the only two polarized caps I left in. While I was at it I replaced the two probe banana jacks since the old ones were corroded.

Some time I might sand down the case and paint it. Maybe even make a custom front bezel instead of the translucent red panel.

Arduino project

I got a nifty idea of a universal device controller to use with the Arduino. It is gear towards test equipment but it could be used for other things. When I say test equipment I’m talking about power supplies, function gen, and maybe even multimeters.
It doesn’t have to be fancy of a setup like a touch screen and I2C hardware but simple a simple LCD and push button switches.

My take on this will use the TVout Library so I can use one of my many 7inch LCDs I keep finding at thrift stores. A simple 4×4 keypad array will work fine and maybe even use a smaller or bigger keypad if needed. There are also a few libraries that use the analog pins of a Arduino to control a keypad via voltage dividers. The fun part is keeping the pin count down to leave for expansion or maybe use some 74HC595 8bit shift registers.

This will be used for my bench power supply and also I plan to build a Function Generator kit to modify as well. In an older post I modified a Voltmeter sketch that uses the TVout library and maybe with some code changes I could add other options and make something of a multimeter. Maybe I could modify a cheap handheld multimeter to use the controller.

So here is the specifications for this project.
TVout support for the video.
PWM to DAC converter to control analog signals.
Keypad controlled with analog pins to keep the digital freed up for more control options.
Relay control for controlling power sources such as AC or output supply from a DC supply.

I’m thinking if I should use a TIP122 Darlington transistor or a ULN2003 transistor array for the PWM to DAC control. I could use the 4n35 opto but I don’t have any in stock.


Building an case for my cheaper transistor tester

I have one of those cheap Chinese transistor tester kits. Today I decided it needs a case. I have a case from a CB radio that will work very well to house the tester. At the moment I have to use cardboard for the front bezel until I can get some plastic or a decent blade for the jigsaw to cut some sheet metal. I plan when finished to paint it to match the rest of my gear or I could keep it with the fake wood panel look.

To make this work I had to modify the circuit board a bit such as replacing the female header for the LCD to make pins so I can use a ribbon cable to extend the LCD. The tactile switch was also removed with some wire to extend the button to the front bezel. While I was at it I removed the brightness pot and put on a typical 1/2 watt 15k pot so I can mount it to the front bezel as well. I just hope after recalibration it will work properly. Right now it sees thins with a 2 ohm resistance.

As you can see I finished it. Some acrylic paint helped a lot on the cardboard bezel. Also recalibrating it worked well. It now rests under my scope.

News 1/31/17

It has been a month with the new job and it is still awesome compared to the old one. Stress free, I’m basically my own boss and I pretty much control my paycheck.

The other day I spent time with the youngest son and both played around with some R/C cars. We got a couple of cheap ones from Walmart. Afterwards I rebuilt my car and made it a hell of a lot faster. Tossed in a old control board I had in my junk box and changed out the odd 4.5 voltage rated caps to higher voltage caps that matched the Micro Farad rating and tossed on a 8 volt battery.

The Desktop CNC project is slow going still. I am taking my sweet time with it so everything will work correctly.

In the mean time I came across a old JCPenny CB radio and stripped it down for parts. The case will be reused for my Component tester. The fake wood and flat head screws have a neat old school look.

Other then that I have been spending more time with the family since I am home in the evenings now.

Table top CNC update.

When building a CNC from wood it can be very challenging. Unless you have a full fledged wood shop. With basic tools that most people like me have it involves tons of sanding to get a level surface. Not to mention a bunch of measuring and remeasuring to make sure you get a proper straight cut. The router table setup I have works well to straighten up most of the cuts but everything else requires tons of hand sanding. I’m tempted to buy a electric sander before doing anything else. Now this isn’t because I went with OSB for the material, this is the challenge for any kind of wood.

I came across an awesome circuit for the Spindle. The circuit ramps up the speed of the motor when the RPMs hit a certain speed. Or should I say when the motor bogs down it adds more juice. I got this from a YouTuber called MrJohhhnnnyyy.

drill  One thing that isn’t shown on the schematic here is you need a Diode to go across the motor terminals for added protection. I doubt 40V is needed to drive the circuit, I am planning to use a old laptop charger to power it. I could always use a small transformer that is rated for 40 volts, an bridge rectifier and boost up C1 to a higher capacity or add in a second filter cap.

I worked on the Y-Axis a bit. The railing I have is too flimsy to use so I found some old drawer handles. I sanded the paint off and polished it with 2000 grit sandpaper. It came out real nice and now I need to mount the railing. Before mounting them I need to find something to use a bushings. I could use some bearings I pulled from old hard drives but it would cause more time to setup for them to run perfectly parallel.

Table top CNC prototype

Today was a good day. I worked on the CNC for a bit while ever so taking my time. When you create a one off prototype you have to really plan things. I don’t like to sit at the computer fighting SketchUp for designs. I prefer to take the material I have and physically draw the layout and put it all together as I go. This way if I run across a problem I can quickly resolve it.

I worked on the Z axis a bit today. When I was thinking of game plan I decided to use a hard wood for the base that will hold the spindle. The vibrations with OSB might loosen things up over time.


Since the back piece of the Z-Axis is OSB I made sure to use the ruff side the glue side so the glue can adhere better. The piece of Oak is pallet wood. I let the glue setup enough that it was squared and remove it from the workbench. I re-clamped to itself and letting it cure overnight so I can pre-drill and secure it with screws. I wish I had enough Oak to do the whole Z-Axis but as long as the spindle mount is solid it should hold up. I’m not planning to use a Dremel or anything like that for milling/routing.

These rail pieces I found last week. I think they were from some can light fixtures. They were sticking in the dirt at a job site. I cleaned them up and filed down the high ruff spots until they slid true. These will be the glide rails for the Z-Axis.

I did a test fit for the X-Axis glide rails. I was a bit off but a simple fix with the router table fixed it. I plan to do a friction fit for the glide rails and cut them to the final size on the final assembly.

I need a base and table to finish the skeleton of the unit before I start on the Y-Axis. I plan to use solid wood for the X-Axis table so I can level it on the first run. I am still going to use a Arduino with GRBL loaded up as the controller. I found a nifty Android app that can control the unit. It costs about $20.00 for the Bluetooth package I might use on the final setup so I can use a old Cellphone or Tablet as a dedicated unit. To power the whole thing I got a 19volt laptop power supply that is rated for 3.5 amps. Should be plenty. I can always add a 5volt regulator to power the Arduino.