Homebrew Z80 update

Since I have an old V-Tech children’s toy that is really a Z80 computer I thought I would reverse engineer it and use it as my Z80 system.

So far things have been working well for it. I have removed all of the ribbon cables and replaced them will headers so now it is a single board. I also changed out the clock crystal from 3MHz to 4MHz. With the stock ROM still on the little theme music plays much faster.

The board has 16KB or SRAM but I am planning out a 64KB module board to replace the old chip. Since the board also has a expansion port for carttages I was thinking to make a backplane board to add a PIO, SIO2, video and maybe a real-time clock module. I got a cigar box to use as a case and mounted 16 switches and LEDs to the lid to attempt to control the Data BUS and Address BUS. I have even probed all of the header sockets back to the main CPU so I can add an HD44780 4×20 character LCD until I can get some proper video output. However I can’t do much software wise until I can get a null modem session connected to it. I might try Grant Searel’s BIOS/ROM image and see how it works out.

Vintage Vacuum Tube equipment

A few months ago I bought a Eico Model 950 Resistance-Capactiance-Comparator Bridge off of eBay.


The seller claimed it was restored IE recapped. When I got it the power cord look a little scary looking so I was going to change it out to a newer polarized cable but quickly found out only one of Capacitors were changed out. 95% of these caps are original from 1964.

Thing is paper based and electrolytic caps dry out and break down. Even if they were never used. When this happens they turn into a short or into a resistor like state.

So now I have to start on the quest of finding some modern caps for this unit.
Vacuum Tube equipment uses high voltages so high voltage caps are needed. Also they’re typically low values and can be a pain since a lot of capacitor values listed are not common anymore so you have to round up or down to the next value. The hardest part honestly is finding Axial type caps. My best bet is to find Mylar or Orange drop caps for this project.

There is also a trap waiting for newbies as well. Depending how old the device you’re working on the Schematics or Service Manual will call Capacitors something else such as Condensers. You’ll also see MFD or MMFD instead of uF, nF and pF. MFD = uF and MMFD = pF. They will list nF values as uF so a 100nF cap would be 0.1uF or in this case 0.1MFD. You can easily obtain a conversion chart from here.

Above was a couple of weeks ago. Since then I have been able to replace most of the caps except a few I don’t have in stock.

Rectifier Tube and two old caps from 1964
The only cap replaced by the Seller was the 4uF cap.
Old cap hiding, needs replaced.
Filter cap that needs to be changed ASAP.

Now the thing that kinda scares me is the third picture of that old wax cap. It’s going from the AC power switch to Ground.

It’s a 10nF cap. I might replace it with a XY style cap. If I use a typical cap and if it was to fail it would cause a short, make the chassis live or a fire hazard.

The new line cord will be easy. Live would go to the rectifier tube and Neutral would go to the switch. I might add a terminal strip for the Line cord so I can put the proper XY safety cap across the line.

I am aslo thinking to lightly wet sand the front of the unit to clean it up. The lettering is engraved and paint filled so it shouldn’t hurt anything. The back side of the case has some rust spots on the bottom but I can easily fix that.

Found memory for my z80

I have been sortting all of my stuff latelty and came across my box of SNES games a friend gave me a while back. 90% of these games are sports games that honestly have no value. I looked them up and also found most of them had 64KB SRAM. So I opened one up and sure enough I found a 28pin 64KB SRAM chip sitting inside along with a 74LS139 Dual 1-OF-4 Decoder/ Demultiplexer IC.

These will come in handy for the z80 project. Too bad Nintendo used PROM chips instead of EPROM/EEPROM. I got a total of six boards that have the memory chips.

Maybe I can desolder the Nintendo D411 security chips and toss them up on Ebay.

News 7/3/17

Finished sorting all of my parts and got a 5000 count trading card box. I was hoping to get a box that held cards sideways but could only find small boxes that did that. The envelopes are a bit tall but fit just fine when tilted.
The envelopes work well but I wouldn’t advise to store IC chips. The pins on IC chips are fragile and bend easy.

The z80 computer project is slow going. Still finding and waiting on parts. Wish I can find wire wrap sockets or stackable headers in larger pin count.

I’ve been checking out thrift stores on my way home from work since nine times out of ten I am on the other side of town or in a different county. I’m hoping to find some retro electronics to tinker around with or add to the collection, so far nothing. There is a YouTuber by LGR that attends to go to Good Will and finds assorts of retro games and such but it seems here in Florida people only donate Clothes, Chachkies or busted up Furniture.
Next payday I might take a trip to a FleaMarket in the next county. Last time I was there I saw some nifty stuff but didn’t have much cash on me and didn’t want to carry it around all day.

Storing electronic parts

If Hobby Electronics has become a somewhat serious hobby you’ll have a huge inventory of parts. Stuffed in storage bins, boxes and so forth. I came across an awesome idea for storing most of my inventory of parts. Just think about it, no more digging for the certain part that got mixed into a bin of similar parts.

It’s a bit overkill but worth it long-term. This mostly works with smaller parts and probably SMD if you’re into that.

All you need is a bunch of #3 Coin Envelopes and a couple of boxes for storing trading cards.

If you watch TV while sorting out everything it will fly by. Just sort out the parts by part number and again for wattage or voltage rating for resistors and capacitors.

Honestly I thought I was doing fine until I sorted out all of my Diodes. My gosh I have tons of different flavors of just Zener Diodes alone.

Here is a photo of my progress.
That’s not even a dent in of the parts I got. All I need to get after the sorting is a few trading card boxes. With the card boxes I can tape on a short inventory list on what is inside it.

I know most people will find this lame but I find this exciting. I can spend less time hunting for a part for a project and spend the time working on the project.