Electronics – Page 2

September 18, 2021October 12, 2021

Hamfest acquisition: Fluke 8600A DMM

The other $5 digital multimeter that I picked up at the Shelby Hamfest was a Fluke 8600A DMM. This one was quite a bit heavier than the 8000A.

Like the 8000A, there’s a label on the bottom that lists the specs and configuration. This unit came with the rechargeable battery option, which would explain the extra heft.

Unfortunately, this one didn’t do anything when the power button was pressed. Getting the cover off the DMM is a simple matter of removing one screw just above the power connector (normally covered by a “Calibration void if removed” type sticker).

The insides are a considerably more complex than the 8000A. A large chunk of the space toward the transformer and power plug end of the board are taken up by the rechargeable batteries. This DMM appears to have had some modifications or repairs made to it.

Inside the Fluke 8600A digital multimeter

A cursory inspection shows one significant issue: leaked batteries.

At some point, one of the D-cell NiCad batteries was replaced with an AA sized NiCad battery.

The manual I found (dated 1981) says the batteries can be replaced, but the ones in this unit have been tack soldered to the terminals. Perhaps the battery options were changed in later models of the meter.

Another IC (possibly a ROM) was also added to the meter and connected to the board with some ribbon cable. The ribbon cable is soldered to a DM47S188AN (256 bit PROM) and the mystery IC is just attached to the RF shield of one of the daughter boards with some electrical tape. Maybe a repair job, or a modification?

There are four daughter boards in the case that attach to the main board via pin and socket connectors. One board is marked as a battery power supply, which I’d guess takes care of charging the NiCad batteries.

Board #2 is marked “OHMS CONV ASSY”

Board #3 was marked “INPUT DIV”

Board #4 was a fairly large one with a large shield on it. No clear silkscreen markings labeling the board like the others but according to the manual, it’s the AC converter board. it also had a holder for a spare fuse. Very thoughtful.

Other than the batteries, I don’t see any obvious problems with any of the other components in the meter. I’m thinking maybe if pull out the leaked batteries, that might get the meter running but I’ll need to study the schematics to see how the batteries are connected to everything to figure out if that will work. This thread on the EEVBlog forum has some potentially useful information.

I’ll put this with the rest of the project items in the stack.

September 18, 2021October 12, 2021

Hamfest acquisition: Fluke 8000A DMM

Two of my Shelby Hamfest acquisitions were a couple of Fluke digital multimeters (DMMs) for $5 each. The seller had no idea if they were working or not, but for $10 I figured they would either be handy workbench instruments if they worked, or fun projects if they didn’t. A brief Google search brought me to a nice tear down/repair attempt blog post.

The Fluke 8000A is a pretty nice looking bench DMM with push buttons for function and range selection, and a handle that also doubles as a stand.

A printed label on the bottom provides specifications and indicates what optional features the meter has. Apparently a rechargeable battery was an available option which would have made it handy for field use. This one didn’t come with a battery.

A label on the top of the unit says this unit was last calibrated in September 1994, 27 years ago. Someone apparently decided at some point the meter wasn’t working properly anymore and wrote “BAD” on the sticker.

Calibration sticker dated September 1994 on the Fluke 8000A

Much to my surprise, the meter turned on when I plugged it in. Testing with some precision resistors I have in my collection gave me some pretty good results.

Measuring the DC voltages of some batteries gave me results that compared pretty well with one of my handheld meters. I haven’t tried measuring anything else yet, but it seems like for $5, I’ve got a pretty decently functioning bench meter (at least for most of the things I’d need to do anyway).

June 6, 2021July 2, 2022

Peeking inside the Altair 680

Let’s have a look inside the Altair 680 that I acquired at the hamfest a couple years ago.

Managed to score a MITS Altair 680 at the Charleston Hamfest yesterday. Was part of the W4RAK collection that was donated to @WA4USN. Looking forward to learning about and playing with it. pic.twitter.com/T5VuSlZ9mU
— imabug (@imabug) February 3, 2019

Undoing four screws at the back of the unit releases the back (surprise!) and top cover to reveal the inside.

Inside the Altair 680 showing the power supply at the bottom, and circuit boards — Inside the Altair 680

The back plate has the power supply consisting of two chunky transformers, a fan, and DB25 connector. There’s also an empty spot for another DB25 connector. Thanks to the two transformers, the rear panel is fairly hefty. The DB25 connector looks like it only has 4 wires. Not sure what would be connected to it, but probably something serial terminal related. There’s not a lot of clearance between the transformers and the SRAM chips on the main board when the rear panel is in place.

Altair 680 rear panel with two transformers, DB25 connector and cooling fan

Altair 680 rear panel. Not a whole lot of clearance between the transformers on the rear panel and the main board. — Altair 680 rear panel with two transformers, DB25 connector and cooling fan

Removing the expansion board (I’ll get to that in a bit) reveals the main board.

Motorola MC6800 CPU in the Altair 680 — Altair 680 main board

The Motorola MC6800 CPU that powers the 680 is up toward the expansion board connector. Toward the bottom rear of the main board are 8 1kbit Intel P2102 SRAM chips that provides the 680 with 1 kB of RAM. Above the RAM is an EPROM (looks like an AM 1702A EPROM). The empty sockets seem like they would provide space for 3 more EPROMs. The other notable chip on the main board is the MC6850 asynchronous communications interface adapter.

Over in the front corner of the board by the big 3.3 mF filtering capacitor, the silk screen says this 680 main board is Rev 1-6.

The only expansion board in this 680 appears to be a RAM expansion board. The riser card has room for 3 slots, but the two other spots are unpopulated.

Altair 680 expansion board with one connector and two unpopulated connector spaces. — Altair 680 expansion board connector

The RAM board contains an 8×4 bank of Semi 4200UCP chips. A Google search didn’t yield a whole lot of information about them, but they appear to be 4kbit SRAM chips, so this board provides the Altair with a whopping 16 kB of additional memory to play with.

Photo of the Altair 680 memory board. A 8x4 bank of 4 kbit static RAM chips provides 64k of RAM. — RAM board

When I plugged it in and turned it on, the fan spun up and some lights came on!

Altair 680 front panel showing some data LEDs turned on.

No smoke released, but the data lights turning on even though most of the switches were in the down position indicates something’s not quite right. Changing the switch positions didn’t affect anything either. The Run light on even though the switch is in the HLT position also suggests something isn’t quite right. Toggling the HLT/RUN switch made the HLT LED blink on once. Toggling the RESET switch didn’t seem to affect anything. The address line switches all seem to work, but LEDs started blinking when A13 was switched on.

Altair 680 front panel with the first 8 address line switches turned on

Altair 680 front panel with the all 16 address line switches turned on — Altair 680 front panel with the first 8 address line switches turned on

Looks like this unit might need some work to restore it to a functioning state. I’ll also need to do some more research to learn about how the 680 works.

Back into the closet it goes for now.

June 4, 2021June 4, 2021

Motorola MC6809 trainer

One of the items donated to the radio club by an SK estate last year (now I’ve forgotten which one) was this microprocessor training laboratory (MTL-1) from Cleveland Institute of Electronics (CIE).

Cleveland Institute of Electronics Microprocessor Training Laboratory

Presumably, if you had an interest in learning computer hardware and programming, you’d enroll in a class and get one of these along with class materials. I’ve seen similar trainers in the past, but never had the opportunity to use them. The breadboard area lets you wire up circuits that let you do things with the 6809.

This one is based on a Motorola MC6809, a contemporary of the 6502 and Z80 microprocessors. After watching the one YouTube video I’ve found along with a bit of random button pressing, I’ve managed to figure out how to to go to different addresses and enter assembly code. I haven’t entered anything that runs though. I’ll have to find some time to learn 6809 assembly first.

Aside from the copyright date on the back, there’s no other indication of when this particular unit dates to. Fun fact: 1984 was my first year of high school,

Surprisingly, I’ve found very little documentation about this particular trainer online. I’ve found one video on YouTube where someone used it to demonstrate testing a RAM chip, and a few photos of similar units on auction sites, but nothing in the way of manuals or even course materials that might have used the trainer.

Labelled regions of the 6809 MTL-1 trainer

This seems like it would be a fun thing to play with, especially for anyone with a retro-computing fetish. The hunt for documentation continues. I’ve already found a few PDFs about programming the 6809, so maybe I’ll be able to figure out enough do fun things with this.

June 2, 2021June 3, 2021

Blinking LEDs with a Z80

In the parts bins that I acquired long ago, four of the many ICs in one of the bins were Z80 microprocessors. At the time, I thought it would be neat to try to do something with them, but had no time and no levels in low level hardware design.

Fast forward to post-PhD time and inspired by Ben Eater‘s 8-bit CPU and 6502 projects and videos, I learned enough about microprocessors and what connections to make with them to try doing something with the Z80. YouTube also presented me with a variety of Z80 related videos to watch. This one by Julian Ilett was one I found particularly informative.

Flashing LEDs – Z80 Style by Julian Ilett

The date codes on the Z80 chips indicates they’re 1987-1988 era chips, and not knowing how they were stored before I got them, I had no idea if any of them even worked. Grabbed one of the Z80 chips, plunked it into a breadboard and started wiring it up.

Spending an afternoon playing with a Z80 on a breadboard pic.twitter.com/bIXBYBeBOW
— imabug (@imabug) May 30, 2021

Off in the corner of the breadboard is a 555 set up as an astable oscillator and functions as the clock for the Z80. The clock goes runs between 6 – 475 Hz depending on the potentiometer setting.

After getting things wired up and applying power, some LEDs would light up, but then all the address bus LEDs would blink on and off but not in the expected binary counting pattern. Seemed like I had everything wired up correctly.

After studying Julian’s video and wiring some more, I tried triggering the reset pin and boom! LEDs blinked off, then came back on and the address bus LEDs started the expected binary counting pattern! Yay, it worked! Added a button for the reset pin to make resetting the Z80 easier.

Blinking the address bus lights on the #Z80! pic.twitter.com/59H7QVcE8S
— imabug (@imabug) June 1, 2021

Getting this going was fun, and a good learning experience. Gained a few XPs.

What next? Well, I could continue on and try to add enough to make a functional computer out of the Z80 and following the techniques in Ben’s 6502 video series. I’ll definitely need some more breadboards, and a few more components. I’ve got a few RAM chips I could try, but no idea of any of them actually work.

It’s kind of cool just turning it on and watching the LEDs blink though.

Making a breadboard based Z80 computer would be a fun project.