On the workbench for some troubleshooting is a 1950s era vintage RCA Victor 45-EY-3 record player. It belongs to someone in my neighbourhood and came to me via a referral from a neighbour who’s familiar with my penchant for tinkering with electronics.
RCA Victor 45-EY-3 record player from 1950RCA Victor 45-EY-3 record player from 1950RCA Victor 45-EY-3 record player from 1950
The owner purchased this recently and had already replaced the tubes, capacitors, a few resistors and some of the mechanical bits before the record player landed on my bench. He said it was sort of working (some mechanical issues with the arm moving), but then stopped turning on. Fortunately, he had a printouts of the service documentation available to look over. On the electronics side, the circuitry is pretty simple consisting of three tubes: rectifier (35W4), amplifier (12AV6), and output (50C5).
RCA Victor 45-EY-3 schematic
After touching up a few solder joints, I found the power switch was kind of dodgy and would work if I tilted it a certain way. I also realigned the muting switch (S2) so that it was oriented the same way as one of the photos in the service manual. That got me to the point where records could play and sound came out of the speaker instead of just resonating through the needle arm. The sound volume was pretty low though, even with the volume pot turned up all the way and there’s also a lot of hum getting into the electronics too. Those are the two main things I need to work on, and the owner will work on the mechanical stuff.
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
Undoing four screws at the back of the unit releases the back (surprise!) and top cover to reveal the inside.
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 fanAltair 680 rear panel. Not a whole lot of clearance between the transformers on the rear panel and the main board.
Removing the expansion board (I’ll get to that in a bit) reveals the main board.
Altair 680 main boardMotorola MC6800EPROM and some SRAM chipsMotorla MC6850
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.
680 Main board 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 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.
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 onAltair 680 front panel with the all 16 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.
