ARP 2600 Voltage Processor The Final Chapter: ARP Electronic Switch: the Workalike Works

 Readers: If you want to build the module featured in this post, please go to PCBWAY's Community pages--a gerber ready to download and fabricate is here. 

Also please visit PCBWAY's site using the link here--it will help this blog. Thanks.

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Hello again! Finishing up ARP 2600 voltage processor clones and workalike modules, this might be the last of the series, which is not a bad thing?  

The ARP2600's unique voltage processor has a click- and pop-free clock-driven analog electronic switch for AC and DC signals. 

You can find the original design on page 29 of the ARP 2600 service manual here.

The 70's era subcircuit used discrete components for its toggle flip-flop and employed LS4392 FETS for heavy lifting.

Best of all: a working Electronic switch completes my ARP2600 voltage processor series. Are we home yet?

Finished ARP2600 Electronic Switch workalike

I decided to use a DG401 IC instead of the FETS--post where I initially experimented with that IC is here with more here.

In short: The DG401 works great--think of a rail to rail version of the venerable 4051 analog CMOS switch; both are useful chips for a lot of what we do.

I laid out PCB's for the workalike and sent them to the blog's patient sponsor, PCBWAY, to create a 5HP Euro prototype.

Let's see if what I designed works....

Happiness is new PCB's from PCBWAY....

The build uses about 2/3rd's SMD components, many of which I don't have in the junk box. I bought the parts from Digikey and posted the BOM on the PCBWAY community page (here).

Unpopulated main board--I am getting more comfortable with1206 SMD components--large enough that I don't need a microscope for a lot of the work. 

Front panel....

Testing....

Did it "work first time"?  

Kinda. 

The switch switched somewhat randomly unless I used a square or pulse wave. During the breadboard stage, triangles and some ramps switched same as a square wave, but in this layout, not as much.

Not sure why....on the bench (breadboarded) here's what I used to buffer the incoming clock signal:

But for this post's Euro module triangle and ramp incoming signals created switching "warbles". 

Not intended, but not a bad thing?  I could have spent days chasing this down--for instance, setting up the breadboard and expermenter's board for the DG401 and double checking my work--redesigned the buffer using an MCU to turn whatever input to a pulse wave--using an op amp with a diode in its negative feedback path--something has to work, right? 

But, naaaaaah. 

Too many other things to do.  

Instead I replaced the .1µF cap with a 1K resistor. That's surprising quick and easy with 1206 SMD parts: heat up both sides of the .1µF capacitor with a soldering iron in quick succession; use tweezers to remove the cap, then solder in a 1206 1K SMD resistor.  

Done.

With that modification square and pulse waves work great to make the DG401 switch.  

Good enough....the workalike switches cleanly, no delays, no clicks, no pops, same to my ears as the ARP2600.   

And! The "any waveform to clock" apparently isn't implemented on the original gazillion dollar collectable 2600 my friend owns....so I could argue that the design flaw makes this workalike more akin to the original.

Uh huh. Righto. Not a feature--a bug. 

Time to move on!  

Next time--no idea. How about this: I'm ditching Eagle for Kicad since Autodesk is raising my annual subscription price 700 percent. So I may do a few less posts in the next few weeks while I learn this new program. Go A's!