ARP 2600 Voltage Processors--Part One--Eurorack Lag Processor

DiWHYers: If you want to build this eurorack module, you can get schematics, gerbers, a BOM, board/schematic layout, and more, from these guys; their community (here), and this project page. 

------------

Hello again, I went to a geeky synth meetup recently--we went through the ARP 2600 patch book. I wasn't expecting much--I already know all this right? Wrong! The patch book is a wealth of great ideas and covers a lot more than patching a vintage ARP 2600 or 2600 clone. For instance, I didn't know you can create a high pass filter by inverting a lowpass filter output signal and summing it with the original? That's in the book...

We ran out of time...I wanted to keep going through the book, but don't have room for a clone or a vintage ARP 2600 (even if I had USD $10K+ to spend....)

I have 2600-like VCOs, VCFs, VCAs, Reverbs and so on, already in my rack however. 

This time: an ARP2600 lag processor clone

But! I don't have the 2600's voltage processors....these subcircuits are an important part of the 2600's cool identity:

Original ARP 2600 Voltage Processor

With that long ado: in this and coming posts, I will create clones or work-alikes for the 2600's voltage processors. This brings some 2600 coolness to my rack without spending a box of money.

This time: the lag processor.

Dual ARP Lag Processor Clone--done, works! 

The original ARP schematic can be found here, go to page 29; subcircuit is in bottom left corner.  

It's a familiar design, "lag" here means "glide". See my portamento post here for more information about how the design works. 

However, two components ARP used in the 2600's lag processor were unexpected--a .1uF cap and an A1M pot. This means very short portamento times even when the A1M lag pot is "turned to 11". 

But on the clone PCB as well as on an original 2600, the "lag" processor sounds good to my ears, an "Oh Lucky Man" outro vibe.

Lag boards! Shout out to my sponsor, PCBWAY, for helping out.....

Time out--I need to do my usual shameless plug for PCBWAY, this blog's sponsor. They never have dictated what I post and are extemely friendly; I owe them my gratitude. Please visit their site and community page--doing this helps me keep creating these projects and posts. 

Back to it....

ARP 2600 LAG PROCESSOR DESIGN

The module clones the original ARP design, with a few minor changes discussed shortly.

If you build this module, you will need 2 PCB's: a front panel and "main board" in 4HP Euro skiff config. Consider Fabricating the front panel in black with white silkscreen; make sure to check the box "do not use an order number on the PCB".

I used a single TL072, the 2600 uses LM301's; I figure any dual DIP op amp will work. 

My clone also uses a 1K resistor in series with the output jack. This is not on the original design; I put it in there to keep a short to ground (a dangling patch cable that shorts against a metal enclousure) from drawing too much current and possibly damaging an op amp or other components.  

This means the voltage at output might diminish a bit as the lag value increases. This is caused by forming a voltage divider between this 1K resistor and the load resistance of whatever you plug the lag processor into--this may not be what you want. 

If not, put some jumper wire in instead of R3 and R10 and try to be neat when patching or (aguably) use 100ohms.  

Using this protection resistor (or not) is more complex a topic than I initially thought. A forum post about this resistor, to use or not to use, what value should it be, etc., is here.

BUILDING THE MODULE

No surprises here, one mistake is on the front panel, LEV2 is the top pot, not the bottom one (I kept changing this and ended up getting it wrong). This is fixed on the gerbers ready for download at PCBWAY (again, get gerbers here). 

Beyond that the dual lag processor can be built in maybe an hour? Something like that. This is an easy build.

Ready to test....

"seems working...."

MODIFICATIONS AND PATCHING IDEAS:

I have to remind myself to use this lag in unexpected places. The patch book has some ideas. 

For longer lag times increase the value of C3 and/or C5, and/or change the value of the "Lev2" and "Lev1" pots. I left mine at A1M pot and .1uF to match values found in the ARP service manual.

There are two lag processors in this 4HP design, the ARP2600 has only one in its voltage processor section. It would be easy to modify the circuit to normal out one to out 2, putting the two in series, giving you longer portamento times. Easier: use a short patch cable. Viola!

 

OUTTRO

Next time I will clone the Mixer/Inverter section of the voltage processor--an interesting single op amp design that is unique to the 2600 and its clones as far as I know. Until then, don't breathe the fumes....

Restoring a Tek 453 Scope

Before we begin: Please visit my sponsor's site, PCBWAY; it makes them happy when their analytics tell them I have readers. Click here to visit their homepage and here to find ready-to-fab gerbers for circuits featured in this blog. Thanks!

============

The story begins in  maybe 1980, my brother found a discarded 50Mhz Tek 453 scope in his employer's garbage dumpster and took it home. It worked!  He used it for a few years but eventually bought something faster, giving me the Tektronix as a hand-me-down. 

I used it until about 2003.

Restored, working scope.  Beautiful in a 1960's industrial sort of way.

Fast forward many years: I use smaller, faster Siglent scopes; the Tek has been gathering dust for over 2 decades.  

Wasteful--I should make sure it works, then, get it to someone who will use it, right? 

It works here, but at first? nope

POWERING IT UP

Nothing! No fans, no sound, no smoke, no traces visible on the CRT: completely dead. 

Fortunately the schematic and user manual are online (here and here). The 453 is solid state. Also i read that the 453 was designed using no unobtainium ICs or components. 

So--assuming the CRT can be replaced, if it's dead--no matter the issue, this scope can be repaired. 

EASY FIX??

I removed its cover to check the power supply. Sure enough, the power supply was dead (nothing, anywhere); mains fuse was good. Checking mains coming into the power supply: nothing, but it didn't appear to have a short. 

Could the power cable be bad?

**Not** the original mains plug

I opened up the Nema 5 connector and sure enough the positive mains lead was disconnected, so the scope was not getting mains power. Doh!

Easily fixed--I screwed the mains lead back to its screw terminal inside the connector and fastened everything back together.

After that it powered on. Amusingly: after 20+ years as a giant paperweight, the scope's basics worked. The CRT was tired--I had to turn "intensity" to about 70% or more to see traces,  but both channels, one shot triggering, X-Y mode, and the trigger out BNC worked great. 

3D PRINTING SWITCH CAPS

On to cosmetics. I removed 20 years of dust and dirt on the scope and its front panel with a handi-wipe. Easy.

The front panel switches had all but one switch cap missing. I figured modeling the only remaining switch cap with Fusion 360 then 3D printing a dozen or so replacements was in order.  

I hadn't used my 3D printer in a long time and forgot pretty much all of it.  

That's why I have this blog, a trail of crumbs right? 

Original 3D how-to post for my bench is here. 

I rewatched the Fusion 360 for beginners video here.  

The Voxel printer uses its own slicing software--Flashprint MP.  As far as I can tell, other popular slicers like Cura still don't work with the Voxel IIIP, you have to use Voxel's slicer. I had forgotten that too.

I wanted to make the replacement caps to look as much like the original as possible.....I couldn't find a switch cap I liked on thingiverse. Time to roll my own.

The original switch cap was carefully measured and drawn out:

Fusion 360 bench notes:

• Save your design before you start doing anything to it.
• Fusion 360 saves everything in the Autodesk cloud--you knew that right? I didn't.  If you want to save the file locally you have to export it.  
• make your origin planes visible--otherwise you can't tell up from down, really.
• You create 2D drawings in Fusion 360, using a"sketch"  You right click on a plane and then choose "new sketch".
• Hitting "S" in sketch mode brings up a very useful dialog: start typing center and find the center rectangle tool, for instance. Now you can draw centered around the plane's origin. Much easier than trying to find things via the menu and icons--beside, Autodesk keeps changing the Fusion 360 UI, so videos showing you where things are are often out of date/inaccurate and thus frustratingly difficult to follow.     
• Hitting "Q" allows you to extrude, turning 2D into 3D.  
• I could extrude holes with offsets from a flat sketch, but not from a curved one. In the case of this part, I had to cut a 50 mil x 135 mil rectangle so the cap could be pressed on the switch.  I did this from the bottom plane with 100 mil offset, extruding the cut to about 200 mils above the top curved surface. The same cut couldn't be done from the curved top down, but this bottom-up approach worked.
• You can scale the entire part--useful for fine tuning its fit. Lasso the entire part with your mouse, click on "base" in the nav tool to the left, then click "Modify" along the top.  In the dialog that pops up, 1.15 means 115% larger than what you have now. The same can be done in the Flashprint-MP software for my 3D printer, which turns the stl file into a gx file to send to the 3D printer. I found the scaling freatures in Flashprint-MP easier to use vs. Fusion 360's scaling feature. Why did I need to scale the part? The caps needed about a 105% bigger cutout to snugly fit on the switches in spite of careful measurements done previously. I figured the extra 5 percent was needed due to slop in the printing process--not sure, but pretty easily fixed--I expanded the entire STL 1.05 before printing; after that the caps fit perfectly. This may or may not be needed for your printer setup.
• Don't use inches or mils when creating 3D models--ever. Use millimeters instead. Flashprint-MP slicing software only works in millimeters, and sending it stl's crafted in inches drives it nuts.  Fortunately when exporting the stl file from Fusion 360 I could choose millimeter scale, even through 3D model was created in mils; now Fusion-MP was happy with the source file.

Let's print!

I was afraid the base of the part was too small to stand upright while being printed but the Flashprint software was smart enough to create a flat base around the part. It printed perfectly:

After printing I tried different means of removing this flash. 

What ended up working best was to cut the base off with household scissors, trim further with an xacto knife, and finish up with a 300 grit filing stick. If I spent time carefully cutting and filing the switch cap came out almost perfectly--very hard to tell from the original. 

But...I didn't end up spending too much time filing to perfection--no point as I saw it....Instead, I got the switch caps to where they were good enough. Time to move on!

PAINTING 3D PARTS

This was something I had never done. Would the paint stick?  Would it turn the plastic into goo? Not sure. 

But--I used ABS (or was it PLA?) filment and read online that painting either type of plastic was easy. 

I used Rustoleum "2x Ultra Cover" black and grey primer--I found this recommendation on reddit--two quick coats and the switch caps looked great.  

Pressed the caps on the scope--done.  

if anyone else needs the stl or gx switch cap files, get them here.

NEW HOME FOR AN OLD SCOPE?

For the antique scope--will anyone want it?  Not sure, but it was fun and easy getting the old scope ready for use again.  

Is antique electronics restoration is more fun then AudioDiWhy? Perhaps. In another universe.  

Ah! Finished.

Until then/there, Tek out, captain.