SA-3A (LA-3A Clone) Support Thread

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andow

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On the Serpent site it says:

"SA-3A PCB’S WILL BE BACK IN STOCK ON MARCH 24th, 2017."

I was thinking about ordering but this makes me a feel unsure...  ???
 

theposterkid

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andow said:
On the Serpent site it says:

"SA-3A PCB’S WILL BE BACK IN STOCK ON MARCH 24th, 2017."

I was thinking about ordering but this makes me a feel unsure...  ???

I ordered (and received) three PCBs in the years after that date -- so I wouldn't put much stock in it. Clearly, they just don't realize or care that that statement is out of date.

I ordered another board last week, but have not heard anything from them. I'll check in today.
 

andow

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theposterkid said:
I ordered (and received) three PCBs in the years after that date -- so I wouldn't put much stock in it. Clearly, they just don't realize or care that that statement is out of date.

I ordered another board last week, but have not heard anything from them. I'll check in today.

Thanks for the reply! Would be nice if you could keep us updated if they are shipping atm...
 

JMan

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Have you tried emailing Mike?  If I’m remembering correctly, I had to use a different email address than the one on the website — apparently he switched to a different email host or server or something (I don’t understand I.T. stuff) and didn’t update the site, so that might be making communication difficult.

Indicentally, I think I’m going to start building mine in the next week or so.  My T4bs arrived, and the transformers are on the way, which I think means I’ll finally have everything I need to get started!
 

JMan

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Went ahead and tested my power supply section this evening.  Everything was good, +35 and +28 were pretty much spot on upon fire-up, and I used the trimmers to dial them in the last few hundredths.  :D

Here's a question: What should I expect to read at V+ test point?  I was getting roughly +45v, but I combed through the schematic and I can't find a reference to an actual number.  I'm struggling to understand how this rail is functioning in the circuit.
 

JMan

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Took a little time yesterday and got all my resistors placed.  I’m glad I’m finally starting this build, even if I’m moving very slowly.

Anyone care to enlighten me about the V+ rail?  (See my question in the previous post)
 

theposterkid

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JMan said:
Anyone care to enlighten me about the V+ rail?  (See my question in the previous post)

There's almost certainly someone else more knowledgeable than me to answer this, but I'll try. On the schematic, you have the toroid transformer, then the power switch, which goes to a bridge rectifier. The V+ is immediately after the bridge rectifier, but before the voltage regulators which supply the +35V and +28V rails accordingly.
 

JMan

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theposterkid said:
There's almost certainly someone else more knowledgeable than me to answer this, but I'll try. On the schematic, you have the toroid transformer, then the power switch, which goes to a bridge rectifier. The V+ is immediately after the bridge rectifier, but before the voltage regulators which supply the +35V and +28V rails accordingly.

Thanks for the reply!  I do see it on the schematic, after C10 on the way to the +28v.  What I'm trying to figure out is what it's doing, and what voltage I should expect there, since the schematic gives no explicit indication.

I think that since my +35/+28/0v are good, the approx. +45v I saw at V+ is probably fine too.  I could try to work it out using the schematic, but honestly, my basic EE math skills are rather subpar.

But like...I am just failing to understand what this is doing, and why we have a separate rail for it being sent over to the second board.  I'm sure the answer is obvious, I am just missing it, and I'm curious.

In the meantime, I finished placing my caps this evening.  Moving along bit by bit, looking forward to when she's done!
 

JMan

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Well, I’ve been slowly continuing with stuffing the boards.  As of last night, everything is placed and soldered except the the interstage and output xfmrs.  I found that a few of the pads on my boards were reeeeally stubborn about taking solder, so I’m hoping all of my joints are good (they look fine, but some of them were just a PITA to get to that point).

I’m still waiting on a back panel from Frank at frontpanels.de which is stuck in pandemic shipping hell at the moment.  In the meantime, I’m going to drill a temporary front panel (I’ll get a nice custom one made once I know everything is working as it should) and start wiring up the controls.
 

JMan

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I spent some time last night wiring up the controls. 

Question: for the 600r/load mod, should I use shielded wire for the run to the front panel switch?  And if so, where would I connect the shield -- just tie it to the shield/ground point of one of the other wire runs (unconnected on the switch end, of course)?
 

ComodoComplex

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JMan said:
Thanks for the reply!  I do see it on the schematic, after C10 on the way to the +28v.  What I'm trying to figure out is what it's doing, and what voltage I should expect there, since the schematic gives no explicit indication.

I think that since my +35/+28/0v are good, the approx. +45v I saw at V+ is probably fine too.  I could try to work it out using the schematic, but honestly, my basic EE math skills are rather subpar.

But like...I am just failing to understand what this is doing, and why we have a separate rail for it being sent over to the second board.  I'm sure the answer is obvious, I am just missing it, and I'm curious.

In the meantime, I finished placing my caps this evening.  Moving along bit by bit, looking forward to when she's done!

Building this for a client right now so I thought I'd share my thoughts! From a quick skim of the schematic, sending the V+ over to the second board is basically unnecessary as it's not used by the second board. One could make an argument that it's best practice to do so, since sending the 35/28V rails without the V+ could cause issues if the regulators are populated on the second board... not sure why this would happen though. Those devices don't like to have voltage on their output and not on their input (they die).

The V+ rail I would call the unregulated supply - it'll vary with the AC voltage pretty much proportionately, though we usually expect to see 120V +/-10%. It's just there to provide a rough DC for the regulator chips to work with and supply the proper voltages to the rest of the compressor.
 

JMan

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ComodoComplex said:
Building this for a client right now so I thought I'd share my thoughts! From a quick skim of the schematic, sending the V+ over to the second board is basically unnecessary as it's not used by the second board. One could make an argument that it's best practice to do so, since sending the 35/28V rails without the V+ could cause issues if the regulators are populated on the second board... not sure why this would happen though. Those devices don't like to have voltage on their output and not on their input (they die).

The V+ rail I would call the unregulated supply - it'll vary with the AC voltage pretty much proportionately, though we usually expect to see 120V +/-10%. It's just there to provide a rough DC for the regulator chips to work with and supply the proper voltages to the rest of the compressor.

Wow, great explanation!  Thank you!
 

ComodoComplex

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JMan said:
Wow, great explanation!  Thank you!

On further thinking I actually think it may be bad practice to send V+ over and use C10 on the second board. The preferred arrangement would be to use double the capacitance for C10 on the primary board (2200uF or thereabouts). It also may work fine with just the one C10 on the primary board! The ripple rejection of the LM317HV's is pretty high especially when dropping as much voltage as they are in this circuit.

The *bad* part about having a C10 on the second board is that it spreads out the topology, while only serving to double the input capacitance of the filter. That means you're gonna have ripple currents charging up that capacitor through the V+ line on the connectors, which could potentially radiate that 120Hz + harmonics to the rest of the circuitry. Going to see how it works once everything's assembled, but I'd be wary of that.
 

Neeno

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Hey guys, does anyone have a copy of the files for this project ?
The dropbox files aren't online anymore...
I tried to PM ruckus328 but his Inbox is full

Thank you for your help !
 

JMan

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Neeno said:
Hey guys, does anyone have a copy of the files for this project ?
The dropbox files aren't online anymore...
I tried to PM ruckus328 but his Inbox is full

Thank you for your help !

I posted them awhile back.  Look at the previous page.  The only docs for this project are the schematic and the BOM, plus the information contained in this thread.
 

ComodoComplex

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Dunno if this helps, but here's a cart with updated items from the original BOM. You'll still need to buy XLR jacks and knobs for the front panel, but this covers the bulk of the parts. Definitely give it a look over before ordering in case items become out of stock/back-ordered.

https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=7bd5b41cd7

 

ComodoComplex

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Just finished up this project and it's working great! In the end, I found that running the V+ between boards using the power link *does* cause hum issues (120Hz)! Rather than disconnect C10 on the second board completely, I shielded this power wiring and that did the trick, though my shield had to be very tight. Ideally one would use proper shielded cable, but most people--myself included--only have 3-conductor cabling...

I tried uploading pictures to no avail. Might try again another day
 

JMan

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ComodoComplex said:
Just finished up this project and it's working great! In the end, I found that running the V+ between boards using the power link *does* cause hum issues (120Hz)! Rather than disconnect C10 on the second board completely, I shielded this power wiring and that did the trick, though my shield had to be very tight. Ideally one would use proper shielded cable, but most people--myself included--only have 3-conductor cabling...

I tried uploading pictures to no avail. Might try again another day

Interesting!  Would definitely be interested in pictures if you can get it to work (I know it's pretty hit-or-miss for me when I try to upload).  Where did you connect the shield?
 
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