LA2A variable capacitors

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xaxxon

Member
Joined
Aug 15, 2017
Messages
16
Location
Western New York
I'm referring to C4 and C14. I'm trying to figure out how they're supposed to be adjusted.

The owner's manual doesn't refer to them. They seem to be a frequency range limiter circuit, but I have no idea how to adjust them, but have an oscilloscope available.

They're supposed to have a range of 50-380 MF, but the nearest I can find on Digikey is a lesser range. 

If anybody has insight on these problems, this newbie to your site would appreciate your help.
 
The one in the side chain is far too subtle to be much use. Use a 150pF silver mica capacitor here it makes very little difference either way. It was even completely bypassed on the very first units ever manufactured - you could do that as well.

Adjust the other cap in the negative feedback loop to the flattest frequency response possible by testing silver micas in the range of 100 - 380pf (or again, a complete bypass). Use A/D converter into an FFT plugin or FFT software for very accurate results.

Or just stick a 330pf silver mica in there and expect "reasonably flat" frequency response depending slightly on your choice of output transformer.

All this is documented in exhaustive detail around the forums already - if a bit scattered around - but can be found with a bit of searching the few long running LA-2A threads.

With the cheap silver micas around I wouldn't bother with the Arcos of the original units even if they are still on sale somewhere.
 
Kingston said:
The one in the side chain is far too subtle to be much use. Use a 150pF silver mica capacitor here it makes very little difference either way. It was even completely bypassed on the very first units ever manufactured - you could do that as well.

Adjust the other cap in the negative feedback loop to the flattest frequency response possible by testing silver micas in the range of 100 - 380pf (or again, a complete bypass). Use A/D converter into an FFT plugin or FFT software for very accurate results.

Or just stick a 330pf silver mica in there and expect "reasonably flat" frequency response depending slightly on your choice of output transformer.

All this is documented in exhaustive detail around the forums already - if a bit scattered around - but can be found with a bit of searching the few long running LA-2A threads.

With the cheap silver micas around I wouldn't bother with the Arcos of the original units even if they are still on sale somewhere.
Click to expand...
Good information - thank-you Kingston!

Mike
 
I know I'm a *bit* late in saying this, but Kingston's answer to my question should be a "sticky" if this forum has that.

By "sticky", I mean a definitive answer to what is likely a common question. Another forum that I'm on (motorcycle) has "stickies" and they're very useful.

I had done searches on this forum on my question, but some of those threads can run 100+ pages (yikes!). Glad I asked my question and glad Kingston answered!
 

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