P2P LA2A build - Meter Fluttering

Help Support GroupDIY:

ron_swanson

Well-known member
Joined
Nov 14, 2018
Messages
138
Location
San Francisco, USA
I've completed a P2P LA2A build following an original vintage schematic that seems to be fully functioning with the exception of the meter display when in GR mode.  It will intermittently flutter up off zero (back and forth and in between ) about 1db (max) and then return to a steady zero db for sometimes quite a bit of time only to later to start fluttering again for a spell.  Fluttering of the GR meter display also occurs when compression is applied, but less so depending on how much gain reduction is dialed  in.

I've successfully built a number of solid state and tube studio devices, but wanted to dive in deeper with this one hoping to close the gap between my theory and my practical application and troubleshooting bench skills which I'm admittedly struggling with.  I worked through a couple of small issues when first powered up, but the flutter meter has me stumped.    Hoping someone here can point to where I should focus as I have run out of ideas short of swapping out components willy-nilly which I don't want to do.

When in either in -10 / +4 output modes, the meter is steady as a rock.  The unit calibrates and compresses as expected and all measurements are reasonably within tolerance from what I undertand tolerances can be within.  I am using a NE-2 which I undertand some say it's better to use a zener instead, but hoping to keep things as near to vintage as possible - if possible.  The voltage at the NE-2 fluctuates between ~64-68VDC when the meter is fluttering.  When the meter is in a steady state, the measurement at NE-2 is a steady ~64VDC.  The meter fluctuates between ~7-8VDC when the meter + is measured directly when fluttering in GR mode.  Something closer to 7VDC when in a steady state. The voltage on the incoming side of R28 is about 255VDC with only a slight fluttering of <0.5VDC.

When passing a 1Khz tone, the audio sounds clean and healthy with no nosies, hums or other artifacts.  Oscilloscope sine wave is clean and steady.  No fluctuation in the sine wave on the oscilloscope when the meter is fluttering as well as no audible volume changes.  Steady tone no mater what mode the unit is in.  I've read that the side chain section can oscillate in builds, but I'm not clear what that means or even if that would show up on the meter as I'm experiencing?  Audibly oscillating or electronically oscillating or both? 

Any input anyone cares to share would be greatly appreciated.  Thanks in advance!
 

gridcurrent

Well-known member
Joined
Aug 28, 2009
Messages
590
the NE-2 is shown bypassed with a 470 pF in drawing C-10953 C  dated 1968.
with the capacitor, the neon can become an oscillator.
early units including those with octal accessory socket have no cap.

changing to a zener as found in the re-issues is highly recommended,
even though the neon is present but not part of the meter circuit.

 

ron_swanson

Well-known member
Joined
Nov 14, 2018
Messages
138
Location
San Francisco, USA
Well then, I'll try replacing the NE-2 and maybe test a zener to see what happens while I have things torn apart.  The NE-2 was in my thoughts, but again trying to just not replace parts without an understanding.  Although, the fluttering seems that it might have a high chance of the NE-2 being the culprit.  Again, just a guess at this point.

I've read that the modern UAD LA2A's have both an NE-2 and a zener installed in parallel, but I can't find a schematic showing that as being a fact.  Either way, even if that is a path, wouldn't that make the NE-2 just a  hidden show piece?

Thanks for all of the responses!
 

gridcurrent

Well-known member
Joined
Aug 28, 2009
Messages
590
ron_swanson said:
I've read that the modern UAD LA2A's have both an NE-2 and a zener installed in parallel, but I can't find a schematic showing that as being a fact.  Either way, even if that is a path, wouldn't that make the NE-2 just a  hidden show piece?
in the UA  re-issue, the neon is powered but not in parallel with the zener.
seems the neon was a  definitely a  "show piece"
it is not in the meter circuit but did examine a re-issue with oscillation due to the powered bulb.
in the new UA's  that I service,
the resistor feeding the neon is removed but the bulb stays, to placate those that might take a peek inside.
 

Latest posts

Top