Author Topic: Old vs New

September 02, 2023, 12:19:48 AM
Read 235 times

craig.donlon@esa.int

  • Newbie
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
Hi:
I've just completed a second 503J unit which works as it should and if it were not for the fact I have a unit built from last year, I would maybe know no better.  Activating the DIST switch bypasses to ground R12 (4.7M).  But, when in tube mode, there is a difference between the Old and the New unit characteristics.

With the DIST switch activated, the new build sucks out more of the high frequencies - muffling the sound compared to the old version (otherwise they are identical in sound). The schematic is the same for both units but I noticed that C6 is a 100V cap in the new build compared to a 63V cap in the old but that shouldn't matter. I checked the DIST switch grounding etc and its all good and the unit is working as it should other than the DIST difference. It was a bit of a challenge to set the 50K TR1 pot with the DIST activated on the new dull sounding build. I swapped over the tubes between units but there is no difference in performance. Voltages on the tube stages are all more or less identical.

What would cause this and how can I get the new dull sounding unit to behave as the old one? Is it tolerances on C6?

Regards
Craig
« Last Edit: September 02, 2023, 12:24:32 AM by craig.donlon@esa.int »

September 02, 2023, 10:27:22 AM
Reply #1

JPK

  • Administrator
  • Hero Member
  • *****
  • Posts: 855
  • Karma: +26/-0
    • View Profile
Do the units sound the same when DIST is off (up)?
If so, the issue is somewhere in TR1, C9, C10, C11, R17, R18, R19, L1.
Check the value and solder joints on these components.
JPK

September 02, 2023, 03:10:32 PM
Reply #2

craig.donlon@esa.int

  • Newbie
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
Hi JP

thanks or the help. C2 and C10 were reversed - everything is fine now

Regards
Craig

September 02, 2023, 03:14:39 PM
Reply #3

craig.donlon@esa.int

  • Newbie
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
By the way - the 503J is an amazing bit of kit that works so well on may tasks - particularly on guitar effects pedals. I am using both with a patchbay setup with stereo pedals.  With the trim pot set correctly, both units are identical in all modes.

All the best
craig

September 06, 2023, 10:08:14 AM
Reply #4

JPK

  • Administrator
  • Hero Member
  • *****
  • Posts: 855
  • Karma: +26/-0
    • View Profile
JPK