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Chameleon 7720 - how to fix auto-release and AC hum
Old 27th January 2020
  #1
Gear Addict
 
Fenris's Avatar
 

Chameleon 7720 - how to fix auto-release and AC hum

I thought I'd post this as a public service to anyone who owns the original Chameleon 7720. I don't like the prices that certain outfits charge to mod them, or the practice of "upgrading" op-amps that don't need to be upgraded. The design flaws with this unit (AC hum, broken auto-release) can easily be fixed by anyone with a soldering iron.

The auto-release never worked correctly, because they copied an old Gyraf SSL schematic that had an error. More info here.

https://groupdiy.com/index.php?topic=63477.0

C1 and C2 are transposed. All you have to do is carefully unsolder the capacitors and switch them, being careful to maintain the correct polarity. The .47 cap should be in parallel with the 91k resistor, and the 6u8 cap should be in parallel with the 750k resistor. I don't recall the component numbers in the Chameleon, but they're easy to find. They're on the same board as the Release control. You have to unscrew all the knobs so you can pull out the board and access the component side.

The other design flaw is audible 60 Hz hum when using the included power supply. The Chameleon CPS-1 is overpriced and not very reliable. I bought a used Power-One HCC24, which outputs the necessary +/- 24v and is very quiet. This model comes in various amp ratings. The 7720 draws around 80 ma. The pinout for the 7720's DC In jack is pin 1 -24v, pin 2 +24v, pin 3 +48v (not used), and pin 4 ground.
Old 27th January 2020
  #2
Gear Nut
 

Thanks!
Old 29th January 2020
  #3
Gear Nut
 

The auto release works great after the cap swap. They weren't marked c1 and c2 though. .47 and 6u8 were good clues as those values were marked on the board. the two lower tantalum caps. in the release circuit are the ones. Thanks again for the tip!
Old 16th February 2020
  #4
Gear Head
 

Quote:
Originally Posted by Fenris View Post
I thought I'd post this as a public service to anyone who owns the original Chameleon 7720. I don't like the prices that certain outfits charge to mod them, or the practice of "upgrading" op-amps that don't need to be upgraded. The design flaws with this unit (AC hum, broken auto-release) can easily be fixed by anyone with a soldering iron.

The auto-release never worked correctly, because they copied an old Gyraf SSL schematic that had an error. More info here.

https://groupdiy.com/index.php?topic=63477.0

C1 and C2 are transposed. All you have to do is carefully unsolder the capacitors and switch them, being careful to maintain the correct polarity. The .47 cap should be in parallel with the 91k resistor, and the 6u8 cap should be in parallel with the 750k resistor. I don't recall the component numbers in the Chameleon, but they're easy to find. They're on the same board as the Release control. You have to unscrew all the knobs so you can pull out the board and access the component side.

The other design flaw is audible 60 Hz hum when using the included power supply. The Chameleon CPS-1 is overpriced and not very reliable. I bought a used Power-One HCC24, which outputs the necessary +/- 24v and is very quiet. This model comes in various amp ratings. The 7720 draws around 80 ma. The pinout for the 7720's DC In jack is pin 1 -24v, pin 2 +24v, pin 3 +48v (not used), and pin 4 ground.
Are you saying the C1 and C2 capacitors should be swapped by each other, or swapped out completely? So the capacitor in the C1 position, needs to be desoldered and soldered into the C2 position, vice verse?
Old 17th February 2020
  #5
Lives for gear
 
gyraf's Avatar
 

Look at the PCB layout, that one was always known working: 750K in parallel with the 6u8 to ground, 91K in parallel with u47 on top of that.

Not rocket science to figure out, and because the PCB is right it was never a problem irl, thus we never bothered to edit the schematics - way of working is clear enough..

For reference look at e.g. the SSL82E27 module: http://gyraf.dk/gy_pd/ssl/ssl_82e27.gif

/Jakob E.
(who made the mistake)
Old 20th February 2020
  #6
Gear Head
 

Quote:
Originally Posted by gyraf View Post
Look at the PCB layout, that one was always known working: 750K in parallel with the 6u8 to ground, 91K in parallel with u47 on top of that.

Not rocket science to figure out, and because the PCB is right it was never a problem irl, thus we never bothered to edit the schematics - way of working is clear enough..

For reference look at e.g. the SSL82E27 module: http://gyraf.dk/gy_pd/ssl/ssl_82e27.gif

/Jakob E.
(who made the mistake)
Interesting that Chameleon Labs put the caps backwards...
Old 4 weeks ago
  #7
Gear Head
 

Can someone help me out? Whats the voltage rating on the capacitors? Would these work https://www.amazon.com/20pcs-Kemet-6...strial&sr=1-20
Old 4 weeks ago
  #8
Lives for gear
 
gyraf's Avatar
 

25V is plenty, I used 16V types.

You probably don't want 20 of them..

Why not just swap the ones already in there..?

/Jakob E.
Old 4 weeks ago
  #9
Gear Head
 

Quote:
Originally Posted by gyraf View Post
25V is plenty, I used 16V types.

You probably don't want 20 of them..

Why not just swap the ones already in there..?

/Jakob E.
Because I was overly aggressive desoldering and slightly tore the leg from the plastic casing on the cap...just to be safe id like to order one and replace it.

Right now it sounds ok, but if the cap was damaged, would I be able to hear anything bad?
Old 4 weeks ago
  #10
Lives for gear
 
gyraf's Avatar
 

..if the 6u8 tantalum of the auto-release was damaged so that it lost connection to a pin, you'd loose the "long" part of the auto time constant and make the fast part very weird:

The autorelease works by having a fast'ish release when only acting sparsely upon dynamic parts of the signal (the 91K in parallel with 0.47uF) - and through this slowly charging a longer release time (750K in parallel with 6u8) when threshold has been exceeded for longer amounts of time.

/Jakob E.
Old 4 weeks ago
  #11
Gear Head
 

I can't tell if that's whats happening due to the meter jumping so weirdly with auto release function on.

Shot a quick video, thanks for your help Jakob.

https://drive.google.com/open?id=1tO...OOM2V4jVJkK63L
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