Solved! Help troubleshooting dbx 266A, high current @ VCA input/ no audio output

[Blue Jinn]

Hi,

I  got one of these cheap to mess around with and FTHOI (for the heck of it) swapped out the VCAs for THAT2180. (Which required removing some parts.)

Left channel doesn't work. I first checked AC voltages from the 5532 (with a Fluke don't have a working scope at the moment) and there was no output on pin 7.  So I though it was just the 5532 that was no good, it ran slightly hot, so I replaced it. Then I discovered for some reason it wasn't getting +15 , (yeah, I know) I couldn't figure out why, so did a solder bridge to a nearby +15v source. Still no output at pin 7. The first stage is working as there is an appropriate AC out at pin 1.

Here's where my knowledge starts to taper off. The VCA has the same DC voltages on pins 5 & 7 of the working channel.  The current at pins 1 & 8 though is:

Working channel VCA:
pin 1  .0118mA
pin 8 .0125mA

Bad side VCA
pin 1 .0370mA
pin 8 0mA

a snippet of the schematic is attached. I swapped the VCAs, both work in the good channel. Testing with a -10dB sine wave.

 

[scott2000]

I swapped the VCAs, both work in the good channel. Testing with a -10dB sine wave.

I've had luck comparing  channels to find out where things are "different"...

 

[Blue Jinn]

Well, actually I should have done more troubleshooting ahead of time with the VCAs pulled out. I must have lifted a trace somewhere, or the channel one is differently laid out, as I had no connection between pins 8 of the VCA and pin 6 of the opamp. I had to make a simple bridge.

Now I gotta deal with the subtle hum, which kinda defeats the purpose.

 

[pvision]

It's a source of continuing amazement to me that dbx could design the 160X then deliver a steaming pile of ordure like the 266. So it's perfect for hacking about

It doesn't react at all well to large amounts of compression or higher compression ratios

Please report back if you get decent results from your mods

Nick Froome