1. I was using a 20k R29 resistor (schematic is 10k) to drop some voltage going to the 12AX7's. To maintain the ~335v I had there, I had to use a 15k R29. The decoupling resistor to the FX Loop is 22k which gives me ~305 volts. The desired range is 285v - 310v so I'm right in there.
Note that if you're installing the zero loss fx loop in a stock post-1986 2204 circuit, you will see ~385v at R29, and have to adjust the loop decoupling resistor accordingly (somewhere around 56k I reckon)
2. The R14 jumper wire is the connection between the master volume and the treble pot wiper. This is where I tapped into the PCB. It makes for a very clean installation.
3. I grounded the in/out wire shield and the FX Loop board at the filter cap, which is where a lead from the PCB is also grounded
4. Drilling the holes. Measure twice, drill once. Take your time and make sure you follow the Metro directions. I clamped a piece of wood in the chassis as recommended which makes for a hole with less burrs. I drilled the "big holes" with a Uni-Bit, then chamfered them by hand. I put the switch opposite of where Metro shows so it wouldn't be through the printing on the chassis. It seems "right" for a hang-down combo chassis
5. FX Loop Level. I used a signal generator - load box - DMM set to A/C. I match the "Loop On" level to the "Loop Off" level. I set the loop level trim pot by measuring the A/C signal going into the load box.
Click on the thumbnails for bigger pics...
FX loop PCB. I doubled up the orange fiber washers for a better fit against the chassis. Note the switch is upside down compared to the instructions. This is because the combo chassis hangs down from the top of the cabinet
Here's the Loop's in/out wires replacing the R14 jumper,
The shield (white wire) and FX PCB ground (black wire with printing) connection to the pre-amp filter cap and PCB ground
Loop decoupling/dropping resistor (22k) connected to R29 (15k in my amp, ~340v)
Jacks & switch,
Chassis, I ran the FX loop power wire along the edge of the PCB and fastened it with zip ties,
I hope this helps
