FET/RACK Troubleshooting Guide

You’re finally at the end of your FET Compressor build and it’s not working right. That sucks. Have no fear; we’ll help you get it fixed.  This guide is set up to help you isolate the issue.

Power Supply

First things first, let’s check the power supply as described at the end of Step 1 of the build guide. It’s crucial that you test your power supply voltages after you build the supply as described in the guide. Make sure your voltage select switch is set to the correct main voltage for your region. If the voltages were correct after building the supply and are now incorrect after stuffing the rest of the board, that suggests that something is wrong with the rest of the circuit and the power supply is likely to be OK.  If your voltages are now lower after stuffing the rest of the components, you should check the DC voltage guide and see if you can locate a damaged or improperly biased transistor.  Also, examine your PCB for shorts or poor soldering. 

If they are correct (+30VDC and -10VDC), walk through the steps below and note where you find discrepancies.

IMPORTANT!!!!

• Make sure your PCB and chassis are properly grounded and all 3 L-brackets are installed securely. Confirm your chassis is grounded by testing the continuity between the chassis and cathode of CR9. This is the side of the diode with a line, opposite to the side marked with the -10VDC test point. You do not require the unit to be powered on when simply testing continuity.
• Make sure you pay attention when you should be measuring for AC or DC voltages.
• Your Digital Multimeter (DMM) has limited capabilities when measuring AC voltages at 1KHz. Most decent meters will do it well, cheaper units may lose accuracy.  Keep in mind that your DMM won’t show you details about the waveform such as oscillation, but it’s a decent quick way to locate signal level issues.
• Take your time 

DC Voltage Guides 

If you’re able to isolate your issue to a particular section of the circuit, check component values, soldering, and continuity between connected pads. If that looks ok, refer to the transistor voltage chart for that section and see if you can isolate the issue further. Check the DC voltages using the chassis as common. 

Revision A DC Voltage Chart

Revision D DC Voltage Chart 

1. QBias Voltage

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: None

With your DMM measure DC at TP18 using the chassis as your reference (black probe). Rotate the Qbias pot (Rev A/D:R59 and Rev F:R81) and note the DCV at each extreme of the pot. Expected negative DC voltages are noted.

 If your voltages are off significantly, you have an issue with your Qbias and you can stop now and consult the forum. If your bias voltage it good, rotate it to the side that is most negative (furthest from 0 VDC) and move to step 2. 

2.  Amplification Stages 

Set your compressor controls:

Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

Input: Set to read 0.100 VAC at TP1
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR

We're going to start by measuring the signal as it passes through the Signal Preamp and Signal Line Amp. With the GR OFF, the sidechain/compression is removed from the equation. With your DMM measure AC at the following test points using the chassis as your reference (black probe). Expected AC voltages are noted below for a calibrated unit. If you're not passing signal, your unit is not calibrated and you can expect AC voltages after TP1 to read a little higher. Don't worry about small differences, just large differences over 20%.

Signal Preamp

We've had two different TPads and an optional mod.  To remove those different from the equation, let's set a starting voltage at TP1. As noted above, turn your input knob until you see 0.100 VAC (100 mV) at TP1. This should be somewhere between 3 and 9 o'clock on the input scale. Now measure your AC through the Signal Preamp stage.

If your signal is significantly lower at any of these points, that will give you an indication of the general area your signal is having issues. Refer to the schematic to identify that are on the schematic so you can check components and solder joints in that area. Also confirm your GR is set to the OFF position on your attack pot.

Signal Line Amp

Moving along let's test the output stage. The output pot is a large pot with tolerance, so there can be some variance. To account for that, lets set starting voltage after the pot at TP17.  Adjust your output pot so you have 0.200 VAC (200mV) at TP17. The pot should be in the middle area of the scale.

If your signal is significantly lower at any of these points, that will give you an indication of the general area your signal is having issues. Refer to the schematic to identify that are on the schematic so you can check components and solder joints in that area.

Now measure your output. The output is isolated from your ground, so you can’t measure it referencing chassis ground. Measure your output between XLR output + and output -.

If these voltages are generally correct you are good to move to step 3. If you have not yet done so, you can set your Qbias as described in the calibration guide now. If you cannot set it, you likely have an issue with your attack/release daughterboard or FET.

• Low voltage at TP1: likely have issues with the input section (T-Pad/Input Transformer/Q1).
• Low voltage at TP15: have issues with the Signal Preamp section marked on the PCB. 
• Low voltage at TP17: have issues around your output pot.
• Low voltage at Output Transformer wire: have issues in your Signal Line Amp section marked on the PCB. 
• Low voltage at Output XLR + and -: likely have issues with output transformer wiring. Confirm you didn't melt wire coating into the solder joint. 

3. GR Control Amp Input

Units that pass nice clean signal but don’t compress, typically have an issue with the GR Control Amp section. This section is marked on the PCB and also includes the ratio switch PCB. The next 3 steps will walk you through testing the GR Control Amp.

Set your compressor controls:

Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

Input: Set to read 0.100 VAC at TP1
Output: 12 O’Clock
Attack: GR ON (fully CW)
Release: Full CW
Ratio: Check All
Meter: GR

With your DMM measure AC at TP22 for each ratio setting using the chassis as your reference (black probe). Expected AC voltages are noted.

Very low voltage here would indicate an issue with your ratio switch PCB AC section. You should also confirm your GR OFF switch on the attack pot is soldered and working correctly.

4. GR Control Amp

This is the amplification section of the sidechain. Controls should remain the same as the previous step.

If your signal is significantly lower at any of these points, that will give you an indication of the general area your signal is having issues. Refer to the schematic to identify that are on the schematic so you can check components and solder joints in that area. 

5. GR Control Amp Output 

This is the output of the amplification section of the sidechain. Controls should remain the same as the previous step.

With your DMM measure AC at the anode of CR3 for each ratio setting using the chassis as your reference (black probe). The anode is the positive lead and is the lead without the line. Expected AC voltages are noted.

 6. GR Control Amp Threshold Voltage

No signal or controls required. The unit just needs to be powered.

Low voltage here would indicate an issue with your ratio switch DC section. You can identify these components in the schematic labeled with resistors R58 and R61-R64. Also, confirm C21 is soldered and oriented correctly.

7. GR Meter Driver Amp (Rev A and D)

If your readings are correct in steps 1-3 you should be able to set your QBias and your unit should be compressing correctly. Problems with setting the Null Adjust or Gain Reduction Tracking would likely be an issue with your GR Meter Driver Amp section.  Review all component values, placement, and soldering in that section as well as the meter switch and PCB. If you’re having issues setting the null adjust confirm you used matched Q12/Q13 transistors.