FET/500 Troubleshooting Guide

You’re finally at the end of your FET/500 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.

Testing Limitations

Troubleshooting a build is tricky, particularly when it's a 500 series build. You'll need to be able to power and send signal to the unit while having access to the PCB. This can be done by using an extender jig or having a multiple space rack you can empty to gain access.

If you don't have a way to safely power and test the unit, consider sending it in for flat rate repair.

Ground

The FET/500 series uses an elaborate grounding scheme for audio and +30VDC rail ground. Using the ground pin on the 500 series edge card will produce incorrect readings. Ground reference for all FET/500 series units can be found at the anode (+) of D4. Unless specified, measure all voltages in this guide using D4 anode as your ground reference.

The Negative Bias Rail and the Meter PCB

The meter PCB contains more than just metering. The negative bias rail circuit for the FET/500 is on the meter PCB. For this reason, the meter MUST be installed for the FET/500 to function.

Voltage Rails

To test the voltage rails, you'll need the unit powered on. No audio signal is needed. As a reminder, use the anode of D4 and your ground/common probe reference. Looking at the top side of the meter board, you'll see the 10 pin connector pins are labeled. Measure for DC +/- 10%.

IMPORTANT!!!!

• Make sure you pay attention when you should be measuring for AC or DC voltages.
• We are in the process of adding test point (TP) designations to our PCBs. We will provide you with the TP designation as well as the location in brackets for PCBs still missing test points.
• 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 

1. QBias Voltage

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: Full CW
Release: Full CW
Ratio: 20:1
GR: Off (Switch pushed in)
Bypass: Off (Switch left out)
Signal: None

With your DMM measure DC at TP18 using the anode of D4 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 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
0dBu is 0.775 VAC across input + and - which are pin 10 and 8 on your edgecard. 
When setting the input level, measure the voltage across the input +/- fingers. Once the level is set, return to using the anode of D4 for ground.

Input: Set to read 0.100 VAC at TP1 (input tx pin directly across from the square input tx pad)
Output: 12 O’Clock
Release: Full CW
Ratio: 20:1
GR: Off (Switch pushed in)
Bypass: Off (Switch left out)

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. Q2 and Q4 in revision A are difficult to measure without producing oscillation so they've been omitted.

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 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.

Touching T15 or Q4 with your probe will probably cause your unit to oscillate and get a high reading here. In that case, skip TP17 and start at the Q5 base and set your output pot so Q5 base reads 0.173 VAC.

Touching T15 or Q4 with your probe will probably cause your unit to oscillate and get a high reading here. In that case, skip TP17 and start at the Q5 base and set your output pot so Q5 base reads 0.195 VAC.

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 D4. Measure your output between XLR output + and output - or edge card pins 2 and 4.

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
Release: Full CW
Ratio: 20:1
GR: On (Switch left out)
Bypass: Off (Switch left out)

With your DMM measure AC at TP22 for each ratio setting using anode D4 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.

Metering

The FET/500 has both a gain reduction meter and an output level meter. The GR meter circuit is primarily located on the main PCB where as the output meter circuit is completely located on the meter PCB. The GR meter circuit is driven by Q11, IC1, and the handful of components around it. 

To test, use the same set-up you would to set the GR tracking. This is probably the point where you first noticed a problem. Start by centering the tracking adjust and zero adjust trimmers as noted in the build guide.

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

Input: 12 O’Clock 1
Output: 12 O’Clock
Release: Full CW
Attack: Full CW
Ratio: 20:1
GR: On (Switch left out)
Bypass: Off (Switch left out)

1. Adjust the Output control so the FET/500 output reads +0 dBu (0.775 VAC across input + and -)
2. Put the unit in GR Off mode
3. Adjust the Input control so the output reads +9 dBu (2.18 VAC across input + and -)
4. Repeat until you see an output of +9dBu in GR OFF mode and an output of 0 dBu in GR ON mode

If you can’t get a 9dBu drop, you don’t have an issue with the meter, you have an issue with your signal amps or GR side chain amp. Revisit the earlier sections of this guide. If you can get a 9dB drop, start by measuring the DC voltage at the “GR SIG” pin on the meter board 10 pin connector (pin 1 which is square) using D4 Anode as your ground reference. You should see a voltage of around +1 VDC in bypass and +0.428 VDC with GR on and a 9 dBu drop at your output. If you are not seeing roughly that voltage, you likely have an issue with your main board GR meter circuit.

Using the numbered schematic and PCB guide, measure the DC voltages in your main PCB GR meter circuit to locate the problem area. Two values for control voltages are shown. The first value is with GR OFF and the second is with 9dB of GR applied. Keep in mind, this is for a calibrated unit so your IC inputs may be mismatched and some and your IC output may differ, but you’re looking for extreme differences like no voltage or reverse polarity. Note the voltage swings positive at the IC output.

GR Main PCB Circuit PDF Download

Once the control voltage reaches the meter PCB, it travels directly to the LM3916 input. Using the anode of D4 as your ground, measure for the following DC voltages at IC204.

 Having no voltage at LED pins when OFF would indicate an issue with the IC205 supply circuit or the LEDs being installed backwards.

Output Level Meter

The output level meter is a typical VU style meter set to show +4dBu as 0 on the scale just like the +4 meter button on the FET/RACK. It also has a peak LED circuit that is set to show when the peak of your signal waveform reaches +16 dBU, the point at which many AD/DA converters may start to clip.

Important!!!
The output level meter uses the traditional ground pin for ground reference. Do not use anode D4 for ground when measuring the output meter circuit. Use pin 7 of the 10 pin connector. It’s labeled GND in the measurement document linked below.

Start by setting you unit to have an output of +4 dBu.

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

Input: 12 O’Clock 
Output: Set for +4 dBu output (1.22 VAC across output + and -)
Release: Full CW
Attack: Full CW
Ratio: 20:1
GR: Off (Switch pushed in)
Bypass: Off (Switch left out)

Using the number schematic and PCB guide below, measure the voltages in your meter PCB output meter circuit. We’ll be working through in a linear path so it should be obvious where the problem area lies. Pay attention when measuring for AC or DC and well as DC polarity.

Output Level Meter Circuit PDF Download

*The transistor base will swing positive when the LED is illuminated.
**LED1 will read about 1/2 voltage.

The +V and –V TL072 supply pins are also labeled. They should read +15 VDC for +V and -15 VDC for –V. Confirm the V+pin of IC202 (see LM3916 diagram above) reads +15 VDC.

 Common errors include:

• TL072/MAX1044 swapped or inserted backwards
• LM3916 driver installed backwards
• LEDs installed backwards or not fully soldered
• Trim pots swapped
• Damaged diodes
• Damaged TL072