FET/RACK Troubleshooting Guide

Posted by Hairball Admin on Nov 14 filed in DIY Resources

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 so you can; a) fix it or B) head to the support forum and let us know where you are having an 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. 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 to measure for AC or DC voltages.
  • Confirm the line on your input and output knob is set to line up with 0 on the scale when fully CW.  It won’t rotate all the way back to infinity but that is fine. UREI used some crazy 320 degree pots back in the day.
  • Understand that these pots have a 10% tolerance, you some of your AC voltages may vary from ours a little, that is ok.
  • 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 (R59) and note the DCV at each extreme of the pot. Expected negative DC voltages are noted.

Test Point Rev A Rev D
TP18 -1 VDC to -1.78 VDC 0 VDC to -2.96 VDC

If your voltages are off significantly, you have an issue with you qbias and you can stop now and consult the forum. If you 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:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

With your DMM measure AC at the following test points using the chassis as your reference (black probe). Expected AC voltages are noted below. “UB” voltages are for an unbiased unit. “B” voltages are for a unit that was successfully biased as described in the calibration guide.

Test Point Rev A Rev D
TP1 0.113 (UB) | 0.113 (B) VAC 0.129 (UB) | 0.129 (B) VAC
TP15 4.28 (UB) | 3.83 (B) VAC 2.50 (UB) | 2.20 (B) VAC
TP17 0.663 (UB) | 0.592 (B) VAC 0.358 (UB) | 0.314 (B) VAC
Output Transformer BRN 10.23 (UB) | 9.13 (B) VAC 6.52 (UB) | 5.73 (B) VAC

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 -.

Output XLR + and - 9.70 (UB) | 8.65 (B) VAC 6.15 (UB) | 5.40 (B) VAC

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 daughter board or FET.

  • Low voltage at TP1: likely have issues with input section (TPad/Input Transformer/Q1).
  • Low voltage at TP15: have issues with the Signal Preamp section marked on the PCB. Check the DC voltage chart for this section.
  • Low voltage at TP17: have issues around your output pot.
  • Low voltage at Output Transformer BRN: have issues in your Signal Line Amp section marked on the PCB. Check the DC voltage chart for this section.
  • Low voltage at Output XLR + and -: likely have issues with output transformer wiring.

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:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR ON (fully CW)
Release: Full CW
Ratio: Check All
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

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

Test Point 22 Rev A Rev D
20:1 0.389 VAC 0.447 VAC
12:1 0.188 VAC 0.215 VAC
8:1 0.125 VAC 0.130 VAC
4:1 0.063 VAC 0.062 VAC

Very low voltage here would indicate an issue with your ratio switch AC section. You can identify these components in the schematic labeled with resistors R19-R22 and R78. You should also confirm your GR OFF switch on the attack pot is soldered and working correctly.

4. GR Control Amp Output

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR ON (fully CW)
Release: Full CW
Ratio: Check All
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

With your DMM measure AC at the anode of CR2 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.

Anode CR2 Rev A Rev D
20:1 3.63 VAC 4.16 VAC
12:1 1.78 VAC 2.08 VAC
8:1 1.20 VAC 1.30 VAC
4:1 0.620 VAC 0.650 VAC

If you’re getting very low readings your unit will not compress. Your issue is likely located on the main PCB in the section marked Gain Control Amp. Check the DC voltage chart for this section.

5. GR Control Amp Threshold Voltages

Set your compressor controls:

Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR ON (fully CW)
Release: Full CW
Ratio: Check All
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

With your DMM measure DC at TP21 for each ratio setting using the chassis as your reference (black probe). Expected negative DC voltages are noted.

Test Point 21 Rev A Rev D
20:1 -5.93 VDC -6.78 VDC
12:1 -3.33 VDC -3.83 VDC
8:1 -2.52 VDC -2.72 VDC
4:1 -1.71 VDC -1.79 VDC

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.

6. GR Meter Driver Amp

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.

Conclusion

We hope this guide has helped you isolate and identify your issue. If you’re still stuck, head over to the GroupDIY official threads and let us know where you’ve isolated your issue.  Information on posting guidelines and official thread locations can be found here. 

http://www.hairballaudio.com/support-forum

We also offer flat rate repair. Information can be found here.

http://www.hairballaudio.com/build-support-services

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