HAMPTONe7. FINAL INTEGRATED TEST
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1. With everything secured, turn the micpre power switch off, and attach the power cord. Connect your meter to the +25.4vdc by connecting the + side to the point where R20 and R21 connect. Connect the – side of the meter to ground, the jumper on the DI jack is a convenient place. Slowly bring the AC voltage on the variac while monitoring the DC voltage on the +25.5vdc supply, it should ramp up and lock in at +25.5vdc just like before.
2. Switch to AC volts, again it should read less than 5vm AC.
3. Move the +terminal of the meter to either of the back two terminals of the phantom power switch (looking at the micpre from the front); measuring DC volts it should be +48vdc.
4. Switch to AC volts, again it should read less than 5vm AC.
5. Refer to the "HJFP2 gain module testing" page in the instructions. Measure "V2" and "V5" for each of the two gain stages in each channel and compare them to the DC values given in the table. If the voltages are close to those in the table, the gain stages more than likely are working properly. If not, test the remaining voltages and trouble-shoot the problem. Basically you are looking for voltages that are way off, then look at that part of the circuit: are the caps in right, are the resistors the right value, are all the parts soldered, is anything shorted?
6. If you have a function generator: put a 100mV rms 1KHz test tone into the DI and measure the gain by measuring "v8" of the first stage at the "SND" to the volume pot, compare this to the value given in the "HJFP2 gain module testing" page in the instructions. Then adjust the volume control so the "RTN" or wiper from the pot reads 0.1v rms. Again measure "V8" on the second stage and compare it to the value given on the "JFET testing page".
7. Shut the unit off, unplug the cord, and allow the unit to discharge for a few minutes.
8. Neatly route the wires away from the heatsinks in the psu and audio section, and tie them together using the tie wraps provided. The point-point wires between the audio and psu were designed to be a couple inches longer than needed, so the audio section can be flipped over without being unwired, if trouble shooting or service is required. Removable Crimp connectors are not used because they are not reliable over a long period of time. Hard wired point-point if done properly will last for decades if not longer, crimp connectors usually fail in less than 10 years, or at a minimum need to be removed, cleaned, and reconnected.
9. Finally, mount the handle (if you want it) to the cover using the 2 x 3/8" #10-32". Make sure these are very tight, if they vibrate loose, they can fall into the circuitry potentially causing a short. Use lock-tite if you have it, or a little fingernail polish on the threads.
10. Put the cover on using the remaining ¼" #4-40 flathead machine screws (there are 3 spare). For the moment I would only put in a few, to keep the cover on. It is best to allow the unit to burn in for a few days, then make a final "tweek" to the +48vdc regulator voltage. Its not critical, so if you want to get tracking, then install them all.
11. MAKE SOME NOISE! Plug a microphone/instrument into the preamp, and give it a listen. If properly assembly, there should be no noticeable noise floor, and should have very wide bandwidth (good lows, clean highs). Try the +48, pad and phase switches to make sure they work. If not, shut the unit off and trouble shoot the problem.
12. If you wish to test the unit more thoroughly, procedures will be provided on our website. These tests will generally require a function generator, and an oscilloscope.