HAMPTONe5. TESTING THE PSU
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Ideally when testing newly built electronics, a variac is used. A variac is a variable transformer that allows you to bring the AC voltage up very slowly when first powering a circuit. The instructions reference the use of one where applicable. If you don’t have one, you will have to just turn the switch on. I highly recommend buying or borrowing one for testing. If you have an assembly problem, it can cause serious damage to the micpre, usually to the power supply. For example, if a cap is in backwards, with a variac you will notice it because the regulator will not ramp up. With the variac at say 5%, if the output is still reading 0, or seems stuck at a volt or two, you can quickly bring the power down typically without the part being damaged; fix the problem, and try again. If the power is just flipped on, parts can be damaged.
1. Connect your volt meter set to DC volts to the green (-), and red (+) point-point wires, making certain the yellow wire cannot make contact with either the red or green wires, or the chassis. Connect the 10k 1/2w resistor across the red and green wires. The LT1086 requires a small load current to properly be tested (this is true of many low dropout regulators). Leave the 10k 1/2w resistor connected across the red/green wires during testing. Turn the power switch OFF. Set the variac if you have one, all the way down to 0 or OFF , then plug in the AC power cord.
2. Turn on the variac, and the power switch on the micpre. Slowly turn up AC power to the transformer input while measuring the +25.5vdc output on the DC voltmeter. If the voltage does not start ramping up after you slowly apply power, turn the unit off, you may have a short, or something mis-wired. Normally the DC output of the regulator will increase as AC voltage is brought up, until it reaches the regulation voltage, where it locks in. Because of the ramping nature of the supply, it takes about 60 seconds to reach its regulation voltage (25.5vdc+-0.25) when hard started.
3. Switch the meter to AC volts, it should read less than a 5mV AC with proper shielded cables. Some cheaper meters don’t read low level AC well, if you are getting high readings, test your meter on a 9v battery which should be 0Vac. If you get something other than zero, your meter is off. You can also test the meter by connecting both leads to ground, again if you read anything other than zero, the meter is not reading correctly, or you are inducing noise into the measurement. Twisted the leads (if they are unshielded) can help considerable. Also, keep the test leads away from the power transformer. If the DC is right, continue.
4. Measure DC volts across D51, it should be 1.25vdc+-0.1vdc. This is the internal reference of the LT1086. This is very important, if it is not very close, the regulator is not regulating.
5. Shut the preamp off, REMOVE THE POWER CORD, turn the variac down/OFF, and monitor the 25.5vdc voltage until it goes to zero (or close to it), this may take a minute of two. IT IS VERY IMPORTANT TO ALWAYS MAKE CERTAIN THE POWER SUPPLY HAS DISCHARGED BEFORE WORKING ON IT. Remove the meter.
6. Remove the psu pcb, and install B30 making certain the polarity is correct (the + mark on the rectifier goes into the square pad). The leads may need to be bent outward slightly to fit the holes. Press the rectifier as close to the board as possible, without straining the leads. Make certain to clip off all the leads. Bolt the psu board back in place.
7. Repeat the above procedure for the +48vdc regulator. This time connect the + lead of your meter to the yellow wire, the – side of your meter to the green wire, and make certain the red wire can’t touch either the green or yellow wires, or the chassis. In the case of the +48vdc, there is a slight delay from when you begin turning up the AC voltage, and you see the output beginning to ramp up. If you have a short, or a cap backwards in this circuit and you turn the power on full without a variac, parts can explode, as well as circuit board traces becoming damaged.
8. With the variac down, turn both the variac and the micpre switches on. Slowly bring up the variac while monitoring DC volts. After the supply has finished ramping, adjust R39 so the output reads 48vdc.
9. Switch to AC volts on the meter, you should read less than 5mV AC.
10. Measure the voltage across D38, it should read 1.25vdc+-0.1vdc.
11. Set the meter back to DC volts, and turn the power switch on both the variac and micpre off, turn the variac down, REMOVE THE POWER CORD. Again waiting until the +48vdc supply has fully discharged, down to less than 1-2vdc.
12. At this point, the power supply is tested and ready for integration with the audio channels.