Follow this procedure to bring up each EHHA Rev A Board. You will need a digitial multimeter. If something goes wrong, disconnect the power connections immediately. You need to be reasonably skilled at using a DVM and accessing test points on a PCB.
Use this wiring digram for two channel operation.
Use this wiring digram if you have hum/buzz that you can't fix through any other means.
- Connect your transformer to the terminal block labeled 6.3-9VAC. Power up the transformer.
- If you have the heater LED installed it should light.
- Measure the voltage at the output block between either G connection and the 6.3VDC output.
- Adjust the trimpot until you get 6.3VDC (or whatever heater voltage is in the range that you want).
- Turn off the power and connect the 6.3VDC and one of the G terminals to the Heater terminals on the amplifer section. Insert a tube and power up the heater supply
- If the tube lights measure the DC output of the supply again. This should be what you set in the previous step.
- If it is, proceed to the next section.
- If it is not, measure the voltage from TP3 on the heater board to ground. This is the input voltage for the regulator. If this voltage is below 8.3-8.5VDC then your regulator will drop out and not regulate properly. Change the transformer and/or diodes.
- Connect the 30-0-30 transformer to the terminal block on the PS board with the same label.
- Turn on the power.
- If you have the rails LEDs installed they should light.
- Measure the +30 and -30 terminals on the block to ground. They should be close to 30V.
- If they are proceed wire the +30 and -30 terminals together between the PS and amp sections.
- Go to the next step.
- If one or both of the voltages are wrong, check all component values and ensure that the right regulator is in the right place.
- Figure out what's wrong.
- Attach the unused G terminal on the heater board to the star ground.
- Attach the center terminal on the PS output to the star ground.
- Attach the ground terminal (the one not labeled) on the amp PS inlet to the star ground.
- Holes are provided on the boards for routing these wires under the board from the terminal blocks.
- Make sure that the heater and rail connections are all wired across the boards.
Amp Power Up
- Set the trimpot P1 to its halfway position.
- Attach a DMM from the OUT terminal to ground set on the 300mV setting.
- Turn on the heater and rails supplies.
- Look for smoke. If there is none, proceed.
- Wait for the output DC to settle into the low mV range. If this does not happen the servo is not working. Stop and turn off the power.
- If the DC offset settles then adjust the bias.
- Attach the DMM from either TP1 or TP2 to the OUT terminal set on the 300mV scale. This measures across the output resistors.
- For the Mosfet version adjust the trimpot until this voltage is 220mV (100mA).
- For the BJT version adjust the trimpot until this voltage is 1V (100mA).
- Measure the other testpoint to OUT. It should be the same.
- After the output heatinks get hot set the bias again because it will change.
- If you get 0VDC offset and if you can set the bias then you can attach headphones and listen.