Hitachi HA-270 Stereo Amplifier -- Bias and Bulbs

Unit had unknown problems.

Unit was slowly brought up to line voltage with variable-isolation transformer. Next, the unit was checked for DC-offset (voltage) at the speaker terminals since this is a direct-coupled amp. Amp is dual-power supply (positive and negative) and push-pull direct coupled, meaning there is no capacitor between the output transistors and the speaker(s).

Bias was adjusted statically (meaning without an input signal) per service manual instructions. Unit was allowed to warm up for a few minutes first. Adjustment is supposed to be 8mV, plus or minus 4mV, meaning anywhere from 4mV to 12mV.

Both VU-meter bulbs were replaced. 12V axial bulbs were too expensive. I ended up putting in 12V, 50 mA, bulbs. VU-meter case is held together by tape, which must be carefully removed or cut at the seams. I used the old tape but also used some new clear tape when I put the cases back together.