• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help with harmon kardon a50k


2011-12-11 8:28 am
Just picked up this integrated.

There's 4x 7355 4x 12ax7 ad 1 12au7

3 of the 12ax7s don't heat/light up

I believe it's v1,2,and3

Everything physically looks ok underneath. Any suggestions on where to stry troubleshooting? I'll attach a link with the schematic

Look at the schematic. That "beast" uses "Cheap Charlie" DC heating of preamp tubes. :mad: Preamp tube heaters are be used in the O/P tube bias networks. While that sort of thing is common in "vintage" units, it's dreadful. :(

Build a proper, regulated, DC supply for the preamp tubes. Separate RC cathode bias networks for each channel are also indicated.

Talk to Jim McShane about tubes, parts, and suggestions. He offers honest advice and fair prices.

I find myself behaving like a "stuck" phonograph record, with amps from this time frame. Replace the source selector switch. Hard wire a single set of I/P jacks straight into the low level mag. preamp circuitry, which gets set up exclusively for RIAA. Switches in mV. signal level circuitry are bad and tape head is passe, in this day and age. Make all the other I/Ps line level. Just under $5 buys a suitable, Lorlin brand, non-shorting, rotary switch from Mouser.
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