Harman Kardon Amplifier Repairs

Harman Kardon Citation 16

Harmon Kardon Citation Sixteen Harmon Kardon Citation Sixteen

This one needed the 4 main caps replacing. After replacing them the quiescent was checked, I couldn't lower the voltage on the resistor below 62 mv, one of the 0.5ohm resistors was 400 ohm, set quiescent to 10mv across left 0.5ohm r seems okay.

Harman Kardon Citation I & II

Harman Kardon Citation I & II Harman Kardon Citation I & II Harman Kardon Citation I & II

The preamplifier needed soaking in switch cleaner (switches & pots) the power amp was a bit low on one channel, found dry joints on the KT88 sockets and various other places, sounded wonderful afterwards.

Harman Kardon HK 670

Harmon Kardon 670 Harmon Kardon 670 Harmon Kardon 670

Blowing left channel fuses, a 2SA1380 had blown, replaced but still blowing fuses. The o\p module PCB is available for £86 advised owner to change it. After doing so it worked fine, noticed 470µF 16V caps on the i\p board had vented and needed replacing.

Harman Kardon Citation 16

Harmon Kardon Citation Sixteen

Power amplifier (from 1977) it needed one channel rebuilding as there was 70V at the output. The input op amp and all the transistors (except the o\p transistors) were changed. I suspect the UA739 op amp had gone faulty and wiped out the rest of the components. These have no DC offset protection for the speakers although they should have been supplied with in-line fuse holders as new, however, these are never passed on to subsequent owners. What happened to the speaker connected to it? I remember 1977, when amps were amps and speakers were nervous! The later version had a protection CCT and a discrete input stage.

Harman Kardon Citation 16

Harmon Kardon Citation Sixteen

This monster from 1977 was received by myself knowing there was a fault on one channel. It had -60V o\p at an unnerving current capacity. Any speaker connected would have vaporised. There are no fuses or offset protection. Looking at the PCB the o\p of the input op amp a ua739 was at -10V, (the supply to it is +/-10V), with the o\p to the speakers -60 V, not quite right! I suspected a fault in the op amp circuit, changing it sorted the o\p offset (the op amp is a high gain video 1MHz bandwidth dual 14 pin, CPC have a stock), but the quiescent current was non existent. All the uniwatt transistors were changed on the PCB and the mje340\350. The two neon on indicators required changing and one of the relay contacts had been bridged with a solder blob. After all that it produced stunning dynamics! For the sake of my speakers sanity I wired a couple of 5 A fuses in o\p's.

Setting the quiescent current to the spec in the service info ie 10 mV across the 0.5Ω resistor on the top of the heat-sink resulted in excess heat in the o\p transistors and it had to be set a lot lower, the resistor was lower in value than the 0.5Ω it should have been.