Wednesday, April 29, 2015

Harman/Kardon 730 Receiver

Harman/Kardon 730 Dual Power Receiver 

I want to take a moment and show off a Harman/Kardon 730 receiver.  Again like the three-thirty from a while back, it has a nice green lighting hue to all the lighting with the black on dial plate. These look just amazing.  Another neat little caveat is that it utilizes a dual power supply/dual rectification stages along with dual outputs; two aux, two tape, two phono and pre-out/main in jumpers!

This particular unit came in for a general maintenance package including new lamps. I did however run across a needed Bias repair /modification outlined below.

The amplification stage is a easy to access in this unit, it uses molex snap connections and just the heat sink mounts.Doing some research I have found many 730 users complain that the Bias was finicky, sure enough I encountered the same issue. On occasion you can clean the bias potentiometers, but more than likely if thats necessary the pot should go. In this case a Bourns precision, multi-turn 3296 potentiometers were installed, the 3296 is preferred because of the 90 degree access on the PCB. 

Another issue to note on a large number of vintage Harman/Kardons, including this one, is the 1S1211 varistors. In the 730 they are located at D407/408, a standard 1N4148 should be replaced here for stability in the bias circuit. The actual bias procedure itself is relatively straight forward, measuring across the collector of the main TO-3 outputs for 35mV. 

Even for a mere 40W/CH rated output the 730 is a damn nice receiver if you can snag one!

1 comment:

  1. have one and they are awesome for a no frills unit,,,I am still having problems locating the 8v 60ma dial pointer lamp,,,a grain of rice bulb