R.Tyner Harman Kardon 930 Restoration

Harman Kardon 930 Receiver

Today were showcasing a Harman Kardon 930 Receiver, which in 1972 was one of H&K’s premier receiver line ups. Like many other H&K these utilizing a dual power supply/transformer array for superior PS designs and simplified NPN gain stages through the Af network. The 930 has a classic warmth one associates with the 70’s era hi-fi and extremely pleasing with our film and low noise compliments. 

Dual Power Supply Source

The HK 930 uses a unique dual power supply, with two separate transformers and regulator circuits per channel. The PS consists initially of two separate bridge rectifiers and a 6.3V lamp tap. Each AC/DC bridge is filtered by two 6800MFD capacitors establishing the 39V rails, one of the primary  B+ rails is then regulated for a 18Vdc AF/RF stage SE supply.

Both bridge rectifiers are comprised of two common cathode/anode devices which were replaced with Ultra-Fast, Soft Recovery On-Semi MUR type TO-220 rectification devices for cleaner DC output and less transient noise. The .1MFD bypass were replaced with high grade MKP polypropylene film capacitors. 

For the regulated PCB, the regulator device was replaced with a new On-Semi TO-220 heavy duty regulator and Wakefield synthetic compound coupled, remaining small signal device was replaced with a TO-92L Fairchild type for better dissipation. New axial viceroy 21V Zener diode was installed. The off-PCB 1000MFD and 12V 470/100MFD capacitors were all replaced with a low impedance, high temp (105C) Nichicon PW with an increase in operating voltages. A new bypass MKP polypropylene film capacitor also installed.

BEFORE 

AFTER

SE 18Vdc Confirmation

Filter Capacitors

The four 6800MFD filter capacitors were increased and replaced with new computer-grade CDE 6800MFD capacitors with an increase in operating voltages. Each of the 5.6K resistors were replaced with 5.6K 1/2W carbon type axial Vishay.

-/+ Regulated Dual B+ Confirmation

PCB 500 PRE-AMP

The pre-amp stage uses a single-ended 18V regulated supply and an entire NPN differential gain stage and coupled output stage, it features dual concentric tone attenuators.

All of the small signal devices were replaced with low noise, modern TO-92 Fairchild devices. Electrolytic capacitors were replaced with audio grade KT/KA and low impedance PW Nichicons with an increase in operating voltages. Mylar and films were replaced extensively with high-grade WIMA polypropylene film capacitors. 

BEFORE 

AFTER

PCB400 RIAA PHONO

The MM phono stage, like the proceeding AF stages is supplied by a SE regulated 18Vdc source and uses a simplified NPN gain-equalization stage. 

All of the small signal devices were replaced with low noise, modern TO-92 Fairchild devices. Electrolytic capacitors were replaced with audio grade KT/KA and low impedance PW Nichicons with an increase in operating voltages, the DC filtering capacitor was increased to 220MFD for better filtering. Mylar and films were replaced extensively with high-grade WIMA polypropylene film capacitors. 

BEFORE 

AFTER

RIAA Curve Response

PCB 600 DRIVERS 

The single PCB driver is mounted with the heatsinks open-booked to the driver assembly with a direct-coupled, push pull gain stages.

Capacitors were all replaced with low impedance PW and KT/KA audio grade Nichicons with an increase in operating voltages. Coupling and bypass films were replaced with a high-grade WIMA polypropylene film capacitors. 

The differential pairs were matched and thermally coupled with performance synthetic wakefield compound unlike the original design. Pre-drivers and drivers were all replaced with TO-92L/TO-126 type low noise, modern Fairchild devices. Epoxy diodes for the bias servo were replaced with modern 4148 general axial Vishay diodes. Both bias adjustments were were replaced with precision Bourns potentiometers.

Differential Boding Confirmation

BEFORE

 AFTER

Driver Bias Confirmation

 

PCB 300/200/100 (FM+RF)

The 930 features a very good, but problematic IC related RF tuner front end. Composed of a single IC front end ceramic filters with a four-gang tuner and FET amplifier differentials. 

All the electrolytic int these stages were replaced with low impedance Nichicon PW capacitors with an increase in operating voltages.

Muting PCB

New low impedance Nichicon PW capacitors installed with an increase in operating voltages along with new high-grade WIMA polypropylene film capacitors. 

BEFORE 

AFTER

AUDIO NOTIZEN