Hitachi HA-6800 (1981)

State of preservation and error analysis

The stereo amplifier from the 80s reminds me of a machine look due to the angular signal displays, which, however, with its silver shine, does not create an obtrusive optical impression. The buttons and controls are also stylishly designed with a fine chrome-plated drawing around the edges. The buttons can be operated pleasantly fine and have a mechanically pleasant feedback.

Repairs recording

• Clean switch and potentiometer
• Fix loose contact of the signal lamps
• Replace indicator lamps with LEDs
• Check ESR of capacitors
• Replace speaker terminals uniformly
• Completely re-solder the main PCB and renew the protective coating
• Power stage FETs: Clean contacts and replace thermal paste
• Dust and clean the inside of the housing and heatsink
• Adjustment and testing of the protective circuits according to the service manual
• Check the heat sink (Heat Loop)
• Clean the outside of the housing

The device can be switched on. However, the controls have poor contact and the left channel has no function. Here it must be checked whether the protective switching relay is still working properly or whether there is another fault. The green and red indicator lights and the signal source buttons are intermittent. There is obviously a problem with the feeding.

Troubleshooting and Restoration

After a careful examination of the main board, the fault of the faulty supply (10.5 V) is soon found. The transistor in position Q801 for the voltage supply of the lamps had cold solder joints. I have the impression that there has been some re-soldering here in the past few decades. However, the soldering process was performed with poor quality with too little heat and the residues were not cleaned at the end of the soldering. The switches and potentiometers now work perfectly again after treating them with contact spray.

The entire main circuit board is resoldered and, as stated in the repair list, the old protective varnish is removed at the end of the soldering process and the circuit board is coated with new varnish. The coating is up to 40 µm thick with an application, which is absolutely sufficient as protection against corrosion. The heat sink is thoroughly cleaned and the thermal paste of the FETs is renewed.

The preamplifier board was also checked and some soldering points resoldered, the 2-channel audio preamplifier ICs "HA1452W" and the dual FET stages of the type "µPA68H (L, M)" were also resoldered at positions FET701 and FET702.

End result

All repairs were successfully completed. The signal lamps replaced with LEDs also work again. By the way, some lamps of the audio source selector were also defective in addition to the faulty supply voltage. Now there are no more contact problems with the switches and potentiometers. All capacitors are still in good condition (ESR).

After completing the repair work, cleaning and testing, the brushed aluminum front shines fresh again and is also almost scratch-free. After more than 4 decades, some patina and traces of use are not a problem and only make the device even more authentic 😉.

The "Main Direct" switch allows the tone control circuitry to be used when required. However, this has no amplification function (0 dB). The main amplifier has a gain of 44 dB.

The speaker connections on the outside (red and black screws) were again replaced with uniform spare parts. The chinch connections and connection contacts have been cleaned and treated with anti-corrosion protection.

Finally, I would like to mention one more unique feature of this amp. Namely, the heat sink of the two output stage FETs has a cooling circuit filled with liquid gas (Freon). The liquid becomes gaseous when heated and then circulates in the cooling circuit, which can thus transport the heat away more quickly. It should also be mentioned that the device gets much warmer than you might be used to from today's devices. The distances for the passive cooling of amplifiers (10 cm above and on the side) should therefore be strictly observed here.