Beyond the list of standard restoration steps detailed on the main page, here are some added notes for this unit :

I acquired this receiver in trade for a repair I did on another KR7600.   

Side note that these are very popular.  I like the KR7600 as it was 2nd from “top of the line” for Kenwood while using standard TO3 output drivers rather than the hard-to-source STK packs used on the more powerful KR9600 outputs.  The KR6600 is a lower power version of the KR7600 with 90% sharing of boards, etc.  The faceplates are even identical (model number on the smoked plastic insert over the tuning dial).  Please also note that the in/out loops on the back of this receiver are NOT preamp out / amp in RCA jacks.  They are labeled “Adapter in/out” for an external quadraphonic processor and occur before the tone control preamp.  These CAN NOT be used as preamp out/ amp in jacks.

Anyway, this was a “parts unit” that was complete, but had 2 of the power supply boards removed with the “bad” ones loose inside (sigh).  As luck would have it, I had yet another KR7600 (4th of this model I have worked on) that I could use (plus schematics) to correctly rewire everything.

The loose power supply boards showed evidence of overheating (not unusual even for “good” boards as Kenwood engineers were OK with rather high, sustained power dissipation for the lower supply rails derived from the +52V amplifier supply).  I checked all the components on both boards and found :

     1) Field service bulletin not applied to +23V supply pass transistor (new pass transistor and series resistor installed per bulletin)

     2) Protection relay coil impedance was way too low which explained upstream driver/resistor failure. (new 24V relay, transistor, resistor installed)

     3) Protection relay kick-back diode removed by someone trying to debug relay coil driver failure. (new diode installed)

     4) 24V Zener diode used to regulate the +23V supply was bad and keeping this rail to ~ 6V. (Zener diode replaced)

After rebuilding these 2 power supply boards, I carefully reconnected all the snipped off wires and applied power.  All rails now correct, but the amplifier was staying in “protection” mode, so this was the next thing to debug.

The rather massive heatsink/board assembly is easily removed (thank you Kenwood engineers…).  I noted that the output drivers on “channel A” were bad (classic collector/emitter shorts).  As is often the case, the pre-driver transistors were also bad as were some resistors that saw too much current.  I used modern equivalent replacements (thank you, AudioKarma) for all of these transistors (on both channels for matching) and the amplifier was now working well.  (Always nice to hear that protection relay “click” on power up.)  

Moving to the tuner, FM reception was good, but only in mono.  Previous experience steered me to the 76KHz VCO which relies on a variable resistor to set the loop frequency.  I cleaned that potentiometer and then tweaked it for 76KHz.  FM stereo came to life, and I completed the tuner alignment.  The tuner dial pointer was loose/bad, so got a replacement which was installed and works fine.

All other functions tested/verified with very nice sound noted (as typical for these KR7600s).

Regarding cosmetics, this unit came with a wood cabinet, but it had nasty scratches.  I first used an iron and wet towel to raise the grain in the scratch areas to reduce how deep I needed to sand to eliminate them.  I then carefully removed the old finish to get down to the walnut veneer, applied stain and multiple coats of polyurethane to get the results you see.  There are a few cabinet blemishes (see first 2 photos above).  2 spots where the veneer was nicked on the end of the thin vent strips and one small spot where someone (not me!) went through the veneer.  Overall, I am very pleased with how the cabinet turned out.

Other blemishes are shown in the second 2 photos above

     1) There are tiny pock marks on the faceplate area and 3 knobs nearest the power switch.  I had to play with the light and camera angle to get these to show up in the photos.

     2) The little plastic insert for the power switch is broken off on one side.

Something I think is interesting is the SSN discretely etched on the back panel (above the power cord entry point).  This repeated on the bottom plate and is typical for stereo gear purchased by military personnel when stationed overseas.