Kenwood Model 600/650 Supreme Integrated Amplifier
I was very excited to get a chance to finally work on one of these, one of Kenwoods TOTL integrated amplifiers, the 600 Supreme. Certainly has an interesting export history.
The unit came in with an issue of going into protection, , as well as a restoration. The protection circuit and power supply/protection had been properly redone a while back so I skipped that after giving a run through. I'll say this, the 600 is not for the faint of heart. To do any front panel pcb assemblies I had to completely remove all the switches/pots etc, and remove the chassis section. Same goes for removing the filter caps. Contrary to a few posts i've read, if the unit was put together correctly at the factory you must undo each power transformer to undo the cap mounting bracket. One thing to note is the amazing Alps pot, its a HUGE enclosed stepped attenuator design, extremely solid construction and action, it would costs high dollar to do better IMHO.
This particular board was slightly different than the documented one in the service manual vs the standard X08-1470-01 PCB. All the lyrics were replaced with Silmic II's and lower rated caps with WIMA MKS2's. This is an interesting pre-amp compared to a lot of Marantz I've seen consisting of 6 FET's for the 4-stage eq phase. This particular board is a bear because of the tight shielded wiring thats rampant here.
Tone Control X11-1310-00
I used a lot of nice MKS2 films and Silmic II's and MUSE KZ, its a little tighter in here as well. The design is a class A ,FET based Peter Baxandall circuit. Again it was easiest to remove the chassis mounts and panel hardware on this board than wrestle inside and potential break something. I much prefer to work outside the chassis anyhow.
Filter Assembly - X12-1130-10
When doing the tone control it was easiest to do the Filter Assembly recap at that time, the boards sandwich each other. Again Silmic II's were used for any lytic.
Power Amp X07-1440-00
The design is a direct-coupled design, it also incorporates the constant current type dual FET stage i've seen in other Kenwoods such as KA-8100, what is interesting is that it does not utilize an initial input coupling cap. The big issue on these boards are the differential pairs (2sa810) and, again, the constant current regulating (2sc1452). If you do your research you note that these heatsinked to-18 type transistors tend to be failure prone. I had one of the 2sc1452 failing not this unit, causing an intermitted protection issue. Remember to match the hfe of the KSA1381's used in the differentials. Not a whole lot of lyrics on this board either, they were replaced with some nice Silmic II's and a single Panasonic ECW PP film along with the main course and BIAS pots were replaced with multi turn Bourns.
Mind the different pin-outs of the new transistors (pictured below)
All four filter caps (18Kuf 75VDC) Chems were used, these are an exact drop in dimension wise, the screw terminal makes for fairly quick work after you add rungs. This is the first time I've tried these particular caps and they seem very solid, one of the filter caps I pulled had a nice big soft spot like a over ripened fruit on it.