My recent DIY DAC was designed for balanced out and already having balanced active crossover/amps I wanted a balanced pre to connect it all up. Ed (Savinon) has been trying to get me Krell-ed for quite some time so I got this old KSL from him complete with KSL phono board.
We are reaching back 23 years with this old Class A design. The pre sounded good, was quiet, but it had some phono stage noise issues on the left channel only with the phono board set to high gain MC. The phono board is a Fairchild J309 jfet N channel RF amplifier / MPS8599 PNP /MPS8099 NPN transistor design. I had feared the worst (bad jfet or transistor) but it just turned out to be a bias issue. Once the phono board was re-biased all was well.
There are different recap camps. Some say only the caps that are bad, many have been telling me get rid of them 23 year old caps right now. OEM caps were Sprague and Rhode. No idea what happened to Rhode, but Vishay bought Sprague and they are just a Tantalum division now so matching brands was out. In circuit testing is not 100% reliable so I pulled some caps to test and found them all over the place. Some were worse than their 20% spec, some were closer. I just decided to recap the entire power, main and phono board to be safe.
Fred was a big help (as usual, thanks Fred!) and had me go with United Chemi-Con caps. That name isn't bandied about but after some research I learnt they are a well respected cap. I used all 105C caps, UCC 1000uf KME's for the main board, UCC KMH for the 4700uf power supply and four extra long life 470uf UCC LXY's for the hot spot between the sinks.
On the phono board I used 105C Vishay AML 138 series. Fred also surprised me with some long OOP 1% NOS Siemens Styroflex polystyrenes to replace the four .033uf poly caps. These are either part of the RIAA filter or some signal path thing. (No schematic available, only $125 an hour for Krell service). I have two Philips .0016uf polystrenes on the way. Word is Siemens and Phillips were the best polystrenes made.
After washing the boards with a 99% isopropyl alky wash and reassembly, I found it survived surgery and is good for another 20 years.