I have L-07M II #4 in pieces here on the floor, and this is the fourth one in six months that I've worked on that has oscillated itself to death The L-07M II (not the original L-07M) is one of the fastest amps that Kenwood ever built. Kenwood rates its slew rate as 170V/µS, and says its 3db bandwidth is DC to 600KHz. Folks...that's fast. This amp was built in the days of the 'TIM' (transient intermodulation distortion) wars, and rather than raw output power, manufacturers were trying to outdo each other with speed. While there's nothing inherently wrong with this (and a lot of things right about it), a very fast amplifier must be very stable. Kenwood has sprinkled a slew of some strange type of 'pf range' caps on the driver board of the 'M II' to ensure that stability. Problem is, these caps are apparently changing value under load and heat conditions, most likely due to age. On all three that I have running at the moment, the output Zobel filter resistors were near-charred, and driver transistors and other parts on the driver board were blown. On one (the one mentioned above), the thermal fuse (Canadian model) is open in the transformer. On another one, four of the six output transistors were blown. These are very special, high-bandwidth outputs, and cannot be sourced (a distributor in the UK says they have them...for about $200 for an amplifier set). All that can be done is to put modern output transistors into the amp, and hope it works well. It certainly isn't going to meet specifications with anything other than the originals. Besides the danger to the output devices, there's a real danger to the transformer if the level of oscillation is high enough. Although the total current flow may not be enough to blow the fuse, the oscillation may well creep under the radar and cause excessive heating of the transformer coils. On a US model, this will eventually mean an open primary winding. On Canadian models, which are required to have thermal fuses in the transformer case, it will mean an open fuse (which cannot be fixed except by a partial rewinding of the transformer). What to do? Scrap those horrid caps on the driver board (and the bias and offset pots while you're in there, and yes...install new electrolytics too, since rising ESR here may well contribute to the problem). What to install? I recommend the most stable 'pf range' caps out there...ones that won't change with age and are as reliable as a brick doorstop: silver mica. On the three that I have rebuilt so far, these are what was replaced - C1 - 100pf C2 - 47pf C10 - 3pf C12 - 33pf C13 - 47pf C14 - 8pf C15 - 4pf C18 - 100pf C19 - 12pf This is all the pf-range caps on the board (the schematic shows others, but on the board these locations are in fact unpopulated). I was unable to find either a 4pf or 8pf silver mica cap, and bought 3.9pf 1000V ceramic caps. The 4pf was replaced directly with a 3.9pf ceramic, the 8pf has one cap installed on the front of the board, and one on the rear in parallel. The stock caps were mounted on very long leads, where the caps themselves were about 2" off the board. I honestly don't know the reasoning behind this, but the replacements are certainly NOT mounted that way. I removed the insulating tubing from the leads of the stock caps, and cut short lead insulators for the replacement caps about 3mm long, so the new caps are just a couple of mm above the board, not 2". I also recommend (as I did earlier) replacing the stock bias and offset trimpots (100 ohm, 2K, and 100K). Simple, single-turn Bourns 3386H-series pots are perfect. Lastly, deflux the back (solder-side) of the driver board before re-installation. This flux may contribute to the oscillation problems. I use Acetone and a cheap paintbrush. Pour a small amount in the bottom of a cut-off 1 gallon plastic milk jug, and use the paint brush to scrub the flux off. Try not to get any on the front of the board (it isn't a disaster, but leaves a white crust after it dries). If you do, just use a paper towel and wipe it off before it dries. To keep the back of the board pretty, spray with WD-40, smear it around with another cheap paintbrush, and pat dry with a paper towel. You also need to examine Rf35 and Rf37 on the mainboard. These are 10 ohm, 3W resistors installed in parallel to give 5 ohms, and connected to ground through a .047µf film cap to form the output Zobel filter. If the resistors show any signs of overheating (they're covered with fabric tube weave, but the light-colored weave will darken if the resistors have overheated) the resistors should be replaced, as well as the capacitor. (A quick mod here while the amp is apart...the output relay is an Omron MY4-02-DC48, a 4PDT, 48V relay. The 48V version of the MY4 is a pain in the ass to find, but you can replace the 48V relay with a 24V relay by changing Rk17 on the protection board from the 820 ohm 1W resistor to a 1.2K 3W resistor. Another quick note...be careful disassembling this amp...it is the Rubik's Cube of amplifiers...and you can easily break the mainboard if you don't know what you are doing.) Unfortunately, L-07M II #4 might be an iffy proposition to get running again. I have sent the transformer to Gary Brown in Orono, ME. He seems to think he can get it working again, but it isn't going to be a cheap operation. He's confident about his chances, I'm not. I'll be pleasantly surprised if he can get it working. And even if he does, I'll have to put output transistors in the amp that are about 1/10 the speed of the originals. So...for those of you using a L-07M II that doesn't quite sound right, or are considering buying one, be aware that they have 'issues'. Without proper maintenance it is entirely possible that your amp might be on the fast track to a big, pretty paperweight. What do I think of the end result after its all done? Damn nice... I'm not tossing my 700M anytime soon, but when everything is doing what it should this amp sounds almost as good as the Sansui BA-F1 or the Accuphase P-300, two of the most transparent solid-state amps that I can recall at this early hour. Well-worth the time and trouble.