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Krell FPB 600c 700cx 400cx Heatsink noises.

In 2001 I bought my complete new Krell cast music system: Krell FPB 600c amplifier, Krell KCT preamp and Krell KPS28c cd-player .Siltech cast interlinks. Speakers are JM Lab Utopia with Transparent ref.XL loudspeaker cables. However I certainly have the feeling that all the manufacturing dollars have been spent on sonically critical design elements,there is a mechanical problem,which is bothering me since the moment of purchase.Two or three times during a listening session the metal construction of the heated amplifier cabinet in operational mode makes a sound (tsssjakkk).Probably because the difference in heat of the heatsinks and/or heavy gauge alluminium chassisplates. When I bring the amp in stand by position after listening,this mechanical sound is there also ,several times. I phoned the Krell service department in 2002.Irving Gross told me that it was a diffraction problem.(traction and contraction of the heated amplifier cabinet.) The service department advised me to tighten all the bolts of the heatsinks. It did not help. During the upgrade from 600c to 700cx,end 2002, I told the German importer about the problem. He admitted that this diffraction problem was a tough one to solve.I had the hope that the problem would have been gone after the X-Upgrade but unfortunately it did not. In the english Hi-Fi News magazine of december 2005 (page 28/Howard's Way) I read an article about a former owner of the Krell 400cx."I sometimes got noise from the heatsinks through heating and cooling." He finally could not live with it and sold the amp. My question is ;does anyone has the same problem with this Krell FPB amplifier(600c,400cx or 700cx)? More important, which owner has a smart suggestion to eliminate this undesirable mechanical'vibration'. Otherwise...I figure I have to live with it, which I can because I simply love the sound of my complete Krell cast system.
paulinemarcel8390

Showing 3 responses by charles_peterson

charles_peterson
11 posts
 
I'm glad to see this discussion. My Krell FPB 300 (not c or cx) was quite noisy. Whenever the music got softer for a bit, I'd hear the ping (or as I called it, klink). I figured that loosening all the bolts would help and was surprised when when a former Krell technician didn't agree and suggested thermal tape instead. I virtually fixed the problem by removing the top cover completely. Many of the top cover bolts were extremely tight, overtightened for sure, and I wouldn't have been able to loosen them without a Wiha handlebar style Torx driver. However it looks like amp should not be moved without replacing top cover since it is necessary for mechanical integrity. Now that I've discovered this thread, I may try loosening all the bolts, though I was afraid of doing anything that might affect output transistors. But right now, I've had one channel go bad anyway, might try that before shipping to Krell for repair or upgrade.
charles_peterson
11 posts
 
I'm back trying to get the noise out of my fpb 300 again. Loosening screws often seems to help. But which, if any, of the outside screws on the left and right side heatsinks actually attach semiconductor devices? I'm not sure I'd want to mess with any of those.
charles_peterson
11 posts
 
I have just now tried the solution being described here, loosening all the external bolts on my Krell FPB 300. All by hand (no torque wrench) I set them all to a snug but not hard torque. If anything, the noises have gotten worse.

It's a pity there is so little information about FPB construction online. I would like to know if the heatsinks, which are apparently attached to 3 rails along the side of the amplifier, can be completely removed. My initial fear that simply loosening the bolts could cause an electronic failure were apparently unfounded. But taking this one step further and actually removing each heatsink to check and clean the rails behind it, I'm not sure if that can be done safely. I'm pretty sure most noises are caused by small movements of the bolts, heatsinks, and rails under varying stresses caused by differential thermal expansions.

I'm going to continue trying different methods of torquing the bolts for now. I'm getting a wrench to set the torque consistently to some level between 10 lbin to 50 lbin. I can try low and I can try high. Also there's a question of HOW you torque the bolts. For example, should you do it when the amp is cold, when it's idling, or when it's going full blast? It's probably safest to do it wihen the amp is cold, but probably more useful to do it when the amp is idling or hot.

The factory and one other service guy has suggested putting heat conducting tape on the rails. At first, I didn't like that idea at all. But I may ultimately have to get that done. Also I wonder about the washers that were allegedly used on later runs of the FPB series, I wonder if I can add those washers to my unit.
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