Krell FPB400CX Restoration

[new2hifi]

On 26/04/2023 at 8:12 AM, Natehack said:

Just like tubularbells said above, the FPB’s are very modular in their design; if you like/liked Legos and logical sequencing of steps you’ll love these FPB’s!! 

To get at all of the axial caps a person does need to remove the board, but removal truly isn’t something that should give you too much trouble. Every connection that needs to come apart/be moved/pushed aside is able to be. 

 

Have fun with it! There will be ‘pulling out your hair’ moments, but have confidence. Even taking my time as a new learner I was able to have this done from first top plate fastener removal to playing music in less than 2 weeks (including waiting for capacitors). Now our FPB throws our Kef Reference 4.2’s around with so much precision and power it’s difficult to see the need for a different pair of speakers!
 

 

Hi Nate:

Can you please elaborate and share any photographs of when you separated the heatsink from the output board? I am also wondering about this step as usually the output transistors on output board are attached to any heatsink using heatsink compound or thermal insulator pads; did you replace them too?

 

There is a very detailed guide by another Krell owner replacing capacitors of monoblocks at https://pages.cs.wisc.edu/~timc/e/krell-cap/index.html . That person also clipped the axial capacitors and then vacuumed the older solder out from top side and never detached the output board from the heatsink just like Doug.

 

I'll also appreciate if you can go in detail for any ‘pulling out your hair’ moments you mentioned.

[Natehack]

12 minutes ago, new2hifi said:

 

Hi Nate:

Can you please elaborate and share any photographs of when you separated the heatsink from the output board? I am also wondering about this step as usually the output transistors on output board are attached to any heatsink using heatsink compound or thermal insulator pads; did you replace them too?

 

There is a very detailed guide by another Krell owner replacing capacitors of monoblocks at https://pages.cs.wisc.edu/~timc/e/krell-cap/index.html . That person also clipped the axial capacitors and then vacuumed the older solder out from top side and never detached the output board from the heatsink just like Doug.

 

I'll also appreciate if you can go in detail for any ‘pulling out your hair’ moments you mentioned.

I referenced the link you provided for my service as well! Nice find isn’t it? Great link.

 

Thinking back, examining the old axial caps, popping off the top cover, and now putting the correct names to the correct pieces of the FPB (because honestly I am a newbie when it comes to how these beautiful pieces work), I absolutely DID NOT detach the output board from the heatsink. I was able to desolder old axial caps AND solder new caps with them connected.
 

The OE axial caps were installed with no bends in the legs so they came out easy. Some had to be reflowed with new solder to grab any residual, but no issues. I just kept my dwell time short, mirrored total leg length and bend location, and they went right in. 
 

Sorry about not knowing what board is named what!! I didn’t take pics of the process, just of important connections for reassembly. 
 

I only pulled out my hair once because I mistakenly ordered too few of one type of cap! Otherwise these amps are a treat to work on. Easy enough for a newbie. Keep your dwell time short (use some heat and flux), take pics of connections, and enjoy the thousands of dollars you can save!

 

Or bring it here and I’ll do it for ya:)

[tubularbells]

1 hour ago, new2hifi said:

Hi Doug;

It seems you never separated the heatsink and just worked on the top side to de-solder and solder the axial capacitors on the output board; right?

 

Correct. I never felt the need to replace the insulation washers for the output transistors as everything looked perfect and the benefit Vs. risk of pulling this area down was simply not worth it for me.

[new2hifi]

Here is the million dollar questions! The most important missing detail. Did it improve the sound? Any changes in sound or performance? Or just the confidence of no dried out electrolytic capacitors? If it did improve the sound then can you please elaborate?

[new2hifi]

Krell FPB Calibration

 

1. Proceed with caution; I am not responsible for any harm.

 

2. If you try and run it in calibration you must do it on a dedicated line 220v preferably or a good solid 15 amp 120 volt line. Once it starts it cannot be interrupted until it is finished. I have no idea what will happen if it is interrupted.

 

3. Disconnect all the cables.

 

4. Turn on the amplifier using front round silver button. When you see triangle of three blue LEDs and here two clicks. Press and hold silver button again until left LED start blinking. Now wait for 15-20 min during which LEDs will come and go. Then all three LEDs will come back. Amplifier is calibrated and ready.

 

5. Amplifier cannot be and shouldn’t be disturbed during calibration process; it cannot be stopped. Thus, only do this process when electricity is stable.

[Allan]

@tubularbells

 

 G'day.. I'm after some feedback please on how your 400cx sounds today, is there anything that stands out in compared to how it sounded prior to the recap?

 

I once owned the 300cx, stupid me.. I've dreaded ever since selling it, was a 1 owner too. In any case I've just taken possession of a mint KSA-150 in original condition other than a soft start repair. It sounds nice but doesn't quite have the resolve I remember of the 300cx or compared to a Levinson No.27 I also have which has had a recap, excellent little amp the No. 27. The KSA is running a little warmer in the left channel and its DCO fluctuates around 5mV whereas I've not seen the right channel DCO move off 0mV., so be nice to see that also addressed.

 

Plan is to recap the KSA-150, sending it off in the next week or so.

 

 

 

 

[tubularbells]

8 hours ago, Allan said:

@tubularbells

 

 G'day.. I'm after some feedback please on how your 400cx sounds today, is there anything that stands out in compared to how it sounded prior to the recap?

 

I once owned the 300cx, stupid me.. I've dreaded ever since selling it, was a 1 owner too. In any case I've just taken possession of a mint KSA-150 in original condition other than a soft start repair. It sounds nice but doesn't quite have the resolve I remember of the 300cx or compared to a Levinson No.27 I also have which has had a recap, excellent little amp the No. 27. The KSA is running a little warmer in the left channel and its DCO fluctuates around 5mV whereas I've not seen the right channel DCO move off 0mV., so be nice to see that also addressed.

 

Plan is to recap the KSA-150, sending it off in the next week or so.

 

 

 

 

 

Hey Allan, in short sensational. after performing major surgery a few years ago its been rock solid without a hint of trouble. In addition its probably one of the best sounding amps ive ever heard with almost limitless power. The ability to listen to my music at highlevels of volume without a hint of clipping, distortion or compression never fails to impress. 

 

Sounds like a similar issue to mine where one channel running much hotter than the other. A full recap will rectify that for you though.

 

Did a full referb of a ML 336 last year which also is a very nice sounding asp (but a nightmare to work on)

 

[Allan]

On 02/05/2024 at 6:48 PM, tubularbells said:

 

 

Did a full referb of a ML 336 last year which also is a very nice sounding asp (but a nightmare to work on)

 

 

 

I must say Krell's modular design used in their amps is outstanding.. bus bars and no wiring, a brilliant design.

 

Biasing my 27, about the only thing that can be deemed easy, though I must say the reward for effort on this amp is world class.

 

Edited May 3 by Allan

[tubularbells]

1 minute ago, Allan said:

 

I must say Krell's modular design used in their amps is brilliant, bus bars and no wiring, a brilliant design.

 

Biasing my 27, about the only thing that can be deemed easy, though I must say the reward for effort on this amp is world class.

 

 

Re-ignighted my nightmares Allan with that pic...

 

[Allan]

^ oh man where do you start.. thats enough for a bloke to develop a phobia. Nice clean example!

 

I think your a bit like me, I find it tough to discard the ol' high current amps particularly when your familiar with what they bring to the table. Had a great session last night with Supratek 6SN7 driving the KSA-150 into some Thiel 2.3's, played Ian Moss's album "Six Strings", a live double album of just himself and 6 strings, majority acoustic. Went to bed thinking man, its been a long time since I got in for free. Without any shadow of doubt, Moss is Chisel.

 

Its going to be interesting to hear what the recap does for sonics. I recently did some testing on a class A amp by varying its filtering capacitance, the lower end resolve being effected. Also having a dedicated 20 amp line put in solely for amp(s). I was going to recap the KSA myself but there's a few upgrades Krell implemented so I figure its best in their hands. The KSA's condition is well above the norm & so worth the investment.

 

 

Edited May 3 by Allan

[tubularbells]

8 hours ago, Allan said:

^ oh man where do you start.. thats enough for a bloke to develop a phobia. Nice clean example!

 

I think your a bit like me, I find it tough to discard the ol' high current amps particularly when your familiar with what they bring to the table. Had a great session last night with Supratek 6SN7 driving the KSA-150 into some Thiel 2.3's, played Ian Moss's album "Six Strings", a live double album of just himself and 6 strings, majority acoustic. Went to bed thinking man, its been a long time since I got in for free. Without any shadow of doubt, Moss is Chisel.

 

Its going to be interesting to hear what the recap does for sonics. I recently did some testing on a class A amp by varying its filtering capacitance, the lower end resolve being effected. Also having a dedicated 20 amp line put in solely for amp(s). I was going to recap the KSA myself but there's a few upgrades Krell implemented so I figure its best in their hands. The KSA's condition is well above the norm & so worth the investment.

 

 

 

You are quite correct in that these out-dated big old "lead sleds" do bring a certain magic/realism to the table particularly when pushing the limit a bit. Even with electronic certificates & a diploma in audio I still dont quite get the whole amp-speaker relationship however I do recognise that when the match is right something special happens.

[Allan]

yes the magic/realism when pushing a bit is exactly what does it for me too. I like the fact that nothing is lost, or gained for that matter. 

 

I place realistic dynamics, retained bass & percussion drive in the same basket as being in tune. Bass is the the soul of all music, anyone who plays an instrument will understand this, its tempo's brother & percussion is the bro-inlaw.  There is music that needs this drive and the difference without it does effect "my" perception of enjoyment. One artists comes to mind, Doug MacLeod. I feel a lot of his music calls for great drive otherwise I get bored very quick and will change the music. Another is the Ian Moss album I mentioned earlier, without great drive Mossy's effort to bring emotion into his music when playing solo would be, for lack of a better word.. monotone.

 

All that aside, I do love the No. 27. It doesn't quite have the drive of say a 300cx or the KSA-150 and neither it should, its only 100w @ 8Ω but it excels in harmonics and overtones. Its extremely sure footed around delicate music and that brings enormous emotion... it'll make you cry!

 

I dont think there is the perfect amp, but its nice to have perfection even if it means two amps and two high currents win, for me.

 

Yep when you find the perfect amp speaker match it is magic all right. I've found it once so far, 300cx and Proac 3.8

 

 

 

Edited May 5 by Allan

[GlassAudio]

Are service manuals with schematics for the old Krells available? I have looked briefly with not much luck.

[tubularbells]

1 hour ago, GlassAudio said:

Are service manuals with schematics for the old Krells available? I have looked briefly with not much luck.

 

There are fragments of info but no definitive cct. diagram that I could find also. For example

 

hfe_krell_fpb-300c_600c_schematics_en.pdf 

[GlassAudio]

Thanks for sharing.