tubularbells Posted July 29, 2021 Share Posted July 29, 2021 Decided to finally document the refurbishment of my Krell 400CX that I picked up a few months ago. Although it worked (and sounded) fine I was bugged by a minor heat issue with the left channel running approx 10 degrees hotter than the right so decided to take the plunge and perform a full recap in the hope that the issue might be related to the aging capacitors which for those who've done it know its is quite the undertaking with over 200 capacitors to replace! So off went the lid for a bit of a poke around. Thankfully no signs of previous repairs or anything dodgy (always a concern with vintage gear) but did notice something interesting that wasn't stock. Krell in there infinite wisdom went through a phase whereby in an effort to limit grey imports devised a circuit to sense the line frequency of the mains and if it didn't match the "region" for which it was initially destoned wouldn't allow the unit to turn on even if you changed the primary transformer taps for correct voltage so in order for it to work you either needed to send it into Krell for a software reflash or had to somehow trick it into thinking it was still in its region of origin. In my case it was the latter with someone installing an oscillator board running at 60hz which is being injected into the line frequency sensing circuit fooling it even though in reality the mains is 50hz. Notice in the pic below the miltipin IC with the sticker stating 60hz operation Next up some disassembly ready for some new caps. Removing the input board and lower regulator board reveals even more bypass caps! First of the main filter caps replaced First regulator board done. Onto the the axial bypass caps on the lower board. Noticed one cap that had gone leaky but surprisingly putting it on my ESR meter it still measured ok! First one done (blue cap) Getting there... And done! So thats where im up too at the moment. For those interested heres a link to my initial parts order. there have been 3 more since then for misc. stuff if not captured in my original order. http://www.digikey.com.au/short/rnmf5brd 19 3 Link to comment Share on other sites More sharing options...
evil c Posted July 29, 2021 Share Posted July 29, 2021 @Tubularbells man that is one massive project, and l have faith this will result in a more favorable outcome- than unmentionable, past traumatizing experiences! 1 Link to comment Share on other sites More sharing options...
metal beat Posted July 29, 2021 Share Posted July 29, 2021 Wow, braver man than I would ever be. I am beginning to think that you like being a tech more than just sitting back and listening As long as you are having fun. 3 Link to comment Share on other sites More sharing options...
tubularbells Posted July 29, 2021 Author Share Posted July 29, 2021 21 minutes ago, metal beat said: Wow, braver man than I would ever be. I am beginning to think that you like being a tech more than just sitting back and listening As long as you are having fun. Its the unfortunate truth MB. My past 18 mths have been plagued with amp issues one way or another. I literally have crates of vinyl purchases that remain unplayed that I hope to rectify soon. 1 Link to comment Share on other sites More sharing options...
Steve M Posted July 29, 2021 Share Posted July 29, 2021 Ambitious resto project! Kudos to you Tb, I’m sure the Duntech Sovereigns will appreciate it. 1 Link to comment Share on other sites More sharing options...
Jakeyb77_Redux Posted July 29, 2021 Share Posted July 29, 2021 Nice work champion! 1 Link to comment Share on other sites More sharing options...
frednork Posted July 29, 2021 Share Posted July 29, 2021 (edited) Nice work TB!! 75% through?? Edited July 29, 2021 by frednork Link to comment Share on other sites More sharing options...
Steam Posted July 29, 2021 Share Posted July 29, 2021 (edited) Really love this. It’s a great amp. Seems to be in amazingly good condition considering age. Edited July 29, 2021 by Steam Link to comment Share on other sites More sharing options...
tubularbells Posted August 2, 2021 Author Share Posted August 2, 2021 Recapping done. Fairly tedious job but hopefully worthwhile. Interestingly all caps I removed and tested measured ok with regards to capacitance and ESR which frankly surprised me seeing as the external heat sinks measure around 60 degrees C (so internals must be higher) at operating temp and the fact this amp is nearly 30 years old. 13 2 Link to comment Share on other sites More sharing options...
frednork Posted August 2, 2021 Share Posted August 2, 2021 IndustrIal art! Looks like an eagle. 1 Link to comment Share on other sites More sharing options...
tubularbells Posted August 2, 2021 Author Share Posted August 2, 2021 7 minutes ago, frednork said: IndustrIal art! Looks like an eagle. Spread eagle 2 Link to comment Share on other sites More sharing options...
frednork Posted August 2, 2021 Share Posted August 2, 2021 18 minutes ago, Tubularbells said: Spread eagle ready to be xxxxxx xx xxx xxx? lets hope not Zed 1 Link to comment Share on other sites More sharing options...
Martykt Posted August 2, 2021 Share Posted August 2, 2021 Nice work TB !! Hopefully the system will be back on track now. 1 Link to comment Share on other sites More sharing options...
Matt Anderson Posted August 2, 2021 Share Posted August 2, 2021 Wow, it looks incredibly clean! I don’t think mine looks that good inside. I can only hope that mine comes up like that when i get it recapped. Great work mate, thanks for posting this. 1 Link to comment Share on other sites More sharing options...
Ooogh Posted August 2, 2021 Share Posted August 2, 2021 Brilliant stuff Doug, looks magnificent, hopefully the channel temperature difference will be gone once it is all bolted back together. 1 Link to comment Share on other sites More sharing options...
Bass13 Posted August 2, 2021 Share Posted August 2, 2021 Amazing work, a credit to your patience & skills especially under these difficult times too, I'm sure she will sing exceptionally well, I guess you will embark on a dusk to dawn music/vinyl marathon once completed. With plenty of your favorite drink of choice. 1 Link to comment Share on other sites More sharing options...
tubularbells Posted August 19, 2021 Author Share Posted August 19, 2021 So to close this one out reassembly is complete and its working (and sounding magnificent) thankfully. Some comments: Dont undertake a project like this unless you know what your doing as it is super easy to screw something up in the process potentially turning the amp into an anchor and certain models have little to no info on the web making diagnosing issues even worse. Make sure you have the right tools for the job. I know this from past failures and in fact had to upgrade my otherwise decent desolder station after not getting great results removing the first handful of caps. There are dual traces everywhere and some are thick so lesser stations wont have the grunt to safely remove old solder before lifting pads and traces. To re-cap or not re-cap? So upon buying the amp it sounded 100% even after close to 30 years and the dozen or so caps I measured were still surprisingly still within spec in both capacitance and ESR however that said my amp was running at temps in the mid 60's Celsius with the left about 15 degrees hotter so clearly something wasn't right and suspect the plateau bias was constantly running flat out hence the heat even at idle. After replacing all caps temps are now in the mid 40's with both channels within a degree of each other so my point is although the amp might still sound ok a recap will help bring it back to original speciation's and get things working as they should. And finally this is what it takes to FULLY recap a Krell 400CX 9 2 1 Link to comment Share on other sites More sharing options...
Yamaha_man Posted August 19, 2021 Share Posted August 19, 2021 @Baldieman right up your alley. 1 Link to comment Share on other sites More sharing options...
BuzzzFuzzz Posted August 19, 2021 Share Posted August 19, 2021 Awesome work there Bro... well done. Looking forward to hearing it, one of these days. 1 Link to comment Share on other sites More sharing options...
tubularbells Posted August 19, 2021 Author Share Posted August 19, 2021 17 minutes ago, BuzzzFuzzz said: Awesome work there Bro... well done. Looking forward to hearing it, one of these days. Better be quick as other amps are on there way... Link to comment Share on other sites More sharing options...
ray4410 Posted August 19, 2021 Share Posted August 19, 2021 3 minutes ago, Doug Johansson said: Better be quick as other amps are on there way... other amps on the way the mind boggles, should be interesting. Link to comment Share on other sites More sharing options...
frednork Posted August 19, 2021 Share Posted August 19, 2021 That is definitely a BOSS level recap that many techs would think thrice before taking on. Well done DJ! 1 1 Link to comment Share on other sites More sharing options...
tubularbells Posted August 19, 2021 Author Share Posted August 19, 2021 38 minutes ago, ray4410 said: other amps on the way the mind boggles, should be interesting. Planning on being my "Summer" amps. A pair of March Audio P701's. Should be interesting as the specs on paper are quite similar in terms of raw power but with a massive difference in weight and power consumption and hopefully not sound quality. I've owned Class D in the past so know already know I like the sonic presentation of the technology. Fun times ahead I hope. 2 Link to comment Share on other sites More sharing options...
ray4410 Posted August 19, 2021 Share Posted August 19, 2021 11 minutes ago, Doug Johansson said: Planning on being my "Summer" amps. A pair of March Audio P701's. Should be interesting as the specs on paper are quite similar in terms of raw power but with a massive difference in weight and power consumption and hopefully not sound quality. I've owned Class D in the past so know already know I like the sonic presentation of the technology. Fun times ahead I hope. 1200watts@2ohms,should suit the dunny's just fine. Link to comment Share on other sites More sharing options...
tubularbells Posted August 19, 2021 Author Share Posted August 19, 2021 Just now, ray4410 said: 1200watts@2ohms,should suit the dunny's just fine. Tell you what I reckon im pushing those levels right now. Transients are shocking! 5 Link to comment Share on other sites More sharing options...
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