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Crown K2 overlap adjustment problem

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Randy Bohannon View Drop Down
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    Posted: 30 October 2022 at 11:17am
Hi all,

I’ve got a K2 that, flush with my success with the Cloud, I decided to crack open and service.

It’s all pretty good in there, the amp has had an easy life by all accounts but there is a problem with one of the output modules in that the pot for adjusting the overlap has no effect whatsoever.



The other channel is fine and I was able to adjust it for the sharper square wave as per the manual, the other channel’s square wave is superimposed on that one in the photo above and working the pot on that channel has no effect on the shape of the waveform.

Any ideas chaps? 
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shaun1264 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 30 October 2022 at 8:28pm
Hi there.  With the K series amps you need to be careful with the overlap underlap adjustment.  I've adjusted many in the past.   You need check how much power its drawing from the mains.  For 240v supply it should be around 90watts at idle. No load.  The square wave should just have a very small amount of rising edge.   If the square wave is too sharp than the amp will start to draw lots of current and will heat up rapidly.   If you have a THD instrument you can measure the distortion with a 20khz sinewave injection to get low distortion when adjusting the overlap underlap and simultaneously checking mains power draw.   

Have you checked the pot to see if it's faulty?  The early K versions had poor quality pots.  Also check the ribbon cables are routed properly as service manual says.  This can affect operation.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 30 October 2022 at 8:31pm
Also I forgot to add, there is two test spades on each module.  Have you tried your probe on the other test point?  I've done that in the past... Where no adjustment could be made untill I tried on the other test point....

When there is too much underlap THD at 20Khz will be just over 1%! And too much overlap will turn the amp into heater with hungry mains current draw! 

It might also be worth checking the EQ correction pots on the input module. So that at 1khz and 20khz it's 0dB.   

You need to very careful adjusting these Class I amps. There is a hell of lot of energy circulating in those output module inductors.  


Edited by shaun1264 - 30 October 2022 at 8:56pm
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Randy Bohannon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 2:06pm
Thank you Shaun, embarrassingly enough the issue was, as you guessed, I was using the wrong test point on the board that "wouldn't adjust"... Embarrassed

I'm grateful for the practical advice, I used one of those cheap plug-top power draw things and set it up so as to have no underlap and drawing about 100W at idle. 

I had to do the auto turn off mod to adjust it, the six turn off is very annoying and for low-level listening too. A 820k resistor was far too much for my unit, I ended up with a 270k resistor and even then it takes more than a minute to turn off.

I only have an ancient HP 333A for distortion measurement, I got for aligning the tuners in vintage receivers and I'm not sure its up to the job with an amplifier like this. If I could find, or build, a suitably robustly protected front-end I would play wit REW and a soundcard for amplifier distortion testing, a project for future perhaps...

I've had a lot of success cleaning pots in an ultrasonic cleaner, not too hot and not too long, followed by cleaning the wipers with Deoxit; the one's in this unit came good.

I recapped the input and main PCBs with Nichicon FG in the direct audio path and a mixture of low leakage and low impedance caps for the decoupling and local filtering caps. 

I took a photo of the ribbon cable position and hopefully have replaced it in the same way, sounds good with no real noise in any event.

I know what you mean about the power of these things, no massive fan and giant live heatsink but massive power from a 2U unit.

I tried it 4R bridged into my load bank and it blew it's 10a main fuse with the power still climbing somewhere around 2500 with no clipping in sight. 

I can't think of another amp this quiet with that sort of HT sub moving grunt.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 4:12pm
Glad you got it sorted.  These amps were Crowns 1st attempt at making a more powerful and lighter 2u switcher. .  Class I topology invented by Crowns chief engineer Gerald Stanley.  A very clever engineering achievement as at the time Crown was not happy with distortion in Class D amps coming from their competitors so they came up with their own invention.   The class I topology must now be in the 7th generation.  After the K series came the CE4000 which used a switching power supply instead of linear as in the K series.  

The K series are very good on bass. High Damping factor and those big inductors in the output modules act like a brake on the back EMF of woofers.  Crown really know how to deal with controlling woofers! 


Edited by shaun1264 - 31 October 2022 at 4:13pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 5:10pm
Thanks for your help Shaun, it's certainly a more daunting prospect to repair than a class AB/G amplifier but at least it's not SMD! 

I'm going to swap into the main system for a critical listen now that it's as good as it can be, be interesting to compare it to some vintage iron as well some modern class AB muscle.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 6:02pm
K2 very nicely put-together amps, something I can't say about what came next. They do use a few custom parts however that are not generally available and do fail. Does anyone else remember the coloured K2's that were put together in a rainbow display for exhibitions? Crown were on top of their game.    
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 6:58pm
I agree, I wasn't very impressed with a XLS 602 I recapped for a mate recently.



 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 7:46pm
A few things to watch out on:

If I remember right the HT rails run at 103v.   The filter caps. ( Two per side but are all in parallel) are quite hard to obtain in the same voltage ratings and physical size.   I've  known people to use 100v rated caps instead!  Better solution would be to use one  150v or 200v 20,000uf on each module off the board using leads to connect to the caps.   I've not known filter caps to fail on these amps yet though considering the temperature in the amp.

There is a good chance caps on the main board will need changing soon .

The way the topology works on these amps the output distortion spectrum is more complex with higher amounts of high order distortion upto around -80db at full power at 8ohms each side which may bother some people if they want very very low distortion amplifiers.   

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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2022 at 8:19pm
The XLS602 aren't really a crown product, they're a C-Audio design (Harman owned both brands). They were made in various issues depending on age and where they were made. Like most C-Audio stuff it was designed and performed well enough but I think eventually suffered from cost-reduced production engineering.

FOr some years the better crown products have been made by chinese/hong-kong company V-Tech and cheaper ones from goodness knows where, including Phonic.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Randy Bohannon Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2022 at 12:13pm
Originally posted by shaun1264 shaun1264 wrote:

A few things to watch out on:

If I remember right the HT rails run at 103v.   The filter caps. ( Two per side but are all in parallel) are quite hard to obtain in the same voltage ratings and physical size.   I've  known people to use 100v rated caps instead!  Better solution would be to use one  150v or 200v 20,000uf on each module off the board using leads to connect to the caps.   I've not known filter caps to fail on these amps yet though considering the temperature in the amp.

There is a good chance caps on the main board will need changing soon .

The way the topology works on these amps the output distortion spectrum is more complex with higher amounts of high order distortion upto around -80db at full power at 8ohms each side which may bother some people if they want very very low distortion amplifiers.   


Interesting comments about the distortion, thank you. I could easily measure -80dB distortion with my equipment, though I've never done it at 600+W stereo... :D

I built a load for my hifi stuff  and thought I was massively over-engineering it by having 16 100W power resistors on a fan cooled heatsink but getting into PA amp repair has found it's limits, time for a bigger version... LOL

The treble of the amp is slightly, what I would call "splashy", you hear it on things like female vocal sibilants, the attack of cymbals, things like that. It's not "HiFi" per se but the bass and the midrange is superb. 

I hasten to add this is being really picky and A/B-ing with some much more esoteric gear.

The criticism of the treble for example is in comparison to gear that is much less efficient, powerful, durable; I would have no problem listening to this amp on the daily.

The main plus for me is how it absolutely savages subwoofers with no fan. A friend had a birthday party in a marquee for his wife's 40th and we set up a nice little basic system of passive JBL EON 15s powered by a QSC 850 (Good reliable amp but I don't like it's sound quality, very grainy and to be replaced by the CLoud VTX 750).

At the last minute the wife ( bless her for this) decided there wasn't enough bass and we hauled the ported JL audio 13W7 from the back of my car and ran it's nominal 3 ohm from the Crown K2 bridged into mono. It was during that really hot weather we had, crossed over at 80HZ pushing it hard enough to keep up with the JBLs it didn't overheat or clip for a party of 50 people. It had to be 2-2.5Kw on the peaks, I was impressed.

I'd love a rack of multicoloured ones! Big smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 03 November 2022 at 1:46pm
This was quite an achievement.... Getting Gerald Stanley to stand there to promote the CE4000!  

I too found the K series top end a bit bright.  A slight rise of frequency response at the very top end inherent of all switching amps or just a very low output impedance at the top end ...I don't know. The input section of the amp is supposed to be an improved version from the  very respected Macro Reference/Studio.  
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