Hi guys,
I bought this on a whim, a version 3, KSA-250 that had hum on one channel. It was a 10 hour drive away so I made the deal on the phone and a friend brought it to me on his trip home. Owner said one channel was fine, the other played music but had some hum. I decided to recap rather than power it up first....
It's a 1995 variation, and had been all recapped EXCEPT for the main power caps, why would anyone pull it all apart and not change the main caps. I thought that would be the issue, easy fix....NOT! When I pulled the cover I discovered on one channel a 1R emitter resistor was missing, and on the other, one was burnt, but still read 1 ohm. I put in a 1R in the missing spot, checked the TO-3 associated with it and it was good. Put in all new main PS caps and powered it up today thinking, ya, it's fixed. Ooops.
So, I haven't delved into troubleshooting it yet, too dejected, will start this weekend, but just a quick summary of what happens. I have meters on both main caps, one left, one right, when I hit the power button, the front board relays click and I get 80VDC on the right, and 80VDC on the left, but then after about 1.5 seconds the amp shuts down and the left channel immediately dropped to 67VDC, the right channel stays at 80VDC. My initial reaction is well it must be the left channel causing the shutdown as there is current drawing the main caps down, but then the 'ol brain says, wait, no current draw on the right, caps staying at 80VDC, maybe it's the right.
At this point, I unplugged and went for a refreshment. I'll pick it up again tomorrow.
As there are no schematics available, I will be troubleshooting this blind . I am an experienced electronics tech of about 50 years, and fixed a dead ML-332 last year, but I had schematics for that, so I am pretty good at troubleshooting.
Just thought I would get the story out there to be digested by all and hopefully a few ideas pop forward.
Any and all help much appreciated.
Cheers!
I bought this on a whim, a version 3, KSA-250 that had hum on one channel. It was a 10 hour drive away so I made the deal on the phone and a friend brought it to me on his trip home. Owner said one channel was fine, the other played music but had some hum. I decided to recap rather than power it up first....
It's a 1995 variation, and had been all recapped EXCEPT for the main power caps, why would anyone pull it all apart and not change the main caps. I thought that would be the issue, easy fix....NOT! When I pulled the cover I discovered on one channel a 1R emitter resistor was missing, and on the other, one was burnt, but still read 1 ohm. I put in a 1R in the missing spot, checked the TO-3 associated with it and it was good. Put in all new main PS caps and powered it up today thinking, ya, it's fixed. Ooops.
So, I haven't delved into troubleshooting it yet, too dejected, will start this weekend, but just a quick summary of what happens. I have meters on both main caps, one left, one right, when I hit the power button, the front board relays click and I get 80VDC on the right, and 80VDC on the left, but then after about 1.5 seconds the amp shuts down and the left channel immediately dropped to 67VDC, the right channel stays at 80VDC. My initial reaction is well it must be the left channel causing the shutdown as there is current drawing the main caps down, but then the 'ol brain says, wait, no current draw on the right, caps staying at 80VDC, maybe it's the right.
At this point, I unplugged and went for a refreshment. I'll pick it up again tomorrow.
As there are no schematics available, I will be troubleshooting this blind . I am an experienced electronics tech of about 50 years, and fixed a dead ML-332 last year, but I had schematics for that, so I am pretty good at troubleshooting.
Just thought I would get the story out there to be digested by all and hopefully a few ideas pop forward.
Any and all help much appreciated.
Cheers!
You may have to bite the bullet and hand draw the schematic. Record the good voltages off the working channel and use that to figure out the problem/s.
check this for ideas
Krell KSA-250 recapping - YouTube
check this for ideas
Krell KSA-250 recapping - YouTube
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Perhaps because this is what Krell used to offer as a standard recap service: all small caps but not the main filters. Those caps just never seem to die. I change them only on sound signature grounds, not because they need changing.
This is only realistic if you have a good idea of what to expect: two discrete servos, discrete input dc detection, multistage bias. The amp is remarkably complex. Circuit is very similar, if not identical to the MDA500 monoblocks.
Hopefully the problem is simple and straightforward. As they usually are unless someone has already tried their hand at repairing and made it much worse.
Thanks guys,
Well, at this point I don't know which is the good channel as the amp shuts down right away, the output relays don't get a chance to close.
We got 5" of snow so I'll be plowing all day...more later.
Thanks!
Well, at this point I don't know which is the good channel as the amp shuts down right away, the output relays don't get a chance to close.
We got 5" of snow so I'll be plowing all day...more later.
Thanks!
Isolating the channels and finding out what fault condition causes the protection to trigger would be my first call. There may also be an issue with the protection circuit itself. Provided you have means of limiting the current disabling the protection PS relay may be a good idea.
Can anyone tell me how many layers the input board is, the one with the XLR and RCA and the DC servo etc.? Reason I ask is there has been a repair on one of the 5 pin headers that join the 2 boards together, and if the plated thru hole has been damaged, those pins may not be making contact with the internal layers, if it's not just double sided. I thought I read somewhere on the web that it may be 4 layers. I don't want to start trying to trace it out if it's not.
Thanks
Thanks
Going back together....again...
I decided to completely dismantle it again and worked my way thru all the boards, looking for cold solder joints on the power board, bad parts etc, none found on the front power board. Then I split apart the input boards from the output boards and started checking all the xstrs I could get at...I wasn't about to unsolder all 56 TO-3 outputs at this point, I suppose I could have removed all 56 emitter R's but again, rather large task.
So what I found was on the right channel, the one that didn't have the damaged 5 pin headers and held 80VDC after power was removed, I found a bad MJF15030 in the driver stage, Q29 for those of you with schematics. I didn't have the flatpack but I had a MJE15030, so I removed the MJF15030 from the relay driver cct, Q37 as it had the same date code as the bad one, put it in and put the MJE15030 in the relay driver cct. using mica washer and grease and a screw insulator, I was able to remove and replace those parts without taking all the outputs off...quite a feat of teeny tiny wiggling and desoldering. I checked all the other devices on board main output and input boards and found no further problems...it's going back together now and should be able to apply power soon....hoping for success.
Cheers
I decided to completely dismantle it again and worked my way thru all the boards, looking for cold solder joints on the power board, bad parts etc, none found on the front power board. Then I split apart the input boards from the output boards and started checking all the xstrs I could get at...I wasn't about to unsolder all 56 TO-3 outputs at this point, I suppose I could have removed all 56 emitter R's but again, rather large task.
So what I found was on the right channel, the one that didn't have the damaged 5 pin headers and held 80VDC after power was removed, I found a bad MJF15030 in the driver stage, Q29 for those of you with schematics. I didn't have the flatpack but I had a MJE15030, so I removed the MJF15030 from the relay driver cct, Q37 as it had the same date code as the bad one, put it in and put the MJE15030 in the relay driver cct. using mica washer and grease and a screw insulator, I was able to remove and replace those parts without taking all the outputs off...quite a feat of teeny tiny wiggling and desoldering. I checked all the other devices on board main output and input boards and found no further problems...it's going back together now and should be able to apply power soon....hoping for success.
Cheers
Attachments
Well, now I know which channel is bad, it's the one that sparked and smoked on power up, the right channel...yup, same Q29 shorted...I pulled all the 1R emitter R's and all outputs seem okay...
I'm going out to the shop to work on the '49 Ford F1...at least with that project there are supposed to be sparks and flames and smoke as I am fabbing some seat mounts.
Cheers
I'm going out to the shop to work on the '49 Ford F1...at least with that project there are supposed to be sparks and flames and smoke as I am fabbing some seat mounts.
Cheers