Krell KSA-250 Soft Start Resistor Relocation

SCompRacer

My old mighty Krell KSA-250. Like me, complicated, old but still useful. Stable to 0.5 ohm, 4kWpc.



One of the common failures is the soft start board. I guess soft start cause of the 4.5kva transformer and four bigger than coke can caps.



One channel output board.



Two 700 Ohm resistors next to some caps on the soft start board get hot. Like 400F hot. The caps dry out and one day, boom.



I've seen one fellow use metal case Dale resistors and attach them to the front panel, connecting them to board with wire leads. For now, I just elected to swap sides of the board they are on. Plenty of room between the board and front panel.

Resistors removed...





Two 700 ohm resistors moved to opposite side of board.



This is better. The resistors have a good standoff to keep from baking the board, but not much concern for those caps when mounted on component side of board. I popped the caps that were next to resistors off to test and they are just within spec. I think I'll be replacing them soon. I've seen pics of caps from soft start boards in these amps badly scorched and discolored.



Like this, burn baby burn. This is from a different version KSA-250 amp where they mounted both resistors on same side of board.



Hot stuff! This should increase the life of my soft start caps and prevent catastrophic failure of the amp.

soundfreak1

Always like the look of a power supply that looks like it could power a small town.LOL

soundfreak1

I bet that amp could double as a hibachi grill. The cat looks like it's warming itself by the "fireplace"

soundfreak1

Btw Your "little buddy" reminds Me of mine.

MrBuhl

Many is the time I wish I had the know-how and 'nads to do that kind of work myself. Looks great!

ken brydson

MrBuhl wrote: »

Many is the time I wish I had the know-how and 'nads to do that kind of work myself. Looks great!

I know right?

honestaquarian

I remember that beast of an amp well. That and a pair of B & W 801 Matrix Series III's in a hi fi shop in Hartford, Connecticut called The Stereo Shop. I just happend to wander in when the late Steve Portocorrero (*a BIG hulk if a muscle man and a hell of a nice guy to boot. He was Krells National sales rep I believe*) was there demoing the Krell to them. Such a synergistic combo those Bower & Wilkins and that Krell KSA-250 were!!! (*this was 25 years ago before they started calling B & W Bower & Wilkins-even though that is what it stood for the whole time*) I popped in a Telarc CD that has Borodin's Polovetsian Dances on it. I think it's the CD that has Stavinsky's Firebird. It is one of the RARE recordings with the chorus. Honest to god I could hear the chorus up on risers behind and above the orchestra. Then I heard the bass drum on the floor behind the orchestra,but in front of the chorus!!!!!
I had never heard that kind of detail before and have not heard it since.
Great piece of kit that.

SCompRacer

Thanks for the kind words. Once you get involved in diy, you get braver and braver until something bites you in the b-u-t-t, I guess. This amp is very complex with a DC servo and autobias circuits. No schematics available to the public for the 250. I have one for the KSA-150 which is a similar/smaller version. Help exists at diyaudio. At 140 pounds plus packaging, shipping costs a bunch. Krell service is like $125 an hour.

It still sounds very, very good though from low to high SPL. It has a cult following and haters too. lol

soundfreak1 wrote: »

Btw Your "little buddy" reminds Me of mine.

Aww...cute kitty. Yeah Jasper likes the warmth.



Hungry too. 10 amps at idle. Need a 20 amp circuit or you'll get close to where a 15 amp circuit breaker trips. Once upon a time Krell would convert these to 500 watt monoblocks. If you bought one MDA-500 they would convert your KSA-250 for free. Ed the SRT speaker king has a pair of the MDA-500's. Sick...lol

soundfreak1

I have a lot of love for Krell. My KAV 250 is a wonderful amp. 500w at 4 ohms driving my RTA12c's (they love it too) As they just sing for me. Even though it it is class A/B it probably plays about 70+% of the time in class A as I don't have to drive it hard enough to go into class B often. And it (and they the 12c's) image so well I find myself listening with my mouth wide open in awe of what I'm hearing. It is truly gobsmaking at times with the right sourcing material. I swear I have too keep saying "excuse me, pardon me, excuse me" as I walk around the room as I fear I am pissing off the band members by walking in front of them as they play. I swear I got hit in the back of the head with a guitar neck once. LOL.

txcoastal1

Wow that's some serious pull. I thought the current draw from Matt's Thresholds 12's was bad. They start up in the 7's each but settle in the 5amp range, but no soft start. Nice mod

agfrost

Electric company should subsidize the purchase price of an amp like that. Everyone wins!

txcoastal1

agfrost wrote: »

Electric company should subsidize the purchase price of an amp like that. Everyone wins!

Probably get fined for not having a good star rating from EPA or some other gov't a$$