Issue with Krell KSA-250

What I would first start to look for:

Check for DC at the wall outlet. Check if the transformer cover can be moved by hand, if so then look at the bolts holding down the cover. Remove the cover and see if the rubber or elastomer bushing is still intact.

That is a 60hz buzz so it’s an AC issue, and it’s either the transformer or the soft start relay coil (those are the only two ac inductors I can think of). If it’s the transformer, then it’s probably the bridge rectifier has an open diode and only 60 hz is getting to the secondary instead of 120hz. If it’s the relay, then its probably a shorted rectifier diode, sending ac instead of dc to the coil and causing the contacts to chatter.

OP, very common issue with this series of amps. My buddy had the kinda rare KSA-250s and he had it repaired for the same issue like 3-4 times in a 10yr period. You could look at George Meyer but they may be backed up as well - https://www.georgemeyer-av.com/

Thanks for all the advice! I ended up taking it into Stereo Rehab in Chicago and the experience could not have been better! Casper obviously knows what he's talking about. Turns out it is a front relay. No problems with the caps or transformer. He took me through the whole unit, explained what was happening and gave me a reasonable estimate to fix it (cheaper than I thought it would be). Just an all around great experience. I should have it back in about four weeks.

Thanks,

Blake

@blakeh Good to see that You already have found the problem. I am late answering but I can give some info. I have several KSA 150 and KSA 250. What 9 times out of 10 goes wrong with them the is the sofstartcard that sits right behind the front cover. Someone mentioned the 700 Ohm resistors next to caps. That was almost right but it is not caps. Krell did a design flaw and placed two big 700 Ohm resistors on the softstartcard just besides the two Omron relays that have plastic covers. What happen is that the resistors get so hot that the plastic relaycovers melt and finally destroys the relays. You are lucky if it is the only problem Your amp has! It seems so because You have been wise enough not to start the amp again. If one forces the amp to start with faulty relays next (it is doable....)  what will happen is that the small trafo on the softstartcard will burn taking other components with it to. That is serious trouble! Those trafos are extremely hard ,if not impossible, to find. To avoid this problem in the future there is a modification I have made on all my KSA 150 and 250 amps (not S series) and it is to upgrade the two 700 Ohm resistors to Vishays with aluminium covers with small cooling fins and mount them on the back of the thick frontplate. You only have to drill two small holes gor each resistor and four small screws to attach them. DO NOT DRILL TO DEEP so You drill through the fronplate. The resistors are available at Mouser. They are quite expensive but is is small, well invested money in Your wonderful amp and You will never have the problem with overhetd softstartcard again! I have had no problems with my 7 modified KSA 150 and 250 since I made the mod. Tell Your tech to do it for You while the amp is disassembled. Good luck!

@dclone I for one thank you for your post. It helps to hear from someone that has hands on personal experience with the Krell KSA-250 Amp.

I do have a few questions about the soft start circuitry I hope you can answer.

It’s a shame with an amp this old the manufacturer/designer never released or authorized the release of the schematic wiring diagram of the KSA-250 amp. Especially because of the problems with the soft start circuit board. Release of at least the wiring diagram of the Soft Start Circuit Board.

Just a guess the two 700 ohm resistors that are mounted practically against the AC power relays are in series (or in parallel), with the hot side feed lead of the 4.5KVA power transformer. (In series with the contacts of one the power relays.) The circuit provides the soft start, therein reduces the voltage fed to the primary winding of the 4.5KVA power transformer and thereby reduces inrush current. Simple Ohms law.

So I imagine when the button on the front panel of the Amp is pushed one of the two relays contacts closes and completes the circuit to energize the primary winding of the 4.5KVA power transformer. The two 700 ohm resistors instantly heat up to over 400 degrees F. (maybe more). The coil of the relay is only energized for maybe a second or so, or maybe not de-energized at all. ??? The second power relay contact closes after the initial second or so startup and supplies full AC power, voltage, to the primary winding of the power transformer of the Amp. The full power relay circuit could just shunt across the two 700 ohm resistors. Basically diverting current from the two 700 ohm resistors to the full power circuit provided by the other relay. Sound about right so far?

So the damage that is done by the heat from the two 700 ohm resistors is momentary, cumulatively, over many years of turning the amp on. Correct?

Scratching my head though... All the above posting is based on the purpose/reason for the two 700 ohm resistors. Looking at the picture of the soft start circuit board the four white color rectangular power resistors on the other side of the two relays look more like what would be used in the soft start circuit. ???

.

Soft start circuit board shown here:

https://www.youtube.com/watch?v=Ng3RlJb1SsU

A close look of the board here;

https://www.diyaudio.com/community/threads/krell-ksa-repair.402617/

Attachments

Click on first photo of the blocks of photos provided.

Note. Four white rectangular power resistors have been removed.

.

Jim