Issue with Krell KSA-250

@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. ???

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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.

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Jim

@jea48 Hi! It looks that You have found exactly the right places to guide KSA 150, MDA 300, KSA 250 and MDA 500 owners in restoring those beautiful amps!

I started by watching Mike at Liquid Audio restoring a KSA 150 which was my first Krell poweramp and it of course blowed the softstartboard after two weeks. Sounded just like the OPs KSA 250 before dying. I am also very lucky to have a friend who is a highly skilled, but now retired, Krell tech. He was head of Krells service, repair and spareparts department in Europe and worked his way up from tech to head of the department. He for sure knows Krells!! He have taught me how to disassemle, reassemble, searching for faults, Krells design flaws better substitutes when changing parts etc, etc, etc. His nickname is "The Krell Doc" in Sweden. A very good friend who has access to all the schematics, of course. I do highly respect him for, not even to me, give copies of those. He is loyal and a man of honour.

I do agree 100% with You that Krell would benefit a lot and we owners even more if they made the schematics public and easy to acess. There is no "tradesecrets" to protect in those schematics. For what I know Krell actually give copies to repair techs which they think can do the work correct. Theu do not give them to private persons only to tech companies who are skilled enough.

One must be aware of, and this is very important, that Krell has so many revisions of both PCBs and values of cf components that they hardly even now for themselves what schematic is for which unit. They must have the unit in front of them to tell. I have seven KSA 150 and two KSA 250 (and a lot more Krell poweramps including the monoversions of KSA called MDA) One thing 150 and 250 (and the MDAs) have in common is the softstarboard and its problems. If I look at my seven KSA 150 I have four different versions of softstartboard and not one single board of all seven has the correct colourcoding printed on them!!! You need to take a lot of photos and as I do, document every single step with pen and paper. Also, just work on one amp at a time!!! The KSA serie are easy to work with (if You now how to do of course) because the way they are built in modules. really high quality parts and not a single unnecessary cable. Instead brass bars are used! Fantastic! 

Under the busboard the 4 big caps are situated. I have change mines (probably not necessary) to Kemets. One really important thing is to discharge the old caps before starting to work on the unit in any way. Even after 3-4 weeks the caps holds lethal amount of power inside them. I discharge them with a big ceramic resistor holding the resistor with leather gloves (heat hurts and give You nasty blisters I have learnt that the stupid way...)

So document, document and take photos. Do the softstartboard modification, change caps and effect resistors and, if You want to do it really well, transistors, (begins to be really hard to find Krell marked Motorolas and there are a lot of fakes on eBay etc.) I even have changed the Neutrik XLR contacts, rubber feets under megasized trafos (necessary because they get dry and loose) and speakerterminals etc. Not a fast job. I have sestored approx 12-13 Krell poweramps and 6-8 Krell preamps during a period of 9-10 years.

I will try to send three photos of one of my KSA 250 currently in use in my small homeoffice room. Hopefully You can see the Vishay-Dale 700 Ohm resistors mounted on front plate, the Kemet caps under busboard and the busboard how it is supposed to look when everything is OK and not overheats.

EDIT: I realise that I can not attach photos. I have to learn how to do it first. But my mod is nearly exactly done as at in one of Your links. That guy have even used exactly the same resistors, aluminium shielded Vishay-Dale military grade and seems to have used the same big Kemet "beercan" caps.

@jea48 

@dclone Thanks for all the excellent information! So glad I started this thread. I hope others can benefit from it as much as I have!

@blakeh said:

So glad I started this thread. I hope others can benefit from it as much as I have!

It would help if you could get the answers to a few questions from the service tech that will be repairing the Amp.

1) What exactly do the two 700 ohm resistors do in the soft start circuit board? What is the power wattage of each resistor? (Hard to judge by a photo. They appear to be maybe 5 watt each.???) Are they wired in parallel or series with one another? Are the resistors wired in series with the Hot lead of the primary winding of the 4.5KVA power transformer, controlled by a set of contacts of one of the two power relays? Therein the purpose of the circuit is reduce, limit, inrush current on cold turn-on of the amp.

2) What are the four square diameter X rectangular long power resistors for, do, in the soft start circuit board. I would think they are actually used to limit the inrush current to the primary winding of the 4.5KVA power transformer. If not what do they do?

If not for the soft start circuit to limit, reduce, current, on a cold start-up of the Amp, the inrush current for a cycle or a few cycles could be as high as 8 to 12 times of the 4.5KVA rating of the power transformer, basically, with its secondary winding shorted because of the four large de-energized electrolytic caps in the DC power supply. Other factors come into play just how much current would travel in the AC power circuit. Like the size of the branch circuit wiring feeding the Amp and the length of the branch circuit wiring. Even the wiring size, type of wiring, (assume aluminum) and length of the wiring from the electrical panel in the house to the utility Power Transformer. As well, the size, KVA rating and the winding’s impedance of the Utility Power Transformer.

Power transformer in the KSA-250 Amp? Reported to be 4.5KVA. 4500W / 120Vac = 37.5A... 37.5A X 8 = 300A possible inrush current. 37.5 X 12 = 450A possible inrush current. No way a 20A circuit breaker would pass 300A let alone 450A without immediately tripping open.

A soft start circuit is a must with this amp...

Here is the soft start circuit wiring schematic diagram for an ARC VTM200 power Amp.

ARC VTM200 (Power Supply page 2.)

Note the power wattage rating of 40 watts for the resistance, ohm-age, used for the soft start circuit current limiting fed in series with the Hot lead, leg, of the power transformer in the Amp.

Note:

R81 A II B

(Quantity 2) 10 ohm 20W WW (Wire Wound)

Wired in parallel = 5 ohms, 40 watts.

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