Issue with Krell KSA-250

@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.

.

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.

Honestly I don't know in what universe it can be considered a good thing to deny folks the means to service or fix the gear they own and paid for.

Well, Krell is good as dead nowadays (sadly), so this point is probably moot anyway

@devinplombier if you ever seen what some people do to them, including some techs, you would know why. These types of repair jobs often used to end up at Krell service. They give schematics to authorized techs.