Repair of Krell KSA-100

[Jackson75]

Hello all.
I've got a job for a friend to repair his Krell KSA-100. I've about 15 years experience from audio amp repair and design, but Krell has never crossed my way before. The amp is all dead, mains fuse OK, powerswitch OK is all that I've checked so far. Slowstart power resistors OK, as far as I can tell. Between the two mains transformers there are a small relay board which I believe may be some kind of starter for the primary side of the transformers. Is that so? Output sections seems OK, measured across some of the final transistors, but that's not the problem. The secondary cap bank never gets charged up so there must a powersupply problem.
I suspect this place is full of people who has worked on Krell's and may have some advice for me.

Best regards
Jackson

[nigelwright7557]

Checked secondary fuses and transformer and bridge rectifier.

[Jackson75]

Well, yes. Obviously, it appears no mains voltage to any of the power transformers.

[embrown057]

Had the same amp before with the same problem, turned out to be that relay was open. I replaced it and amp was go to go. I did replace a lot of the old cap as some were leaking. I would say check there first, no pri B+ sounds the same to me but never know.

[LineSource]

Take a look at the restoration and schematic info on the web.

Krell KSA-100 Mk II restoration
Krell


I would:

1) connect a 100 watt light blub in series with the AC power line to protect against amplifier shorts from destroying additional components or blowing a home circuit breaker.

2) The KSA100 has a power up circuit which has a slow start resistor shorted out by a relay after the main power capacitors ramp up to full voltage. It is common for these relays and power-up resistors to fail. There are a few different simple circuits used to sense the capacitor voltage and trip the shorting relay.

3) Check the leakage on the main power capacitors before you attempt to power up. You want to check for both shorts and high leakage. I put a 1k resistor in series with each capacitor and monitor the resistor's voltage drop(charging current) using a lab power supply. If the capacitors are more than 10 years old, I would probably replace all of them as part of the rebuild/chean-up.

4) Since one common output power supply runs both channels, disconnecting each channel from the large output power caps can isolate a bad channel. Now you can use the good channel as a reference.

5) If the predriver PC board circuits do not properly power up, the output power supply will not ramp up on the MK II revision.

[Jackson75]

Thanks for the post. Unfortunately, there are no schematics available on the restoration site.
Just to make clear for my mind:
Both powertransformers are connected to the mainspower as soon as the powerswitch is ON through series resistors, these resistors are shorted out when cap bank has reached a threshold voltage.
So, obviously, when there's a short in the amplifier modules, this will make cap voltage decrease and make the softstart relay go open, and the softstart resistors will bang open.
What I was uncertain of, if the power transformers had a start-up relay which was needed to close before transformers got power, but this seems not to be the case.
I measure resistance across the powercord, powerswitch ON, there's all infinite. I would expect to read the softstart resistors at least.
Anyone got the schematics for this beast, or the powersupply part at least?

[AndrewT]

I think all the data is already on the Forum or linked from within the Forum.

[Jackson75]

It appeared the 5ohm/50W primary softstart resistor was open.
Replaced this and unit is up and running again.