Krell KAV 250a deader than a dodo

[digitallake]

Andrew LOL - despite appearances I'm not that stupid!

I'm just a wee boy you know

 

[K-amps]

Hi K, Pinkmouse

OK - so the transformer is ok!

I tested it out of the chassis with a lamp lead and no glow - the secondaries read 65V each.

I tested the rectifiers and caps both are ok, so I put in back in the chassis, connected the rectifiers back up and tested it again with the lamp lead.

On first switch on the lamp glowed full brightness, I immediately switched it off. I double checked everything, seemed ok, so I switched it on again. This time the lamp dimmed over about 2-3 seconds then went off. I tested the DC rails at this point and they were reading +/- 92V so I guess it's ok, then???

Confused as to why the lamp glowed in the first place, I can't get it to do that again, but the filter caps were completely discharged first time round.

Confused. But happy

Question is why it happened... Perhaps the AC mains fuse was fast acting? The caps completely discharged? The current inrush limiter not as cold resistive as should have been? Some PSU connection shorting to the chassis ground?

Jim: The trannies are being packed today. Sorry for the delay.

 

[im1663]

Last week, My output transistor of 250a were all shorted, I replace it with MJ15024/25 and it work properly, the off-set output voltage is below 100mv

 

[AndrewT]

Hi,
have you compared the difference in current through each output device?
Have you monitored the way output offset now varies with temperature and during the warm up phase?

Does the 250a have an adjustment to reduce output offset?

 

[im1663]

Hi,
have you compared the difference in current through each output device?
Have you monitored the way output offset now varies with temperature and during the warm up phase?

Does the 250a have an adjustment to reduce output offset?

There are two port for adjustment but I didn't adjust it.
The offset voltage drop to 40mv for 10 hour continous working
I haven't compare the current thr each output device, it is a good question.

By listening test, I can't notice the difference.

 

[im1663]

Does the 250a have an adjustment to reduce output offset?

Yes, there are two trimmers in the amp, the one near MPQ6600A1 (Quad complementary pair) is for adjust the offset.

Another one near the bias transistor 15031 mount on the heat sink is for adjust bias.

 

[carz]

There are 2 jumpers on the power supply board, J2 (Power- Mains) and J3 (ON/OFF - Manual). Do you guys know what these are for ?

Also what is voltage value for TP1 and TP1 and in relation to what ?

 

[anatech]

Hi carz, all,
Without the service information, I would recommend that you should leave their products alone. I would also support the same action in the retail stores. Leave Krell products alone to sit on the shelves (don't buy them).

If you can find an authorized service shop that is reasonable to deal with, allow them to do just that. I feel the possibility for abuse from Krell and their network is high, so maybe that one good shop can help ...

As for poking around taking measurements, don't. One error and you could be in for an expensive repair job. Even the shipping on these is terribly expensive.

-Chris

 

[peter460]

Any one know what the bias voltages on the emitter resistors should be on KAV 250 ? One channel of mine is 7 mv and the other 15 mv . Rail current on both 220mA
PEter

 

[dvavc]

Jim:

... Don't use the 15025/5's either, they don't sound as nice as the newer devices...

K-amps, so which one you recomend should (and may) be used on original Krell KAV 250A board for better sound? 21195/6?
Tks.

 

[dvavc]

Excellent. ... Don't forget to change the resistors as well, and get some new isolation pads for the TO3s.

pinkmouse, what resistors need to be changed?