This time it's dead "and" buried!!!
Hi Everyone
Been a while
After the rebuild was completed I sold the amp to a hi-fi enthusiast I know...needed the money!
I've now bought it back off him for peanuts because he managed to BLOW UP THE MAINS TRANSFORMER and, if that wasn't enough, he also managed to fry the right channel in a similiar fashion to the left channel which I spent months repairing with all your help
So, the left channel is still ok, the other one needs replacement output devices, of type MJ21195/6 (5 pairs) to get it working and to match the other channel. Grrrr!
Don't know what to do about the mains transformer it's a custom made job, in the order of 2kVA or more, I measured the DC on and off load when I had the amp in bits last time, it's +/- 90V off load so the AC is probably about 60V - I'm thinking of using 2 transformers instead.
I can only guess why the transformer failed, there is no soft-start so the switch on transient, over time would have stressed it, the primary is reading 1 ohm....
K-AMPS - my saviour last time - if you've still got some ON semi transistors knocking around for sale how much do you want for 5 pairs??
Thanks, Jim
Hi Everyone
Been a while
After the rebuild was completed I sold the amp to a hi-fi enthusiast I know...needed the money!
I've now bought it back off him for peanuts because he managed to BLOW UP THE MAINS TRANSFORMER and, if that wasn't enough, he also managed to fry the right channel in a similiar fashion to the left channel which I spent months repairing with all your help
So, the left channel is still ok, the other one needs replacement output devices, of type MJ21195/6 (5 pairs) to get it working and to match the other channel. Grrrr!
Don't know what to do about the mains transformer it's a custom made job, in the order of 2kVA or more, I measured the DC on and off load when I had the amp in bits last time, it's +/- 90V off load so the AC is probably about 60V - I'm thinking of using 2 transformers instead.
I can only guess why the transformer failed, there is no soft-start so the switch on transient, over time would have stressed it, the primary is reading 1 ohm....
K-AMPS - my saviour last time - if you've still got some ON semi transistors knocking around for sale how much do you want for 5 pairs??
Thanks, Jim
Yes I got th MJ's Jim. How many do ya need?
I think it is a 1 or 1.2 kVa part from Plitron. You probably need a 64-0-64 sec. 60-0-60 will yield only 83vdc's. Maybe even 65-0-65. The max rating is 1800va, but a 900va toroid can peak out at 1800va on audio duties without issues.
I think 1kVA should work, I do not think the amp would be hard pressed to blow a 1kva part, however you might get some voltage sagging for an off the shelf part. So do 66-0-66 to compensate.
I have read readings of 96vdc on my KAV-250a, so 66-0-66 should be ok.
Best,
K
I think it is a 1 or 1.2 kVa part from Plitron. You probably need a 64-0-64 sec. 60-0-60 will yield only 83vdc's. Maybe even 65-0-65. The max rating is 1800va, but a 900va toroid can peak out at 1800va on audio duties without issues.
I think 1kVA should work, I do not think the amp would be hard pressed to blow a 1kva part, however you might get some voltage sagging for an off the shelf part. So do 66-0-66 to compensate.
I have read readings of 96vdc on my KAV-250a, so 66-0-66 should be ok.
Best,
K
No idea on the prim impedances... Check the bridges.
I would decouple the secondaries from the bridges, then power on the amp with a 100w bulb in series with the mains. If no load, you know the donut may be ok. I cannot believe a toroid blowing on a commercial amp, especially a Krell. I think it's the bridge/or cap
I would decouple the secondaries from the bridges, then power on the amp with a 100w bulb in series with the mains. If no load, you know the donut may be ok. I cannot believe a toroid blowing on a commercial amp, especially a Krell. I think it's the bridge/or cap
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
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
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