Fellow hifi nerds,
I'm kinda sorry I joined this forum mainly to ask for help, but it's the only forum I've found on the web where knowledgeable people have in the past discussed this beautiful machine 😉.
What happened? While rearranging my room I managed to short my speaker cables (hey, haven't we all done that at some point?), resulting in a blown mains fuse. Replacement fuse blew immediately, so I decided to buy new mosfets IRFP240 (and an "audiophile" fuse 🙂). Now the amp powers up nicely, but the sound is completely distorted, maybe because there is zero idle current, unless my multimeter is lying to me. I've read up on all old threads regarding the Delta 290 but I'm afraid I'm at a loss: What exactly are those "drivers" that might be shot? Can someone point me towards the parts responsible for delivering the necessary idle current? I'd be grateful for any thoughts.
I'm kinda sorry I joined this forum mainly to ask for help, but it's the only forum I've found on the web where knowledgeable people have in the past discussed this beautiful machine 😉.
What happened? While rearranging my room I managed to short my speaker cables (hey, haven't we all done that at some point?), resulting in a blown mains fuse. Replacement fuse blew immediately, so I decided to buy new mosfets IRFP240 (and an "audiophile" fuse 🙂). Now the amp powers up nicely, but the sound is completely distorted, maybe because there is zero idle current, unless my multimeter is lying to me. I've read up on all old threads regarding the Delta 290 but I'm afraid I'm at a loss: What exactly are those "drivers" that might be shot? Can someone point me towards the parts responsible for delivering the necessary idle current? I'd be grateful for any thoughts.
Just start measuring all components backward from the outputs - including resistors.
Thanks guys, I do have the SM but still I'd love to not measure all components backwards - I can check resistors and diodes but that's about it. No bias current after a short - doesn't that at least limit the options?
You need to take voltage measurements. If there is no bias current (does it adjust at all?) then see what voltage you have between G and S of each FET which will give a clue as to how close to conduction they are.
I know Mooly has remote repaired more than one Arcam 🙂. Unfortunately neither channel is working properly, which seems odd to me, with the short having occurred on one channel only.
In case anyone remembers the Delta 290 very precisely: My new power mosfets have slightly thicker legs than the old ones, which means they touch the heatsink which has only very tight holes for them. Could that maybe, just maybe, be ... not good?
In case anyone remembers the Delta 290 very precisely: My new power mosfets have slightly thicker legs than the old ones, which means they touch the heatsink which has only very tight holes for them. Could that maybe, just maybe, be ... not good?
All three leads must be isolated from the heatsink. Resolving this issue is first order of business.
Yes, very odd that damage to one channel takes out both. I can only speculate that the damage propagated via the power supplies.
My first instinct is to remove the output FETs from the second channel until the first is again working properly. Protection circuits will probably intervene.
I’m going to defer to Mooly, but will follow with great interest.
And welcome to the forum!
Yes, very odd that damage to one channel takes out both. I can only speculate that the damage propagated via the power supplies.
My first instinct is to remove the output FETs from the second channel until the first is again working properly. Protection circuits will probably intervene.
I’m going to defer to Mooly, but will follow with great interest.
And welcome to the forum!
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Thanks BSST, I guess I'll have to drill larger holes into the heatsink then. Probably next weekend :-/ ...
Just to keep you posted: It wasn't mosfet legs touching the heatsink, would've been too simple ....
Voltages between G and S of the output fets: 5, 2, 4, 2 V. Does that tell me anything of interest?
Voltages between G and S of the output fets: 5, 2, 4, 2 V. Does that tell me anything of interest?
Not sure about the gate voltages--- the 5V and 4V might be a bit high. Maybe Mooly will advise. I suggest identifying the order of the FETs you've mentioned to eliminate guess work.
I'll suggest some measurements to pave the way. I'd confirm proper supply voltages first. What are supply voltages at the DC servo opamp? What output voltages do you observe at PA output? Do you observe any bias current at R1? Does it respond to RV1 adjustment? Careful ! (Same questions for each channel.)
Other measurements that will give a broad overview: voltages at PA output, bases of Q10, Q11, and servo out at pin 6, both channels.
Thanks.
I'll suggest some measurements to pave the way. I'd confirm proper supply voltages first. What are supply voltages at the DC servo opamp? What output voltages do you observe at PA output? Do you observe any bias current at R1? Does it respond to RV1 adjustment? Careful ! (Same questions for each channel.)
Other measurements that will give a broad overview: voltages at PA output, bases of Q10, Q11, and servo out at pin 6, both channels.
Thanks.
Around 4 volts would be needed to get the FET into conduction, 5 volts sounds high though. The IRFP data sheet only starts at 4 volts Vgs at which point Drain current is zero. So give or take around 4 volts would be expected.
If one FET is high in the pair and one low then no bias current will flow because one of the pair is still cut off. I see gate protection Zeners. Could one of those be leaky and pulling the voltage down.
Having said that its odd both channels are down. That doesn't really compute. As @BSST says, check the rails first.
If one FET is high in the pair and one low then no bias current will flow because one of the pair is still cut off. I see gate protection Zeners. Could one of those be leaky and pulling the voltage down.
Having said that its odd both channels are down. That doesn't really compute. As @BSST says, check the rails first.
Guys, thanks a bunch for thinking along with me, but I'm afraid this is over my head - and over my time budget .... I managed to grab a Hitachi HA-6 for pretty little money, a very decent MOSFET amp in its own right, and for the time being it'll do. Successfully replacing those output FET with the bent legs was about the limit of what I'm able to do with a soldering rod 😕
Ok fellow members, this amp has landed on my desk.
I will be asking for advice, all support will be appreciated.
But it will be a "slow waltz".
I will be asking for advice, all support will be appreciated.
But it will be a "slow waltz".