Evening all.
I recently bought a (non-functioning) NAD 7020e on flea bay for a smallish sum and, on examination, the primary of the main transformer (it has a smaller one to power the tuner) is O/C.
I pulled it out and had a look for the (now legendary) thermal fuse, to no avail. However, in addition to the wired in primary black/red wires there are, on the same side of the transformer and of the same gauge, a white/blue pair which were just blanked off with plastic doo-dahs.
My hope is that this is a separate 120v primary which I can drive with a suitable step-down. I shall (after disconnecting all the secondaries of course) squirt in a small voltage AC and see what comes out but my question is: does anyone know the nature of this thing? Is my hope foolish? I've been trying to source a replacement/s for the transformer and it's not looking good...
Cheers, Jon.
I recently bought a (non-functioning) NAD 7020e on flea bay for a smallish sum and, on examination, the primary of the main transformer (it has a smaller one to power the tuner) is O/C.
I pulled it out and had a look for the (now legendary) thermal fuse, to no avail. However, in addition to the wired in primary black/red wires there are, on the same side of the transformer and of the same gauge, a white/blue pair which were just blanked off with plastic doo-dahs.
My hope is that this is a separate 120v primary which I can drive with a suitable step-down. I shall (after disconnecting all the secondaries of course) squirt in a small voltage AC and see what comes out but my question is: does anyone know the nature of this thing? Is my hope foolish? I've been trying to source a replacement/s for the transformer and it's not looking good...
Cheers, Jon.
Yeah, you're probably right, 120Vac between RED and BLUE wires, check continuity.
The smaller (12V?) transformer went on my unit so no power up lights, tuner. O/C primary not a good sign...
The smaller (12V?) transformer went on my unit so no power up lights, tuner. O/C primary not a good sign...
According to 7020e service manual, the 240V version has a power transformer tapped also for 220v but not 110 or 120V. The US/Canada version has a different power transformer that conversely, is only suited to 120 or 110V supplies. To top that complexity, the main secondary windings are tapped at different voltages too, in order to get the most from either 4 or 8 ohm speaker loads by switching the amplifier's power supplies to suit. I imagine that could easily lead to damage if you used the amplifier on 4-6R speakers when set to 8R and perhaps that's what happened when the previous owner figured that a louder amp must be better but .....oops! What's that smoke mean?
I suggest you don't try wiring those tappings to a 110V transformer or any other AC supply in the hope of finding something that works by chance. It's more likely that you'll do further damage that ends in further losses and frustration.
I suggest you don't try wiring those tappings to a 110V transformer or any other AC supply in the hope of finding something that works by chance. It's more likely that you'll do further damage that ends in further losses and frustration.
Thanks for the responses.
Spot on about the 220 tap @IanFinch but I'm pretty sure it will be OK to use 240. The output stage is the one we know and love from 3020 and indeed 7225 and has voltages from +/- 23 up to +/- 41 applied to it with same semis and just tweaked cap voltages. They all seem to be fine for the extra 10% or so that will come with the changed primary. The regulated supply side already deals with a pretty hefty drop from +/- 46v (I think, my scan is a bit unclear) down to 27v but it's pretty low power and there are hefty resistors in series so it's not all dropped in the regulator transistors. I'm going to give it a shot. I'll pop the onboard fuses and check the AC voltages first and see if it all seems reasonable. Powered through a 60w bulb of course.... Wish me luck!
Spot on about the 220 tap @IanFinch but I'm pretty sure it will be OK to use 240. The output stage is the one we know and love from 3020 and indeed 7225 and has voltages from +/- 23 up to +/- 41 applied to it with same semis and just tweaked cap voltages. They all seem to be fine for the extra 10% or so that will come with the changed primary. The regulated supply side already deals with a pretty hefty drop from +/- 46v (I think, my scan is a bit unclear) down to 27v but it's pretty low power and there are hefty resistors in series so it's not all dropped in the regulator transistors. I'm going to give it a shot. I'll pop the onboard fuses and check the AC voltages first and see if it all seems reasonable. Powered through a 60w bulb of course.... Wish me luck!
First, just think; the 220V tapping is at about 90% of the full primary winding, so are you sure the failure is in the last 10% between the 240V and 220V connections? If otherwise, it won't work any better than previously. It's easy enough to check the tappings if you remove the mains power and disconnect both 220 and 240V primary taps from all else. You can then use a simple continuity test or a low resistance measurement on your DMM to verify where the failure is - assuming you have some such way of checking things.
If NAD is true to form with this model, the transformer will still be on the cheap end of the scale from small to way too small so use it with caution and set for 4R speakers. Consider that it's already been burnt in (most likely) prolonged overload usage so try to avoid a repeat performance.
If NAD is true to form with this model, the transformer will still be on the cheap end of the scale from small to way too small so use it with caution and set for 4R speakers. Consider that it's already been burnt in (most likely) prolonged overload usage so try to avoid a repeat performance.
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Yes, thanks, I'm capable of thought. I had already established continuity on the unused taps and was still hoping for a conveniently placed thermal fuse to have blown leaving the unused winding operable.
Sadly not to be. On testing, although the higher-voltage pre-amp supply was in the right ball-park ~ 70V ptp, the main power side is only about 30 so I suspect something messy in the windings. Bah. Resume search for replacement.
Sadly not to be. On testing, although the higher-voltage pre-amp supply was in the right ball-park ~ 70V ptp, the main power side is only about 30 so I suspect something messy in the windings. Bah. Resume search for replacement.
So the primary is OK but it's the main secondary winding which is now suspect. It's centre-tapped and has switched tappings for 8/4 ohm speakers so it should be easy to check for balance between the + DC or -DC sides of the winding at either load setting which would nail the problem with more certainty.
It may be that there is still some useful life there. BTW, is there much of a hum or vibration from the transformer when you power it?
It may be that there is still some useful life there. BTW, is there much of a hum or vibration from the transformer when you power it?