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Old 23rd September 2021, 07:16 PM   #51
DonaldStott is online now DonaldStott  Scotland
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On the basis that I've nothing to lose I've popped off the metal cover and will have a go at exploratory surgery tomorrow.

I fully appreciate that this is a one-way street and that I probably won't find the fault or be able to reassemble the transformer. I'm doing it for my own entertainment and education and will post some pictures if people are interested?
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Old 23rd September 2021, 08:06 PM   #52
indianajo is offline indianajo  United States
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Okay that explanation of power cord makes sense. Blue & brown, yes that could be right.
I won't need pictures, been there, done that. I can't repair EI transformers, except maybe if the studs are loose & the laminations hum. My rewound secondary was smaller than original since copper instead of aluminum, and I still couldn't make it fit or keep it from shorting to the laminations.
There are way too many output voltages from this transformer to replace. That goofy impedance switch is a cheapskate way to get away with $3 heatsinks on the output transistors. Maybe sell off the boards for parts or repair, some people will buy anything.
I'd scan gumtree & ebay for something easier to fix. I picked up a PV-4c for $15 last month, +80 freight from AZ. Banana jack broken off, flange bent from dumpster trip, dirty inside as the inside of a vacuum cleaner. Transformers didn't stink. Freight shouldn't be that high in UK. Nice simple 120 w/ch 8 ohm, 200 w/ch 4 ohm. I fixed the PV-4c that was stolen 9/20 for $80, new output transistors, rail caps & a dozen little electrolytic caps. That one cost me $20 + $20 freight from the next state.
Got a PV8 mixer for $40 +$25 freight, only thing wrong was wall transformer was missing. Replacement $3 @ charity resale shop. (12 vac instead of 16 vac, but works for signals <10 v)
I just have to be patient, sometimes there is no amp under $200, like this week. Crowns & QSC's are also easy to get schematics for, beside Peavey.
Waited 18 months to replace my $300 each SP2-XT speakers stolen, freight from Massachusetts would be $1000. Then a pair of SP2(2004) came up 120 miles away for $400, $600 each new. Guy wanted a new guitar, NOW! They just barely fit in the wife's Hyundai Elanta.
Happy repairing.
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Last edited by indianajo; 23rd September 2021 at 08:11 PM.
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Old 24th September 2021, 01:53 PM   #53
DonaldStott is online now DonaldStott  Scotland
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I've taken the non-destructive disassembly of the transformer as far as I can and there is no evidence of any fault! There may be an internal fuse or break in one of the leads inside the transformer but I can't find it? Any further disassembly would have to be destructive and there seems little point in proceeding further.

Thanks to everyone for their helpful contributions

Time for a change of direction - is there a WANTED section on this Forum on the off-chance that someone may have a transformer for a NAD 7020i? Worth a shout ... ?
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Old 24th September 2021, 02:03 PM   #54
Alan4411 is online now Alan4411  Wales
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I suspect very little chance of a replacement, but worth a try at least.
Left hand side, 'Comercial Sector' then 'Swap Meet'.
WTB = Want To Buy.

Good luck, Alan
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Old 25th September 2021, 08:52 AM   #55
DonaldStott is online now DonaldStott  Scotland
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Thanks Alan - worth a punt but after that it's up for sale for parts!

WTB : Power Transformer NAD 7020i
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Old 26th September 2021, 09:02 AM   #56
DonaldStott is online now DonaldStott  Scotland
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I've tracked down someone who says that he can reverse engineer the existing transformer and rewind on a more modern core.


He does, however, need to know the voltages on all those Secondaries - can anyone help me decipher the NAD schematics?
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Old 26th September 2021, 09:41 AM   #57
Alan4411 is online now Alan4411  Wales
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Reverse engineering is just that.
Take apart what you have turn by turn and section by section. Then you actually do not need to know the secondary voltages. Just a tedious operation.

To build a replacement without taking the original apart you do need to know the secondary voltages. There are no AC voltages on the NAD circuits I have access to, but once removed from the amp you could try powering the transformer bypassing the 'fused' line and measuring the secondary voltages.

Finally - Have you considered there is a fault on the amplifier that has caused this problem?
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Old 26th September 2021, 10:46 AM   #58
DonaldStott is online now DonaldStott  Scotland
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Thanks Alan for your ongoing support and interest.


You mentioned 'powering the transformer bypassing the 'fused' line and measuring the secondary voltages' - any advice on how I could do that?


And of course at the back of my mind is that there may be a fault with the receiver which has caused this problem - but that's a bit 'chicken and egg'?
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Old 26th September 2021, 11:55 AM   #59
indianajo is offline indianajo  United States
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Quote:
Finally - Have you considered there is a fault on the amplifier that has caused this problem?
The NAD 7020i power page schematic I have, there is no primary fuse or circuit breaker. There are fuses on most secondary windings, but not on +5.6 v & +12 v windings. Thus NAD has set this receiver up to burn transformers fuse if an electrolytic cap in that section or something else shorts in the tuner. Great way to sell transformers, if the company was still operating.
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Old 26th September 2021, 12:57 PM   #60
NareshBrd is offline NareshBrd  India
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If you do have the schematic, give the ratings of the secondary side, to help him make another transformer.
And OP, please put fuses everywhere!

Last edited by NareshBrd; 26th September 2021 at 01:00 PM.
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