Hi,
I have a NAD 3020A on the bench; it's a friend's amp that I've agreed to look at. But it seems to have some significant issues.
On arrival it would instantly blow fuses F3 and F4 (powering the main output stage) on power-up. I replaced the 4A bridge rectifier and all initially seemed well, until a wisp of smoke from R640 (right channel collector resistor for Q606/Q608) when testing at low volume levels (no speaker loads). Measuring quiescent voltages on the Right channel power amp, I found that voltages at the bases of Q608 and Q610 were way off; -9V at the base of Q608 (should be +2V) and -11V at the base of Q610 (should be -0.6V). Initially I thought that Q610 may have failed B-E short, but swapping it with the Left channel part (Q609) made no difference. I was planning to check around Q613 for issues there when R651 (in the Left channel) suddenly went up in smoke and flames.
So there are clearly at least two issues going on here; a significant quiescent issue on the Right channel, and a sudden spectacular burnout on the Left channel. Any pointers would be most welcome! I am slightly out of my familiarity zone with power amps; most of my repair work is in analogue synthesisers.
Many thanks!
I have a NAD 3020A on the bench; it's a friend's amp that I've agreed to look at. But it seems to have some significant issues.
On arrival it would instantly blow fuses F3 and F4 (powering the main output stage) on power-up. I replaced the 4A bridge rectifier and all initially seemed well, until a wisp of smoke from R640 (right channel collector resistor for Q606/Q608) when testing at low volume levels (no speaker loads). Measuring quiescent voltages on the Right channel power amp, I found that voltages at the bases of Q608 and Q610 were way off; -9V at the base of Q608 (should be +2V) and -11V at the base of Q610 (should be -0.6V). Initially I thought that Q610 may have failed B-E short, but swapping it with the Left channel part (Q609) made no difference. I was planning to check around Q613 for issues there when R651 (in the Left channel) suddenly went up in smoke and flames.
So there are clearly at least two issues going on here; a significant quiescent issue on the Right channel, and a sudden spectacular burnout on the Left channel. Any pointers would be most welcome! I am slightly out of my familiarity zone with power amps; most of my repair work is in analogue synthesisers.
Many thanks!
Before continuing, go for an incadescent lamp of about 100W (no led, no fluorescent, no CCFL, only filament of your corresponding voltage rating), a socket and a piece of wires. Remove the fuse (preferably the one in the live wire if dual fuse is found) and hook the bulb in place of the fuse.
This way you have a current limited supply.
Under normal conditions, after power up the set, you should see a fast flash while charging up the lytics in the PSU. But after this transient, filament must be dark or very poor red. If it is visible, the set is under overload or shortcircuit.
For more details, a schematic would be interesting for guidance.
This way you have a current limited supply.
Under normal conditions, after power up the set, you should see a fast flash while charging up the lytics in the PSU. But after this transient, filament must be dark or very poor red. If it is visible, the set is under overload or shortcircuit.
For more details, a schematic would be interesting for guidance.
Hi Osvaldo,
With the bridge rectifier replaced, there are no more blown fuses, so my attention is now on the issues within the power amp. Apologies, I should have provided the schematic first time round; it's attached here.
With the bridge rectifier replaced, there are no more blown fuses, so my attention is now on the issues within the power amp. Apologies, I should have provided the schematic first time round; it's attached here.
Ok. But to protect the amplifier and yourself, I remark the use of the lamp. I'm 53 and use it from my 16's when I learn about its use.
do what Osvaldo is telling you, best damage preventer tool you can have - also helps if/when your meter probe slips and shorts something, at least it will help it's only for an instant.
Google Dim Bulb Tester for a graphical schematic of what Osvaldo is recommending.
Carefully check resistors, NAD is known to use underrated wattage ones in the amp and power supply sections that open up after a few years of being run at 80% of their current capacity. But sounds like you are on the right track checking the TO-92 and TO-220 transistors, but likely have at least one back output as well, so work methodically and take good notes of voltages/resistance.
Google Dim Bulb Tester for a graphical schematic of what Osvaldo is recommending.
Carefully check resistors, NAD is known to use underrated wattage ones in the amp and power supply sections that open up after a few years of being run at 80% of their current capacity. But sounds like you are on the right track checking the TO-92 and TO-220 transistors, but likely have at least one back output as well, so work methodically and take good notes of voltages/resistance.
💡I ran a cold-junction check on all the output stage transistors in both channels. In the Left channel there is carnage (see attached schematic); five transistors failed in varying ways (mostly short). The Right channel fares better; the main PNP output transistor MJ2955 has an open-circuit emitter, which may explain why all the quiescent voltages were skewed so low.
So, various replacements to do; I'll also check thoroughly for dry joints, open-circuit resistors etc.
2SD669 and 2SB649 seem hard to find these days, but there is a version of the NAD schematic that suggests 2SC3421 and 2SA1358 as equivalents.
So, various replacements to do; I'll also check thoroughly for dry joints, open-circuit resistors etc.
2SD669 and 2SB649 seem hard to find these days, but there is a version of the NAD schematic that suggests 2SC3421 and 2SA1358 as equivalents.
Attachments
one thing to check on these while you are doing everything else is check the regulated supply transistors, they are normaly loose/cracked/dry joints
You can buy decent copies (Unisonic) of these original Hitachi driver types, right in your own back yard, so to speak. There are several other, less certain quality copies too but try Profusion Ltd at Southend-on-sea, UK. That's the same outfit who market Exicon branded Lateral Mosfets.
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Oh yes, 'forgot to mention that all NAD 3020 models were originally fitted with old technology silicon power transistor types (hometaxial) and you can't simply drop in the newer 2N3055 NPN types (epitaxial) without changing the bias resistors to different, suitable values. There are many old threads here discussing this same issue, as you'd expect with the most common integrated stereo amp. of all time.