0:10 Intro 0:48 Service manual and Schematic / Circuit Theory 29:14 Adjusting the regulated +/- 54 Volt power supply - FAIL! 31:13 Troubleshooting continues 33:48 Testing the power amp board on the bench 39:49 Testing the power supply after repair 45:55 Adjusting the +/- 54 Volt Power supply again - SUCCESS! 47:53 First test of the repaired amplifier 49:32 Adjusting DC offset of the flat and EQ amp boards 57:11 Replacing the broken headphone jack and power indicator lamp 58:20 Testing amplifier performance parameters 1:10:09 Sound test 1:13:22 Final thoughts / comments
Really interesting investigation of a pretty unusual amp design Tony, thank you! The analysis and solution to the "current source dilemma", added a level of clarity to my understanding of circuits in general, appreciate it! Very clever. - JRH
Hi Tony 👋👋👋Attention 👋👋👋 This amplifier design to achieving low noise , SNR 110 . The reasons they use to bias zener diode is to get higher PSRR low noise and low crosstalk between channel. You can find about it in Walter Jung ,analog device legend . Usual regulator such ad 78xx are too noisy. They have noise floor around -60dB . I use in all my amplifer design current source with mosfet . You can use DN2540N8-G . cascode configuration of two will get noise floor around -140dB. They are cheap too. This amplifer is designed for low noise . Those Opamp are DC servo. Work as integrator.
I have seen speaker relay contacts cause distortion even when not visible in the waveform. A quick thump on the relay while monitoring the distortion will expose this.
A good tell that this design wasn't so great is that nobody latched onto it and either did it themselves or made a variant of it. I wonder if they didn't start out with good intentions and in the end realized that it wasn't any better (and in fact worse), but they were already invested in this unique design. It only having a 1-year availability is another tell. Honestly, I didn't even know that NEC got into the HiFi market like that. Did the power LED/lamp end up working? I don't think your video ever show it lit (could have been the camera angle. Thank you for your pain on this.
Hi Tony! After all, we're only human and seeing such a bad design, so difficult to troubleshoot and service sure gets one to exhaustion. I do admire your pacience! Thank you!
A+++++ for effort. Thank you, Tony. For the reserve cap system... Remember how you taught us that you want to keep the voltage rating on a replacement cap to be within a reasonable range of the original, in order that the cap be formed correctly to operate correctly? I wonder if the reserve system is intended to have the caps operating within their best voltage region, in order to achieve the best ripple rejection? I only have a hobbyist knowledge base, so I could be off base. And, I probably did not explain it so well. I hope you got my meaning. Cheers.
Excellent Series on this amp. I work on a lot of vintage audio and really enjoy your videos. ( I'm even a subscriber LOL ) But honestly , on this unit , I would have given it back to my customer as needing too much work. And your right , I would not put that much work and time and money into an amp like this. Keep up the Great Work Buddy !
Great work on this beast. I totally agree with you that this integrated amp was way overengineered. And in my years of servicing audio equipment, when something is way overengineered, serviceability is an afterthought. I feel that the overengineering was to produce a product with good specs on paper, but leaves the integrated amp to sound clinical.
Thanks a lot Tony for those videos. Vor many years this NEC Amplifier was on my wishlist, but also the prices on the aftermarket and his rarity blocked these wish until today. And now you blown all the positive attractivity away. 😂 The Design is freaky and as you declared it over designed. The only positive point on the amplifier, which alived, is the optical design. I like it. Kind regards from Germany Marcel
I think that's a great call Tony, with its 'Out there 70s design because the transistor was a new fangled thing and let's give the marketing guys something to get their teeth into approach to say 'That's it not worth any more time and money" is the right thing....cheers.
"Rube" and "Murphy" were both Optomistic. Next stop for this brick would be next to the "Pyramid", wherever you packed it away. Otherwise good learning stimulation, thanks Tony.
The NEC A-10X is equally weird, doesn’t have the heat issues of this one but weighs roughly 30kg, dual mono design yet is rated at 60Wpc. Such weird beasts!
yes over engineered like the "cube" you mentioned amps like that may sound really good brand new but as they age they suffer from all the complications that were added and difficulty of getting something that complicated into a box that will fit on a shelf in an ordinary house, making it almost impossible to work on. I fear we they are doing similar a thing now with the use of surface mounted devices space is no longer the constraint it once was so bring on the overly clever overly processed. seems like the idea of keeping it simple has been forgotten. thanks for working on the thing and talking about it as you go along working. So long as the camera is not swinging all over the place it is OK you are clearly not faking it, it is all good!
hi tony . i think fast recovery diode don't need small caps on them ?. NEC used transistors which are fast recovery. Remember hifido sells this amp at very high rate like sansui au-717, au-919 . i think hifido guys know what they are doing .
43:00 I'm sure you know this Tony, I would use tiny buck converter to lower the voltage without wasting too much power. You may add a voltage regulator if required.
Great description well explained ! maybe NEC had a 'shit-ton' of those JFETS lying around, I mean 22 on one board !!? lol Talk about over engineered 15v power supply.....cheers.
NEC A-11E was sold from 1984-1985 and retailed for $799. (Source: Orion Audio Bluebook 1997) Consider a Denon PMA-500V integrated 80wpc amp 87-88 retailed for $370, and a Pioneer A-77X integrated 95wpc amp 85-89 production run with three transformer multi-supply retailed for $500
I don't want one, but looked on e-bay out of curiosity, no 11's but a few 10's there asking real money. Hope the early model had a better design and layout.
I am suprised they used so many J-FETs to drop the voltage. I wondered if putting in a 7815 voltage regulator for the +15V and 7915 for the -15V supply would have worked well.
I know I could get it to be perfect, but to me, it isn't worth the amount of effort required. I think this design had some great proof-of-concept circuitry in its design, but there was a lot of room for improvement.
@@xraytonyb - yeah, I give the engineers the benefit of the doubt in that there must have been sound engineering reasons for some of the design choices but they make it extra difficult to troubleshoot. And the layout of the boards and connections between them seems to have completely ignored those of us who want to work on this amp.
They current sources to bias zener diode by this why you get lower noise compared to resistors. If you use resistors for biasing zener diode then you need to get high value but you increase noise too.
This thing is the overengineering/underperforming poster child -- definitely not competitive with its counterparts from other Japanese makers, not to mention a huge PITA to service.
It is not a constant current source, but it doesn’t really matter: The current will vary with the input voltage, but if the input voltage is roughly constant, the current is roughly constant. Another option might be to replace the 12V zener diodes with ones with a higher voltage rating (and a appropriate power rating), while using 30V JFETs. But Opamps usually have a high PSRR, so using a resistor should be ok.
Why so high THD at high vol ,is it not strange ? . Can you explain tony its not because jfet are replaced with resistors ? Again i remind to upload service manual where any body can download , from hifi only few already registered guys can download those may not have this amp.
I'm pretty sure, as another viewer mentioned, that this is partially due to the speaker relay contacts. They are most likely tarnished by now. I don't have a replacement for this type in stock and I don't want to make a special order for new ones.
I got the impression you didn't like working on this amp. It seems to leave a lot to be desired from the service side of things, especially access while setting up the DC offset. Who'd have thought all that over-engineering would lead to crappy THD levels. NEC - Not Everybody's Choice.
Also, I agree with Mark (another viewer) that the speaker relays are most likely tarnished by now and are not making good contact. That's why the distortion gets worse with higher volume, even though it isn't clipping.
