My NAD 3020 project: list of things done (long)

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This project was a lot of fun, and was more of a restoration/upgrade project than a series of modifications. The results however, are quite stunning from where it started, which was all original, and a little beat up. With each thing I did, there was usually a couple of weeks that passed before I tried something else. Also, I actually have 2 of these amps, one was in kind of rough shape, so I used it as a quinea pig before trying it on the better one. Funny thing is, the "guinea pig" now sounds a LOT better than the other one, but I haven't done the last modification to it yet (operative word is "yet"). Anyway, in order (If I remember correctly), here are the things I changed/switched/modified:
1. Cleaned all the pots and connectors with de-oxit.
2. Replaced the ps caps, and caps on the regulator board, with like values (Nichicon PW), and also put polyester-film bypass caps on the reservoir caps, and the larger ones on the regulator board. (This was a big improvement over where it started)
3. Replaced power switch with a new one, and replaced the filter cap on it was well. (No difference in sound really, but the other one was getting nearly non-functional)
4. Replaced all the pre-amp section caps with Nichicon muse, like values, but the higher (up to 50v) voltage rated ones were easier to find, so I used those. It was a tough fit for some of them, but do-able. Also, put bypass caps on C517-518, as described on "http://gnu.295.ca/peak/audio.html" (Sound overall was definitely clearer afterwards)
5. Replaced all the caps in the power-amp section with Nichicon muse, again used like value, but higher voltage rated caps because they were easier to find. C615-616 (1000uf) could not be fitted down on the board because of size, and C616 had to be tilted slightly to clear the fuses on the regulator board, but I made sure the leads were the same length on both caps, and they were hot glued in place, and both were bypassed on the bottom of the board with a 0.1uf polyester cap. (Sound got a little clearer, but had a lot more depth)
6. Decided to go back to the power supply, added a heavier-duty rectifier, creatively mounted, and replaced the reservoir caps again with Nichicon muse this time, since I had some left from another project, and added an additional 1000uf cap on each side in addition to the 2200uf caps. (Bass had more punch, and was clearer)
7. At this point I started getting brave, but out of necessity. When I started to adjust the DC offset, the original trim pot crumbled. Replaced those with nice cermet multiturn trimpots of the same value (they were Vishays I think). With these, I managed to get the DC offset down to about 0.05mv. Even with multiturn pots, this took patience, because there were a lot of teeny turns and waiting involved. (This made a BIG difference in clarity and detail, mainly because it was REALLY out when I measured the DC offset before even trying to adjust with the original trimpots in)
8. Got even more brave, decided to replace the output transistors with newer, more modern ones. Since I was at it, replaced the driver transistors for the outputs as well, and the transistors that feed the signal into the output transistors. All the transistors are modern cross-referenced equivalents from Onsemi:
original/new part #'s:
2N3055/MJ15001
MJ2955/MJ15002
BD139/BD139
2N6554/2N4919 (for Q613-614)
2N6551/2N4923 (for Q611-612)
If you try this, note that the pin-outs for the new Q613-614, and the new Q611-612 are NOT the same, you can't just plug them in (you have been warned). The new ones also have no heatsink tab, so I added small heatsinks to them.
Here's where things got comical. Switched amp on for the first time with the new transistors (no load), and it powered up, and no sparks or smoke came out of it. Quickly swtiched it off and hooked it to a set of bookshelf speakers I use for test purposes. (I might be stupid, but not stupid enough to plug a freshly modded amp into my beloved Clements speakers). Switched the amp back on, and slowly turned the volume knob up, with no source connected as yet. The NAD 3020 really has a nice tone when it turns into an oscillator :). It was QUICKLY switched off. I was all set with the DMM to check the idle current when i first switched it on, but was afraid to switch it back on. I measured the resistance on the fixed resistor that sets the idle current, and went off to buy a couple more multi-turn trim pots that encompassed that value. Bad move, I had forgotten that there were two more resistors in parallel with RX1 and RX2, and bought a trimpot that was nowhere near enough resistance. Didn't realize it until I switched the amp back on. For fuses labelled "slo-blo", they sure went up pretty quick, and in spectacular fashion. Went out and got 10k trimpots this time, not a 1k, and turned them to full resistance, instead of halfway through their range like I did with the 1k's. But, I had turned them all the way to one end before i rigged them as trimpots, and wasn't paying attention when I rigged them and put them in, set to 0hms (yes, I am diagnosed ADD, lol). Two more fuses later, and with them rigged properly this time, turned the amp on (no flashes of light from the fuses this time), let it sit for about 5 minutes, and tried to get a reading across the output emitter resistors. No reading at all. Turned a few turns, still nothing, kept turning, still not reading anything, even after going quite a few turns past the halfway point. "Ok, maybe 10K is too much". Now with 5k trimpots in place (I payed attention this time), I switched it back on (no smoke, yay), and started adjusting, but could still get no reading, either voltage or current, no matter how I connected the leads across that resistor. I didn't adjust very far, and decided to turn the resistance back up, and plug in my test speakers, and a CD player. The amp played, but was lifeless and dull, and very muted. This is where the faint of heart, or people with a strong knowledge of electronics, may want to stop reading. With one hand on the heatsink, and one eye on the fuses, I turned a half turn and waited, listening to a CD the whole time. I went like this for some time, turning a half turn, one channel then the other, feeling for warmth out of the heatsink, which was no more than room temperature for awhile, even 15 minutes into the CD, and 3 whole turns total. About this point, the sound started getting a little better, and the heatsink started getting a little bit warm. I kept going a half turn at a time and waiting about 1 minute before turning again, and things steadily inproved, until all of a sudden the sound completely opened up, and was vibrant (obviously the point where the transistors were biased enough to stay on). The heatsink was not uncomfortably warm to the touch at this point, so i decided to keep going a little at the time, really keeping an eye on the fuses now (well, I had half a box of them left, why not?). Things started heating up, and the sound kept getting even better, until a certain point. There seemed to be a little graininess getting into the sound, and the heatsink was really hot, but not so hot you couldn't stand to touch it, but I backed it back down to a half-turn less than when the graininess went away. Took the amp into the listening room, plugged in into the good CD player and speakers, and I was blown away. It had MUCH more dynamic range than before, and even more detail and clarity than the new NAD C320BEE I had borrowed, and sounded better playing softer than the C320BEE did. My wife, who is also a musician, was impressed by the sound also. She agreed, and was much more pleased than I, that it sounded better than the new amp, because the transistors were a LOT cheaper than the new C320BEE I had been begging her to let me buy.
Now, if I can just get rid of that last teeny bit of barely audible hum . . .
BT

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Topic - My NAD 3020 project: list of things done (long) - BrassTeacher 10:46:53 07/10/05 (2)

wow, what a project! - BillyBuck 23:12:36 07/10/05 (0)

Re: My NAD 3020 project: list of things done (long) - M3 lover 21:16:36 07/10/05 (0)

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