Nad 3020 work including idle current adjustment

[J. Schaeffer]

Hello. I'm new to the forum here but not audio as a whole! It's nice to see a forum like this which seems to have very good useful information.

I have an original version nad 3020 that my father bought in the late 70's that has had no work performed on it to date. I figured it was time to go through it and do some preventative maintenance- re cap all electrolytics, replace the power switch and snubber caps, replace the speaker output terminals, re-flow the entire unit, set the dc offset and idle current, do some paint work and clean those nasty boards covered in old skotchkote which can eventually become conductive! Needless to say I have been at work for a couple weeks.

I started with the circuit board cleaning and power supply capacitors. Bought some MG chemicals safety wash and an aerosol contact cleaner. I cleaned the circuit boards with the MG and a 1" paintbrush and used the spray cleaner to rinse THAT off. I also cleaned all of the switches and pots. Worked well but took a few cleaning cycles to get things really clean. Ordered Nichicon 63 volt 2200 uF low-impedance low leakage caps for the power supply. They were the only cap in mouser's inventory that maintained the dimensions(although there is room for height) and fit the bill electrically having both low-impedance and low leakage current. The ripple current rating is 3.2A on these. Not as if THAT alone will amount to anything sonic, but I suppose it's nice to know I have it. Ordered a new alps power switch from Ebay and two 300vdc orange barrel-style 4700 pF polypropylene caps for snubbers. The amp had the classic power switch noise. Found a sato brand high quality spring terminal on Ebay and studded it to the circuit board. I know, 5-way binding posts, but I thought these were more fitting and didn't want to modify the backplane . Besides a modification to change the color ordering on the terminals, they fit perfectly requiring no modifications on lug centers and have high spring pressure. They were literally the highest-quality(the only quality)spring terminal I could find anywhere after hours of research. I have ordered a cap kit from Ebay having all 43 electrolytic caps and 6 tantalums. Have yet to install them as they haven't arrived.

Now, on to the idle current... lol! I figured I would practice while waiting on my cap kit. I literally racked my brain on seeing 5 or more different service manuals until I found the closest one(original, A, B, I, etc.). I have the first version with the p/s caps behind the headphone jack and the phono power supply board mounted on the inside of the back plane. My amp calls for the procedure of using 1K(not 1 ohm) resistors across R653 and R654 cutting the trace under each since they're bridged(not in the procedure) with a trace, not soldered, as delivered. Next I take it you're supposed to measure across the 1K resistors with a meter for mV. I did install 25 turn 2k ohm 1/2 watt trimmers in place of both RX resistors which were factory installed with 1K and 1.2K resistors(reason for the 2K's). I installed everything and measured 215 and 179mV across the resistors. Far from the 5 to 11mV range specified. I think I followed the procedure properly but I also think the procedure may have a typo. I read the later 3020A procedure(I believe) and did ohms law a billion times trying to translate the 1 ohm(not 1K) procedure and 30 to 60mV from this later model. Through my math I believe 200mV with the first procedure equals 20mV for the later procedure with the 1 ohm(not 1K). I decided to go with it and without bridging the 1K test resistors, powered down and removed the trimmers to make sure they were not miles from the original 1 and 1.2K. Had 960 ohms on the right and 1.4K on the left trimmer as adjusted from above. I think I'll take it! Not 100% sure but it should be right. Re-installed the trimmers and folded the 1K test resistors down against the board and bridged across them with some cut off leads. I can remove them after I do the extensive re cap and final adjustment. Was able to get the offset within 0.2mV on both channels and boy are those adjusters sensitive. Almost the wrong trimmer design used in this amp altogether.

I suppose what I'm saying is I just intend to post my progress on here for everyone to see. I have some electronics experience but NO nad 3020-specific repair experience. I welcome everyone's opinions and advice as that I what i am here for, advice. I screw up too and may have missed something but there is definitely an issue with the service manual. Maybe there was an engineering revision that wasn't annotated or something on my amp. Maybe the remaining bad caps are skewing my measurements but I don't think by that much on both channels too. I noticed the main power supply rail runs a little hot, 32vdc on both sides and polarities all the way from the rectifier output to the output transistors. The print says 28vdc. The 6vac secondary to the power meter is at 7.8vac. The phono section voltages are close to print. Maybe the 32 instead of 28 volts thing is skewing my measurements also. I don't know.

Fired the amp up on some crappy speakers from an old rack stereo and it sounds real nice. Only modest but equal temps on the outputs thru the heat sink at maybe 5 watts output on the power meter for a while. Could be the speakers but the mid range sounds a little overly present. Still has a very quiet line frequency hum thru any of the outputs in any configuration, the reason for all this work. Lol! Forgot to mention that...
It does the hum thing (which is quiet, maybe 5db) no matter if the pre amp is coupled, uncoupled, through the headphones and speakers, and no matter what switch configuration is used with either previous combination. So hopefully this 49 capacitor kit does the trick! The caps are at least 40 years old now!

 

[mbz]

Try and stay away from ebay, many fakes however sometimes...
Cap kit will be over priced but it's a learning experiance, NADs are good to learn on.
Most things easily accessible despite the birds nest of wiring. Have fun, learn a little...

 

[sregor]

No emitter resistors in original, so only can use original type outputs. (2n3055 and mj2955)
Ohms law says 1mV= 1mA when using a 1 ohm resistor. 1V =1 mA using a 1k resistor, so you are actually running less than a mA - but without emitter resistors, you will have lower distortion at low idle current than a design with emitter resistors, and reliability is better at low currents. If you ever change outputs, it is recommended to use emitter resistors. My 2 cents.

 

[Goldie99]

Agree with Steve - the 215 & 179 mV measurements across the 1k resistors you used would correspond to only ca. 0.2mA idle current.

You can find a 'correct' bias adjustment procedure (from 'Leesonic'), for your specific amp, in post # 43 of the following thread.

http://www.audiokarma.org/forums/index.php?threads/nad-3020-rebuild-and-upgrade.460678/page-3

 

[J. Schaeffer]

Agree with Steve - the 215 & 179 mV measurements across the 1k resistors you used would correspond to only ca. 0.2mA idle current.

You can find a 'correct' bias adjustment procedure (from 'Leesonic'), for your specific amp, in post # 43 of the following thread.

http://www.audiokarma.org/forums/index.php?threads/nad-3020-rebuild-and-upgrade.460678/page-3

Thank you! I see that the manual says to adjust for a certain voltage drop across these resistors, it dosen't mention the actual idle current spec(it says mV not mA) therefore if I adjust for what they say per the procedure, the amp would have between 5 and 11 micro amps idle current? (.005-.011 mA) I realize by adjusting and measuring across these resistors you're affecting something elsewhere in the circuit, but Nad's "as installed" Rx resistors that were in this amp were way off in my amp's case. This is what I'm stuck on.
I see that this subject has come up many times in the past and you guys are probably sick of answering the same question over and over, so I appreciate it!

 

[J. Schaeffer]

Try and stay away from ebay, many fakes however sometimes...
Cap kit will be over priced but it's a learning experiance, NADs are good to learn on.
Most things easily accessible despite the birds nest of wiring. Have fun, learn a little...

Yes I've heard of this before... counterfeit transistors and what not. This kit was expensive I suppose at 35 dollars shipped. Didn't feel like doing the research on every single can for replacements so I suppose I got lazy in a way. Thank you for the words!
Edit: I am having a great learning experience for sure! Luckily I haven't smoked anything in the unit yet! I have noticed one thing, the amp's workmanship isn't the best at all. If I had a few months I would desolder and resolder every component until they were straight and symmetrical. I did remove all that nasty brown adhesive that was slopped all over the insides!

 

[Goldie99]

The manual procedure assumes R653 (R654) are installed, e.g., that the amp is set up with 0.22 Ohms emitter resistors. It then asks you to install 1k resistors in parallel to the R653 (R654) - the effect of that is (would be) to give you a combined resistance of 0.2199 Ohms... in other words, for measuring mV drop across the resistors, it makes no difference at all (provided the 0.22 Ohms resistors are present).

That being the case, the procedure is then looking for 5 - 11 mV across the combined resistors, which is equivalent to an idle current of 22.7 - 50.0mA (Ohms law: mA = mV / R).

The last one I did, I simply cut the relevant traces, installed 0.22R resistors into the R653 & R654 positions, e.g., bridging the new trace cuts, and soldered a couple of extra wires on each side of the new cut traces, to be able to remotely monitor the mV across the new resistors without having to squeeze multimeter leads / clips into what's already a tight (and 'risky') area. I didn't bother with 1k resistors at all. The photo's should show what I mean better - circled in red is the new R653, and the 2 extra wires are going to the ends of the cut trace, which also corresponds to both ends of the new R653.



To set the idle currents, it's then simply a question of monitoring the mV across the new R653 (or R654), and setting them to 5 - 11mV using the new trimmers. Once they'd been set, I just removed the extra wires, soldered bridges back across the traces that were cut, and left the new R653 / R654 in place for next time they might be needed.

NOTE - to other users - this is specific to the early NAD 3020 models, as per the model the OP has.

 

[J. Schaeffer]

The manual procedure assumes R653 (R654) are installed, e.g., that the amp is set up with 0.22 Ohms emitter resistors. It then asks you to install 1k resistors in parallel to the R653 (R654) - the effect of that is (would be) to give you a combined resistance of 0.2199 Ohms... in other words, for measuring mV drop across the resistors, it makes no difference at all (provided the 0.22 Ohms resistors are present).

That being the case, the procedure is then looking for 5 - 11 mV across the combined resistors, which is equivalent to an idle current of 22.7 - 50.0mA (Ohms law: mA = mV / R).

The last one I did, I simply cut the relevant traces, installed 0.22R resistors into the R653 & R654 positions, e.g., bridging the new trace cuts, and soldered a couple of extra wires on each side of the new cut traces, to be able to remotely monitor the mV across the new resistors without having to squeeze multimeter leads / clips into what's already a tight (and 'risky') area. I didn't bother with 1k resistors at all. The photo's should show what I mean better - circled in red is the new R653, and the 2 extra wires are going to the ends of the cut trace, which also corresponds to both ends of the new R653.

View attachment 1437589 View attachment 1437590

To set the idle currents, it's then simply a question of monitoring the mV across the new R653 (or R654), and setting them to 5 - 11mV using the new trimmers. Once they'd been set, I just removed the extra wires, soldered bridges back across the traces that were cut, and left the new R653 / R654 in place for next time they might be needed.

NOTE - to other users - this is specific to the early NAD 3020 models, as per the model the OP has.

Thank you so much for de-mystifying that! It makes sense now. There is certainly an incorrect call out in the parts list in service manual as far as the uninstalled .22r resistors(one of the position numbers is incorrect). I will give this a shot and I'll de-solder and verify the trimmer adjustment before I re-bridge the traces. Maybe I'll push my luck with the components and de-solder all the crooked components and re-solder ALL these huge "mountain" high resistance solder joints I have. This amp I have has come a long way, it was really a mess before I started working on it. I was using it as a preamp with my 2600 power envelope previously before this noise started. Thanks again!

 

[J. Schaeffer]

Got the recap done tonite, it took 6 hours which included 1 hr setup time. This included straightening more componentry and measuring/setting the idle current and dc offset. All electrolytics and 6 tantalums were replaced. The 60 hz hum problem(with octave harmonics/sibilance) is gone! Also the idle current procedure works, I had it previously set to .15mA as stated in above replies. It makes sense since the unit had 1k and 1.2k RX resistors originally. Thank you all again!

 

[Goldie99]

Sounds like a good result

How did the eBay cap package turn out in the end ? e.g., were they all the correct sizes, correctly spec'ed for the circuit locations, and decent makes, etc. ? I've never tried any of the eBay recap packages - I prefer to purchase direct from the likes of Mouser, Digikey, etc., with known quality and no unnecessary added costs.

 

[J. Schaeffer]

Sounds like a good result

How did the eBay cap package turn out in the end ? e.g., were they all the correct sizes, correctly spec'ed for the circuit locations, and decent makes, etc. ? I've never tried any of the eBay recap packages - I prefer to purchase direct from the likes of Mouser, Digikey, etc., with known quality and no unnecessary added costs.

I ended up with about 10 extra caps I did not need, it was a kit for specifically a 3020...
The seller also has them for 3020b. I had a couple of each left over- 220 uf, 330uf, 47uf, 4.7uf, 1uf. I didn't get the Panasonic kit but the standard one. I cant speak for impedance, ripple, leakage, etc but they were mainstream brand manufacturers. 6.3v up to 25v, 25v caps to 63v, 35v caps to 63v, all the tantalums were 35v. He just provided higher voltage and of course new caps at around 35 dollars shipped.
Edit- I had enough for all electrolytics and tantalums. Also noteworthy, by "extra" leftover from the kit I am referring to caps that were required for the job AND values my paticular amp doesn't use.

 

[J. Schaeffer]

Decided to mess with idle current adjustment tonite because I noticed harshness in the midrange testing on a set of small speakers. I installed 220 ohm across and went from a 150mV reading across the meter(my initial out-of-the gate setting) to 1 volt(translates to about 5 mA current on the test point). The 4 amp fuses did their job quite nicely. Luckily, they sank the current out and blew slowly. Lol
Therefore I've concluded that even the earliest iteration of the manual's procedure is n/a. It's certainly the early version missing 4 resistors near the sink and also missing diodes in that area which are jumpers instead from the factory. I counted turns back to about where I thought they was in turns, turned it on, new fuses, and set it for 150mV(600 micro amps) across the 220 ohm testers. Yep, I bought the wrong ones. But the math works out to 600. Reset the offset to within .2mV with the o.e. trimmers and tried it out. Midrange problem solved, sounds really nice now.
Does anyone have any thoughts on this? Thanks.

 

[Goldie99]

Something's wrong here - the 4A fuses are for the unregulated PS to the main amp drivers & outputs only. There's no way you should be blowing 4A fuses unless you've installed something in the wrong place, have a short or solder-bridge somewhere, are measuring incorrectly, or....(?). I'd advise to use a DBT for now, until the problem's found - it doesn't sound as though it is yet ?

Can you maybe post clear photo's of the top / bottom of the pcb, showing the areas where you've installed R653, R654, & the cut traces ? Also the area in front of the new trimmers, showing the 'missing diodes' area. When you measure the 150mV values you referred to, is that with one meter probe directly on each side of R653 (or R654) ?

As a side note, it's very common that (from the factory) it's missing the 4 emitter resistors, R653 - R656, and has wire jumpers instead of D605 & D606 - I assume those are just production changes that weren't sorted in the manual.

Hopefully someone else will also 'chip in' - I'm maybe not explaining it well, and will be traveling a lot in the next days....

 

[gpounce32768]

Have to be careful adjusting idle current, its easy to blow driver transistors- I did that to an Onkyo I was working on a while back. Its helpful to do the adjustment in 2 passes, the first iteration done with the system on a dim bulb which will prevent damage if you go to far, get it close. Then after the unit is warm, refine the adjustment on the mains instead.

 

[J. Schaeffer]

Thank you for trying to help me through this... there wasn't a problem after the work I did, I was just trying to put the theory into practice on my paticular amplifier. As with probably most of us audiophiles on the forum I have paticular hearing skills, or so it's in my head. Therefore when I auditioned the amp I noticed some midrange "tension" in the sound so decided to check the idle again cause it's been the one source of question in the project. When I noticed the sound, I immediately unhooked the amp and hooked it up with my main speakers, a pair of L-36's which are paticularly efficient as a side note. I also drove my audition speakers with my 2600a which is normally connected to my L36s. Sound followed the amp. All other aspects fine with the amp, drove the 36s at around 20 watts(about 1 or 2 o'clock position on the volume knob) for a while and there was only a small warmth top of the case thru the vents.
My small so-called "audition speakers are some minimus 7As walnut that have upgraded crossovers, the typical ebay kit...
New huge inductor and film cap for the woofers and 2 films for the tweeters. Nice quality kit. Therefore they are inefficient, and due to the size seem to be "midrange heavy" which I'm not used to because I just bought them. I also mixed some fiberglass resin and applied about a 1/16" coating to all surfaces inside the enclosure. Also put an additional sheet of insulation on the backboard just opposite of the baffles to help compensate for the decreased enclosure displacement due to the larger crossovers. Re-rubbered the surrounds because one of them had a stiffer mechanical compliance of the surround. To note, the original surrounds seem to actually be vinyl.
End result seems to be that I wasn't used to the midrange of a small bookshelf speaker on my 3020 which I just totally changed up. Go figure, I suppose. The amp still sounded noticeably better after the 2nd setting in which the 1st setting wasn't really bad sounding at all to begin with. Just myself being overly critical about things, I think.

 

[leesonic]

On ALL of the NAD 3020s that I work on, from the early version to the 3020B, I now do the same procedure. If it doesn't have 1 ohm resistors in-line with the outputs (like the early ones), I install them, cutting the track shorting out the resistor position. I then fit 1.2K resistors when the adjustment resistors are, in parallel with these are 1K Bournes trimmers set to around 500 ohms. I then dial the bias in to 30mV measured across the 1 ohm resistors. Once this is done, I bridge across the 1 ohm resistor with a piece of scrap wire. For models that have a 180 ohm resistor at the adjustment point, I take this out and use a 1.2K resistor. I've found with the 180 ohm one, the bias tends to ramp up too quickly.

Lee.

 

[J. Schaeffer]

Something's wrong here - the 4A fuses are for the unregulated PS to the main amp drivers & outputs only. There's no way you should be blowing 4A fuses unless you've installed something in the wrong place, have a short or solder-bridge somewhere, are measuring incorrectly, or....(?). I'd advise to use a DBT for now, until the problem's found - it doesn't sound as though it is yet ?

Can you maybe post clear photo's of the top / bottom of the pcb, showing the areas where you've installed R653, R654, & the cut traces ? Also the area in front of the new trimmers, showing the 'missing diodes' area. When you measure the 150mV values you referred to, is that with one meter probe directly on each side of R653 (or R654) ?

As a side note, it's very common that (from the factory) it's missing the 4 emitter resistors, R653 - R656, and has wire jumpers instead of D605 & D606 - I assume those are just production changes that weren't sorted in the manual.

Hopefully someone else will also 'chip in' - I'm maybe not explaining it well, and will be traveling a lot in the next days....

I'm measuring in parallel with the resistor, directly across it on its legs over the cut trace while adjusting a 2k trimmer set to a basic resistance of 1.2k on one channel and 1k on the other; which is the same value as the original RX resistors(my amp had 1k and 1.2k RX resistors factory).
I think the amp blew the fuses because I turned transistors on full current with the adjustment that I made. I don't think anything shorted or went to ground and it was a result of the adjustment that the fuses opened up. When I solder or re-solder boards I'm in there with a pick scraping around in between all joints and removing the rosin while I'm at it... while referencing the schematic. I don't believe I have a bridge anywhere, but Ill be checking it anyway, I think! I didn't get pictures of the underside but I can say the trace modifications are in the exact same spot as in your pictures, in between the 28 volt rails and the collectors of the npn's.

 

[J. Schaeffer]

On ALL of the NAD 3020s that I work on, from the early version to the 3020B, I now do the same procedure. If it doesn't have 1 ohm resistors in-line with the outputs (like the early ones), I install them, cutting the track shorting out the resistor position. I then fit 1.2K resistors when the adjustment resistors are, in parallel with these are 1K Bournes trimmers set to around 500 ohms. I then dial the bias in to 30mV measured across the 1 ohm resistors. Once this is done, I bridge across the 1 ohm resistor with a piece of scrap wire. For models that have a 180 ohm resistor at the adjustment point, I take this out and use a 1.2K resistor. I've found with the 180 ohm one, the bias tends to ramp up too quickly.

Lee.

I think that I may try it this way! What you've said here is the same as all threads that I've looked in on this subject with the exception of the 1.2k in parallel with the installed trimmer. Of course I may have missed or misapprehended it reading through threads. This is 353 ohms resistance in parallel with the trimmers at 500 ohms, way off the original 1k and 1.2k. However, I think that in conjunction with a 1 ohm resistor across the cut trace being measured might make a difference! Thank you for the information. I'll install the 1.2 k's in parallel with my current trimmers WITHOUT changing the adjustment and measure across the 1 ohmers just to see how close I have it now.

 

[gpounce32768]

The NAD 3150 I finished a couple months ago tended to run pretty hot- OEM heatsinking was inadequate in several cases, the chronic overheat of the protection relay series resistor notwithstanding. I was wondering how the 3020 tends to run, heatwise.

 

[Goldie99]

Cheers Lee - I'm using basically the same procedure as you (hardly surprising since I discussed it with you originally ), with the only differences being due to what I had on stock - so I used 2k trimmers; 0.22R resistors instead of 1R; and I didn't bother changing out the 180's; otherwise it's all the same, just explained clearer by you.

I did think of adding a screenshot of the relevant schematic area, highlighting the exact parts / modifications needed as a visual illustration, but I don't have suitable pc access at the moment.... just thinking it might provide a better long term reference for others in future, as it's a common question.

@J. Schaeffer - I thought the traces you'd cut were the same as in my photo, but I couldn't see clearly enough so thought it worth checking - in any case, just for reference, I think you'll find the trace cuts to fit R653 / R654 are actually on the emitter side of the 2N3055's, not the collector side (later versions of the 3020 used the collector side).