I thought my NAD 7020e was working 100% but I spoke too soon...
Originally my NAD 7020e was working 100%. Decided to recap some of the caps (mostly everything except the tuner section) following some internet guides where some caps are overspec'd and some replaced with non-polar WIMA caps. After the mods, a loud repetitive thumping from high bass would occasionally occur. While investigating the problem, I checked the DC offset and bias. That's when I accidently shorted the DMM leads and blew some transistors which I have only fixed this week. Oddly the loud repetitive thumping seemingly disappeared. Or so I thought. It's back and is more regular than before replacing the transistors.
Anybody have any ideas what could be the cause? The overspec'd electrolytic caps? Using unpolarized WIMA caps (I believe they are restricted to the tone circuit but I need to recheck this)?
I think I could just backtrack the overspec and nopolar changes in caps but if I could pinpoint where to begin, I might be able to minimize the backtracking and keep most of the changes. Others who have followed these guides/threads seem to have not run into this problem though.
Any insights, hints?
Thanks.
Originally my NAD 7020e was working 100%. Decided to recap some of the caps (mostly everything except the tuner section) following some internet guides where some caps are overspec'd and some replaced with non-polar WIMA caps. After the mods, a loud repetitive thumping from high bass would occasionally occur. While investigating the problem, I checked the DC offset and bias. That's when I accidently shorted the DMM leads and blew some transistors which I have only fixed this week. Oddly the loud repetitive thumping seemingly disappeared. Or so I thought. It's back and is more regular than before replacing the transistors.
Anybody have any ideas what could be the cause? The overspec'd electrolytic caps? Using unpolarized WIMA caps (I believe they are restricted to the tone circuit but I need to recheck this)?
I think I could just backtrack the overspec and nopolar changes in caps but if I could pinpoint where to begin, I might be able to minimize the backtracking and keep most of the changes. Others who have followed these guides/threads seem to have not run into this problem though.
Any insights, hints?
Thanks.
Certainly this could be the problem. Did you install any capacitors that were different in value from the originals?
In standard commercial products like the NAD, parts are never over-specified, due to cost constraints.
In standard commercial products like the NAD, parts are never over-specified, due to cost constraints.
Yes. Most of the caps had higher capacitance values though not all by the same percent.
Is there a "safe" percentage over-spec that I can use as a guide in choosing which caps to revert first?
Is there a "safe" percentage over-spec that I can use as a guide in choosing which caps to revert first?
You cannot change the power supply capacitor values very much. Too small could cause instability, and too large could cause damage.
The original values were chosen for very good reasons, such as these. Maybe the internet guys never heard of Mr. Bode and Mr. Black.
If any of the tone circuit values were changed, that would alter the way the tone controls were intended to work.
The original values were chosen for very good reasons, such as these. Maybe the internet guys never heard of Mr. Bode and Mr. Black.
If any of the tone circuit values were changed, that would alter the way the tone controls were intended to work.
These PCBs are not of good quality and you must be SURE that all the solders and tracks are in good condition.
I will look at the side of the muting circuit (its power supply circuit) and the general switch.
I will look at the side of the muting circuit (its power supply circuit) and the general switch.
Thanks for the tips. I will retrace and revert the recap starting at the power supply section. Also I will test traces near the swapped caps and then reflow the solder points nearby.
@rayma: I have some regret for following these guides. I, and likely the writers, are/were probably under the spell of capacitor fetishism, the pitfall of half-bright, somewhat competent with a soldering tool, laymen caught in the charms of modifying electrical circuits without realizing the relative simplicity of their acts. Hopefully there might be enlightenment. 🙂
Thanks again!
@rayma: I have some regret for following these guides. I, and likely the writers, are/were probably under the spell of capacitor fetishism, the pitfall of half-bright, somewhat competent with a soldering tool, laymen caught in the charms of modifying electrical circuits without realizing the relative simplicity of their acts. Hopefully there might be enlightenment. 🙂
Thanks again!
Yes, revert the power supply first. This will almost certainly exorcise the low frequency instability.
Post back here if the problem remains. The remaining new parts may be able to stay in place.
Post back here if the problem remains. The remaining new parts may be able to stay in place.
You had a perfect working amp. Then you change some component values, deviating from those once carefully choosen by the designers. And now it's not working properly anymore. Doesn't take much head scratching what the first next step should be.
Change all component values back to original and start from there. Enlarging coupling caps can very well cause low frequency instability in a feedback loop. Enlarging ps caps can cause regulators to go into lf oscillation.
Change all component values back to original and start from there. Enlarging coupling caps can very well cause low frequency instability in a feedback loop. Enlarging ps caps can cause regulators to go into lf oscillation.
Everything's working fine now! Thanks for the pointers.
I recapped this about three years ago for preventative maintenance. It has taken me this long to get back to it (mainly because of the transistor issue). Had to dig up my old notes. Judging by the suggested upgrade values in the guide, if I didn't know better, you'd think someone was out to get me 🙂
I think that there were two sources of the oscillation. Changing back to stock power supply caps values made the oscillation/instability occur (at high bass) after a longer period of time and it could be triggered at flat bass but by playing with the balance knob. I targeted the largest out of spec coupling cap upgrade (C315, C316) and reverted these. That stop any remaining problems.
I recapped this about three years ago for preventative maintenance. It has taken me this long to get back to it (mainly because of the transistor issue). Had to dig up my old notes. Judging by the suggested upgrade values in the guide, if I didn't know better, you'd think someone was out to get me 🙂
I think that there were two sources of the oscillation. Changing back to stock power supply caps values made the oscillation/instability occur (at high bass) after a longer period of time and it could be triggered at flat bass but by playing with the balance knob. I targeted the largest out of spec coupling cap upgrade (C315, C316) and reverted these. That stop any remaining problems.