advertisement


Ncc200 caps.

Edwar65

pfm Member
Hi, I wish to change C3/C6 caps on my ncc200 boards, they will be swapped out for a 10uf wins in C3 and 100uf Sprague wet tantalums in C6.
Is it necessary to check anything on the boards when they are powered back up, if so, what and how? I have access to a few multimeters.
Also, do I need to be careful with regard to static as I don't have Access to a wrist band at moment?
Many thanks.
 
As with any change to the board you should check the bias current, 36mA and the DC offset, <50mV.
 
Hi garf, thanks for reply. How does one check and adjust these and at which point on the boards do I take readings for the bias/DC offset?
I didn't build the boards,the amp was bought complete,so am a novice with regards to electronics,I am a qualified electrician though!!!
Thanks.
Tom
 
You could use 2 multimeters to set bias on both channels at the same time to speed things up ( you need to give the current time to stabilise ) as it's a bit long winded doing one channel and waiting for the supply to discharge before doing the second one. I believe it can be a bit risky disconnecting the +ve supply whilst there's still charge left on the -ve supply. For the same reason make sure your meter is firmly connected and can't slip off the +ve supply lead and the corresponding tag on the amp board.
Check the voltage on each channel output terminals when you've finished ( you can't adjust it, it's set by the relative gain of TR1 & 2 - Les will have matched them during construction ).
 
Yeah I would always recommend discharging everything + & - side before connecting / disconnecting just in case it blow's something, and some people have reported exactly that happening.
Always connect the ground lead's first, then Neg then pos, that's my own little routine but it works for me :rolleyes:
Alan
 
Sorry if I'm missing something, but There's no mention in above diagram of disconnecting the ground? Also, can I just allow a period of time to let the capacitors discharge, say an hour?
Is it possible to measure the voltage in the caps with the multimeter to prove dead?
Thanks.
 
Sorry if I'm missing something, but There's no mention in above diagram of disconnecting the ground? Also, can I just allow a period of time to let the capacitors discharge, say an hour?
Is it possible to measure the voltage in the caps with the multimeter to prove dead?
Thanks.
If you're going to replace capacitors on the NCC200 boards you will be removing the boards from the amplifier and so you will be disconnecting ALL the flying leads from the boards (including the ground leads) and removing the 2 screws that secure the amp modules to the chassis. Then you can lift out the NCC200 boards and replace components on the boards as you like (Les chose pretty good parts in the first place!).
Once you've done that you will replace the boards (with some fresh thermal paste between the heat sink and the chassis) and reconnect the flying leads onto their tags.
By the time you've removed the boards the mains supply will probably have been isolated long enough for the power supply to have discharged, when you re-install the boards connect the leads as before but with the +ve supply connected through a milliameter as described above. Once you've set the current on one (or with 2 meters both) channels turn off the mains and watch the current on the meter.
The current will drop rapidly at first then reduce to a slower rate until the current drops to a few ma. If you leave it for 30 minutes or so it should have dropped to very little and then it's safe to remove the multimeter and reconnect the leads.
Static shouldn't be a concern with these sort of circuits, just do it!
 
I would always discharge the power supply caps into a hundred or so ohm wire wound resistor.
Not doing so can cause substantial damage and discharging only takes a minute even with multiple supplies.
Cheers Andy.
 
I use a bed side lamp (with bulb in) with plug cut off. One wire on 0V and the other to positive or negative depending on which you are discharging.

Works for me
 
I've noticed that when checking bias after 20mins or so,mine are at about 34mA but are increasing minutely as time increases?
Do I just set at 36mA at bang on 20mins,as after this time it's going to increase possibly??
Thanks
 
Have yo got the lid in the amp as that also makes a difference
I check mine after 20 mins, it usually needs a tweek but then its another 20mins
I used to set mine at 34mA for that reason

Alan
 
Hi, no lid is off. Had to drop it slightly after 25-30 mins but it doesn't seem to stabilise entirely. I'm using 2 dmms so I can check both board in my dual.mono setup.
 
There's nothing magical about 20 minutes - leave it until the current stabilises, then adjust the bias if necessary.
Usual advice is to set current to 36 - 38mA but not to exceed this for fear of thermal runaway (above reproduction of Les's instructions quotes 40mA max, as does Accoustica website for Naim power amps).
I'd set it toward the top end of the 36 - 38mA range as low bias can reduce sound quality - I bought a NAP180 last year that sounded very poor but it turned out the bias was less than 31mA and once I raised that to 37.5 it sounded like a different amplifier (I note this phrase is often used by people commenting on the results of having their amp serviced - could it be that the bias was wrong prior to the service?).
It sounds like you are on the right track!
 
Its harder to get it to stabilise with the lid off, just try breathing inside the amp and watch the bias go up and down
You need to set in its operational condition with the lid just laid on top and the meter outside the case is all I do.

Alan
 
Hi Alan, Do you set a time limit for the amp to stabilise with the lid on top? Mine has been on for around an hour or so at 32/33 mA and it's very steadily increasing by a couple of mA every 10mins or so, I don't want to increase anything too much beyond this,as with the lid on and in operation I assume it will increase slightly above this?
Thanks.
 


advertisement


Back
Top