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0mv bias voltage after NAP 135 recapped

A

azman

pfm Member
Hi PFM members, I need help on this issue.

I received 6 x NAP135 own by a friend, he just new to Naim and start collecting Naim olive boxes from different age. He run it with 52/52PS. 2 of them have HF trafo and using NAPA 4 series board. One of this pair developed distortion (Right amp) during power ON until several hours, then the distortion slowly reduced. Hook the amp to oscilloscope, and sure enough the signal is distorted.
After reading in this forum, I ordered the axial 10uf 63v cap for the regulator board. After assembly, I set the voltage to +39.2 and - 39.8 as in Colasblue thread. DC offset is 8mV and 9mV respectively. Hook to the scope and the signal is now clean. However, when I set the bias across 4 output resistor it give 0mV, but if across 2 resistors it give 4.3mV and can be trim using trim pot..but the 0mV stay, even when I adjusted the trim pot. What is wrong with my method? Measuring points as show below (photo from Colasblue)
scope by Azman Mahmod, on Flickr

measuring points by Azman Mahmod, on Flickr

WhatsApp Image 2023-07-05 at 08.00.14 by Azman Mahmod, on Flickr

WhatsApp Image 2023-07-05 at 08.00.17 by Azman Mahmod, on Flickr
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WhatsApp Image 2023-07-05 at 08.00.16 by Azman Mahmod, on Flickr
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Your mention of NAPA4 rang a bell. Here is @misterc6's pic of where the test points are for NAPA5/4 boards...

NAPA%2525205%2525EF%252580%2525A24%252520bias.jpg


According to Malcolm, these points are related to Naim's method from an official Naim "Set up Information" leaflet dated 1981.

Les W of Avondale has long suggested using the following setting standing current method.

SetNaimBias.jpg
 
Thanks Craig, I will look again on this. What is the bias value for this setup, still 7.2mV or else?

Thanks again
 
You should have 3 0R22 resistors. One goes direct to the output tag, looks like the bottom one in your pic, the other two have a common point. Measure the bias voltage from the other ends of those two, so you are across 0R44 in total. If you figure it out like that by examining the board and measuring with a meter, you won't be wrong.

I think somewhere around 9mv when nice and warm is standard.
 
Please report about the sound differences and let us know the power transistor type and transformer of the pair that sounds best to you!
 
I was going to say - how nice to see new members engaging here - but your profile suggests you've been here a long time member of pfm @azman

So - it is really lovely, appreciated, to have you engage here in the DIY area. A fine set of projects there; please do let us know, how these turn-out, for you & yours. And - do engage with the huge variety of experience here, so that it, 'we' can support you and all learn from doing-so; that's the joy of pfm-diy

ATB!
 
jpk said:
Please report about the sound differences and let us know the power transistor type and transformer of the pair that sounds best to you!
Click to expand...

I use to have NAP 250..this have Holden Fisher transformer, fitted with kendel caps (last serviced by Witchhat in 2015) ..just sold few weeks ago after completed mono 140s. During this conversion, I bought 3 pcs of 140.

The 1st one is very old HF 140 with NAPA 5/4 board, look like it was converted from CB140 as the serial number is around the bottom of the back panel not at the top right. It has NA001 o/p transistor SM3282.

The 2nd has Nuvotem trafo with NA001 SM8927 o/p transistor
The 3rd one has Nuvotem with Sanken 2SC2922 o/p transistor.

Comparing all 3, I like the 1st one the most, then 2nd one. The bass slam from Sanken sound the weakest. I like the sound of NA001, regardless 3282 or 8927.
Holden Fisher trafo is very quiet, Nuvotem from 2nd amp hum more mechanically. All have 240V primary.

So for mono 140s, I choose Napa 5/4 board with new 300VA 30-0-30 V transformers bought from Element 14.

Now I prefer the sound of mono 140 over the 250 so I sold it to fund other project :)

For bedroom system, I use CB Nait 2 with Ergo-IX..this system is sweet sounding. Never been serviced, still sound good. Just received 1 olive Nait 2 for recapping from another friend (cd version). Comparing this, the olive one sound dull from cold, but after 3 hours or so, keep improving..so I guess the new caps will help. Will start new thread on recapping this and also will install Rk27 Alps pot as the original pot is in-balance at low volume.
 
azman said:
I also tried Phoenix board, this has mix feeling in sound. I like the overall presentation, but like the naim original bass slam. Phoenix bass is fuller but lack of slam. Finally, the phoenix is now installed in it's own enclosure with diy pre-amp based on nac 42.

phoenix 2 by Azman Mahmod, on Flickr

phoenix 1 by Azman Mahmod, on Flickr
Click to expand...
Interesting what you’re witnessing with the Phoenix boards.......
I have 2 NAP 110 here, 1 with Phoenix boards and the other one with Avondale boards and power supply.
The Phoenix sounds more detailed to me with more natural human voices but the Avondale is the one that kept 100% of that famous Naim’s PRaT.
I'm keeping both but if I had to choose, the Avondale would be the keeper.
 
jpk said:
Please report about the sound differences and let us know the power transistor type and transformer of the pair that sounds best to you!
Click to expand...

Have time during the weekend to compare all 3 pairs of 135s.

They are sounding vey close to each other, the Nuvotem trafo and SM8927 o/p transistor one sound very2 similar. The Holden fisher with Kendeil caps sound slightly darker and more focus in bass. I will choose this pair but can live will all. Good sounding amp these 135s...
 
...new to Naim and start collecting Naim olive boxes... 6 x NAP135 ...

That's what I call jumping in the deep end. ;)
 
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