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Naim NAP-140 DC Offset High!!!!

F

Flaxmill19

Member
Hi All,

I am new to this forum and new to Naim Audio gear. I have picked up a Naim NAP-140 and NAC-62.

On the NAP-140 I checked and adjusted the Bias to the recommended 7.0 to 7.2 mV (it had been running way high, about 18mV). I then checked the DC offset and the left channel was 0.245mVDC and the right channel was 0.765mVDC, both I consider to be very high. So I thought it will be the tantalum capacitors leaking dc, so I have replaced the 10µF with 10µF WIMA Films and the 47µF with 47µF Elna Slmic’s. Retested the DC offset and found virtually no change in the off set.

My thoughts are now turning to the semiconductors, with the paired inputs to start with, but I am amazed that the changing of the capacitors made no difference, should I be looking at something else?
Any assistance on this would be appreciated.

Tony
 
Could be one of many things:
  • The vbe bypass cap - the 47uF tant that bypasses the trimpot
  • The two IN4148 diodes in the CCS (front right of the board)
  • Leaky feedback cap
  • One (or both) of the LTP transistors on the way out (as you suggested)
  • Output or driver transistors...
 
I don't remember the "normal" figures for DC offset but unless I'm reading this wrong or unless you misplaced a decimal point what you have should be very low, so OK, not high.

But I am not very competent, wait until someone competent responds.
 
Ah, Neil is competent! But still, I'd check that because it is easy to get confused with decimal points and millivolts.
 
0.765 Milli Volts ?

Paul beat me to it. If so that's amazingly low.

0.765 Volts is HIGH and 76mV about maximum I think - 50mV would be more like it.

However, do let the unit warm up - lid closed etc it may stabilise.

And do take care taking these measurements.
 
trancera said:
0.765 Milli Volts ?

Paul beat me to it. If so that's amazingly low.

0.765 Volts is HIGH and 76mV about maximum I think - 50mV would be more like it.

However, do let the unit warm up - lid closed etc it may stabilise.

And do take care taking these measurements.
Click to expand...

Hi All,

Thanks for your comments, good spotting, the DC output exceed my limit on my MM mVDC scale, so switched to VDC, so should read 0.765VDC.

Tony
 
Did you replace both 47uF on each board (four in total)? The 47uF Vbe bypass capacitor close to the trim pot is often the culprit for high DC offset.
 
misterc6 said:
Did you replace both 47uF on each board (four in total)? The 47uF Vbe bypass capacitor close to the trim pot is often the culprit for high DC offset.
Click to expand...

Hi Mister6,

I replaced both the 47uF's on the left channel board, I haven't replaced anything on the right board. So there has been no change in the DC offset on either channel. I thought I would try and get one board right and then do the other board.

Tony
 
Just a note that I've read many folks have replaced tantatum with film caps in Naim amps and been disappointed with the change in sound.
 
If somehow your measurements were an order of magnitude out, so left was 24mv and right 76mv then left would be in the ballpark and right might need a capacitor change. You've changed the caps in the left, resulting in no change because it's not the caps.

Otherwise I would leave it to warm up for a while with the lid on, then recheck everything. And then I'd measure lots of voltages around the amp, and write the values on the schematic, looking for things that don't addup. And I'd also pay attention to the claimed values of the resistor in the CCS for the LTP and the one between the rail and base of the Vas transistor.

Finally, a stupid question, is the preamp connected?

Paul
 
Paul R said:
If somehow your measurements were an order of magnitude out, so left was 24mv and right 76mv then left would be in the ballpark and right might need a capacitor change. You've changed the caps in the left, resulting in no change because it's not the caps.

Otherwise I would leave it to warm up for a while with the lid on, then recheck everything. And then I'd measure lots of voltages around the amp, and write the values on the schematic, looking for things that don't addup. And I'd also pay attention to the claimed values of the resistor in the CCS for the LTP and the one between the rail and base of the Vas transistor.

Finally, a stupid question, is the preamp connected?

Paul
Click to expand...

Hi Paul,

As per my previous post, I made an error when initially showing the results, the DC outputs are definitely 0.245VDC & 0.765VDC. To me there is something seriously wrong with this amp, to me the logical assumption based on peoples comments is that the capacitors are most likely to be the problem areas. That doesn't appear to be the case for me, so I need to keep looking. Once I have sorted the problem I can start looking at sound and capacitors. But as this is the only Naim system I have listened to I need to get the basics right and then work on the sound by changing capacitors, at the end of the day I may re-fit all the original capacitors to achieve the sound that suits me.

PS: The pre amp is not connected during the DC Offset checks.

Tony
 
Hi All,

I have removed, tested and replaced both the input transistors on the left and right channels. With the transistors removed, I tested these and found the HFE to be all over the place, left channel 142 & 476 and the right channel 51 & 648. I replaced these with BC237BG’s from Digi-Key, the four measured 277 to 279 and the channel pairs had identical curves.

With the new transistors installed, the DC offset was left 35mVDC and the right 20mVDC, great result.

I have now fitted the same capacitors to the right board as I installed on the left board. The amp is now running and sounding not too bad and with the amp warmed up I adjusted the bias 7mVDC.

To me it sounds pretty good, a little boomy (or soft at the bottom end, hasn’t got that nice slam in the base), but I will give the amp a few more hours before making any changes.

My thoughts are to replace all the transistors (and maybe get the DC offset even closer to the magical 0mVDC) and then look at changing the capacitors in the NAC 62. That should ensure the amp & pre-amp are good for another 30 odd years.

Thanks for everyone’s help.

Tony
 
Once again there is nothing magical about zero offset, it's not the Holy Grail. Some DC offset is inherent in all designs based on the original RCA circuit. A small amount of DC offset is perfectly acceptable and has no effect in the performance of the amplifier. Anything less than 50mV is fine and you don't have to match the offsets of the two channels. It doesn't matter if one channel has a positive offset whilst the other is negative. Time and effort spent trying to reduce the excellent figures you now have will be totally wasted.

If the offset is substantially higher than 50mV there is a risk of damaging your speakers. The values you now have are unlikely to change over time. I've worked on dozens of vintage Naim amplifiers and all of them were within their specification of 30mV offset.

Do give your capacitors a chance to burn in before making any conclusions on amplifier performance.
 
The woolly bass will be the silmic2 in the feedback position IMO, change it for a mil spec wet tant 68 or 82uf see if that cleans it up
If it still sounds to smooth change the input film for a 10uf wet tant or BC 128

Alan
 
Hi All,

My restoration of my NAP-140 and NAC-62 continues, I believe I have the amp sorted and I have recapped the NAC-62, excluding the NA 323-2 phono boards. All sounded good.

I then hooked up a turntable and played through the NA 323 boards and the sound was terrible, scratchy, and what music sound there was, sounded very muffled, along with was a nice good feedback hum.

I pulled the NA 323 boards and recapped these, replacing the 47uF/40V ROE’s with 47uF/50V Nichicon Muse’s and the 10uF/35V Tantalum’s with 10uF/50V AVX Corp Tantalums.

Reconnected the boards and played through the phono boards, a lot of the crackle had gone, but I notice when you touch the Normal, Mute, Monitor toggle switch a lot of noise is created, I Deoxit the switch but that doesn’t seem to have made a lot of difference, as I don’t intend to use the switch I think I will put two jumper wires into the circuit to cut the switch out, that should improve things a little.

But the sound is still muffled and this where I hope someone may be able to offer some assistance as where I should start looking, could it be a bad transistor on the NA 323 board? If so, it may be in both channels, because both channels sound about the same.

Any help would be appreciated.

Tony
 
My knowledge of Naim stuff is limited but I do know that the NA323 is a MC board. If you have a MM cart, you will need NA322 boards as the loading is quite different for these different types of cart.

AP
 
a.palfreyman said:
My knowledge of Naim stuff is limited but I do know that the NA323 is a MC board. If you have a MM cart, you will need NA322 boards as the loading is quite different for these different types of cart.

AP
Click to expand...

Hi AP,

Thanks for your input, I think you are onto something, I am using a MM cart., but I am finding that I am using about half the volume to get a reasonable level of sound (although muffled), I would have thought that it would have been the other way around with the MM cart. having a much higher output than a MC cart. Or am I missing something?

Tony
 
Flaxmill19 said:
Hi AP,

Thanks for your input, I think you are onto something, I am using a MM cart., but I am finding that I am using about half the volume to get a reasonable level of sound (although muffled), I would have thought that it would have been the other way around with the MM cart. having a much higher output than a MM cart. Or am I missing something?

Tony
Click to expand...

No

Cheers, Harry

;) :) ;)
 
Flaxmill19 said:
Hi All,

My restoration of my NAP-140 and NAC-62 continues, I believe I have the amp sorted and I have recapped the NAC-62, excluding the NA 323-2 phono boards. All sounded good.

I then hooked up a turntable and played through the NA 323 boards and the sound was terrible, scratchy, and what music sound there was, sounded very muffled, along with was a nice good feedback hum.

I pulled the NA 323 boards and recapped these, replacing the 47uF/40V ROE’s with 47uF/50V Nichicon Muse’s and the 10uF/35V Tantalum’s with 10uF/50V AVX Corp Tantalums.

Reconnected the boards and played through the phono boards, a lot of the crackle had gone, but I notice when you touch the Normal, Mute, Monitor toggle switch a lot of noise is created, I Deoxit the switch but that doesn’t seem to have made a lot of difference, as I don’t intend to use the switch I think I will put two jumper wires into the circuit to cut the switch out, that should improve things a little.

But the sound is still muffled and this where I hope someone may be able to offer some assistance as where I should start looking, could it be a bad transistor on the NA 323 board? If so, it may be in both channels, because both channels sound about the same.

Any help would be appreciated.

Tony
Click to expand...

I had this with my '62 - the switch was ****ed, so I rang Naim and talked to the service department.

Two days later there was a plop on the doormat; a new replacement switch FOC . . .

That's called after-sales service, Naim style. Much better than wiring the bugger out.

Ring 'em on 01722 426 600; ask nicely and they should do the same for you.

Cheers, Harry

;) ;) ;)
 
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