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Old 20th May 2023, 8:37 pm   #1
yestertech
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Default NAD amplifier with faulty muting circuit

I can't quite fathom this. The amp output is silent, caused by the FETs in the muting circuit ( see extract below ) not being 'on'
There is signal at their input but nothing at the preamp out, which is linked to power amp in.
( The power amp was tested separately and works fine )
I can't quite understand the voltages given on the circuit diagram or quite how it works.

The line shown entering bottom left connected to C532 emanates from one AC input from power transformer to bridge rectifier. It then goes through what seems to be a voltage doubler to arrive at C533 at around -62 volts. This is then connected to the network R549 C531 via R550. Switch '1' is part of the mains on off switch which I think activates the muting at power on/off.
At the output of C531 a voltage of +20.8 is given. Given there is an additional input at this point from a +28.5 v supply via a potential divider, does this seem in order ?
The +20.8 volts is then fed via diode D507 to the gates of both muting FETs. The voltage given at the gates is only +0.5.
My measured voltages are wrong at this point. at the junction of R549 /C531 I get around-48 volts. The +28.5 volts entering R547/8 is present and both resistors check OK ( R548 is shown as 68K but 150K is fitted )
The gates of the FETs are sitting at around -0.44 volts and so are 'off'.
Inserting a meter probe onto either gate will switch the FET's on and restore output.

I've replaced C532 and C533 as their ESR's were poor, also diode D507. Other components seem to measure OK.

I'm now rather stumped....

Andy

PS Looking at this again, could this be caused by SW-1c failing to open ??
Attached Files
File Type: pdf mute cct.pdf (415.8 KB, 84 views)

Last edited by yestertech; 20th May 2023 at 8:43 pm.
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Old 21st May 2023, 7:43 pm   #2
DrStrangelove
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Default Re: NAD amplifier with faulty muting circuit

The 0.5V on the gates of the E111 fets indicates that the gate is forward biased which really turns it on.

The 10M resistors limit the gate current to something acceptable.

Is there 20.8V on the anode of D507?

When S1-C is closed the -62V is connected via ?R49? 390 ohm resistor which will reduce the 20.8V to something negative wrt gnd turning the fets off, presumably via the reverse leakage of D507.

When S1-C opens, then C531 charges up slowly and the anode of D507 presumably reaches 20.8V.

Closing S1-C discharges C531 through the 390 ohm resistor and turns the fets off a good deal faster.

Vgs(off) for the E111 is -3V min to -10V max. I wonder if they were selected.

The equivalent of the E111 is the J111-18 according to an ancient Siliconix Fet book, the -18 bit means it has a different pinout to the bog standard J111 to be pin compatible with the E111.
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Old 21st May 2023, 9:53 pm   #3
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Default Re: NAD amplifier with faulty muting circuit

Try changing C531, if it's the original it's probably leaking - I use a 100V cap, 63V is too low.
R548 being 150k is not unusual, it's not critical for the circuit, and the +20.8V is wrong anyway - with R547 / 548 both 150k, it'll be closer to +14-15V once it's settled, namely ca. half the +ve supply.
I also find that version of the schematic confusing; the voltages shown are obviously with the amp ON, but SW-1c is shown in the OFF position. While it may be 'technically' correct, it looks confusing.

I should still have an LTSpice simulation for that circuit somewhere, will have a look in the morning.
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Old 21st May 2023, 11:28 pm   #4
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Default Re: NAD amplifier with faulty muting circuit

Agree, C531 is the obvious candidate for the fault. (R548 is 68K but the 20.8V still seems a little high)
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Old 23rd May 2023, 11:14 pm   #5
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Default Re: NAD amplifier with faulty muting circuit

Thanks to everyone who has contributed here.
Alas, I made an error in my bench notes and C531 HAS already been changed.
I've also rechecked the replaced parts and everything in the path from the AC input to the gates of the FET's - only 2 suspect items have not beenr eplaced so far - C532, between the transformer AC out and D506, and D505/506 themselves. These latter two check OK with the diode check on the Fluke. All Other resistors in the chain measure OK

I still can't fathom what's occuring. Sw1 -c is working OK. Yet, on power up the voltage at the junction of the potential divider/anode of D507 goes straight to -48.7 and stays there.
When switched off it decays fairly quickly
The voltage at the input to the potential divider is showing + 31 volts. I've rechecked polarity on C531 and also re measured with the ESR meter and it measures OK ( I've used 68u as I've run out of 47/63v caps )

Weird !


Andy
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Old 24th May 2023, 9:52 am   #6
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Default Re: NAD amplifier with faulty muting circuit

The only way I can see anything like -48V on the D507 anode side of C531 is if the -62V 'supply' is somehow leaking thru' C531, or the switch SW-1c is still at least partly 'closed' (it should be open for normal operation). There should be no other source for a negative voltage as 'high' as that, all the normal supplies are much lower voltages. Could the 2 wires going to SW-1c possibly have been reversed, or put to different switch terminals ? The switch needs to be 'open' when the amp is on - if not, the -62V from the voltage doubler will simply 'swamp' the +ve supply, and give a fixed -ve voltage at D507 anode (that's how the turn-off muting works).

Try disconnecting one of the 2 wires that go to SW-1c - you should then be measuring ca. -62V on one side of C531, and ca. +15.5V on the other (if you have R547 & 548 = 150k, as you said previously). If so, the problem may be the switch, or the wires to it.
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Old 24th May 2023, 11:26 am   #7
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Default Re: NAD amplifier with faulty muting circuit

If the doubler is not working properly then it is possible that AC is getting through via C531. I would replace C532 and C533 and check they are the right way around.
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Old 24th May 2023, 10:29 pm   #8
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Default Re: NAD amplifier with faulty muting circuit

Quote:
Originally Posted by Goldie99 View Post
The only way I can see anything like -48V on the D507 anode side of C531 is if the -62V 'supply' is somehow leaking thru' C531, or the switch SW-1c is still at least partly 'closed' (it should be open for normal operation). There should be no other source for a negative voltage as 'high' as that, all the normal supplies are much lower voltages. Could the 2 wires going to SW-1c possibly have been reversed, or put to different switch terminals ? The switch needs to be 'open' when the amp is on - if not, the -62V from the voltage doubler will simply 'swamp' the +ve supply, and give a fixed -ve voltage at D507 anode (that's how the turn-off muting works).

Try disconnecting one of the 2 wires that go to SW-1c - you should then be measuring ca. -62V on one side of C531, and ca. +15.5V on the other (if you have R547 & 548 = 150k, as you said previously). If so, the problem may be the switch, or the wires to it.

I'll go back and recheck but my notes tell me that the switch was checked early on, checking for continuity between the 1K2 and the 390R 'closed at power off/open at power on'
I'm wondering now if I've installed the replaced C532/533 the wrong way around....
It's odd that the fault is precisely the same now as before I started
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Old 25th May 2023, 11:02 am   #9
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Default Re: NAD amplifier with faulty muting circuit

What happens if you remove C531 completely?
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Old 28th May 2023, 9:56 am   #10
yestertech
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Default Re: NAD amplifier with faulty muting circuit

Well Well - I've finally tracked down the culprit ! Not what I expected at all

Although I'd already tested the action of the contacts on the mains switch used to discharge C531 (sw1-c ) in both positions and found they were operating correctly, I unsoldered one of its leads and took some more measurements just to be sure. This time, the voltages were all correct and the FET bias correct for turn on. Output restored.
In theory, this is exactly the same electrically as operating the power on switch, but with the switch isolated.
Switching to a higher ohms range and re-testing across the switch contacts revealed some resistance present, constantly changing with each operation of the switch, at times as low as 11K.
Having ruled out switch replacement, as it has signal and mains sections, and not wanting to disable the mute circuitry as there is no output protection relay, I found a spare pair of contacts which measured fine, so these were reconnected in place of the 'leaky' ones.
Power up is now fine, with approx. an 8 second delay before the FET's switch the preamp output.

This proved to be another of what I call the '1% likelihood' faults, of which I seem to have had rather too many of late !

Thanks again to everyone who has contributed to this thread and in particular Goldie99 who had in effect identified the cause in post 6 !!

Andy

Last edited by yestertech; 28th May 2023 at 10:03 am.
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Old 28th May 2023, 10:18 am   #11
Goldie99
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Default Re: NAD amplifier with faulty muting circuit

Cheers Andy - good to hear it's sorted ok

In case you ever do need it, the only (new) replacement switch I've come across, with both the AC & DC switching sections, is the NE1839EE from C&K - although they're getting hard to find in stock now.
https://www.mouser.co.uk/ProductDeta...tl4FQ0uQ%3D%3D

Alan
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