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Old 12th Dec 2016, 12:20 am   #21
paulsherwin
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Default Re: NAD 7020 weak channel on FM stereo

How does this design mute David?
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Old 12th Dec 2016, 12:30 am   #22
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Default Re: NAD 7020 weak channel on FM stereo

I don't know. There is some monkey business going on in the emitters of those two transistors being biased by a connection on the schematic which jumps off to a point "H" which I haven't found, and there is a bizarre variable resistor AC coupled between their emitters boosting their 'differential' gain Huh? some sort of width enhancer?

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Old 12th Dec 2016, 12:33 am   #23
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Default Re: NAD 7020 weak channel on FM stereo

Thanks for all the advice so far. Monday beckons, but I'll get on with applying the guidance asap.
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Old 12th Dec 2016, 12:41 am   #24
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Default Re: NAD 7020 weak channel on FM stereo

Ah, found it. point 'H' is a -15v supply from a zener located at IC103 so the buffers are always on.

There doesn't seem to be muting in the HA1156, but it's got to be somewhere...

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Old 12th Dec 2016, 11:20 pm   #25
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Default Re: NAD 7020 weak channel on FM stereo

Well there is something odd about the buffer amplifier in the weak channel. When I was comparing the voltages between the two, the base of the weak channel emits a hum when touched by the DVM probe, whilst the base of the good channel does not. I reflowed the solder connection, but it made no difference. I'll get a replacement, unless there is another explanation for that symptom.
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Old 12th Dec 2016, 11:37 pm   #26
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Default Re: NAD 7020 weak channel on FM stereo

Have you compared the circuit voltages around the buffer transistors for both channels?

You should be able to easily desolder and remove the transistors to eliminate a transistor fault, which is unlikely but possible. You can use any general purpose type as a temporary substitute - BC108, 2N3904, anything you have to hand. Just make sure that the leadout connections are correct.

Look for an electrolytic coupling capacitor immediately before the buffer transistors. Try bridging it with another capacitor - the value may have gone low.
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Old 12th Dec 2016, 11:44 pm   #27
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Default Re: NAD 7020 weak channel on FM stereo

ah-ha!

There's a 4.7uf electrolytic feeding each base C219 and it looks like C218. also C222 and C226 and R225 and R224.

Something along this route in the bad channel may be open circuit.

The hum makes me suspect that the transistor is transistoring OK, and that the feed of signal to its base is open circuit, leaving it as quite a high impedance and much more open to hum pickup.

Just to double check... you now it's the bad channel that hums when prodded because it's out of the other speaker than the one which can produce music?

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Old 12th Dec 2016, 11:46 pm   #28
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Default Re: NAD 7020 weak channel on FM stereo

I agree with David (I still don't have a circuit and am using guesswork regarding components).
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Old 13th Dec 2016, 12:39 am   #29
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Default Re: NAD 7020 weak channel on FM stereo

While you gents were sending your last posts, I followed up Paul's tips in #26. I popped in an old AF6 (BC107 equivalent) the nearest I had. No change. But when I checked the four 4.7uf coupling caps before the buffer transistors, one was dud.

As you both suggest, the transistor is probably okay, and it was just the open circuit cap.

As luck would have it, I had just one such cap to hand. I popped it in and all's well! Thank you Paul and David for your interest and assistance. I've learned a lot.

As a final twist, after I inserted the replacement cap, one channel seemed to have gone completely, but then I noted that my headphone socket was half unplugged!

And so to bed.
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Old 13th Dec 2016, 12:42 am   #30
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Default Re: NAD 7020 weak channel on FM stereo

Good stuff.

If I were you I'd change that cap in both channels while you have easy access.
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Old 13th Dec 2016, 12:45 am   #31
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Default Re: NAD 7020 weak channel on FM stereo

Noted Paul. Thanks again.
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Old 13th Dec 2016, 12:48 am   #32
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Default Re: NAD 7020 weak channel on FM stereo

I think there are four of them.

Best put the original transistor back. It's not a well-designed bit of circuitry, that buffer amp and its bias current is rather dependent on the transistor gain. Crappy design to save one resistor but usually winds up forcing the use of selected transistors. Bit of a home-goal really.

The crap on the board may have been capacitor guts. A good clean on both sides might be advisable.

Have a good snooze, you've earned it!

David
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Old 13th Dec 2016, 9:07 am   #33
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Default Re: NAD 7020 weak channel on FM stereo

Yes, I assume you've refitted the original transistor if there's nothing wrong with it.
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Old 13th Dec 2016, 10:59 am   #34
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Default Re: NAD 7020 weak channel on FM stereo

Not refitted yet, but I will. The other three caps measured okay in circuit. Not sure if they need to be replaced. Of course, when you look at all the aged components on the PCB, you realise that they are all growing older gracefully together. A bit like the rest of us.
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Old 13th Dec 2016, 11:16 am   #35
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Default Re: NAD 7020 weak channel on FM stereo

There is an argument for changing all electrolytics of a particular value and voltage rating if one fails, or at least changing the equivalent caps in both channels where the circuit conditions are the same. It's partly a matter of personal preference and partly down to how easy it is to do.

There's nothing wrong with just changing the faulty one if that's your decision. Changing more is just preventative maintenance.
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Old 13th Dec 2016, 5:56 pm   #36
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Default Re: NAD 7020 weak channel on FM stereo

It's worth looking around for traces of leakage and prodding around with an ESR meter, then there is some information to use to make a decision.

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Old 13th Dec 2016, 6:17 pm   #37
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Default Re: NAD 7020 weak channel on FM stereo

I did check the other three caps and they measured okay. However, I've picked up three 4.7uf caps today. It's easy enough to put new ones in now.
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Old 14th Dec 2016, 12:55 am   #38
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Default Re: NAD 7020 weak channel on FM stereo

Quote:
Originally Posted by Radio Wrangler View Post
Best put the original transistor back. It's not a well-designed bit of circuitry, that buffer amp and its bias current is rather dependent on the transistor gain. Crappy design to save one resistor but usually winds up forcing the use of selected transistors. Bit of a home-goal really.
I did put the original transistor back and replaced the three old but working capacitors. With this problem fixed, I wondered about the very soft putt-putting sound that is audible on all inputs when listening with headphones. As it is present on all inputs, I realise that this is not related to the FM stereo circuit, and it's not a big deal, but while we are talking about this circuit, I am curious about where is this oscillation likely to be located? Any thoughts?

I observe that it goes away if the 'low level' function is pressed, even if the volume is then increased to the original level.
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Old 14th Dec 2016, 10:03 am   #39
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Default Re: NAD 7020 weak channel on FM stereo

It's called 'motorboating' and is an indicator that some power supply decoupling capacitors have gone low in value, high ESR, or open circuit.

You can go round them all with an ESR meter and replace only the duds, or go for just swapping the lot, whichever approach you subscribe to. Where they're all of the same age, I'd suspect they're going to all be close to end of life, and go for a preemptive strike. I don't rip all the capacitors out in the hope that some new magic brand will sound better. The mass swap happens only on evidence of a general trend needing a general solution.

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Old 14th Dec 2016, 10:33 am   #40
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Default Re: NAD 7020 weak channel on FM stereo

I routinely replace grid coupling caps in valve amps, but I'll need look at the schematic and check with you which ones these decoupling caps are. Thank you for the steer.
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