NAD 1020 Preamp Problems

I am having problems with a NAD 1020 Preamp. The unit powers up fine and the 120V plugs in the back of the preamp also work as I can power a phono or CD.

The problem is that I get no output signal. If I turn up the volume all the way, you get some very distorted and scratchy thumps and pops that is generally coincident with the bass of the music being played.

I popped the unit open and there is nothing that looked obviously cooked.

Does anyone have any suggestions as to where the problem may be?

It does not seem to be a power supply problem. I fear that one of the transistors may be cooked in which case I doubt I would be able to track the bad one.

Any help would be appreciated.
 
anatech said:
Yup, power supply or muting circuit. Power supply could cause that as well.
Follow the signal with a 'scope (got to love those things).

-Chris


I measured 50VAC from the transformer and 63.5VDC after rectification. So there are no obvious problems with the power supply. I will have to look at it in more detail. Could bad powersupply caps be the problem?

I listened very carefully with headphones and there is a very very
weak (almost inaudible), but clean signal. However, when you turn the volume way up, the signal remains very weak, but you get the ocasional loud distorted thump and pop that is coincident with the loud parts of the music.

I have a two channel scope, but only one probe. I have some basic electronic skills, but I am still learning how to use the scope. Do you have any tips or suggestions on how I should go about tracking down this problem?

I am also starting to think it may be a muting circuit problem, but that does not explain the loud pops? Could a faulty ground or a short going to the ground also cause this problem.
 
Re: mute?

planet10 said:
Not the 1020 manual, but the 1020 is the preamp out of a 3020 so this may help

http://homepage.mac.com/planet10/NAD_3020_ServiceManual.pdf

dave


Cobra2 said:
I think it has a mute-relay, that keep outputs muted for some seconds after power-up.
Listen for a small click within 10 sec.
I have had other NAD preamps with faulty delay/ relay-driver-circuit.


Arne K


Dave, thanks for the manual.

Cobra2, there are no relays. You get sound as soon as you power up. The tone, bass and balance controls work on the very weak signal I am getting. If you turn up the bass, the distorted thumps and pops occur more frequently.

I'm not really sure where to look from here.
 
anatech said:
There may be a transistor muting circuit. It would be triggered from an AC detect circuit. Look for a single diode from the secondary to a cap around 1uF. Check the ripple.

-Chris

The preamp section of the 3020 is not the same as the 1020 so the schematic does not help much.

Checked the regulators and the voltages seem ok +24 and -21, a bit of a bigger difference than I expected, but I don't think the few volts are the problem.

It does not make sence why both channels are out, so some sort of muting transistors may be the problem. However, without the schematic, I can't figure out where the muting circuit is.

I had planned on upgrading the pot and capacitors .. perhaps its time to get something newer.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi GG,
Sorry, this would be much easier wit ha schematic. Look for 2SC2878 transistors - classic muting transistors. If you find a bad one, they must be replaced by the same thing.
However, I strongly feel you have a supply voltage fault. Possibly affecting the muting circuit (if there is one).
-Chris
 
anatech said:
Hi GG,
Sorry, this would be much easier wit ha schematic. Look for 2SC2878 transistors - classic muting transistors. If you find a bad one, they must be replaced by the same thing.
However, I strongly feel you have a supply voltage fault. Possibly affecting the muting circuit (if there is one).
-Chris

Measurements relative to the ground on the chassis:

DC +32.4 and -31.9

Orange (zener?) diode in -20.3V, out -2.5V

Regulator transistors <23.4, 31.6, 24.0> and <-20.3, -31.2, -20.9>

Blue diodes, most less than 3V (some near zero?) and the others at
about 20V. Will have to look at these more closely.

I think the orange diode drops the voltage down to 2.5V for the LED
power indicator which is right by this orange diode. This is the only
thing that makes any sence since there is only one of these orange diodes.

To me it looks like the power supply is functional. I will have to look at it again tommorow with a clear head.
 
This is very weird. If I pull any one of the two fuses, the problem remains the same. That is a very very faint signal with occasional loud pops. It does not mater which fuse you remove, as long as one of the two fuses remains in place. It looks like there is one fuse for the +AC and one for the -AC (center tapped transformer).

Does this mean that there is a short somewhere on the circuit crossing the +DC and the -DC?
 
GG said:


The preamp section of the 3020 is not the same as the 1020 so the schematic does not help much.


I have to take that back. Most of the line amp section is similar. There is an extra transistor pair in the lineamp stage. However, I have a signal beyond that point so I assume there is not problem. The phono section is also a little different (extra transistor pair plus variable input capacitance).

The 3020 schematic shows only a preout. However, in addition to the preout, my 1020 preamp also has a high level preout. As far as I can tell, after the treble, bass and balance controls, the signal goes to the headphone amp and from there, it goes to the preout, high level preout and headphone. Now, there is no signal going to the headphone amp, but there is power, so I assume that the headphone amp is also functional. The headphone amp is also not shown on the schematic as the 3020 does not have one.

When I trace the signal back from the headamp, I get to another transitor pair which also is not on the schematic. I'm not sure what the purpose is of this transistor pair ... perhaps this is a muting protection circuit that kicks on for a few seconds when preamp is turned on that others have mentioned. However, this circuit is seperate from the audio mute button which is working.

Here are the details for the transistors (Q509 and Q510 which are not shown on 3020 schematic)

B231 (or 8231) two pins at zero voltage, pin marked "G" -10.7V

B221 (or 8221) two pins at zero voltage, pin marked "G" -9.7V

It is funny that the board is marked "G", typically for transistors, they show "C" "B" "E"?

Also, I have signals at the bass and treble controls. No signal at the balance control, but the balance control is connected to the transistor pair I just mentioned.

I guess the options I have are to try and bypass this transistor pair and balance control. If so, where should I pick up the signal (R537, R541 or R543). If I ran this signal into the headamp, I should still be able to use both the preout and high level preout, provided I don't encounter any more problems? Can those transistors I am thinking of bypassing be important?
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi GG,
Now those sound like a muting circuit. There are probably junction fets. Desolder them from the circuit. You may need to short the source and drain pins if they are used to pass signal. Take a picture of the circuit before you pull them with a digital camera if you can.
The negative gate voltage would keep them in cut off. Things would be a lot better with a sensitive relay in this circuit.
So if you find the unit works with these bypassed, you have a problem in the muting circuit. Bettcha it's an open small capacitor used for AC detect. They may be using a set of contacts on the power switch for this too. Easy to check by looking at the switch.

-Chris
 
anatech said:
There may be a transistor muting circuit. It would be triggered from an AC detect circuit. Look for a single diode from the secondary to a cap around 1uF. Check the ripple.

-Chris


anatech said:
Hi GG,
Now those sound like a muting circuit. There are probably junction fets. Desolder them from the circuit. You may need to short the source and drain pins if they are used to pass signal. Take a picture of the circuit before you pull them with a digital camera if you can.
The negative gate voltage would keep them in cut off. Things would be a lot better with a sensitive relay in this circuit.
So if you find the unit works with these bypassed, you have a problem in the muting circuit. Bettcha it's an open small capacitor used for AC detect. They may be using a set of contacts on the power switch for this too. Easy to check by looking at the switch.

-Chris


Hi Chris,

Thanks for your help thus far. I am starting to think it is the muting circuit causing the problem.

Now, I have located a single diode close to these two transistors. The diode is at about -30V. However, the nearby capacitors are in pairs and 47uF. There are also some green mylar capacitors nearby, but also in pairs.

Aside from the AC, there are also two sets of DC wires hooked up to the switch.

If I opt to bypass the muting circuit, should I remove those transistors, or just leave them in place?