NAD 7020 repair: emitter resistors

[kmatch]

I had a NAD 7020 dropped on the bench and one of the outputs blew when I was adjusting the idle current with various fixed resistors.

So I decided to follow some of the suggestions from other folks (including @leesonic) that have repaired these units and thought I would document it here for posterity.

1. Replaced RX1/2 with 25 turn 1kOhm potentiometers
2. Replaced R643/644 with 1.2 kOhm resistors
3. Replaced blown outputs with MJ15015G/MJ15016G with new mica insulators and thermal paste
4. Cut the output transistors’ emitter traces on the PCB
5. Installed 3 Watt, 0.22 Ohm resistors on each output emitter

I placed the emitter resistors on a breadboard and used a couple nylon spacers from Lowe’s as standoffs and screwed it onto the heatsink using one of the existing screw holes where the u-channel fins are attached. I needed to use two longer sheet metal screws 3/4 inch long.

I tested for about an hour and the idle current and DC offsets are all stable. Yay!

Thanks to y’all for being a good resource. Here a couple photos of the install. I used a conveniently-located existing hole in the PCB to route the emitter resistor wiring.

 

[leesonic]

Just wondering why you installed emitter resistors in the first place? They never had them from the factory. The Interweb is full of stories of how these amps can go into thermal runaway. People don't know what they are talking about. Case in point, I had a 3020 on my bench this afternoon. I had it driving some 4 ohm car speakers until the heatsink was almost too hot to touch. Thermal runaway is a condition whereby an amplifier keeps getting hotter and hotter until something blows. I turned the volume down to more respectable levels, and after 5 minutes or so, the heatsink was barely warm. If it was going into thermal runaway, it would have kept getting hotter.

Interweb stories also tell how the hometaxial made transistors didn't need emitter resistors, but the later epitaxial ones do. Except they are missing the fact that manufacturing went over to epitaxial in the mid-1970s, before this amp was manufactured.

https://en.wikipedia.org/wiki/2N3055

I'm not trying to rain on your parade, just thought I'd throw my 2 cents in. Your work looks very neat and tidy, can't wait to see pictures when you recap the whole thing.

Lee.

 

[petehall347]

why are those resistors on the back side of the board waiting to cause a short ?

 

[kmatch]

@leesonic thanks for the note. I'm fairly new and learning fast, but main question is how to separate the Internet wheat from the Internet chaff.

Just some background on this unit. I followed the idle current adjustment procedure and ended up burning R633, R655 and Q617. Here's the sequence of events.

1. Initial situation: RX1 = 470 Ohms. In parallel with R643 (180 Ohm) gives an overall RX1||R643 resistance of 130 Ohm.
Idle measurement shows 6.1 mV on the left channel across the 1 Ohm collector resistor (below the target of 30-60 mV).

2. Removed RX1, so overall RX1||R643 resistance of 180 Ohm.
Idle measurement shows 3.2 mV.

3. Installed RX1=220 Ohms, giving an overall RX1||R643 resistance of 99 Ohms.
Burned R633, R655 and Q617.

After blowing the output, I figured this is pretty twitchy, so started perusing the forums. Three main things I saw:
1. Folks saying "Add emitter resistors and then it should be more stable".
2. Add multi-turn pots to make it easier to adjust the bias gradually.
3. Add base stopper resistors.

I poked my mentor/trainer a few times on the way to go and they said to add the pots and then try it out. But the smell of magic smoke from burned resistors still rattled around in my nostrils. After seeing a (relatively) easy way of adding the emitter resistors by using the existing PCB hole to route the wires, I decided to add #1 and #2, the idle-current pots and the emitter resistors.

[Also, I replaced the blown output transistors with the higher voltage rated parts, since the 60V parts seemed way too close to the total rails voltage.]

It may not have been necessary but this unit was already a customer return and thought a little extra work would reduce the chance of another return on this unit.

I suspect that 99 Ohms blew the outputs with excess idle current, probably since the idle current goes exponential with voltage, I assume that I took too big of a change in RX1. If that is the sole problem, then the fine adjustment pot would fix that problem.

Any feedback is appreciated, and that's exactly why I posted my summary, so I could get inputs on whether I was barking up the right tree, or was overdoing it.

 

[kmatch]

why are those resistors on the back side of the board waiting to cause a short ?

Sorry I did't include the details. The standard procedure for setting the idle current is to install remove a solder jumper and temporarily install a 1 Ohm resistor (in series with the output transistor's collector). I put them on the back of the board so that I could easily clip my DMM on them. The service manual gives the target idle current that is monitored using this 1 Ohm resistor. After the bias is set, then the resistors can be removed and then the solder jumper is added back.

For my case after adding the emitter resistors, the use of the 1 Ohm resistors is unnecessary. The emitter resistors can then be used to monitor the idle current. However, please note that the idle "milliVolt mV" target values must be adjusted to correspond to the emitter resistance that is installed.