NAD 7020i sudden death

[Ian Finch]

this is a similar device to what you may be looking for:

[zenzaman]

well, after a few days with the amp gutted on my dining table ) I found the following:

• both breakers show continuity - in-circuit, unpowered (they are marked E-401 and E-402 in the schematic)
• both output transistors of the right channel (2N3055 and MJ2955) are shorted across all terminals
• all other transistors read ok in-circuit. I've pulled a few that I wasn't really sure about and all measured ok off-circuit as well.
• all diodes in the amplifier stage read ok
• the preamplifier stage and radio work fine when preout and main in jumpers are removed and connected to another amplifier
• the voltage of the power supply feeding the amplifier is around +-24V when the amplifier section is disconnected

regarding the dead transistors, what kind of modifications should be made if replacing them with new 2N3055 and MJ2955 (by On Semiconductors that I can find on Mouser)? and what does actually happen if I simply replace them?

since no other parts show signs of damage (resistors, caps, diodes etc), is it possible that these transistors simply failed of old age?

thank you!

[zenzaman]

LE: were you talking about the emitter and base resistors for the output transistors mod?

[indianajo]

Output transistors are usually blown by shorted speaker wiring, as a 1/4" phone plug pulled partially out. Or a bad speaker, say a high value crossover capacitor allowing current to flow to both woofer & tweeter at too many frequencies. Or a shorted turn in one driver.
Or the mica washer under the TO3 transistors could have shorted across. These are 30 year parts, and should be replaced with the transistors. Silicon rubber insulators require no heat sink compound, but cost more than mica.
2n3055 and MJ2955 are rather minimal these days. I'd upgrade to MJ15015 MJ15016, or MJ21194 MJ21193. Note new TO3 packages come with smaller holes, and require 3 mm or #4 screws instead of #6 or 4 mm. Try to get the screws in the same box for the same E9 freight.
Double check the emitter resistors on the output transistors. They frequently blow also.
BTW, the modification Ian Finch was talking about necessary for faster modern output transistors, is a 10 ohm 1 to 3 watt resistor between the driver transistors and the bases of the output transistors. You may need to cut the leads and drill tiny holes in the boards to install these resistors. The legs of the new parts can be bent down to cross the old leads, and soldered to them. I use a general brand pin vise to drill these holes, usually a #41 or 45 (US number drills) to drill the hole. These tiny drills will not fit in 3/8" chucks of electric drill motors. The inductance of these resistors if wirewound or metal film is usually enough to keep the output transistors from oscillating in the RF band.

[zenzaman]

well I'd exclude a shorted speaker wiring as I didn't touch the cables and it worked fine with them for a month prior to the incident. the speakers are working fine as well.

I'm either missing something or there's no emitter resistor on the output transistors in the schematic of the 7020i. the emitters are linked together...

[Ian Finch]

As posted already, the replacement output transistors need to be hometaxial type to match the sound quality of the original amplifier, as you can hear from the working channel. Sure, you can modify both output stages to suit newer epitaxialtransistors of almost any complementary epitaxial TO3 transistor type but that does not quite restore the original NAD 3020 sound quality. It may be good enough though.

As I understand the OP, you bought the amplifier to hear the original NAD sound quality. It's seems you were misled by the seller if the second channel doesn't work now but you must have shorted the output somehow if its burnt now. Anyway, I don't think you are going to be satisfied with modern 2N3055/MJ2955 substitutes as I found from 2 customers, when I replaced them after similar damage to one channel.

There are several threads about the problem because so many amplifiers were produced. Here's a good example with an explanation of the problem. Look particularly at post #2 here: NAD 3020 (& related NAD output stages)

Edit: I guess you now understand why there are no emitter resistors.

[indianajo]

As that thread indicates, RCA quit making homotaxial transistors in the late 70's. others a few years later. The RCA NJ factory is now a county park. I think Comitern in Eastern Europe made the last of them. You are not going to find any. Of course, you can buy any number you want on aliexpress or ebay. See "my transistors are they fake" at the head of parts forum. I am trying to repair a replacement swell engine board where the factory tech installed 27 "transistors" of the same number as the print required. Only they aren't transistors, they are little black pieces of plastic with 3 wire legs.
And add the emmiter resistors. I use .47 ohm, most use .33.
If you want a particular sound, buy a graphic equalizer or a DSP. Frankly, the Dynaco ST120 with homotaxial transistors 1966-1972 was panned by the critics, sounded much worse than tube amps of the time. The "TIP3055" mod of 72, which replaced the homotaxial output transistors with epitaxial TI ones, produced a lot less harmonic distortion of high frequencies. As proved by a sim run by a diyaudio poster. I confirm, with an additional mod to keep idle bias current up to 20 ma, my ST120 sounds much better on high frequency sources like piano and tinkly bells than my tube amp, a ST70.

[mjona]

There is no emitter resistor.

The supply rails for the power amplifier section should be +/-28 volts if you measure those on the cases of the 2N3055 and MJ2955 in the power amplifier channel that is still working.

I had a friend who had a similar experience to you when switching digital sources like CD players only he was less lucky as this caused a two channel power IC module to fail.

You could have been loading one CD player output momentarily with the second CD player output when switching between these where the inter-reaction has caused an oscillation that has migrated into the failed channel of your power amplifier.

The service manual for this amplifier shows the output transistors as plain 2N3055 and MJ2955 if these were Hometaxial types that would be denoted by the suffix H which would also be stamped onto the transistor cases. If otherwise they are Epitaxial types which is a later and cheaper manufacturing process with superior high frequency characteristics.

I have no problem with your buying On Semiconductor replacements as you propose.

[Ian Finch]

indiana, might your "little black pieces of plastic with 3 legs" just be a different type of device, such as a "digital" or pre-biased transistor for switching purposes? These add a couple of resistors on the chip to ensure precise switching action without the need for additional components. The major semi brands all now churn out several grades and package types of these by the truckload.

[zenzaman]

While all components on one channel seem to read ok (most readings in-circuit), the sound is extremely distorted and muted. Hopefully it has something to do with the present imbalance of the two channels, otherwise there's something wrong there that I haven't found too.
But I just checked the service manual for the original 3020 and the parts list specifies MT2955 and 2N3055 for the output transistors (no H suffix). Is there any other way to tell on from the other?

Originally Posted by Ian Finch

As I understand the OP, you bought the amplifier to hear the original NAD sound quality.

While it's true I wanted (and still want - but now I see things with a different set of eyes) a 3020 to experience the 'legendary' sound, I do admit my ears are not the sharpest out there, and, except a true A/B test, I wouldn't probably tell one from the other.

Right now I'll just consider it a project from which I can also learn a thing or two (since I'm totally new to electronics). Of course, since my aim is to make it sing again, I might as well do it as good as I can (albeit in a 'monkey sees, monkey does' manner).

Thank you!