NAD 7020i sudden death

[mjona]

Originally Posted by Ian Finch

Nice video and professional attitude. Note that it's a different PCB and layout to the OP's and he's most likely replaced the original RCA hometaxials with those epitaxial ST Micro versions - a mixture of types that may actually work OK up to medium power. You don't see or hear it delivering more than perhaps a couple of watts so we don't know what happens at higher power when the power devices become hot. I've learned not to return amps to the owner in that shape without a full measured power check. Otherwise they can bounce.

You are suggesting here the 2N3055s are hometaxial.

If so the gain bandwith product would differ in each output half, and the amplifier. There would be competing poles.

You cannot compensate for dual poles so in that situation the amplifier would be unstable even with no input.

I don't see this amplifier achieving the claimed THD spec of <0.02% unless the outputs are epitaxial devices.

[zenzaman]

Originally Posted by indianajo

You have the idea.
Parts soldered to the trace on the surface tend to fall off when the product is jostled in handling. 3 to 5 watt resistors are not light.
Parts on the opposite side of the board with a lead run through a drilled hole and bent over the trace then soldered are better supported against jostling. A kink can be bent in the lead on the component side to clamp the part in place mechanically from both sides.
The interference with the heat sink is the reason I suggested putting the extra resistors over in the blank area with the two big holes. Wires can be run over there. 2 wires instead of one. My ST120 amp the entire transistors are remote from the driver board, 1" away. Emitter resistor flies in the air.

Got it now. Thanks!

[indianajo]

Originally Posted by mjona

You are suggesting here the 2N3055s are hometaxial.
If so the gain bandwith product would differ in each output half, and the amplifier. There would be competing poles.
.

Mr. Finch has clearly told you the units he repaired did not have RCA parts after the very early ones. MJ2955 did not have to come from Motorola, the part number was not copyrighted. There were lots of little semi houses pushing obsolete parts 1978-1985. Homotaxial MJ2955 would be such an obsolete process, and quite profitable to NAD that covered the earth in their trashy emitter resistor lacking cheap amplifiers. Sourcing homotaxials would be a serious competitive advantage, at a time inflation was cramping everybody's style. Bookshelf speakers were all the rage then, 1 to 3 watts were great for Simon & Garfunkle or Joan Baez LP's. 3 W resistors didn't fit the same pick & place machine head as 1/4 watt resistors, and avoiding that changeover of process machine probably saved $2 a unit production cost. NAd saved another $3 by not having a idle bias pot adjustment that had to be handled by a human being.
People that are willing to pay $3300 for a NAD on Ebay (last week asking price) are preserving the delusion of "quality".

[mjona]

Originally Posted by indianajo

You have the idea.
Parts soldered to the trace on the surface tend to fall off when the product is jostled in handling. 3 to 5 watt resistors are not light.
Parts on the opposite side of the board with a lead run through a drilled hole and bent over the trace then soldered are better supported against jostling. A kink can be bent in the lead on the component side to clamp the part in place mechanically from both sides.
The interference with the heat sink is the reason I suggested putting the extra resistors over in the blank area with the two big holes. Wires can be run over there. 2 wires instead of one. My ST120 amp the entire transistors are remote from the driver board, 1" away. Emitter resistor flies in the air.

This advice is short on detail. There is nothing on resistor values, if these are wire wound, how these will affect power output and how the collector and emitter currents will be laid out to cancel radiated fields due to output stage switching. This looks too much like a Heath Robinson fix.

[indianajo]

In post 17 I suggest .47 ohm emitter resistors. I used .51 in my AX6. There are successful epitaxial transistor amps with .33 ohm, .22 ohm, and possibly even .1 ohm emitter resistors.
You can see pictures of how I flew the wiring from driver board to output transistors on the heat sink in this AX6 post #212: Retro Amp 50W Single Supply - Page 22 - diyAudio
Keeping the OT wires 1" away from each other, and keeping the temperature sense wires 2" away from output transistor wires, are key IMHO avoiding oscillation. You'll see my base & emitter wires were 4" long so I could flip the driver board over for work on each side. The high current stuff is IMHO, not sensitive to position or length inside the RF free case. Of course, the NAD has no temperature sense diodes or transistor to cause oscillation with the VAS.
Note nigel7557 or whatever has had trouble with oscillation in Maples (?) 1970's amps that came with homotaxial transistors. His solution was to put 10 ohm 1 watt resistors in the base line to the output transistors. I did that as a damage limitation during meltdown practice, when I replaced the 1970 RCA transistors (burned ) with NTE60's (MJ15003 white box?) epitaxials in my dynakit ST120. I replaced the wires from the driver board to the bases of the transistors over on the heat sink, with 10 ohm 1 watt resistors. Flying right out in the air. If the OP gets oscillation, I suggest that is the next step. As I suggested in post 14.

[Ian Finch]

Yes, also look at this thread: NAD 3020 (&amp; related NAD output stages) . Does the OP there seem familiar?

This same problem with the 3020 series NAD design and the simple emitter resistor solution with suggested resistor values have been covered here time and again (particularly around holiday time ) for the 20 years the forum has been running and in my amateur repairer experience at least, its been done successfully for even longer.

[indianajo]

Originally Posted by mjona

if these are wire wound, how these will affect power output and how the collector and emitter currents will be laid out to cancel radiated fields due to output stage switching. This looks too much like a Heath Robinson fix.

BTW, my 50 w retro AX6 amp,with single pair NTE60 output transistors & .51 ohm wirewound emitter resistors (added), a 70 v rail, put out 24 VAC for 5 seconds at a time on SP2-XT "8 ohm" speakers. That is 72 watts. Test done with Simpson 266-XLPM analog voltmeter which measures average AC voltage. I do have heat sinks on the drivers & vas, which Apex doesn't specify on the AX6. VAs was a GE D44R4 from the '70s and drivers were On semi MJE15028/29. All 30 mhz Ft TO220 transistors. Idle bias current was adjusted to 20 ma, so there was no output transistor switching.

[mjona]

Originally Posted by indianajo

Mr. Finch has clearly told you the units he repaired did not have RCA parts after the very early ones. MJ2955 did not have to come from Motorola, the part number was not copyrighted. There were lots of little semi houses pushing obsolete parts 1978-1985. Homotaxial MJ2955 would be such an obsolete process, and quite profitable to NAD that covered the earth in their trashy emitter resistor lacking cheap amplifiers.

The comment was made in the context of the NAD 3020 in the youtube video where the output devices were made by RCA and ST Micro inferring a hometaxial device in harness with an epitaxial one.

The RCA hometaxial was rated at 150W and used in consumer and industrial products and probably within the military.

There would be need to service older appliances and equipment relying on these in workplaces etc. You would have a situation of supply and demand and hoarding by the bigger players whom NAD as a relative infant would never have been able to compete with.

[PRR]

> The RCA hometaxial was rated at 150W

RCA 1966 (hometaxial) 2N3055 was rated 115W.

This part shows ~~1.3Vbe at 5 Amps Ic, so "acts like" 0.14 ohms internal emitter resistor due to significant parasitics.

2N3771/3 was rated 150W.

[mjona]

Originally Posted by indianajo

In post 17 I suggest .47 ohm emitter resistors. I used .51 in my AX6. There are successful epitaxial transistor amps with .33 ohm, .22 ohm, and possibly even .1 ohm emitter resistors.

Of course, the NAD has no temperature sense diodes or transistor to cause oscillation with the VAS.

See attached image of NAD7020 circuit for elaborated amplified diode formed around Q609