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NAD 7020i sudden death
NAD 7020i sudden death
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Old 18th January 2021, 05:44 AM   #41
Ian Finch is offline Ian Finch  Australia
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NAD 7020i sudden death
Re: thermal properties of Hometaxial transistors.
We've been advised several times in NAD threads over the years, that epitaxial transistors absolutely need emitter resistors to maintain bias stability. Meaning if you don't fit them when you change epi for hometaxials, your bias will drift all over the place and out of control as temperature changes. This could just mean harsh sounding due to not enough bias or it could also end in thermal runaway and failure.

In my ignorance when I first tried replacing original semis in NADs with epitaxials, it was only a matter of minutes before a repeat meltdown occurred - this was not only embarrassing but expensive so it really is unwise to try your luck.

In contrast, hometaxial transistors do not need emitter resistors for stable bias current and like many early power amplifier designs where they were used, they are generally safe without them. That's what the 3020 was designed around, hence I don't see anything questionable in Jaycees's reply.

For those interested in the historical facts and figures of early silicon audio power, here's some dirt on 2N3055, with its various known US and European manufacturers and reference part numbers of unspecified "taxy". 2N3055 - Wikipedia
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Old 18th January 2021, 07:42 PM   #42
mjona is offline mjona  New Zealand
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In post 28 on page 3 zenzaman said he would rather not mess around with the pcb. I take this as fitting emitter resistors would be the last resort.

He managed to set the output current to specification but he was concerned the drivers were too hot to touch.

These also appear to have survived the fault which took out the outputs and there is no mention of these having been replaced.

I suspect the base emitter junctions in the outputs developed a brief short before going open circuit which likely compromised the drivers.

Last edited by mjona; 18th January 2021 at 07:49 PM.
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Old 19th January 2021, 01:49 AM   #43
Ian Finch is offline Ian Finch  Australia
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NAD 7020i sudden death
Agreed. Neither the OP or anyone else would choose to hack the PCB with impunity but is there any option if we want a reliable amplifier from our economic DIY fix? Here in Oz, the original hometaxial parts are now effectively unobtanium because stocks either no longer exist, they're hoarded by collectors or they are too difficult, risky or expensive to buy in small quantity as replacement parts.

If I were determined to get the original hometaxial versions where zenzaman is located, I'd look at the old local sources of 2N3055 equivalents that are known to have been produced in Eastern Europe long ago (see the 2N3055 wiki for clues) but I wouldn't hold much hope for the PNP (MJ2955) equivalents.

As an aside, its also interesting that member John Ellis is named as co-author of a 2N3055 case study doc. I searched while checking sources of info. Maybe I should be paying more attention to the comments and suggestions made here .
The 2N3055: a case history - IEEE Journals & Magazine

Last edited by Ian Finch; 19th January 2021 at 02:07 AM.
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Old 19th January 2021, 04:02 AM   #44
mjona is offline mjona  New Zealand
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@zenzaman,

You might be interested in removing the fuses protecting the output transistors, connecting a speaker to the output and see if you can match what can be seen from just before the 6 minute mark in this youtube video

NAD 3020 Helpdesk: Blown Fuses and Output Transistors Repair, Fix - YouTube. That should tell you whether or not your drivers are working OK. If you are worried about this fit a resistor say 100R in series with the speaker for a start.
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Old 19th January 2021, 05:36 AM   #45
Ian Finch is offline Ian Finch  Australia
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NAD 7020i sudden death
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.
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Old 19th January 2021, 06:07 AM   #46
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Ian Finch View Post
Agreed. Neither the OP or anyone else would choose to hack the PCB with impunity but is there any option if we want a reliable amplifier from our economic DIY fix? Here in Oz, the original hometaxial parts are now effectively unobtanium because stocks either no longer exist, they're hoarded by collectors or they are too difficult, risky or expensive to buy in small quantity as replacement parts.
As far as I can see On semiconductor have a 2N3055A but the complementary 2N2955A is no longer being made.

Going back to 1978 RCA sold a hometaxial 2N3055 as well as the epitaxial version but as long ago as that there was no complementary hometaxial MJ2955.

Do you have a wizard in oz where you can get help
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Old 19th January 2021, 06:53 AM   #47
zenzaman is offline zenzaman  Romania
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Quote:
Originally Posted by mjona View Post
He managed to set the output current to specification but he was concerned the drivers were too hot to touch.
When I first started to identify all parts gone bad, the only shorted transistors were Q416 and Q418 (2N3055 and MJ2955). Everything else checked ok, but I also replaced all BD139 transistors with similar new ones ( https://ro.mouser.com/ProductDetail/511-BD139-16/ ), 2SD669A with https://ro.mouser.com/ProductDetail/512-KSC2690AYSTU/ and 2SB649A with https://ro.mouser.com/ProductDetail/512-KSA1220AYS/

After seing that Q408 and Q407 were getting a lot hotter than all the others I also mounted some salvaged heatsinks on them (couldn't find locally any heatsink for this package).

I've attached a drawing with my understanding of the mod needed. I'm inclined to take the easier route, with cutting the traces in the right places, 'surface mounting' the resistors and connecting them with leads, rather than drilling the PCB (as that would require removing the heatsink and outputs again). Besides, I don't really understand the advantages of drilling holes for the resistors, since there are no unused, isolated copper traces for them to be soldered to, and also there seems to be plenty of solder (thickness wise) to cover the resistors' legs really well).

I'm sure I've seen a couple of pics with this mod somewhere around but can't find it anymore (they were for earlier models but could have helped me as inspiration)
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File Type: jpg 139917881_1397673310582961_5052293969930139984_n.jpg (116.8 KB, 96 views)
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Old 19th January 2021, 01:29 PM   #48
indianajo is offline indianajo  United States
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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.
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Old 19th January 2021, 02:47 PM   #49
Ian Finch is offline Ian Finch  Australia
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NAD 7020i sudden death
Yes, don't rely on solder alone to mount power resistors. That's only done with low current SMD parts that would be unlikely to even get warm.
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Old 19th January 2021, 04:09 PM   #50
Ian Finch is offline Ian Finch  Australia
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NAD 7020i sudden death
Quote:
Originally Posted by mjona View Post
As far as I can see On semiconductor have a 2N3055A but the complementary 2N2955A is no longer being made........
.....Do you have a wizard in oz where you can get help
I think you mean MJ2955 as there there never was a TO3 2N2955 that I'm aware of. You get hits with a browser search but when you follow the links, this is what you actually get: datasheet 2N2955
I don't repair old solid state gear now but if necessary, I would be pragmatic and just use epitaxials with emitter resistors as we're discussing. It's not quite the same sound quality as original, and that's what you need to make valid comparisons but most people have been pleased just to have their old treasure working again.
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