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NAD 7020i sudden death
NAD 7020i sudden death
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Old 20th January 2021, 06:05 AM   #61
PRR is offline PRR  United States
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NAD 7020i sudden death
Quote:
Originally Posted by PRR View Post
RCA 1966 (hometaxial) 2N3055 .... 2N3773 ....
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File Type: gif 2N3055-1966-42.gif (59.3 KB, 62 views)
File Type: gif 2N3773-1966-42.gif (61.6 KB, 51 views)
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Old 20th January 2021, 06:46 AM   #62
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by PRR View Post
> 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.
You are right, that figure was quoted in another source. I should have checked this in the RCA Handbook. There is no MJ2955 hometaxial complementary device in this Handbook.
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Old 20th January 2021, 06:52 AM   #63
mjona is offline mjona  New Zealand
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@zenzaman

If you still have the defunct power transistors could you post an image or otherwise tell us what brand these are and the full part numbers.
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Old 20th January 2021, 06:32 PM   #64
mjona is offline mjona  New Zealand
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Originally Posted by indianajo View Post
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.
]
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.

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.
Replacing the RCA devices with MJ15003 was the key move which increased the stability margin in your amplifier.

The MJs have higher fT than the RCAs which means that decline in gain and the point where phase shift starts is moved to a higher frequency.

Your amplifier is compensated for the lower frequency point where phase shift starts with RCAs.

I would not rule out aging of components for their burn out.

On radiated emissions consider the primary of an ignition coil where the current is turned on and off. You have the same effect with wire wound resistors and variations in current passing through these.

You can check out from other sources what I said about running collector and emitter leads closely to cancel these effects. If you space these out there is a wider radiation field inside your chassis.
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Old 20th January 2021, 06:48 PM   #65
indianajo is online now indianajo  United States
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Quote:
Replacing the RCA devices with MJ15003 was the key move which increased the stability margin in your amplifier.
The MJs have higher fT than the RCAs which means that decline in gain and the point where phase shift starts is moved to a higher frequency.
Stability of the ST120 was not a problem. Use of just enough aluminum heat sink to pass the FTC 1 hours 60w/ch test was. Lots of comments about "the worst amplifier ever sold" out there. Also lack of output transistor idle bias current when the music was soft caused crossover distortion. The HD tests are at full power, so this didn't show up in the advertising. Just in the terrible magazine reviews. 200 khz Ft output transistors on 1966 build didn't help high frequency IM distortion. My use of MJ15003 solved that problem. Dynaco went to TIP3055 or 2n3773 in 1970.
I was lucky that the MJ15003 worked with no emitter resistor on the lower transistor, although the idle bias current was set by measuring the current through the .51 ohm resistor between upper OT emitter & lower collector. When the PC15 driver became unrepairable due to patches caused by unavailability of TO5 drivers, an AX6 board (point to point) took over the job with emitter resistors on both upper & lower output transistors. Both PC15 side with djoffe closed loop idle bias control modification (7 transistors), and AX6 side, sound great 1/8 watt or 72. Two PCAT fans solve the heat problem.
Spacing the OT b & e leads 1" apart works great. Radiated emissions are more a problem from 1000 w CB radios driving by emitting barking dogs performing Dixie or an AM radio station. Solved by changing input cap on AX6 from 47 pf to 82. Unit already had a output coil, wound around the output capacitors. PC15 side had 150 pf input cap.
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Last edited by indianajo; 20th January 2021 at 06:59 PM.
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Old Today, 02:26 AM   #66
mjona is offline mjona  New Zealand
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Default Facing some Facts

You say the output transistors used in the Dynaco ST120 output did not help IMD - officially quoted as less than 0.5% the same as for THD.

The 1978 RCA Power Devices handbook gives fT as 800kHz rather than the 200kHz you quoted. The latter figure would align with the RCA 2N3773.

If we accept 800kHz for the Dynaco and ditto for the NAD 3020 "speculative" hometaxial output transistors, the constraints on the gain bandwidth level and distortion results would be similarly affected.

That is not the case when one compares the specifications attached for the NAD 7020 which speak for themselves.

I have searched for detail in a NAD 3020 youtube re the maker and date codes of the output transistor pairs - these RCA devices described earlier as hometaxial.

The giveaway to this theory is the devices have 1985 date codes. If anyone wants to check that see Vintage NAD Amp teardown - YouTube

There is a hometaxial 2N3055 in the 1978 RCA Power Devices Handbook which recommended for Quasi Complementary Output stages.

That there is no hometaxial PNP of any description to match this in the handbook presents as an inconvenient fact which people choose to overlook.
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File Type: png NAD7020spec.PNG (109.8 KB, 29 views)
File Type: jpg NAD3020.jpg (110.8 KB, 27 views)

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Old Today, 02:43 AM   #67
indianajo is online now indianajo  United States
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The NAD might have been built in 1978 or later.
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If we accept 800kHz for the Dynaco and ditto for the NAD 3020 "speculative" hometaxial output transistors, the constraints on the gain bandwidth level and distortion results would be similarly affected.
My ST120 was built in 1970, going by date codes. The original units in magazine reviews were sold in 1966. A lot of progress was made in power transistors 1966-1978.
In RCA 1972 databook for power transistors there are no TO3 pnp transistors at all. All the homotaxials are npn.
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Old Today, 05:46 AM   #68
mjona is offline mjona  New Zealand
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I noted your comment about the lack of an emitter resistor in the lower NPN output transistor.

It is more usual in conventional designs to leave that out and put a suitable value in series with the collector where the vbe will not be reduced by local feedback.
Dynaco have not followed that approach either.

The upper NPN has an emitter resistor connecting to multiple points where local feedback signals will arise and putting aside the usual thinking of current flowing from positive to earth I looked look at paths for the charges which flow from in the opposite direction. These are capable of changing the base emitter voltage of the upper NPN transistor and act as controls.

One of these paths is the 7 Watt 300 Ohm resistor between the upper NPN transistor collector and earth which is a clean source of electrons and the voltage drop across the resistor could be regarded as linear.

To me this kind of use of local feedback is an interesting approach.

Last edited by mjona; Today at 05:48 AM.
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Old Today, 03:53 PM   #69
indianajo is online now indianajo  United States
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Quote:
Dynaco have not followed that approach either.
The upper NPN has an emitter resistor connecting to multiple points where local feedback signals will arise and putting aside the usual thinking of current flowing from positive to earth I looked look at paths for the charges which flow from in the opposite direction. These are capable of changing the base emitter voltage of the upper NPN transistor and act as controls.One of these paths is the 7 Watt 300 Ohm resistor between the upper NPN transistor collector and earth which is a clean source of electrons and the voltage drop across the resistor could be regarded as linear.
That dingbat $1 300 ohm 7 watt resistor and the $.75 non-polar cap it feeds are the problem on dynaco ST120. There is minimal idle bias current on output transistors at low wattage until louder output charges up the capacitor. The djoffe mod uses 7 transistors to measure the idle current across the .51 ohm emitter and force 20 ma. Dynaco Stereo 120...can be beautiful - diyAudio
I built two and the mod forced the dynaco design to sound great at all wattage. But the add on has a tendency to blow the sense transistor emitter junction & go 200 ma idle current. Fortunately my fans blew off the excess heat without output transistor damage. The AX6 dispenses with all that and sounds good with only 6 transistors 18 resistors and 6 e-caps. Retro Amp 50W Single Supply - Page 22 - diyAudio
Dynaco should have cut up more Armstrong 621 or Leak delta 70. Which came first? ST120 was 1966.
One dynaco board lost too many lands due to damage from replacing TO5 drivers with TO220 drivers. So I built a point to point AX6 and replaced it. The AX6 pcb artwork won't fit in the 3.5" tall ST120 case. Other than a slight gain difference covered by adjusting the balance pot on the preamp, the two channels sound the same - as good as my CS800s at 1/8-70 watts.
To your point about no homotaxial pnp, you're welcome to buy an NAD7020 and repair it without emitter resistors. As many times as necessary. Mr. Finch and others advocate that epitaxial output transistors last longer with emitter resistors. Also they say the idle bias current is easier to set right with emitter resistors. I'm not buying a NAD, ebay NAD's here cost 5 to 310 times what Peavey products cost with big honking .5 ohm emitter resistors OEM. Listening to a MMA-875t now, cost $35, monaural with speaker safe transformer output. An actual stereo M-2600 came in from ebay last week, $105, also works and sounds as good as these **** $3 speakers I'm having to make do with can show.
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Last edited by indianajo; Today at 04:21 PM.
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