The part number is PTH487A01BC471TS, is PTH a positive thermistor, if so my previous theory is wrong.
Craig
Craig
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I would also suggest you read this and write to Piercarlo. He has responded and kindly helped me. He knows the NAD circuit perfectly. Keep in mind that this happened a few years ago, but with a try you do not lose anything...😉
Elettronica Audio - I progetti: NAD 3020 - Un "Classico" molto poco classico
Elettronica Audio - I progetti: NAD 3020 - Un "Classico" molto poco classico
A little bit like that. Its not as definite as a switch though which is either on or off. This will pass through a 'zone' that (I think 😉) will start to reduce the audio rather than abruptly shut it off but its not a low distortion method.
When the FET's begin to turn off they will also generate distortion as they begin to attenuate the audio level.
You mentioned earlier that 'it works' with the thermistor shorted. Do you mean you had audio coming through or just that nothing went bang ?
Quick test as to what to do:
With thermistor out and the amp on measure the DC voltage on the anode (non striped end) of D603 (the one that goes to the FETs).
If you have 20 volts or so then leave the thermistor out.
If the voltage is negative then short the thermistor and see if the voltage comes up to around 20v.
It does appear as though a PTC is needed. Q517 has to be turned off for the amp to work and only a PTC could do that. As the amp gets hot Q517 begins to conduct and starts to turn on Q516 pulling gates negative very gradually.
Looks the same i proposed before.
The PTFM04BE471Q2N34B0 from Murata.With the "471" in the code they are both 470Ω PTC resistors.
More chance to find one or the other (at a decent price).
Mona
No audio without the wires shorted - audio shorted out. I've pm'ed someone on the forum who fixed this issue a few years ago but waiting on him getting back in touch.
Ill rig up a light bulb safety device and fit the new transistors then test the voltages.
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There's a few available on Ebay with different temperatures.
BD - Curie Point 80°C
BE - Curie Point 70°C
BF - Curie Point 60°C
BG - Curie Point 50°C
That confirms it is a PTH (PTC) type. Unless you could be sure of getting an exact replacement then its debatable whether its even worth bothering to try and fix this aspect of it simply because any variation in characteristics will alter the way the circuit responds to heat.
What you don't want is for the device to begin to limit audio when its not necessary as that will almost certainly be introducing unwanted distortion at lower levels.
I would be very tempted to leave it non operational.
Yep. Maybe this circuit is just a 'fail safe' -- for those poor units that have found themselves stuck in a big, closed, glass-front entertainment center . . maybe with a big, hot TV (pre-flat screens).
Or has become a handy place for a stack of magazines.
Good luck!
Or has become a handy place for a stack of magazines.
Good luck!
That's much to high.The transistor chips are getting much hotter.150°C is the max, with 2°/W thermal resistance chip to sink (probably more) you get there with 20W dissipated in the chip.
70°-80°C is a more realistic value.
Mona
Why not avoid unwanted distortion and keep the temperature safety feature as well: Is there any engineering reason that prohibits a temperature switch in series with a small resistor in the "hot" range of the thermistor?
There would be no distortion up to the switching point and lower volume / temperature protection beyond.
Regards,
Rundmaus
EDIT: To resemble the thermistor more closely, one could also bypass the temperature switch with a high resistance in the "cold" range of the thermistor.
There would be no distortion up to the switching point and lower volume / temperature protection beyond.
Regards,
Rundmaus
EDIT: To resemble the thermistor more closely, one could also bypass the temperature switch with a high resistance in the "cold" range of the thermistor.
If this thermistor is so rare that no one really knows what it is - is it really necessary anyway? How many amps have been running perfectly fine without it?...
From the 7020/ series service manual, the thermistor part # is most likely the same PTH487A1D1BC471TS ...got that 
?
Of course not and neither does it show up on a Google search so that doesn't help. What's more, it is shown as a PTC type, having a positive thermal coefficient rather than the usual semiconductor NTC (negative coefficient) type so you may appreciate that apart from being obscure, the odds of finding a similar substitute are also very slim. Your best chance is to find a wreck to cannibalise for parts. I actually had a few burnt-out 7020/7225 series wrecks some years ago but they were unreliable beasts even when working properly and all finally went to the recyclers.
As you may be suggesting, since none of the manual documents for 3020 models show the thermistor, why not simply remove the extra parts fitted to your 7020-like version and replace any others remaining in that area that have amended values? I suspect the addition of a tuner in the 7020 series increased the temperature by restricting airflow and necessitated some extra safety measures. This was probably the reason such a protection feature was needed but as a basic integrated amplifier, that problem may not occur.
Here in Oz, hot running internals are a problem and many UK products have wound up as toast early in life, despite conservative use. It pays to keep power devices well ventilated and unless they have massive external heatsinks or there is a forced air cooling system, don't use them in cabinets or close fitting shelving.
? Of course not and neither does it show up on a Google search so that doesn't help. What's more, it is shown as a PTC type, having a positive thermal coefficient rather than the usual semiconductor NTC (negative coefficient) type so you may appreciate that apart from being obscure, the odds of finding a similar substitute are also very slim. Your best chance is to find a wreck to cannibalise for parts. I actually had a few burnt-out 7020/7225 series wrecks some years ago but they were unreliable beasts even when working properly and all finally went to the recyclers.
As you may be suggesting, since none of the manual documents for 3020 models show the thermistor, why not simply remove the extra parts fitted to your 7020-like version and replace any others remaining in that area that have amended values? I suspect the addition of a tuner in the 7020 series increased the temperature by restricting airflow and necessitated some extra safety measures. This was probably the reason such a protection feature was needed but as a basic integrated amplifier, that problem may not occur.
Here in Oz, hot running internals are a problem and many UK products have wound up as toast early in life, despite conservative use. It pays to keep power devices well ventilated and unless they have massive external heatsinks or there is a forced air cooling system, don't use them in cabinets or close fitting shelving.
Read post #48 I don't think its necessary.
Yes, I was thinking along those lines. One alternative (if you really wanted the protection) would be to find a thermal switch of a style that could be attached to the heatsink and use that to offer 'on/off' muting in the event of high temperatures. Something like this with a 60C trip and 45C reset could be ideal.
THERMAL SWITCH, N/C 60C 45C-RESET - HONEYWELL | CPC
I'll likely fit a thermal switch if it's running hot. Just need to see if I can get it working first. I've posted another thread regarding MJ15003/MJ15004 to replace MJ2995/2N3055 and associated emitter resistors.
NAD 3020i MJ15003/MJ15004 replace MJ2995/2N3055
NAD 3020i MJ15003/MJ15004 replace MJ2995/2N3055
With the usual cost-cutting measures in electronics, I would really doubt that any manufacturer puts something not visible to the customer inside a device that is not really necessary...
Regards,
Rundmaus
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