The NAD 7225 service manual shows the power amplifier supply is +/-41.1 volts i.e. 82.2 volts rail to rail. Seems to me this is too high to guarantee reliability with 2N3055/MJ2955 as the output pair as they have Vceo = 60 V and Vcer = 70 V so conditions could arise where the Vce ratings of these transistors are exceeded. (The Vce ratings for the driver pairs are over 100 V so no problem for them). Perhaps back in the day some brands of these transistors were known to exceed their datasheet voltage ratings.
@keithfisk5641 indeed. I agree that the supply rails exceed the transistors maximum ratings. Back in the 70s and 80s, the original pair easily allowed the maximums to be exceeded (only by 5V or so). The newer ones, nope.
If the transformer is good, then nothing is unrepairable. Degeneration resistors are really only necessary on parallel output stages. But if the video were to cook some transistors, then sweet 😂
From what I've heard on forums, etc., a lot of people believe the brand to be junk. If it was designed right, maybe used much higher Vce transistors, it'd be alright. The -15V DC I'm getting across the Vbe multiplier, btw, is because the O/S isn't working.
@@AstrosElectronicsLab A unit that has lasted for over 40 to 50 years isn't junk. It's the new replacement parts that are junk. You are lucky if what you buy now is really meeting specs or aren't fakes.
@@AstrosElectronicsLab a hifi repair channel i watch says nad usually use cheap parts but bear in mind the 7225pe is a vintage amp and nothing lasts forever.
@@AstrosElectronicsLab still, its given 40 odd years of service. that repair channel had a recent(5-7 years old) £600 fancy schmancy cambridge audio amp that he couldnt repair because CA didnt stock the propietry parts so a 600pound amp was a boat anchor, if i was its owner i would be pissed AF 🤬
Pity the NAD isn't yours. I'd seriously consider making you an offer for it and then repairing it to give it a new lease on life. I fully understand your reasons for not investing anymore time or money on it, but your idea of ratting the 20k tapped volume potentiometer to put it in another amp that requires a 50k pot IMHO isn't a good idea. Sure, it may function but certainly not as intended. The loudness bass and treble boost at low volume settings will not follow the design curve and I suspect that there will also be a premature roll off of the bass by using the 20k pot in a circuit designed for a 50k unit. By ratting the pot from the NAD you've pretty well much ruined any chance of restoring it to its former glory and thereby committed it to landfill in favour of a cheap Proton amp which would be better off just having its faulty pot replaced with a standard 50k dual gang unit and the user just advancing the bass and treble controls a bit to compensate for listening at lower volume levels. Just my 2c worth mate.
It's not my idea to "rat" the volume control out of it. I'm personally against it. I have explained to my housemate that you can just put a standard pot in and defeat the loudness circuitry (no one uses the stupid thing anyway, except to boost the bass, lol). However, this won't end up in landfill and will eventually be used for parts.
@@AstrosElectronicsLab Have you considered taking the pot out of the Proton and see if you can take it apart and fix it? It is after all already broken...
The NAD 7225 service manual shows the power amplifier supply is +/-41.1 volts i.e. 82.2 volts rail to rail. Seems to me this is too high to guarantee reliability with 2N3055/MJ2955 as the output pair as they have Vceo = 60 V and Vcer = 70 V so conditions could arise where the Vce ratings of these transistors are exceeded. (The Vce ratings for the driver pairs are over 100 V so no problem for them). Perhaps back in the day some brands of these transistors were known to exceed their datasheet voltage ratings.
@keithfisk5641 indeed. I agree that the supply rails exceed the transistors maximum ratings. Back in the 70s and 80s, the original pair easily allowed the maximums to be exceeded (only by 5V or so). The newer ones, nope.
If the transformer is good, then nothing is unrepairable. Degeneration resistors are really only necessary on parallel output stages. But if the video were to cook some transistors, then sweet 😂
NAD amps seem to be problem devices in general, aren't they ?
From what I've heard on forums, etc., a lot of people believe the brand to be junk. If it was designed right, maybe used much higher Vce transistors, it'd be alright. The -15V DC I'm getting across the Vbe multiplier, btw, is because the O/S isn't working.
@@AstrosElectronicsLab A unit that has lasted for over 40 to 50 years isn't junk. It's the new replacement parts that are junk. You are lucky if what you buy now is really meeting specs or aren't fakes.
@@AstrosElectronicsLab a hifi repair channel i watch says nad usually use cheap parts but bear in mind the 7225pe is a vintage amp and nothing lasts forever.
@@The_Penguin_Overlord I agree.
@@AstrosElectronicsLab still, its given 40 odd years of service. that repair channel had a recent(5-7 years old) £600 fancy schmancy cambridge audio amp that he couldnt repair because CA didnt stock the propietry parts so a 600pound amp was a boat anchor, if i was its owner i would be pissed AF 🤬
Pity the NAD isn't yours. I'd seriously consider making you an offer for it and then repairing it to give it a new lease on life. I fully understand your reasons for not investing anymore time or money on it, but your idea of ratting the 20k tapped volume potentiometer to put it in another amp that requires a 50k pot IMHO isn't a good idea. Sure, it may function but certainly not as intended. The loudness bass and treble boost at low volume settings will not follow the design curve and I suspect that there will also be a premature roll off of the bass by using the 20k pot in a circuit designed for a 50k unit. By ratting the pot from the NAD you've pretty well much ruined any chance of restoring it to its former glory and thereby committed it to landfill in favour of a cheap Proton amp which would be better off just having its faulty pot replaced with a standard 50k dual gang unit and the user just advancing the bass and treble controls a bit to compensate for listening at lower volume levels. Just my 2c worth mate.
It's not my idea to "rat" the volume control out of it. I'm personally against it. I have explained to my housemate that you can just put a standard pot in and defeat the loudness circuitry (no one uses the stupid thing anyway, except to boost the bass, lol). However, this won't end up in landfill and will eventually be used for parts.
@@AstrosElectronicsLab Have you considered taking the pot out of the Proton and see if you can take it apart and fix it? It is after all already broken...
@@geirendre it's a possibility, however it's the wiper (center) that's open.
Little advice - speak slower please