NAD frying sound... mean anything to amp experts?

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Hi, an 80's vintage NAD 7175 I have is making a "frying" static noise intermittently in the left channel; volume of noise is independent of music volume level, and a test woofer I had hooked up to it exhibted DC-like cone excursions (i.e. would jump forward or backward 1/4in or so).
My question is: for an electro-ignoramus like me, is there likely to be anything I could identify/replace that would cure the problem?


The problem is that many different types of components can cause this problem, including bad solder joints. If you don’t have any audio equipment repair experience I would suggest that you take it to a reputable service shop. Such a shop however may be hard to find since most people now days do not have equipment repaired, they just replace it due to the high cost of good quality work and the low price of equipment sold by major chain stores..

Finding a intermit noise problem can be quite time consuming and thus substantial costs can be incurred. These costs will likely exceed the value of the item. My advice is to replace this particular item rather than repair.

John Fassotte
Alaskan Audio
I would start by looking for a noisy differential transistor. If that does not solve the problem you could swap parts between the left and right channel until the distortion switches sides and identify the bad part.
This can be time consuming but if you have nothing better to do...... Also inspect the board for defective solder connections like John suggested.
As usual all the usual safety precautions should apply.

Hello VDoran,
Your NAD is now pushing 20 years old.
These machines were Taiwan built using European transistors and asian sourced passive components.
Taiwan solder process sucks, particularly in NAD.
In my experience only sure cure is to blanket resolder power supply and output stage pcbs, clean the pcbs (isopropyl alcohol, brush and tissue paper) and see what you get.
If still noisey go for input diff pair transistor.
If diff pair is 5 pin dual transistor suspect it to be faulty/noisey straight away.
Also electrolytics are likely to be suspect by now too - best to replace with modern SMPS low ESR type.

Maybe this helps,
Regards, Eric.
Some help

I had a Nad3020 actually. quite beaten inside. Well what i would advise is to disconnect the speakers and turn on the amp. Measure the dc voltage at the speaker jacks. Most probably the Dc level is above 100mv. If so you might have to look for a pot controling the dc level. The pot would be(might be) a cheap pot and dust level might have creep in and change the resistance value . You might want to recalibrate the pot or change it to a multi-turn cermet of the same value. This pot is more reliable. The caps you could change it as i heard that electro's dry up or something after a long time. the output transistor your could change since they are the ones always working at higher temperature. If it is an intergrated amp try changing the balance and volume pot as dirt might have entered. This is evident when you shift the pots, crackling sounds could be heard. And finally get more advise on your particular amp, i am just shooting from my amp point of few. Yours could be different. You could try mailling Nad for the schematics. This could help you check the values
Thanks guys,

Appreciate the advice. Ditch and replace is probably the sensible course, but since I could use a little education in this area, I think I'll try some of the suggested replacements, driven by the channel-channel swaps.
The net has allowed some great things to happen in the pooling of knowledge.

Nah, it ain't a bad transistor. Most likely a bad connection. Broken solder joint, dirty connector, poor splice. Look for water (humidity) corrosion. Maybe even dust or other contaminants.

Heck, soak the whole thing in isopropyl for 1/2 hour, then dab at it with a paint brush to try and remove all the nasties.

Could possibly be a leaky electrolytic.
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