Diagnosing and repairing NAD amplifier

[natlus]

I'm just starting to get interested in DIY and maintaining audio myself, so please accept my apologies in advance if my questions seem naive. However, I recently purchased an old NAD 7225PE receiver, am hell bent on making it work, and need help. I'll give a brief outline of the
problem first, and then I'll pose specific questions at the end. I welcome your input.

In first testing the receiver, I discovered only one channel worked. The sound sometimes went off completely, but then seemed to come on and improve as I adjusted the volume up. It also intermittently distorted.

If had not been used for several years, and a huge amount of dust etc. had accumulated inside the receiver housing. I brushed out the debris, used forced air to clean it out more, and used a combination of alchohol and DEOxit cleaner and then power enhancer on as many contacts and pots as I could find.

The metal strip holding the pots for treble, base, and balance, seemed to have been bent backward, and I noticed the tab-like housing for the balance had been broken.

I ran the receiver for several hours bursts after the cleaning, and viola, no more distortion with changing the volume. After several more hours, the second channel came on! Distorted, but audible nonetheless. I found by pushing the balance control in slightly, I could get more sound out of the second channel.

However, after running the receiver for the better part of 3 days, the other channel disappeard once again, with the other channel sounding fine.

So:
-overall diagnosis of problem--balance pot?
-If replacing the balance pot, where to get parts and any other recommendations for how-to
instructions, ie, websites. What would I need to do that work?
-are there other components to replace/repair?
-I've heard some different ideas about how pots should cleaned--does the housing/large part(name?) of the pot also get injected with cleaner/enhancer?

Thanks

[Jace]

ok. this sounds sort of like an issue i had with a super old jvc stereo receiver i rebuilt. i had one channel that would just cut in and out randomly with static and if you messed with the balance by rotating it a lot of pushing it in, it would sometimes fix itself to some degree.

i am pretty sure its a bad pot. it could just need cleaning as you say or it could just need replacing. if you plan to replace it, you need to find the model (should be written on it somewhere) or even just the size and style of pot it is (i.e. 10K, 100K resistance etc and wether or not it is linear or logorithmic) if you know that and go to an electronics store and get the replacement. the best option would be to write down all the pots info that you can come up with and bring that to the electronics store and ask them if they have that specific one or one with the same specs.

to do a proper job of replacing it, you will need a soldering iron and a solder sucker and solder of course. have you soldered or removed solder before? if not, i can try to describe the process and/or find a site that shows you how.

as for cleaning, i am not sure as to the usual procedure for cleaning them. i tried to clean the pot on the receiver i had, but that didnt help so i know mine needs replacing.

anyway, hope some of this helps you out.

-Jace

[natlus]

ok. this sounds sort of like an issue i had with a super old jvc stereo receiver i rebuilt. i had one channel that would just cut in and out randomly with static and if you messed with the balance by rotating it a lot of pushing it in, it would sometimes fix itself to some degree.

i am pretty sure its a bad pot. it could just need cleaning as you say or it could just need replacing. if you plan to replace it, you need to find the model (should be written on it somewhere) or even just the size and style of pot it is (i.e. 10K, 100K resistance etc and wether or not it is linear or logorithmic) if you know that and go to an electronics store and get the replacement. the best option would be to write down all the pots info that you can come up with and bring that to the electronics store and ask them if they have that specific one or one with the same specs.

to do a proper job of replacing it, you will need a soldering iron and a solder sucker and solder of course. have you soldered or removed solder before? if not, i can try to describe the process and/or find a site that shows you how.

as for cleaning, i am not sure as to the usual procedure for cleaning them. i tried to clean the pot on the receiver i had, but that didnt help so i know mine needs replacing.

anyway, hope some of this helps you out.

-Jace

Thanks Jace--that definitely helps. I have not soldered/removed solder before, though I used to be a residential contractor, and witnessed many-a-plumber putting joints and so forth together. A web site or a walk-through would still be great, though, on how to. Also, where to get replacement pots and tools--on the web or in the NYC area?

Natlus

[Jace]

ok. im in one of my electronics labs right now so i wont be able to give you a full description of the process until i get home. ill see what i can find though. i am from ontario, canada so im not sure what electronics stores are in the NYC area, but ill see what i can do. you could do a search for electronic component suppliers on the web. ill also do a search and see what i can find for your area.

just a thought. does NAD sell replacement parts? you could try emailing them and asking them if they would consider sending you a replacement pot for your specific model of receiver.

anyway, until i can give you more info, good luck with your search and take care.

-Jace

[Haoleb]

nad probbly wont sell you replacement parts. If it is indeed the balance pot you could either find a replacement, like said above it should say on the pot what the rating is. Or you could just bypass it out of the circuit completely by simply connecting the wires that connect to the leads to their corresponding wires. And just leave the pot in place so you dont have a hole in the faceplate.

Soldering in electronics is alot different than how plumbers solder together water pipes. No blowtorches here :) A decent digital multimeter would also be helpfull. There are many soldering how to's and such online, to go this link and check out the different pages, quests page has some info that you might find helpfull on soldering and such.

http://www.bottlehead.com/loosep/loosep.html

Its hard to try and diagnose and fix something through forums so your situation could be different. I dont know what your talking about when you say the tabs are bent. But whatever you do make sure you mark every wire you desolder and such and be patient.

[Jace]

ok. like i said before, you will need a soldering iron (http://store.tubedepot.com/tl-wp35.html) you will also need a solder sucker (http://www.mainelectronics.com/soldersuck.htm) (the links are just to give you examples of irons and suckers. i have never bought from either place shown so i cant recommend them).

anyway. here is a link for some tips on soldering.

Soldering Tips (http://www.elexp.com/t_solder.htm)

i suggest you practice a bit first before you actually try it. also, since i dont think the link has any examples of solder sucking, ill explain the process. first, you use your soldering iron to melt the solder on the joint (shouldnt take long) with the solder sucker locked and loaded (youll figure this out the moment you buy one and youll be doing it just for fun for a while too!). once the solder is liquified, place the tip of the sucker right beside the iron's tip and press the button. it will suck up the liquid solder. if you dont get it all the first time, do it again. so ya, try soldering some pieces of wire or something together and desoldering them until you are comfortable doing it. then you are ready to try it out on your balance pot.

some things that im not sure the soldering article mentions is that a good way to tell if your soldering joint is good is by its shininess. if it is shiny, it is a good connection. if your solder is dull and lifeless, it is whats known as a cold solder joint and will not conduct properly and wont be physically strong. also, i recommend 60/40 solder for people who are new to the process of soldering as it gives you a little more playtime between solid and liquid states (it mentions the different types in the article). also, if you are getting flux as well (not really needed for what you are doing because most solder has a flux core), dont get any acid based flux. it is for use with plumbing only and will not be too friendly with your circuits.

finally, i would not recommend replacing the pot with a wire. this is a bad idea as it basically shorts out the pot. a pot is an adjustable resistor and its resistance is not only used for adjusting balance, but also has an effect on the current in your device. shorting it will up the current. depending on the circuit, this could potentially fry things. what you can do, however is find out the total value of your pot and find two resistors, each half the value of your pot and run one from where each outer pin should be to where the center pin should be (after disconnecting the pot from the circuit). still, if you are gonna do this, you might as well just get the new pot!

also, i still havnt found any stores in your area. i hope you had better luck with that than me.

if you have any more questions, let me know.

-Jace

[Haoleb]

In most cases bypassing the pot would not be reccomended, like if it were a volume pot as it would always be full tilt. In this case when the balace is set to zero the resistance should be zero which of course makes both channels come through with the same amount of signal. I do not see how it is going to fry anything.

[Jace]

in this specific case, it probably wont cause damage (so you are right to a point, sorry for contradicting you, i was in the middle of someting when i wrote my last post so i wasnt totally thinking properly), but your logic is still flawed. at center (equal balance point), the pot would be at half resistance, not zero (it would be zero in the case of a volume pot or when you have your balance turned fully to one side or the other which produces full resistance for one channel and no resistance for the other)

with this said, shorting the outer pins of the pot to the center pin will give you an even balance, but there will be zero resistance there for both channels. so, as i previously mentioned, there will be a current increase. again, this probably wont cause damage due to the fact that the balance knob turned to either extreme will produce zero resistance for one channel anyway.

again, back to haoleb said about getting a mutlimeter. this is a very useful tool for stuff like this. with it you would be able to tell what voltages are at different locations and the resistances of parts (only when they are not attached to anything if you want an accurate reading). still, it is good for verifying if things are working (like pots).

finally, to be honest, without a decent background of electronics, it would be simplest to just find a replacement pot and since we dont know what you are dealing with without seeing it or the scematic, we cant 100% garentee what whill happen when you short the pot out.

-Jace

Edit: i had to edit this post a bit after reading what i wrote and thinking about it.

[natlus]

in this specific case, it probably wont cause damage (so you are right to a point, sorry for contradicting you, i was in the middle of someting when i wrote my last post so i wasnt totally thinking properly), but your logic is still flawed. at center (equal balance point), the pot would be at half resistance, not zero (it would be zero in the case of a volume pot or when you have your balance turned fully to one side or the other which produces full resistance for one channel and no resistance for the other)

with this said, shorting the outer pins of the pot to the center pin will give you an even balance, but there will be zero resistance there for both channels. so, as i previously mentioned, there will be a current increase. again, this probably wont cause damage due to the fact that the balance knob turned to either extreme will produce zero resistance for one channel anyway.

again, back to haoleb said about getting a mutlimeter. this is a very useful tool for stuff like this. with it you would be able to tell what voltages are at different locations and the resistances of parts (only when they are not attached to anything if you want an accurate reading). still, it is good for verifying if things are working (like pots).

finally, to be honest, without a decent background of electronics, it would be simplest to just find a replacement pot and since we dont know what you are dealing with without seeing it or the scematic, we cant 100% garentee what whill happen when you short the pot out.

-Jace

Edit: i had to edit this post a bit after reading what i wrote and thinking about it.

Thanks to Jace and Haoleb--now time to roll up my sleeves and work. And yes, you're right, I imagine plumbers solder alot differently than you guys (and guaranteed to charge at least several hundred dollars more a day).

Natlus

[Jace]

hey man, let us know how it goes and if you get bogged down at all. i hope fixing the balance does the trick man.

-Jace