Hey hey, early this year I repaired my NAD C350 with a lot of help here, and today I finally started to rehab this:
It came to me from the original owner, and it works and sounds okay, but there's quite a bit of hiss when I listen through headphones, and of course the controls are very, very scratchy.
Earlier this year I went after the switches and pots with DeoxIT, and that seems an ongoing project. Every time I think I've got them, I walk away for a while, and when I return they're noisy again.
I've spent a lot of time studying the service manual from HiFi Engine, and reading threads on this and other forums. The service manual is excellent. It's so cool how open to service electronics manufacturers were in the old days. After ordering a box of electrolytics and transistors earlier this summer, I finally started today, replacing the 2SC458LG transistors and caps on the Head Amp Unit (phono/tape preamp). I'm using KSC1845FTA as a replacement transistor for those.
So far so good. I started with this section of the amp thinking it would be fun to hook up to a turntable after just this bit is done to see if I like the phono stage any better than I remember. I'm not sure what I'll do after that, but I have new filter caps (going up to 2200µF from 1000) and output coupling caps, so might do those next.
I do have a question about the noise the amp puts out, but I'll save that for later.
It came to me from the original owner, and it works and sounds okay, but there's quite a bit of hiss when I listen through headphones, and of course the controls are very, very scratchy.
Earlier this year I went after the switches and pots with DeoxIT, and that seems an ongoing project. Every time I think I've got them, I walk away for a while, and when I return they're noisy again.
I've spent a lot of time studying the service manual from HiFi Engine, and reading threads on this and other forums. The service manual is excellent. It's so cool how open to service electronics manufacturers were in the old days. After ordering a box of electrolytics and transistors earlier this summer, I finally started today, replacing the 2SC458LG transistors and caps on the Head Amp Unit (phono/tape preamp). I'm using KSC1845FTA as a replacement transistor for those.
So far so good. I started with this section of the amp thinking it would be fun to hook up to a turntable after just this bit is done to see if I like the phono stage any better than I remember. I'm not sure what I'll do after that, but I have new filter caps (going up to 2200µF from 1000) and output coupling caps, so might do those next.
I do have a question about the noise the amp puts out, but I'll save that for later.
Stuff built back then were in chassis that stand the test of time. Polished alum, wood, heavy. Worth keeping even if you totally replace the internals.
All the caps I've replaced consistently read about 50% above their stated capacitance, but their ESR values seem good. So far these caps seem in good shape for being ancient, but it's just the phono/tape preamp I've worked on.
So here's my question: in addition to a small amount of 60/120 Hz hum (not really enough to worry about at this point), there's steady white noise through portions of the volume control. As I turn the knob, noise appears around 20%, fades a bit as I continue turning, comes back louder, then fully fades away as I approach full volume. It's not terribly loud, but it needs to be seen to. Below, VR1(alb) is the volume control.
Anyone have advice on the potential source of this noise?
So here's my question: in addition to a small amount of 60/120 Hz hum (not really enough to worry about at this point), there's steady white noise through portions of the volume control. As I turn the knob, noise appears around 20%, fades a bit as I continue turning, comes back louder, then fully fades away as I approach full volume. It's not terribly loud, but it needs to be seen to. Below, VR1(alb) is the volume control.
Anyone have advice on the potential source of this noise?
Attachments
Welp, I've replaced almost all the electrolytics, except a couple of axials that I couldn't get replacements for, as well as 17 transistors. I upped the capacitance of the filters from 1000µF to 2200µF, but otherwise tried to match existing parts. I used high-temp capacitors on the power supply and otherwise went for the "audio" versions, more than necessary (e.g. the filters in the photo below). I made adaptors from PVC plugs to fit the new output coupling caps into the original clamps.
I listened to the unit (always powered through a dim bulb tester) often as I went, and I think there was improvement in the sound, but it's impossible to know when one does it so gradually over many weeks. It does sound very good on the work bench, except for that one issue I mentioned in the previous post: there is white noise, and it varies as the volume is adjusted. It does not increase with volume. As I turn the knob, it comes up, then drops, then comes up again, then drops again. At full volume I hear a faint bit of hum, but the white noise is gone.
I'm wondering, should I be looking at the volume control and the components adjacent to it? That seems like the obvious answer. I hoped that updating caps and transistors might eliminate it somehow, but no dice. I've applied DeoxIT (both spray and the precious red liquid) enough times that I'm sure that's done all it can for what's reachable on the volume pot. At this point I feel like if the issue is in the volume control, I should just pull it out and really get after it. I haven't even looked to see if replacements for these are gettable, since if I disassemble it, I probably might as well replace it. Suggestions?
I listened to the unit (always powered through a dim bulb tester) often as I went, and I think there was improvement in the sound, but it's impossible to know when one does it so gradually over many weeks. It does sound very good on the work bench, except for that one issue I mentioned in the previous post: there is white noise, and it varies as the volume is adjusted. It does not increase with volume. As I turn the knob, it comes up, then drops, then comes up again, then drops again. At full volume I hear a faint bit of hum, but the white noise is gone.
I'm wondering, should I be looking at the volume control and the components adjacent to it? That seems like the obvious answer. I hoped that updating caps and transistors might eliminate it somehow, but no dice. I've applied DeoxIT (both spray and the precious red liquid) enough times that I'm sure that's done all it can for what's reachable on the volume pot. At this point I feel like if the issue is in the volume control, I should just pull it out and really get after it. I haven't even looked to see if replacements for these are gettable, since if I disassemble it, I probably might as well replace it. Suggestions?
After spending some time with the volume control, and solving nothing, I thought I should also try to address a small level of hum through headphones. When I touch a bamboo skewer to the inputs of the "control amp unit" (and a few other locations, I think) the level of the hum increases on whichever side the skewer contacts (can somebody explain why?) That hum, and all audible hum in fact, can be eliminated by doing either of the following: 1) touching a finger to the chassis; or 2) connecting a ground wire between the chassis and my house ground. It seems opinions differ on whether a device made in 1971 with its original two-prong non-polarized plug should have that plug converted to a polarized and/or a 3-prong plug. It's pretty tempting right now given that I can silence the hum by grounding the chassis, but is it correct that this is kind of cheating, and not ideal if the unit is going to be connected to devices lacking 3-prong plugs, and that it is better and possibly safer to find out where there is a voltage difference between different grounding points on the receiver and tackle its cause?
Finally, FYI I measured the AC voltage between the neutral side of the outlet I'm using and the ground, and it's about 35 mV.
Finally, FYI I measured the AC voltage between the neutral side of the outlet I'm using and the ground, and it's about 35 mV.
I looked for other places where a touch with a wooden skewer would cause hum, and could not find any, apart from the control amp unit inputs, which are connected to the volume pot.
There's one other related oddity I'm hoping to resolve. On the bit of schematic I posted earlier, two 0.22u/25V capacitors (C17 and C18) are shown. They're drawn as electrolytic and listed that way in the service manual, but they look like this:
Similarly, the 0.47u/25V capacitors at the input of the "Control Amp Unit" (C601 and C602) are listed as electrolytic in the manual and here they are (the electrolytics in this photo have all been replaced since I took this photo):
These look like mylar capacitors which I assume to be non-polar, but throughout the service manual, in all the schematics and parts lists, they are shown as (polar) electrolytics.
Similarly, the 0.47u/25V capacitors at the input of the "Control Amp Unit" (C601 and C602) are listed as electrolytic in the manual and here they are (the electrolytics in this photo have all been replaced since I took this photo):
These look like mylar capacitors which I assume to be non-polar, but throughout the service manual, in all the schematics and parts lists, they are shown as (polar) electrolytics.
Ah, so I guess those must be tantalum electrolytic caps. Learning lessons all over the place. I did replace the 0.47u ones with regular Al electrolytics, but it made no difference. Possibly will try replacing the 0.22u as well , though I expect that's not going to do the trick. I found an eBay seller who offers new replacements for this volume control (they cite it as being for the Pioneer SX-770, which lists it as the same part number C82-038-0 in its service manual), $90 a pop. Possible next step, though that would about double the cost of this repair.
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Erm, metal front panel and not earthed? That's a no-brainer, 3 cord plug and cable - you should have protective earth if the metal is exposed.
I deinstalled the volume pot (part C82-038-0, 250k dual) so I can thoroughly clean it, de-corrode its contacts, and give the insides a good whack with deoxit fader. I don't think it would be a good idea to pull it apart, so not planning to do that. I'll be a little surprised if this eliminates the white noise problem, but it's worth a try.
It looks nice, but no luck. After plenty of deoxit and some elbow grease, I'm still getting the same hiss at 90° and 180°. I should add, the hiss is audible in speakers, but far more noticeable through headphones. It is present on all input selector settings.
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One thing I noticed about the hiss... when it is present, turning on the high cut switch reduces it on the right channel, but not on the left. If I flip the high cut on and off, on the left it sometimes seems to reduce the hiss for just a moment, but then it returns. With the high cut turned off, it is present, and about equal, in both channels.
I have not replaced the transistors in the control amp unit. I noticed that some have replaced these (2SC870 and 2SC871) with KSC1845FTA, and I do happen to have four of those... unless someone talks me out of it, I'm going to swap those in before I order a new volume pot.
I have not replaced the transistors in the control amp unit. I noticed that some have replaced these (2SC870 and 2SC871) with KSC1845FTA, and I do happen to have four of those... unless someone talks me out of it, I'm going to swap those in before I order a new volume pot.
My bet the hiss you're hearing is current noise in the transistors and its amplitude will vary as the source impedance changes with volume control position. It will be lowest at 0 and full volume because source impedance is low at both extremes of pot position. It will tend to be better where the wiper is positioned at the loudness tap where R17 reduces the driving impedance; it will be worst when the wiper is midway resistively, i.e. where the pot resistance is maximized. An exact replacement pot will not help this issue, but if you impedance scale the volume pot to a lower resistance, you'd get some improvement. (You'd also have to impedance scale associated passives to maintain the same frequency response.)
If the stage following used a JFET, you might get lower hiss, but that's quite a departure...
If the stage following used a JFET, you might get lower hiss, but that's quite a departure...
Thank you for this response. I don't fully get it but will work on that. Here's a graph of my resistance measurements. P = "power", S = "signal" (the wiper), G = "ground", and "T" is the loudness tap. The actual rotational positions were just eyeballed.
probably follow the input signal path.
and find where the noise stops and starts in circuit.
My guess would be noisy ceramic cap/cap in signal path.
or transistor on its way out making noise.
Eliminate guessing would have to follow signal path.
if not in pathway even a constant current source transistor
been cooking all its life.
and find where the noise stops and starts in circuit.
My guess would be noisy ceramic cap/cap in signal path.
or transistor on its way out making noise.
Eliminate guessing would have to follow signal path.
if not in pathway even a constant current source transistor
been cooking all its life.
Nice empirical data and graph work!
If you're into experimenting, I can suggest a couple of off-the-board experiments that may be insightful. This can be a real experiment, or simply a "thought" experiment.
If you measure the resistance from T to G and from T to P, you'll be able to determine exactly where "electrically" the loudest tap is connected along the resistive element.
From the perspective of source resistance, the G and P terminals are effectively connected to each other, since both ground and the P terminals have low impedance. So tack a wire between P and G. If you measure resistance from Wiper to P,G you'll find a maximum resistance of about 62.5K--- i.e. 1/4 of the element resistance.
Now add a 12k resistor from the T lug to the P,G connection and again measure wiper resistance. You'll find a two-humped variation in maximum wiper resistance as you rotate the control. If you note these two resistance maxima (perhaps with magic-marker scribes on bushing and shaft), I predict these two positions will be where you observe maximum hiss when you put the pot back into service.
If you're into experimenting, I can suggest a couple of off-the-board experiments that may be insightful. This can be a real experiment, or simply a "thought" experiment.
If you measure the resistance from T to G and from T to P, you'll be able to determine exactly where "electrically" the loudest tap is connected along the resistive element.
From the perspective of source resistance, the G and P terminals are effectively connected to each other, since both ground and the P terminals have low impedance. So tack a wire between P and G. If you measure resistance from Wiper to P,G you'll find a maximum resistance of about 62.5K--- i.e. 1/4 of the element resistance.
Now add a 12k resistor from the T lug to the P,G connection and again measure wiper resistance. You'll find a two-humped variation in maximum wiper resistance as you rotate the control. If you note these two resistance maxima (perhaps with magic-marker scribes on bushing and shaft), I predict these two positions will be where you observe maximum hiss when you put the pot back into service.