Hi everybody,
Again, a newbie's question
I got myself an old NAD 314 amplifier, with a dead right channel, and I'd like to try fixing it.
With a source connected (on any channel), the right speaker is dead, but when I plug my earphones - I can hear on both sides.
Seems like the power amp section is good, since when i connect a source directly to the power-amp in sockets, both channels function well.
This leads me to believe that the problem is between the pre and the power amps, but I don't know how to trace it from here.
I have the service manual and schematics, but frankly - I'm lost. Any help / hint would be much aporeciated.
Thanks,
- Barak
Again, a newbie's question
I got myself an old NAD 314 amplifier, with a dead right channel, and I'd like to try fixing it.
With a source connected (on any channel), the right speaker is dead, but when I plug my earphones - I can hear on both sides.
Seems like the power amp section is good, since when i connect a source directly to the power-amp in sockets, both channels function well.
This leads me to believe that the problem is between the pre and the power amps, but I don't know how to trace it from here.
I have the service manual and schematics, but frankly - I'm lost. Any help / hint would be much aporeciated.
Thanks,
- Barak
Welcome to the hobby. More fun than crossword puzzles, but takes a few more tools.
You'll need at least an inexpensive DVM. Also some alligator clip leads to connect the black lead of the DVM to the speaker ground.
You should never check any voltage with 2 hands or do any work with 2 hands, with the power on or sometime thereafter. Voltage >24 v from one hand to the other can stop your heart.
You should never wear jewelry on hands, wrists, or neck. Voltage over 1 at large current through a ring etc can burn your flesh to charcoal.
Wear safety glasses. Parts explode sometimes. Solder can splash in the eyes, especially desoldering.
Okay, first thing to check with the DVM is DC voltage on right channel inductor L302. Same voltage as connection of D332 & D334 if that is more convenient. Should be less than 200 mv. > 200 mv, output transistor problems (pretty common).
If no DC, possibly output relay 701B is not conducting. Another common problem in old units. Best way to determine that is look for AC voltage, music if an input is on, @ L302. Unfortunately cheap DVM lie a lot on music, they are for 60-50 hz power frequency only. 2 cheap ways to go. Cheap salvage speaker L302 to speaker ground, or AC scale of cheap (<$30) analog VOM with a 20v or 50 VAC scale. these read fine on music. The negative probe of an analog VOM should be connected to speaker ground with a .047 or .1 uf capacitor, preferably film preferably rated 100 v or higher so you can use in future for probing more powerful amps. If only rated 63 v, that is enough for this amp.
Other cheap tests with DVM only, +-54 v & +-38 v power supplies at various points in the amp shown on schematic. Probably there, but PS supplies are high on the list of failed parts.
3rd likely problem is C302 the input cap is bad, pretty common with old units.
BTW, at this point some people like to buy cheap scopes. Great devices; most cheap ones are rated 50 v max. Even this amp has 58 v. 10x probes are expensive and break if stepped on. Also scopes are full of electrolytic caps, that go bad in 1 to 30 years depending on the initial quality bought by the manufacturer. My 1985 analog VOM is still working great, with no electrolytic caps in it. "PC scopes" using a sound card, are an extremely cheap way to blow up a $500 PC or cell phone. Amps are full of uncontrolled voltages, 58 in this case, 180 in the case of an amp of mine, and PC or cell phone sound inputs are made for 2 v line level signals. Only. Keep your cell phone away from your bargain amp full of suspicions voltages. I exercise my amp with a $1 battery FM radio fished out of the trash & the battery clip or volume pot replaced. Blow that up, I'm out a dollar and a couple of hours.
Happy hunting!
You'll need at least an inexpensive DVM. Also some alligator clip leads to connect the black lead of the DVM to the speaker ground.
You should never check any voltage with 2 hands or do any work with 2 hands, with the power on or sometime thereafter. Voltage >24 v from one hand to the other can stop your heart.
You should never wear jewelry on hands, wrists, or neck. Voltage over 1 at large current through a ring etc can burn your flesh to charcoal.
Wear safety glasses. Parts explode sometimes. Solder can splash in the eyes, especially desoldering.
Okay, first thing to check with the DVM is DC voltage on right channel inductor L302. Same voltage as connection of D332 & D334 if that is more convenient. Should be less than 200 mv. > 200 mv, output transistor problems (pretty common).
If no DC, possibly output relay 701B is not conducting. Another common problem in old units. Best way to determine that is look for AC voltage, music if an input is on, @ L302. Unfortunately cheap DVM lie a lot on music, they are for 60-50 hz power frequency only. 2 cheap ways to go. Cheap salvage speaker L302 to speaker ground, or AC scale of cheap (<$30) analog VOM with a 20v or 50 VAC scale. these read fine on music. The negative probe of an analog VOM should be connected to speaker ground with a .047 or .1 uf capacitor, preferably film preferably rated 100 v or higher so you can use in future for probing more powerful amps. If only rated 63 v, that is enough for this amp.
Other cheap tests with DVM only, +-54 v & +-38 v power supplies at various points in the amp shown on schematic. Probably there, but PS supplies are high on the list of failed parts.
3rd likely problem is C302 the input cap is bad, pretty common with old units.
BTW, at this point some people like to buy cheap scopes. Great devices; most cheap ones are rated 50 v max. Even this amp has 58 v. 10x probes are expensive and break if stepped on. Also scopes are full of electrolytic caps, that go bad in 1 to 30 years depending on the initial quality bought by the manufacturer. My 1985 analog VOM is still working great, with no electrolytic caps in it. "PC scopes" using a sound card, are an extremely cheap way to blow up a $500 PC or cell phone. Amps are full of uncontrolled voltages, 58 in this case, 180 in the case of an amp of mine, and PC or cell phone sound inputs are made for 2 v line level signals. Only. Keep your cell phone away from your bargain amp full of suspicions voltages. I exercise my amp with a $1 battery FM radio fished out of the trash & the battery clip or volume pot replaced. Blow that up, I'm out a dollar and a couple of hours.
Happy hunting!
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Hi!
I have some updates, and some follow-up questions, if it's ok
I did some more digging, and decided to check whether signal reaches the pre-out connector (which is bridged to the power-in) and guess what - no signal reaches there, so the problem is in the preamp!
I have a signal generator, so connected it to the cd input (20Khz, 0.3Vpp),
and tried tracing the signal along the way. The problem is that my "oscilloscope" is a handheld "all-in-one" device, and apparently it can't sense voltages lower than 0.4V while in oscilloscope mode.
I did find out that signal sometimes reaches R170, but it gets distorted when I change the volume. I compared it to the signal on R169 (for the left preamp), and I saw that I get a very nice amplification without distortion on the left channel). In addition, the Vpp is lower on this channel by 0.05V than on the left, but I don't suspect it being too low (yet) because increasing the volume increases this voltage.
I continued tracing, and found out that the signal does reach R158 (but get distorted when volume is changed). Unfortunately, I couldn't get a reading on R144 and R146 because the scope is not sensitive.
What am I doing wrong? How can I find the defective part? I know I can replace them all, but I won't learn anything by doing so.
Any help would be much appreciated. Thanks!
I have some updates, and some follow-up questions, if it's ok
I did some more digging, and decided to check whether signal reaches the pre-out connector (which is bridged to the power-in) and guess what - no signal reaches there, so the problem is in the preamp!
I have a signal generator, so connected it to the cd input (20Khz, 0.3Vpp),
and tried tracing the signal along the way. The problem is that my "oscilloscope" is a handheld "all-in-one" device, and apparently it can't sense voltages lower than 0.4V while in oscilloscope mode.
I did find out that signal sometimes reaches R170, but it gets distorted when I change the volume. I compared it to the signal on R169 (for the left preamp), and I saw that I get a very nice amplification without distortion on the left channel). In addition, the Vpp is lower on this channel by 0.05V than on the left, but I don't suspect it being too low (yet) because increasing the volume increases this voltage.
I continued tracing, and found out that the signal does reach R158 (but get distorted when volume is changed). Unfortunately, I couldn't get a reading on R144 and R146 because the scope is not sensitive.
What am I doing wrong? How can I find the defective part? I know I can replace them all, but I won't learn anything by doing so.
Any help would be much appreciated. Thanks!
I sounds as if the volume pot is bad. Pretty common in old units. Other common problem is bad board interconnects especially if push in flat cable. Other common problem is bad solder joints. Resistors don't go bad much in preamp section, neither do film caps or ceramic caps.
I warned you about cheap "scopes". Many digital devices can't handle 20 khz. try 1 khz to see if it works.
You need an analog voltmeter with a 2 VAC scale at 5000 ohms/volt also 20 vac scale. You block the negative probe of those with .047 uf cap >63 v, so they don't show AC reading on a DC voltage. I have a $260 simpson 266-xlpm from 1985, but there are import knock offs for about $30.
RMS AC digital voltmeter like fluke will work on basic music tracing, but they miss signals >7 khz so they can't diagnose an oscillation problem.
I warned you about cheap "scopes". Many digital devices can't handle 20 khz. try 1 khz to see if it works.
You need an analog voltmeter with a 2 VAC scale at 5000 ohms/volt also 20 vac scale. You block the negative probe of those with .047 uf cap >63 v, so they don't show AC reading on a DC voltage. I have a $260 simpson 266-xlpm from 1985, but there are import knock offs for about $30.
RMS AC digital voltmeter like fluke will work on basic music tracing, but they miss signals >7 khz so they can't diagnose an oscillation problem.
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TL;DR: Success + aa floowup question
indianajo - you rock!!!
Problem was indeed the volume pot !
I connected the signal generator to the input, and even without turning the amp on I took a measurement at the volume pot. Signal was distorted.
I used some contact-cleaning spray (well, I used it heavily) and sprayed inside the pot while turning it, and VIOLA - signal is not distorted anymore, and the right channel is back on !!!
Now to the followup question (as I'm doing this as a hobby and out of curiosity):
Before closing the lid, I tried following the idling-current setup.
The service manual says that I should measure the DC voltage between two test-points. 2 things happened here:
1. The measurement was not constant - it was fluctuating between positive and negative voltages, so I figured out I'm measuring AC, but the manual says that I should read DC. What am I doing wrong?
2. When I measured the TP for the right channel, I accidentally touched something with the probe, and a huge spark (and "bang" sound) came out.
It was such a strong spark that is scored the probe !
There is not visible evidence on the board / parts (at least, to my untrained eyes), and the amplifier seems to ne working just fine.
What should I check in order to make sure I didn't harm anything?
Again, thanks a lot! Meanwhile I connected the amp to my computer and
I've already started enjoying it (but would still like to understand and continue learning, so please don't tell me to forget and enjoy )
- Barak
indianajo - you rock!!!
Problem was indeed the volume pot !
I connected the signal generator to the input, and even without turning the amp on I took a measurement at the volume pot. Signal was distorted.
I used some contact-cleaning spray (well, I used it heavily) and sprayed inside the pot while turning it, and VIOLA - signal is not distorted anymore, and the right channel is back on !!!
Now to the followup question (as I'm doing this as a hobby and out of curiosity):
Before closing the lid, I tried following the idling-current setup.
The service manual says that I should measure the DC voltage between two test-points. 2 things happened here:
1. The measurement was not constant - it was fluctuating between positive and negative voltages, so I figured out I'm measuring AC, but the manual says that I should read DC. What am I doing wrong?
2. When I measured the TP for the right channel, I accidentally touched something with the probe, and a huge spark (and "bang" sound) came out.
It was such a strong spark that is scored the probe !
There is not visible evidence on the board / parts (at least, to my untrained eyes), and the amplifier seems to ne working just fine.
What should I check in order to make sure I didn't harm anything?
Again, thanks a lot! Meanwhile I connected the amp to my computer and
I've already started enjoying it (but would still like to understand and continue learning, so please don't tell me to forget and enjoy
)- Barak
indianajo was giving you good advice regarding test leads, clips leads etc. Never use long, exposed probes if you can avoid them because new and occasional DIYS, even pro technicians occasionally, can all make slip-ups, drop the probes and short things, sometimes with disastrous results. All you need to supplement your probes is a cheap but durable pair of meter leads (Ebay, AliExpress) terminated with IC clips (short hook probes). Also, slide some tight-fitting PVC wire insulation onto your existing probes, leaving just a few mm exposed.
Idle current means the DC bias current flowing through the output transistors. It is measured with no signal so we disconnect all inputs, turn volume control right down, disconnect speakers and measure the tiny voltage across large emitter resistors R359 (L) and R360(R). Current is calculated by Ohms law, I=E/R but the voltage measurement is sufficient to compare with the recommended value in the service manual.
If you have fluctuating voltage there, either you still have a signal connected or there is instability or a malfunctioning power control circuit. Take care, this no conventional amp. It's known as class G type which has switching power rails as shown on the schematic. I suggest you don't try to change things there unless you really know what you are doing.
If you have fluctuating voltage there, either you still have a signal connected or there is instability or a malfunctioning power control circuit. Take care, this no conventional amp. It's known as class G type which has switching power rails as shown on the schematic. I suggest you don't try to change things there unless you really know what you are doing.
Hi again and thanks for the help and the advices.
I've learned my lesson and I'm more careful now.
I connected small isolated alligator clips to the test points,
and I surely got some AC there. The voltage fluctuates between positive
and negative. No source is connected, no output is connected, and the volume knob is set to minimum.
Does this mean that there's a problem with the power supply?
Maybe a filter?
As before - any help will be appreciated
Thanks !
- Barak
I've learned my lesson and I'm more careful now.
I connected small isolated alligator clips to the test points,
and I surely got some AC there. The voltage fluctuates between positive
and negative. No source is connected, no output is connected, and the volume knob is set to minimum.
Does this mean that there's a problem with the power supply?
Maybe a filter?
As before - any help will be appreciated
Thanks !
- Barak
Frequency of plus to minus voltage matters. If it is 1 hz or 1/2 hz your DVM might follow it. However a dual slope integration DVM average the signal over 2 to 4 seconds, so what reading you get of AC depends on where the sample ended versus the phase of a low hz cycle.
If you are still using a DVM to measure AC voltages, I can't help you. My DVM makes up random numbers at AC frequencies that aren't 50 or 60 hz. That number varies all around at the frequency you suggest. Perhaps your meter is as pure as the driven snow, but if it is not a Fluke RMS AC meter, then I suspect the readings as fake. My Sears DVM was showing varying AC voltage out of a Hammond organ, and those tones on flute stop are very nearly sine waves at fixed music frequencies. And since the Fluke RMS AC meters are blind to frequencies over 7000 hz, I won't allow one in my repair area. 1 mhz up oscillations occur that an analog AC meter will see, but not a cheap DVM and not a Fluke RMS DVM either.
The test points don't show on my copy of the service manual. If it is across R358 and across R359, then those should never go negative with no input signal.
You'll have to trace back through the circuit to see where an oscillation is occuring. Likely points are VR303 & VR304 the bias adjustment pots.
Another likely culprit is RV301 & RV302 the zero adjustment pots.
Pots tend to oxidize the wiper after some years and lose the adjustment.
If those aren't it, then you'll have to check through the circuit for the initial point for some voltage varying at the same frequency as the plus & minus signal you are seeing on the collector resistor of Q329 or Q330. Something is wrong in the first stage where varying voltage occurs with no input signal.
If you are still using a DVM to measure AC voltages, I can't help you. My DVM makes up random numbers at AC frequencies that aren't 50 or 60 hz. That number varies all around at the frequency you suggest. Perhaps your meter is as pure as the driven snow, but if it is not a Fluke RMS AC meter, then I suspect the readings as fake. My Sears DVM was showing varying AC voltage out of a Hammond organ, and those tones on flute stop are very nearly sine waves at fixed music frequencies. And since the Fluke RMS AC meters are blind to frequencies over 7000 hz, I won't allow one in my repair area. 1 mhz up oscillations occur that an analog AC meter will see, but not a cheap DVM and not a Fluke RMS DVM either.
The test points don't show on my copy of the service manual. If it is across R358 and across R359, then those should never go negative with no input signal.
You'll have to trace back through the circuit to see where an oscillation is occuring. Likely points are VR303 & VR304 the bias adjustment pots.
Another likely culprit is RV301 & RV302 the zero adjustment pots.
Pots tend to oxidize the wiper after some years and lose the adjustment.
If those aren't it, then you'll have to check through the circuit for the initial point for some voltage varying at the same frequency as the plus & minus signal you are seeing on the collector resistor of Q329 or Q330. Something is wrong in the first stage where varying voltage occurs with no input signal.
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