I wonder if anyone can help diagnose this.
I have an intermittent crackling noise on one channel. It's in the power amp stage as it happens even with the preamp disconnected. It's not in the speakers or speaker cables as it stays on same channel when speakers are swapped.
The music still comes through at the same volume as the other channel.
The crackling occurs even when the input is grounded but is a bit louder when input is floating. There is also a slight hiss when input is floating but not when grounded. (The other channel never has this hiss).
I have looked carefully for any cracked solder joint and have tried tapping each of the components to see if I can make the crackle come on demand, but it seems to be independent of any tapping.
I have looked for any dirt shorting across tracks the circuit board.
It's not in the "power envelope"� circuitry as it still happens when this is disabled by removing the fuses on the +/-71V supply.
It's not in the speaker selection switch as I have tried connecting a speaker to the output track before this switch.
Any ideas for more tests I could try?
I attach a schematic.
I have an intermittent crackling noise on one channel. It's in the power amp stage as it happens even with the preamp disconnected. It's not in the speakers or speaker cables as it stays on same channel when speakers are swapped.
The music still comes through at the same volume as the other channel.
The crackling occurs even when the input is grounded but is a bit louder when input is floating. There is also a slight hiss when input is floating but not when grounded. (The other channel never has this hiss).
I have looked carefully for any cracked solder joint and have tried tapping each of the components to see if I can make the crackle come on demand, but it seems to be independent of any tapping.
I have looked for any dirt shorting across tracks the circuit board.
It's not in the "power envelope"� circuitry as it still happens when this is disabled by removing the fuses on the +/-71V supply.
It's not in the speaker selection switch as I have tried connecting a speaker to the output track before this switch.
Any ideas for more tests I could try?
I attach a schematic.
Try freeze spray on one part at a time to see if the noise is changed.
Or try tapping around with a plastic/wood tool. Use some junk speakers just to be safe.
Using Freeze Spray to Diagnose Faulty Electronics | Techspray
Most likely to be a solder joint, resistor, or transistor.
Or try tapping around with a plastic/wood tool. Use some junk speakers just to be safe.
Using Freeze Spray to Diagnose Faulty Electronics | Techspray
Most likely to be a solder joint, resistor, or transistor.
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It could be one of many things. Knowing what we know about NAD you need to be 100% sure it is not a dry... which you seem to have done 🙂
Other suspects would parts that run hot such as Q414 and Q416 (channel dependent of course) and also any stressed resistors such as R442, R448 and R450. Anything that gets hot is suspect for failing. Resistors need checking with one end isolated. If any are high then change them.
Transistor faults for this type of fault probably would not show on a DVM but might on a scope if you know how to interpret results. You would see the 'crackle' but you would need to look for unexpected higher than normal Vbe voltage on the transistor when the spikes occur. That is much more advanced fault-finding. Other than that its a case of swapping the parts for new. Swapping between channels isn't recommended because the heating of the part can temporarily 'fix' it.
Faults like this are not easy to diagnose.
Other suspects would parts that run hot such as Q414 and Q416 (channel dependent of course) and also any stressed resistors such as R442, R448 and R450. Anything that gets hot is suspect for failing. Resistors need checking with one end isolated. If any are high then change them.
Transistor faults for this type of fault probably would not show on a DVM but might on a scope if you know how to interpret results. You would see the 'crackle' but you would need to look for unexpected higher than normal Vbe voltage on the transistor when the spikes occur. That is much more advanced fault-finding. Other than that its a case of swapping the parts for new. Swapping between channels isn't recommended because the heating of the part can temporarily 'fix' it.
Faults like this are not easy to diagnose.
If you’re absolutely 100% sure you’ve eliminated dry joints on the main board , try delicately pushing and prodding at various semiconductors
But I’d triple check for drys you haven’t spotted (yet) - with a very strong light and a loupe if possible
You can also try pushing at the actual board in various places (til it bends, but not until it breaks ) , it might be worthwhile actually blanket re-soldering the board in certain areas
But I’d triple check for drys you haven’t spotted (yet) - with a very strong light and a loupe if possible
You can also try pushing at the actual board in various places (til it bends, but not until it breaks ) , it might be worthwhile actually blanket re-soldering the board in certain areas
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Excess noise effected by source impedance and crackle sounds like a failing small signal transistor in the front end. A drop of freezer spray on each transistor in turn and when it hits the offending device, the crackle will instantly stop. If you have a heat gun for re-flow soldering you can use this instead to carefully heat one transistor at a time, which when applied to the offending device will cause its behaviour to change (get worse or better).
I haven't tried this but judicious use of a can of lighter gas should work just as well as freezer spray if you can find a way of dispensing it in liquid form to one transistor at a time.
I haven't tried this but judicious use of a can of lighter gas should work just as well as freezer spray if you can find a way of dispensing it in liquid form to one transistor at a time.
I suggest looking at VR401 or V402 as required since airborne contaminants can cause deterioration in wiper contact with the carbon track.
These components connect to inverting amplifier inputs so and issues in that regard will be amplified.
Since these components also adjust dc output stage dc offset it is likely after using a spray lubricant you may have to reset this afterwards.
It would be as well to check the resistors and capacitor in the negative feedback decoupling arm to earth - especially the capacitor for a.c. connectivity.
These components connect to inverting amplifier inputs so and issues in that regard will be amplified.
Since these components also adjust dc output stage dc offset it is likely after using a spray lubricant you may have to reset this afterwards.
It would be as well to check the resistors and capacitor in the negative feedback decoupling arm to earth - especially the capacitor for a.c. connectivity.
Hi johnmath,
Noisy transistors can cause this, but do not always react to cold. Sometimes heat, and sometimes they don't react at all. Resistors can certainly do this as well. But, bugging the front end can cause sudden large DC offsets. This is best done looking with a scope and not listening with a speaker (maybe a cheap one that you have more of!). I have seen, very rarely, cheap ceramic capacitors that have caused noise. Best to keep an open mind when troubleshooting.
That's why a good technician is worth the money. This stuff isn't always cookie cutter type repairs, and the small stuff will bite you.
Hi mjona,
The element in a trimmer or pot that goes noisy is the slip ring connection, not the carbon track. Just for reference.
-Chris
Good lord! Don't do that, why would you even recommend such a dangerous practice???
Noisy transistors can cause this, but do not always react to cold. Sometimes heat, and sometimes they don't react at all. Resistors can certainly do this as well. But, bugging the front end can cause sudden large DC offsets. This is best done looking with a scope and not listening with a speaker (maybe a cheap one that you have more of!). I have seen, very rarely, cheap ceramic capacitors that have caused noise. Best to keep an open mind when troubleshooting.
That's why a good technician is worth the money. This stuff isn't always cookie cutter type repairs, and the small stuff will bite you.
Hi mjona,
The element in a trimmer or pot that goes noisy is the slip ring connection, not the carbon track. Just for reference.
-Chris
Traditionally freezer sprays were just liquified petroleum gas AKA propane / butane mixes, although many freezer sprays now contain tetrafluoroethane (an ozone depleting gas).
Butane (AKA lighter gas) is a commonly used propellant in household and industrial aerosols. It's evaporation rate and dispersal in the atmosphere means it won't normally sustain a flame when used, for example, for freezer spray.
I should have been more specific and said to use contact cleaner made for electronics for this job and confine such to VR1 and VR2 so you have covered both channels.
Carbon resistors- normal for old valve equipment are known to go high in resistance. In use these are subjected heating due to sustained high voltages across them.
The circuitry involved in this piece of equipment is complex and it could end up badly for you to go beyond the elementary steps like point using contact spray and the service manual adjustment for dc offset and output standing current.
The advice about getting qualified help from a service technician looks pretty good to me.
Carbon resistors- normal for old valve equipment are known to go high in resistance. In use these are subjected heating due to sustained high voltages across them.
The circuitry involved in this piece of equipment is complex and it could end up badly for you to go beyond the elementary steps like point using contact spray and the service manual adjustment for dc offset and output standing current.
The advice about getting qualified help from a service technician looks pretty good to me.
They are non-flammable.
Butane is not allowed as a propellant here.
Why on earth would your introduce any flammable liquid / gas into an energized circuit??? You have got to use your common sense, especially if you are giving advice.
-Chris
Butane is not allowed as a propellant here.
Why on earth would your introduce any flammable liquid / gas into an energized circuit??? You have got to use your common sense, especially if you are giving advice.
-Chris
Hi Mjona,
For all controls, the source of noise is commonly the slip ring and contacts. A tiny amount of zero residue cleaner will fix that. If the carbon track is damaged - it's over. When too much cleaner is applied, it attacks the protective lubricants on the carbon track and also in the bushing. Never use more cleaner than needed, and don't clean controls that don't need it. 99% of people out there don't know how to clean a control.
-Chris
True, mostly. They can go down quite a bit depending on their starting resistance. Pots often go high resistance where the end contacts are. You can't repair that, the conductive coatings are a short term fix.
For all controls, the source of noise is commonly the slip ring and contacts. A tiny amount of zero residue cleaner will fix that. If the carbon track is damaged - it's over. When too much cleaner is applied, it attacks the protective lubricants on the carbon track and also in the bushing. Never use more cleaner than needed, and don't clean controls that don't need it. 99% of people out there don't know how to clean a control.
-Chris
This.
Sometimes you’ve gotta approach these as if you’re approaching some wildlife with a camera….😱
Delicate approaches , trying various tricks …. Physical prodding , a bit of cooling , sometimes a bit of heat from the top of an iron.
This sort of job can soak up many time units …. Even for a many years seasoned engineer …. Without even trying , sometimes putting it to one side and reapproaching another day with a fresh head can yield results.
But don’t go in all guns blazing….. creep up on the problem component.
Even a simple poor connection can hide for several attempts.
The freezer spays I have on my service bench are listed as flammable, but they don't list ingredients. I can't say that I know too many service technicians that incinerated themselves; have you heard of any? AFAIK from my time as an OHS materials safety officer the main reasons behind removing LPG products from aerosols are misuse (inhaling), and danger of the container exploding in fire. So far no country has removed butane from lighter gas refills. 🙂
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You might want to check that.
"Based on information reported pursuant to section 71 of the Canadian Environmental Protection Act, 1999 (CEPA 1999) (Environment Canada 2007), the major use patterns for butane and isobutane include solvent/carrier, propellant/blowing agent, fuel or fuel additive and formulation component. End-use fuels (e.g., motor vehicle gasoline, liquefied petroleum gas) formulated with butane and isobutane will be addressed under the Petroleum Sector Stream Approach of the Chemicals Management Plan. In terms of non-fuel uses, butane is used in various applications including insulating polyurethane foam, aerosol sprays and coatings, paint dyes and automotive spray waxes in which the concentration of butane can range up to approximately 60 w/w%. Isobutane is used in consumer products such as cosmetic/beauty preparations, air freshener, cleaners, activator/primers and various coatings in which the concentration of isobutane may range up to 70 w/w%."
Chemical Abstracts Service Registry - Butane - Environment Canada
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The stuff I use is safe, but is freezing spray. I rarely need to use it, but I know techs who go through case lots.
I have seen a cleaning spray ignite. A relay opened without a cover - pooof!
Anyway, never recommend using a product that isn't specifically designed for a purpose. The butane content make vary a lot, so using it straight is unwise.
Also, as MikePP said, take an open approach with your eyes open and brain active. That is a skill many do not have. Use methods that are less dramatic to start, reaching for a can of something, or the soldering iron or heat gun isn't done as a first step unless you have very good information to suggest it may be effective. Patience, observation and think first.
I have seen a cleaning spray ignite. A relay opened without a cover - pooof!
Anyway, never recommend using a product that isn't specifically designed for a purpose. The butane content make vary a lot, so using it straight is unwise.
Also, as MikePP said, take an open approach with your eyes open and brain active. That is a skill many do not have. Use methods that are less dramatic to start, reaching for a can of something, or the soldering iron or heat gun isn't done as a first step unless you have very good information to suggest it may be effective. Patience, observation and think first.
Thanks all for the suggestions.
I have now tried freezing each component with lighter fuel which worked well, and heating each with a hair dryer with a paper cone attached to make it a fine jet, which also worked. But no change to the noise. Also tried chilling and heating the solder joints to see if it triggered any change, but nothing.
Checked the VRs too and cleaned them with fader lube.
I tried testing a few of the higher wattage resistors by removing one end - all OK.
I haven't tested the transistors as I don't have a scope.
Guess I'll replace some of the early stage transistors next...
Thanks again.
I have now tried freezing each component with lighter fuel which worked well, and heating each with a hair dryer with a paper cone attached to make it a fine jet, which also worked. But no change to the noise. Also tried chilling and heating the solder joints to see if it triggered any change, but nothing.
Checked the VRs too and cleaned them with fader lube.
I tried testing a few of the higher wattage resistors by removing one end - all OK.
I haven't tested the transistors as I don't have a scope.
Guess I'll replace some of the early stage transistors next...
Thanks again.
Sorry, you said you haven't tested the transistors? The early stage transistors are often the culprit in this amplifier especially given your description of the fault. Freezer is normally the way to identify the faulty ones. To chill them they need to be coated with white icy frost. You'll normally hear the crackles reappear as the frost disappears.
You can get an indication whether or not a transistor is working by measuring the base to emitter voltage which should be in the vicinity of 0.6 Volts.
To do this use your multi-meter with the black probe connecting to earth and use the red to probe the base and emitter leads separately and record the plus or minus differences for NPN or PNP readings.
If this amplifier uses banana style screw style output connectors you can do this by putting the black probe into the small hole on the black connector and turn the screw lightly to hold.
There is less risk of an accident inside the chassis with only one hand and the red probe in play.
I believe your pcb is a phenolic type and this type of board has some degree of flexibility and which can develop cracks if it is not thick enough. These can arise some distance away from the point of tapping on components.
Applying gentle pressure to a pcb over a wider area at a time, an index finger can feel where there may be too much give and help discover where cracks may lie.