noise.wav - 0.88MB
It comes through one channel only on my amp when the volume is altered, I recorded this noise from the pre-amp section out which rocking the volume knob back and forth. I started a thread assuming that it was noise from a worn out pot but it persists despite having replaced it. I also replaced the two 5.6k resistors on the board the pot is on with a better matched pair, this has not helped.
Categorized Schematics and Service Manuals for free download The schematics for my amp can be found there.
Any advice would be greatly appreciated, I'm incredibly curious as to what exactly is causing the issue, it seems so bizarre that an issue of this type wouldn't have been solved by replacing the pot.
Thanks!
It comes through one channel only on my amp when the volume is altered, I recorded this noise from the pre-amp section out which rocking the volume knob back and forth. I started a thread assuming that it was noise from a worn out pot but it persists despite having replaced it. I also replaced the two 5.6k resistors on the board the pot is on with a better matched pair, this has not helped.
Categorized Schematics and Service Manuals for free download The schematics for my amp can be found there.
Any advice would be greatly appreciated, I'm incredibly curious as to what exactly is causing the issue, it seems so bizarre that an issue of this type wouldn't have been solved by replacing the pot.
Thanks!
It sounds as if you may have DC on the pot. Check for leaky electrolytic capacitors around the pot as well as placing a voltmeter across it. You can then compare left and right channels to see if they are identical. Does this symptom appear without any input leads connected to the preamp input?
Regards, currentflow
I would initially check C501 (Left channel) & C502 (Right channel), both are 4u7 / 25V. In any case, there should be no DC at the preamp output sockets - test with tone defeat switch in both positions.
Regards, currentflow
Regards, currentflow
The issue is before the volume board, thus it's definitely not those caps. I elimited that possibility first by swapping the output channels, the noise then presented on the right channel. I then switched the inputs to the board then the noise returned to the left channel.
This indicates the issue comes before the volume board yet is triggered by altering the volume.
The issue comes through both channels regardless of tone defeat and when it's set to mono predictably the noise comes through both channels.
This indicates the issue comes before the volume board yet is triggered by altering the volume.
The issue comes through both channels regardless of tone defeat and when it's set to mono predictably the noise comes through both channels.
IC801 is the only device in the signal path between the input sockets and the volume control which is why I suggested looking for DC offsets at the volume pot. S501 should be in the 'stereo' position.
Please confirm that the noise occurs with the input sources ALL unplugged. One of your source inputs may have DC on its output and this would be passed directly to the volume control via IC801.
Does this happen with any input selected, or just one?
Regards, currentflow
I have no inputs plugged in and the noise presents no matter what input is selected. S501 is indeed set to stereo, I merely toggled it to establish that the issue persisted on either setting.
The IC seems to be available still, but it's not particularly cheap so if it needs replacing it's doable.
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Have you measured across the volume control yet (both channels)? DC will make such a sound.
Yeah, there's like 15V on one channel! This is troubling. I measured this between 1 and E on TX-457.
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Then it seems highly likely that you will have to replace IC801. However, it would be a good idea to check the supply rails are correct on pins 9 and 22 (+18.2V and -18.2V respectively) of this chip before condemning it to the bin.
Regards, currentflow
As curentflow says, it could well be a faulty IC but first measure on EVERY pin of the IC before making a decision.
Too many years spent as a bench tech and the saying "it's never the big IC" come to mind.
Could it be spillage ?
Too many years spent as a bench tech and the saying "it's never the big IC" come to mind.
Could it be spillage ?
I was going to also suggest checking resistors R811 (47K) and R813 (4.7K) are the correct values and that their PCB tracks & joints are intact.
At least the left and right channel pin voltages can be compared to one another as Mooly suggests.
Regards, currentflow
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Just a thought, but I seem to recall this issue presenting around the same time I first tried connecting my xbox 360, it outputs an obscenely high line level and distorted to all hell, is it feasible that this damaged the IC?
Seems like I can get a replacement fairly cheaply so replacing it would be no trouble.
For now I'll take some measurements around it.
Seems like I can get a replacement fairly cheaply so replacing it would be no trouble.
For now I'll take some measurements around it.
R811 reads a few k then climbs up to 26k and then my meter reads rather erratically, the corresponding R810 reads 26k.
R813 and R812 read 4.7k fine.
R813 and R812 read 4.7k fine.
A high line level wouldn't damage anything... but equipment powered by SMPS's (TV/DVD/ Xbox???) could possibly.
Hard to explain quickly... in a nutshell equipment powered by switching PSU's can appear to have almost a "live chassis" due to leakage currents. No shock hazard as the impedance is high but it can be of the order of several hundered microamps or more. You can certainly draw an arc off many an older TV sets aerial socket and ground (aerial lead) and it's certainly enough to make you let go quickly if you happen to have damp hands.
If anything like that were connected while it was on then maybe it could damage the IC as there is no external series resistors on all the inputs.
I'm aware hence unsurprised that one read 26k rather than 47k, the significant point is that the corresponding resistors do not measure the same.
Removing the resistor would take several hours and require me to take the amp apart. Given the chip costs all of a few pounds I think I'll buy one, take the amp apart to that point replace the chip and the resistors nearby, check the work as best I can and hope.
If dismantling is a problem then replacing the IC probably best.
Meter lead polarity can alter in circuit readings 🙂
Meter lead polarity can alter in circuit readings 🙂
Not so much a problem, I'm just lazy. I'd really rather have to only do it once and take a shotgun approach to component replacement, I may even see about recapping it while it's all apart.
hmmm according to schematic at the output of the IC should be 0 volts ...so the capcitor is there to prevent a few millivolts of ofset that might be catastrophic for the amplifier but other than that not actually needed .
There is a chance though that something is wrong in the power supply and absense of ground at some point will drive the op amp to full dc output
verify all voltage according to the schematic arround the IC area (also make sure that you and the Ic share the same ground IE your ground point for measuring might not be the same used by the ICs )
There is a chance though that something is wrong in the power supply and absense of ground at some point will drive the op amp to full dc output
verify all voltage according to the schematic arround the IC area (also make sure that you and the Ic share the same ground IE your ground point for measuring might not be the same used by the ICs )
It was the big IC, had to order one from spingapore and it took it's sweet time to get to me but it's all fixed now.
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