sweetbeats
Reel deep thoughts...
Set your DMM to DC volts and measure across the shield and signal conductor of the RCA STEREO OUT. Let me know if that doesn’t make sense.
sweetbeats
Reel deep thoughts...
Also, do you have a frequency counter function on your DMM? If so can you measure the frequency of the hum?
Also does the hum occur with only either the R or L STEREO OUT jack connected, or do both have to be connected along with the MONITOR out?
Also does the hum occur with only either the R or L STEREO OUT jack connected, or do both have to be connected along with the MONITOR out?
B
BMG123
Member
Good evening Sweetbeats, sorry for the delayed response. I'm the tech director for a high school and our musical opens in less than a week. I've been overtime getting things ready for the show. Anyways, its Saturday and I'm back in front of the 388.
first: set your DMM to DC volts and measure across the shield and signal conductor of the RCA STEREO OUT. Let me know if that doesn’t make sense.
Not 100% sure but I'm guessing that would be both the inside and the outside of the RCA connector?
I did try that and the reading stays at 0.00 (if I measured it correctly)
second: Also, do you have a frequency counter function on your DMM? If so can you measure the frequency of the hum? It does not but I added a sound clip below
third: Also does the hum occur with only either the R or L STEREO OUT jack connected, or do both have to be connected along with the MONITOR out?
I hear the hum in either speaker if I disconnect one or the other. So if I leave the right in I hear it in the RT speaker, if I only have the LFT in I hear it in the left speaker.
That is the case weather the monitor is plugged in or out.
If I leave the monitor plugged in and listen through the headphone amp AND unplug the Main Stereo out its quiet (if I'm completely honest there is a very quiet version of the hum in my headphones with just the monitor outs plugged in but its acceptable for a headphone mix)
The minute I plug in the main stereo outs the noise triples in volume and is completely unacceptable.
Just one final test to make sure you know I'm trying everything. I wanted to rule out my amp as a source of the issue. I hooked up a different amp and have the same problem.
I also took the RCAs from the monitor out to the headphone amp and plugged them into the STEREO outs and I had the loud hum in the headphone amp then.
So it seems like an issue with the stereo outs more than the monitor outs.
What I really don't understand is how my computer can have an affect on the noise.
If I hold down the "backspace" key I'll hear a ramp up in the sound as I delete a sentence.
Really hoping there is a fix. While the rest of the deck seems to be in prefect running condition, the noise level makes it un-usable.
first: set your DMM to DC volts and measure across the shield and signal conductor of the RCA STEREO OUT. Let me know if that doesn’t make sense.
Not 100% sure but I'm guessing that would be both the inside and the outside of the RCA connector?
I did try that and the reading stays at 0.00 (if I measured it correctly)
second: Also, do you have a frequency counter function on your DMM? If so can you measure the frequency of the hum? It does not but I added a sound clip below
third: Also does the hum occur with only either the R or L STEREO OUT jack connected, or do both have to be connected along with the MONITOR out?
I hear the hum in either speaker if I disconnect one or the other. So if I leave the right in I hear it in the RT speaker, if I only have the LFT in I hear it in the left speaker.
That is the case weather the monitor is plugged in or out.
If I leave the monitor plugged in and listen through the headphone amp AND unplug the Main Stereo out its quiet (if I'm completely honest there is a very quiet version of the hum in my headphones with just the monitor outs plugged in but its acceptable for a headphone mix)
The minute I plug in the main stereo outs the noise triples in volume and is completely unacceptable.
Just one final test to make sure you know I'm trying everything. I wanted to rule out my amp as a source of the issue. I hooked up a different amp and have the same problem.
I also took the RCAs from the monitor out to the headphone amp and plugged them into the STEREO outs and I had the loud hum in the headphone amp then.
So it seems like an issue with the stereo outs more than the monitor outs.
What I really don't understand is how my computer can have an affect on the noise.
If I hold down the "backspace" key I'll hear a ramp up in the sound as I delete a sentence.
Really hoping there is a fix. While the rest of the deck seems to be in prefect running condition, the noise level makes it un-usable.
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B
BMG123
Member
Sweetbeats, I know my DMM doesn't have a frequency counter but here is a short audio clip from my phone. You can hear me move from the woofer to the tweeter as I had it in record. Not sure if this tells you anything (besides how annoying this sound is) but I thought I'd try.
Brian
Brian
sweetbeats
Reel deep thoughts...
If it was me I’d be flipping it on its side, pulling the bottom cover back off, powering my scope back up and probing the audio power rail outputs and looking for AC components…starting with the basics. That sounds like 120Hz. Make absolutely certain your power rails are clean DC and the right voltage. And to be honest I’d probably check the other DC rails too just to make sure they are all clean and the right voltage.
B
BMG123
Member
Thanks! So this goes back to the original thought that AC current is getting through the power supply, correct? I don't have access to an oscilloscope but was looking on line to see how much they cost. I saw some small versions of "pocket" oscilloscopes for $60 to $80. Here is a link: https://www.circuitspecialists.com/...cYVIG2hrzOZx5_-nnEfCBtgnyUjlrmGkaAj6YEALw_wcB
Would something like this be sufficient?
Id be happy to pick something like this up and go through the learning curve to check if you think this would work.
sweetbeats
Reel deep thoughts...
Oh shoot I got confused with another poster that had done work on their power supply board and had a friend with a scope.
Yeah I suppose that handheld scope you linked would work. It’s really low bandwidth so limited in what it could do, but certainly for the sake of the current need it should work fine.
The other thing you could do is set your DMM to AC volts and measure each DC power rail output to its ground. There should be no AC. And then measure DC volts to make sure the voltage is correct. You can access the solder joints for the power supply board’s outputs on the associated motherboard when you remove the bottom panel.
The OTHER other thing you could do is just recap the power supply. The other poster I was thinking about, IIRC, was having him issues and what was happening is the filter caps had drifted enough below spec they weren’t able to keep up and filter the AC out of the rectified inputs to the power supply board. So there’s that “shotgun” approach. If it’s not been recapped it would be good to do.
Yeah I suppose that handheld scope you linked would work. It’s really low bandwidth so limited in what it could do, but certainly for the sake of the current need it should work fine.
The other thing you could do is set your DMM to AC volts and measure each DC power rail output to its ground. There should be no AC. And then measure DC volts to make sure the voltage is correct. You can access the solder joints for the power supply board’s outputs on the associated motherboard when you remove the bottom panel.
The OTHER other thing you could do is just recap the power supply. The other poster I was thinking about, IIRC, was having him issues and what was happening is the filter caps had drifted enough below spec they weren’t able to keep up and filter the AC out of the rectified inputs to the power supply board. So there’s that “shotgun” approach. If it’s not been recapped it would be good to do.
B
BMG123
Member
Nope, that other poster is me 
No confusion on your part.
I jumped over to this thread after I recapped the power supply. I ended up reading most of the pages in the "388 story" and picked up here.
I'll see if I can get those measurements myself. I do have a friend with an oscilloscope but everyone is super busy right now. I'm hoping to not have to get this out of the studio and take it to him simply because of the size and weight of the 388.
If I can verify things with my DMM that would be great. I assume I'll be using the contact points on the underside of the board that the power supply plugs into?
Brian
No confusion on your part. I jumped over to this thread after I recapped the power supply. I ended up reading most of the pages in the "388 story" and picked up here.
I'll see if I can get those measurements myself. I do have a friend with an oscilloscope but everyone is super busy right now. I'm hoping to not have to get this out of the studio and take it to him simply because of the size and weight of the 388.
If I can verify things with my DMM that would be great. I assume I'll be using the contact points on the underside of the board that the power supply plugs into?
Brian
sweetbeats
Reel deep thoughts...
I wouldn’t say you can “verify” things with the DMM…if you have a problem with a bad rectifier and/or primary filter cap on a linear regulated DC power supply, checking for AC volts at the supply output with a DMM will at least indicate there’s a problem if one exists, but that’s about it. And if there is something like a regulator oscillating it won’t identify that. It’s kind of like using a code scanner to find out what’s triggering the check engine light on a car…the scanner tells you the code, you can get online and find out what that code means, but the identification of the code doesn’t tell you the cause. Scoping the output can tell you more about the cause by being able to see the waveform if there are AC components at the output. So go ahead and measure with your DMM, just do it armed with the knowledge there could still be a problem with the power supply if you don’t measure any AC at the output, but you’ll have at least ruled out a bad rectifier or filter cap, which is more typically something that would cause hum in the audio output; extreme AC components in the DC power rail, too big for local filter caps to filter out or active devices (opamps) to reject.
And yes you can access the power supply outputs via the contact points on the underside of the board the power supply plugs into. That’s what I meant when I said “You can access the solder joints for the power supply board’s outputs on the associated motherboard when you remove the bottom panel.”
And yes you can access the power supply outputs via the contact points on the underside of the board the power supply plugs into. That’s what I meant when I said “You can access the solder joints for the power supply board’s outputs on the associated motherboard when you remove the bottom panel.”
B
BMG123
Member
Perfect! I think I have a shot at getting my buddy with the oscilloscope to come by here next week. We will review all your suggestions and hopefully get to the bottom of what's going on. Now the hunt
is really on, I won't give up till we find and fix the issue.
I'll keep you posted on our progress.
Thanks again for all your time on this!
Brian
B
BMG123
Member
Sweetbeats, I ended up taking the 388 to my buddy's and there is no AC present across the power rails. So at least we can rule that out.
But this gets more interesting... With just the Stereo outs plugged into the amp, it's dead quiet. So I figured I'd start adding things and see when hum gets introduced.
1. First I added a mic snake. I have a robust 16 channel, mogami snake i bought from a studio. I plugged the first 8 in and started to hear hum! (no mics plugged in yet)
I unplugged and tried a mic straight into the 388 and it's quiet. I also have a Hosa Lil-buddy 8 ch snake and that too was dead quiet! so I'm guessing something is up with my first snake.
2. I have two external mic-pres. I plugged the mic into the pre and the out from the pre into the line-in on the 388.
one of the pres is silent and the other generates hum. I tried swapping cables and checking all 8 line jacks and it had the same result. Hard to believe that pre is "bad" its brand new in December but whatever.... it's currently a source of noise
3. I then moved on to the PGM outs. I want to send these 8 outs to an 8 ch AD convertor. This will send all 8 channels to the computer, basically using the 388 as 8 mic pres/mixer directly to the DAW. This too generates a low amount of hum.
4. I plugged in an external reverb unit to the EFX send/rtn and that is quiet.
5. Last thing was some rack mount compressors running through a patchbay into the Send/RCV on the individual channels.
#1,3,8 have noise and 2,4,5,6,7 are silent.
Bottom line... it plays nice on its own. If I plug a mic in and record, it's quiet. once I start adding things, it tends to get noisy. Why some channels snd'rcv introduce noise and others don't is a mystery and why one mic pre would be noisy and the other quiet is also a mystery.
I borrowed an isolation transformer from my buddy and any piece of gear that introduced hum, I'd try plugging into that first and it didn't stop the noise.
I'd really like to take advantage of the PGM outs but, like I said above, that also adds hum.
I get the fact that its 35+ years old. I'd just like to get all aspects working like new if possible and eliminate the noise.
Wanted to give you an update and curious to see if you have any thoughts on anything listed above.
As always, eternally grateful...Brian
But this gets more interesting... With just the Stereo outs plugged into the amp, it's dead quiet. So I figured I'd start adding things and see when hum gets introduced.
1. First I added a mic snake. I have a robust 16 channel, mogami snake i bought from a studio. I plugged the first 8 in and started to hear hum! (no mics plugged in yet)
I unplugged and tried a mic straight into the 388 and it's quiet. I also have a Hosa Lil-buddy 8 ch snake and that too was dead quiet! so I'm guessing something is up with my first snake.
2. I have two external mic-pres. I plugged the mic into the pre and the out from the pre into the line-in on the 388.
one of the pres is silent and the other generates hum. I tried swapping cables and checking all 8 line jacks and it had the same result. Hard to believe that pre is "bad" its brand new in December but whatever.... it's currently a source of noise
3. I then moved on to the PGM outs. I want to send these 8 outs to an 8 ch AD convertor. This will send all 8 channels to the computer, basically using the 388 as 8 mic pres/mixer directly to the DAW. This too generates a low amount of hum.
4. I plugged in an external reverb unit to the EFX send/rtn and that is quiet.
5. Last thing was some rack mount compressors running through a patchbay into the Send/RCV on the individual channels.
#1,3,8 have noise and 2,4,5,6,7 are silent.
Bottom line... it plays nice on its own. If I plug a mic in and record, it's quiet. once I start adding things, it tends to get noisy. Why some channels snd'rcv introduce noise and others don't is a mystery and why one mic pre would be noisy and the other quiet is also a mystery.
I borrowed an isolation transformer from my buddy and any piece of gear that introduced hum, I'd try plugging into that first and it didn't stop the noise.
I'd really like to take advantage of the PGM outs but, like I said above, that also adds hum.
I get the fact that its 35+ years old. I'd just like to get all aspects working like new if possible and eliminate the noise.
Wanted to give you an update and curious to see if you have any thoughts on anything listed above.
As always, eternally grateful...Brian
W
wkrbee
Active member
Sounds like you have a ground loop with the devices you have plugged into 1.3.8. Plug them in one at a time without the patchbay. Hum yes or no?
sweetbeats
Reel deep thoughts...
Well shoot…
Regarding #2 are you sure it’s the outboard unit and not the 388? The 388 seems to be the problem here…I would t assume it’s the outboard preamp.
Did you measure for AC components in the DC power rails *while* the noise is present? And was a scope used?
Did you ever measure for DC offset at any output that carries the noise? Like get the noise to happen, put a ‘Y’ cable on the noisy output, and set your meter to DC volts and measure between the signal conductor and the shield of the output?
I’m spit-balling here because there’s no clear answer and this is not a common problem at all. Have you pulled the bottom cover back off and carefully inspected for any pinched wires? You’d be surprised how easy it is to do that…sharp metal edges…
I think if it was me, after I did all the above, would be to pull what PCBs out that I could to limit the assemblies involved and see if that mitigated the issue. The problem is with the 388 you have to be careful which ones you take out of the picture because of how power is distributed. I would only operate the unit with the mixer input cards removed. And secondly the way the 388 is constructed this isn’t a simple task…but it would be one thing to try. Remove the input PCBs, see if you can get the noise to happen using a couple of the BUSS IN jacks and the STEREO and MONITOR outputs. No noise? Add one card back in at a time until it happens.
Regarding #2 are you sure it’s the outboard unit and not the 388? The 388 seems to be the problem here…I would t assume it’s the outboard preamp.
Did you measure for AC components in the DC power rails *while* the noise is present? And was a scope used?
Did you ever measure for DC offset at any output that carries the noise? Like get the noise to happen, put a ‘Y’ cable on the noisy output, and set your meter to DC volts and measure between the signal conductor and the shield of the output?
I’m spit-balling here because there’s no clear answer and this is not a common problem at all. Have you pulled the bottom cover back off and carefully inspected for any pinched wires? You’d be surprised how easy it is to do that…sharp metal edges…
I think if it was me, after I did all the above, would be to pull what PCBs out that I could to limit the assemblies involved and see if that mitigated the issue. The problem is with the 388 you have to be careful which ones you take out of the picture because of how power is distributed. I would only operate the unit with the mixer input cards removed. And secondly the way the 388 is constructed this isn’t a simple task…but it would be one thing to try. Remove the input PCBs, see if you can get the noise to happen using a couple of the BUSS IN jacks and the STEREO and MONITOR outputs. No noise? Add one card back in at a time until it happens.
sweetbeats
Reel deep thoughts...
Yeah that’s what I was thinking too but if you read back a ways you’ll see we did some things to rule that out. I had a hunch it was a “pin 1” problem revealed by a voltage differential between two or more floating ground chassis. But Brian tested that out per my instructions and it didn’t mitigate the issue. So then I thought bad filter caps, but he’s recapped the power supply PCB assembly. So now I’m wondering about possibly a bad rectifier or regulator or something else…that it’s an issue somewhere that doesn’t manifest until there’s a load.
W
wkrbee
Active member
Reading what he said about some devices adding noise as he plugs them in tells me there might be a ground loop happening in the patchbay between some of the devices, especially if they are in a metal rack with all the chassis grounds tied together through the metal rack. Removing all of them and plugging them in one at a time would isolate the problem .I had a problem with a old DOD rack mount delay. Fine on it's own, but plugging in other things made it hum. I finally lifted the shield on the input cable at the device end to break the ground loop. He could try doing that with the offending device. It would be easy to fabricate a short Female to Male cable with the shield lifted to verify it. I used to carry around a cable like that when I was doing a lot of Field Service for the company I worked for to show the client it wasn't the mixer or R2R that was the problem- it was his wiring or patchbay.
sweetbeats
Reel deep thoughts...
The problem happens without the patchbay.
W
wkrbee
Active member
Reading his last few comments, he still hasn't connected his effects one at a time to see if one of them- or more- have ground loop problems. If he isolates it to one or more, he could still lift the ground at one end of the cables to see if it makes a difference, or make a bunch of female to male cables with the ground lifted and see if that helps. A/C cables connected to different outlets or branch circuits can also cause ground loops. It sounds like the owner has not been methodical enough in his troubleshooting and documenting his troubleshooting process.
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B
BMG123
Member
Hi Sweetbeats and wkrbee (thanks again for the hook-up on the manual) so reading your responses I ran across a video talking about eliminating ground loop noise. this guy suggested a product made by ifi-audio called a ground defender link: https://ifi-audio.com/products/gnd-defender/
It's a bit pricy at $99 but if it works I thoght it would help clear up some ideas.
The offending device needs an IEC connector. My external preamp and the A/D convertor both have one (and both were causing noise when connected)
First the preamp - worked perfectly! no noise!!
Second the convertor - worked perfectly! no noise!!
Not sure what voodo is inside this little box but it cleaned up those two issues. So it seems I've determined that I am getting a ground loop noise with certain devices.
Not sure if the items in my rack with the patchbay will work because most dont have an IEC plug.
I have a couple 24 channel pathbays. I was hoping to use a single pathbay and have 1-8 for snd/rcv and 9-24 for outbaord gear. I was hoping I coud plug any peice of gear in 9-24 into any channel on 1-8
I doubt this will help but I'll try moving the outboard gear to a different patch bay above the snd/rcv and see if that make any difference.
Any thoughts on that little device from ifi? and, now that we know this fixes the noise, maybe there is something I can do with the studio wiring to fix this problem once and for all?
Thanks, Brian
It's a bit pricy at $99 but if it works I thoght it would help clear up some ideas.
The offending device needs an IEC connector. My external preamp and the A/D convertor both have one (and both were causing noise when connected)
First the preamp - worked perfectly! no noise!!
Second the convertor - worked perfectly! no noise!!
Not sure what voodo is inside this little box but it cleaned up those two issues. So it seems I've determined that I am getting a ground loop noise with certain devices.
Not sure if the items in my rack with the patchbay will work because most dont have an IEC plug.
I have a couple 24 channel pathbays. I was hoping to use a single pathbay and have 1-8 for snd/rcv and 9-24 for outbaord gear. I was hoping I coud plug any peice of gear in 9-24 into any channel on 1-8
I doubt this will help but I'll try moving the outboard gear to a different patch bay above the snd/rcv and see if that make any difference.
Any thoughts on that little device from ifi? and, now that we know this fixes the noise, maybe there is something I can do with the studio wiring to fix this problem once and for all?
Thanks, Brian
W
wkrbee
Active member
As I suggested make a short audio cable with the shield lifted at one end. Find one of the devices that hums, plug it in and see if the hum goes away. Isolate everything in the rack with plastic washers and don't let the chassis touch each other. When I was doing some tech work- time code bleeding into audio at well known film composer Frank Serafine's- RIP- studio, the fix was wooden rack rails, and spacing rack mount stuff so there was no contact between the chassis of the rack mount stuff.
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Mark7
Well-known member
After watching some videos on other prosumer reel to reel multitrackers I think it's a shame Tascam never thought to include a spooling option for tails out storage (something they recommend in the manual).