Tascam 388...Sounds like a ground hum?

B

BMG123

Member
Hi all, first time asking a question here. I've recently acquired a 388 and have been going through the deck cleaning and demagnetizing heads, cleaning pots and switches etc.
I have what seems to be an intermittent hum that sounds similar to a ground loop hum. It gets louder with raising the master fader and is completely out when the master is pulled down. I say intermittent because there have been times when the "noise" is barely there but if its been on for a while it gets up to a consistent volume.
I hear it in the headphone jack and stereo outs through monitors.
Things I've tried: 1.flipping the plug 2. running on a different circuit in the room 3. plugging into a power conditioner.

I'm starting to think this is not a ground issue but just sounds like that.

I'm planning to use the mixer portion as 8 mic pres for both tape and to the computer with the PGM outs to a set of convertors.

I just need to get this thing quiet and any help/insight is GREATLY appreciated.

All my best, Brian
 
Hi, Brian.

Do you have access to and know how to use an oscilloscope? When I have hum like that, like when nothing is connected to the device, the first thing I do is check the power supply. The purposes of a regulated DC power supply are to take AC power, rectify it to DC, and ensure a constant DC voltage. One sign of a failure of a regulated DC power supply is hum. This happens when the rectifier or part of the rectifier fails and no longer converts the AC waveform to DC, so you’ll have a partial waveform that gets through the rectifier and the amplitude is too great for the regulation circuit to filter. Scoping the output of the audio power rails and looking for AC at the outputs tells you there is a power supply failure if found. You *can* also use a multimeter. This is not ideal, but if the failure is bad enough (and if the hum is caused by a failed rectifier it usually is) a multimeter is usually sufficient. So if using a multimeter you set it to AC volts, and measure for AC across the +15V output and the 0V reference (ground) at the output of the power supply, and then do the same for the -15V power rail. Those measurements should be at or very close to zero volts AC.
 
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Sweetbeats, that description makes perfect sense. Thank you for the detailed description! I have a buddy that can bring an oscilloscope to check the power supply. If we find find a failure in the DC power supply, is there a fix? or is the best/only option to replace the power supply? I don't know how available a power supply is for a unit of this era.

Thanks!
Brian
 
BMG123 said:
Sweetbeats, that description makes perfect sense. Thank you for the detailed description! I have a buddy that can bring an oscilloscope to check the power supply. If we find find a failure in the DC power supply, is there a fix? or is the best/only option to replace the power supply? I don't know how available a power supply is for a unit of this era.

Thanks!
Brian
Click to expand...
Yeah it’s not likely you’ll be able to easily find a complete replacement power supply. But it is a plug-in card…super easy to remove and service. The trick will be diagnosing what has failed because it’s difficult to probe components when it’s mounted. You can tip the 388 on it’s side, remove the bottom panel and IIRC once you do that you can at least access the solder joints for the power supply outputs via the motherboard connector, so that’s how you can probe with the scope to check for AC on the +/-15V rail outputs…you can find the pinout in the service manual in the schematics. But in-circuit diagnosis and troubleshooting are more difficult unless you have the unobtanium extender card, or make a set of extender cables which is what I did back when I had my 388. Short of that, if you have significant AC in your audio power rail outputs and you want to take the shotgun approach is start by replacing the bridge rectifier…it is a single part on the board (as opposed to four discrete diodes), and if that doesn’t do it then recap it. A recap is kind of a good idea at this point anyway FWIW.
 
Perfect! My buddy with the oscilloscope will be more familiar with this process. I'll forward him your message and we'll get to work on this. I'd like to recap as much as we can get to just to get things back to when it was built and avoid chasing any problems because of a dried out capacitor. Am I correct in thinking we should pull the cards behind the VU meters and recap those too?
 
Don’t go nuts with the recapping. Look at some of the cards. If you see bulging or leaking caps then it’s definitely time. Power supply caps experience more heat and stress than most of the other caps in the unit, so their life cycle is shorter. So it’s always a good idea, if you’re going to recap, to start there. And in your case it may be necessary. But I’d just start there. Sometimes you might be surprised how much of a noticeable drop in the noise floor you experience with a power supply recap.
 
I hope that's all it takes! I'm in my 50s and was lucky enough to have a dad that was into recording when I was a teenager. He bought a 388 new and in 1985 I was the coolest kid in my circle of band friends. I never got rid of those tapes and have enjoyed hearing all the stuff we recorded over 30 years ago! It's so nostalgic to use this machine. Just the sound of the transport brings back memories. I'm hoping to make this the centerpiece of the studio and use the 8pres even when recording into the computer. I know there are much better options out there but for me, they sound just fine. I really appreciate your time and detailed response to these questions. You're a wealth of knowledge and I feel lucky you saw this thread! I'll let you know how things go and thanks again, Brian
 
Well Sweetbeats nailed it again! I recapped the power supply card and the 388 is silent. I cranked the apm and there is no noise...zero...none!
Thanks for the diagnosis and appreciate your time on this forum.
Brian
 
Well that’s fantastic!

Thanks for the follow up.

So there’s only one other time I’ve run up against that issue and it was with the power supply on a vintage CRL analog stereo multiband dynamics processor I have…I got it for relatively cheap, which usually means one thing: needs work. It did. It didn’t pass signal. I found the culprit: a bad source switch which switched between the input terminals and the internal noise generator used for calibrating the unit with the rest of the broadcast preparation equipment. Replaced the switch…passing signal only with a horrible hum. Otherwise it seemed to work. It’s really nicely built and designed for serviceability…easy to isolate the power supply. So I did that and put the scope on the outputs of the +/-15V audio power rails. +15V was fine…-15V was running about -9V and had a horrific amount of AC components at the output. I thought “Ah…easy. Needs new rectifiers.” So I replaced all four diodes…still bad. So I dug a little deeper and started tracing with the scope. It was bad right after the main filter cap where it should be a lot cleaner. And next came the regulator. It didn’t make sense that it could be dirty upstream of the regulator if the regulator was bad but I was thinking in a box and I’d had regulators go bad, and the caps looked good and the +15V supply was fine…well, long story short I replaced both regulators and of course it was still bad…I was going to recap it anyway so I did that and *boom* it was fine. Clean outputs, clean sound. I didn’t understand what was going on though so I reached out to a smart friend and he surmised the main filter caps were a bit smallish in capacitance to start with, and the one on the -15V rail drifted enough off spec that it couldn’t keep up. If I understand correctly filter caps work like a balloon on a T on a garden hose. Like, cut a garden hose, and insert a T…put a balloon on the T, and now start turning the water on and off and on and off. The balloon expanding and contracting will smooth out the pulsing of the water on and off at the end of the garden hose. Try it. But now if the balloon is too small, and can’t absorb the pulsing, the pulsing will be evident at the end of the hose. So that’s like the cap that’s too small in capacitance. So when I recapped, I increased the capacitance value with the caps I used to replace the factory caps. Maybe that’s what happened with your 388 PSU.
 
Could be...That's a really interesting story. It's also why I'm fascinated with vintage electronics. I think it's great that we can trace all this down and get things working like new. My buddy keeps a decent inventory of caps in stock. We matched everything with original specs except the 25v caps (four of them) he had higher value 25v caps and we decided to try that. This machine is now shockingly quiet. It's a real joy to use. I really did a number cleaning all the pots. those stacked eq pots were slow and felt a bit gummy. I had to repair a couple faders as well. I'm guessing something got dropped from above on a couple sliders (tracks 1 and 4) and the rails along the bottom of the fader got partially pushed away from the metal housing. so I carefully pried back the tabs and repositioned the board back in place and they work perfectly.

Now the faders are great, the eq works perfectly and I'm listening to these old tapes!

Trying to understand the send and receive on the individual channels. Unless I'm doing something wrong, it seems as if I can only affect the signal on the way in (in mic mode) If I try to apply effects on play back (rmx) it's not affected . Does that seem correct? I was hoping to add a compressor to individual tracks after recording if possible.

Id LOVE to get my hands on an actual manual but cant find one for sale anywhere!!

Thanks again for all your input and if you have any insight on the send/rcv I'd appreciate it.
Brian
 
The SEND/RCV access points are post input select and post EQ. You should be able to apply insert processing regardless of the channel source (MIC, LINE, RMX). I suppose somebody could modify things so in RMX it bypasses the access points, but that would be complicated because the source select switch is ganged and what follows is a singular signal pathway. I’d have to study the schematics, but I’m pretty sure active components would have to be added. It wouldn’t be as simple as adding some jumper wires. Let me ask you this…is the EQ section active when in RMX mode?

Just keep checking eBay and Reverb for a manual. They do turn up every now and then.
 
Thanks! This could very easily be operator error and I just didn't have sends routed correctly. The EQ is definitely active in rmx mode. Having the bottom off and extensively looking around I don't see any sings of modification. I'll hook up an obvious effect (like reverb) and make sure I've got things sending and receiving right and see if it is applied on mic, line and rmx.

I doesn't seem intuitive to only apply an effect on the way in. That's really commiting to your source. I'm sure its just me but now I'm more curious than ever to get things right and ill check back in with you. Thanks for riding along with me on this!
 
I think I have the very same problem with PSU noise. I’m trying to follow your instructions to diagnose. Any tips on how to safely remove the modules?
 
Thank you so much! I have a Fluke 87V and an old analog scope. I've been trying to find a video on youtube showing something similar to what you are describing to use as a model. No chance you know of something that might serve as a useful video guide?
 
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