crackles and pops. re-cap time?--388 related

W

williambslack

New member
I recently opened up my tascam 388 and reseated a bunch of the cards. I highly recommend this to 388 owners experiencing hard-to-solve problems with the machine.

That said, I think that there's still some occasional pops and crackles when the DBX is on (perhaps when it's off too, though i usually use the dbx). I've been thinking that, with a machine this old, it might be smart to re-cap the power supply. Could this potentially solve the problem? As capacitors begin to fail, could they cause these quiet pops?

fwiw, these sounds aren't too loud yet. They're really only a problem during very quiet passages.

Thanks.
 
They're just kind of random or do they occur when you are switching functions?

are the noises present in all outputs and present regardless of the source or the monitor mode?
 
They don't seem to be the result of switching. I think they might be present at all time. I get the impression that they're just inconsequential pops, except if they happen while recording, the get printed.
 
Set up a recording session and get someone to go around the house turning lights and appliances on and off, if you're in a house.

I know I used to get clicks and pops when the boiler engaged, until I rewired.
 
I'll try that---could be the problem, though I'd imagine that the sound would be just one pop, but this wil be a few little pops and a fizz (best I can describe it).

I rent an old farmhouse, and while the wiring is up to date, I wouldn't be surprised if the house's electrical played into it.
 
That does sound to me like a bad cap. You need to isolate where its coming from which can be done by using the routing features and i/o jacks to narrow down the section and how widespread it is.

Obviously if it is getting printed to tape then it is at or upstream from the record amp, and you can work backwards from there.
 
Sweetbeats, I may need you're guidance in isolating the problem.

So, working backwards from the record amp, would I first test signal sent to the PGM INs? These go to the pgm faders, then to the recorder, yes? Is there a more direct path to the recorder?
 
Actually, just start with channel 1 in the mixer section...

1. Plug a cable into the SEND jack of channel 1 and connect the other end to a monitor, monitor amp or another mixer or something that you know operates cleanly. The point here is to avoid the monitor section of the 388.

2. With nothing plugged into the mic, line or RCV jacks set the source to mic and turn up your external monitor level and then turn up the trim level on the 388. Do you hear the fizzy-pops?

3. Switch the source to line...any fizzy pops?

4. Switch the source to rmx...fizzy pops?

5. Do the same for channels 2~8 and document the results.

This test determines if the source of the noise is coming from any of the mic amps or the eq sections on the channels. By testing this way we can rule out those sections and then move on to the fader boosters and buss amps. If all those test clean then it isn't the mixer but something in the play/rec amp section or possibly the monitor section.
 
Sweetbeats, I ran the test. Now, this being an intermittent problem, I'm hesitant to say I found the only source, but I can say I found one problem.

All channels were fairly consistent running your test--the mic mode was noisier, the line and RMX were quieter (this was with m monitor cranked up). Channel 6, however, may have the issue. I almost missed it, but then I noticed that the selector switch for mic/line/RMX had a louder electrical pop then on other channels, so I left it in RMX a bit longer and sure enough the odd pops and fizzes began, along with a faint 60hz buzz. I'm not hearing the pops in Line mode or in Mic mode.

EDIT: I just rechecked the channels and I heard some slight, only VERY occasional pops in channel 1 on RMX. These were nowhere near as consistent or as loud as those on channel 6.

So, what does this tell you? The problem is in the EQ section?
THANKS!
 
No, not the eq section. If it was the eq section it would have manifested regardless of the input source.

Rmx mode sources the playback amplifier.

What I'd do now is swap the record/play amp card that has channel 6 on it for another one and see if the problem follows the card.
 
Follow up:

Circuit-wise there is NO difference between the line and rmx inputs as far as the mixer card goes and that's why I'm looking at the amp card now, because said channel 6 was okay when sourcing the line input. if its okay in line mode it should be okay in rmx mode as far as the mixer channel card. Swap amp card and see what you get.

Its normal for there to be more noise between a line input and a mic input if there is no mic plugged in so don't worry about that. I don't know how MUCH more noise you experienced, however...if you wanted to be sure things are okay you could put the tails of a 150 ohm resistor between pins 2 and 3 of a mic jack and the noise floor should drop to something closer to the line input...or just plug in a mic. The noise will diminish. My hunch is that all is well there and we're chasing a failed component on an amp card.
 
OK, I think I'm understanding.

I'll say this, I think I heard some quieter pops on channel 6 when in Line mode, but it didn't persist or come back, so I can't say for sure.

I'll swap the cards BUT...I'm not really sure how to do this. These are the cards directly behind the channel VU's, yes? How do I know which one, and how do I pull them out? I ask because last time I tried to pull these to have a look at them, I couldn't get them to budge and was scared to force them.

THANKS for all the help so far.
 
OK, after looking in the service manual, I'm pretty sure that I know the rec/play card is one of two on the right side, but I don't know how to pull it. Just force it? there's no way to really get a good grip on it.

THANKS.
 
There are 4 rec/play amp cards. One for channels 1 and 3, one for channels 2 and 4, one for channels 5 and 7, and one for channels 6 and 8. Look in the service manual. There is a page that shows what's what in the cardbay. IIRC there are small holes in the corners of the cards. You can make a bale out of wire to help grab the card like a handle and gently pull the left and then the right and then the left and so on to work the card out of its sockets.
 
Ok, I finally got the cards out--they really took some muscle.

I swapped the CH. 6/8 card and the CH. 5/7 card. After doing this, I first expected to find the problem in channel 5, but I found no audible problem. I checked 6 through 8, leaving each channel on for a while but heard no problems. I then checked 1 through 4, and still nothing wrong.

THEN, I swapped the cards back to their original positions, and the fizz/pop immediately returned to channel 6, and ONLY channel 6.

What does this tell us? I would have thought the problem would have either followed the card to channel 5 or else persisted on channel 6, but not that it would have disappeared, then returned back to 6.

Thanks.

edit: Not sure if this helps or further confuses, but---i left the machine on listening to the send from channel 6 for about 20 minutes. After 5 or so minutes of the expected fizz/pop sounds, they seemed to go away and have yet to return. Is that indicative of anything in particular?
 
Last edited:
remove all the amp cards as well as the dbx cards and see if the problem persists. Give it time. Make sure the machine is cold (i.e. leave it down for awhile) before power it up to check for noise after the cards have been removed. I'm wondering about a hairline break in a trace on the motherboard.

If there is no noise then start adding cards back in one at a time, powering down in between and then checking for the noise in between each one.

If you have another mixer this could help (i.e. connect the SEND jacks of each channel to 8 inputs on an external mixer so you can listen to all channels at once and then SOLO to find which channel or channels is/are making the noise).

When you start getting the noise then gently push around on the card or cards installed to apply some pressure to the motherboard to try and make the noise occur...this *could* help to narrow down if it is a bad trace or bad solder joint.

There could simply be a cold solder joint on one of the sockets for the 6/8 amp card which would involve pulling the bottom cover off the machine and reflowing the solder joints for each of those pins.
 
dang, its getting serious!

so, do i check all the sends with the cards out, then turn off the machine, insert a card, power up and check all the sends again?

When in this process should i add the dbx cards back in?


THANKS!
 
Yeah, that's what I'm suggesting. Pull all the cards, check all 8 sends. Add a card, check all 8 sends...annnnnnnd so on. Document your findings. Doesn't really matter what card you install when in the process. We are just looking to see if the problem occurs with those cards out, and if certain card *changes* things when you put it in. My hunch is that you'll have some random results which points to a broken/intermittent solder joint.
 
You must log in or register to reply here.
Back
Top