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

  • Thread starter Thread starter williambslack
  • Start date Start date
Sweetbeats, THANKS for the pic of the signal chart. Now I see that page in the manual.

A new development:

All play/rec cards and DBX cards are still out. The problem is still occurring at the channel sends, and at the stereo outs. I checked the monitor outs and headphone outs, and those ALSO have the problem. But, when I rechecked the tape outs, i had a new issue. Before, when checked, the tape outs were quiet, free of crackle and pops; now there's just a loud 60hz ground sound coming from these outs! Any ideas why this would be?

My notes aren't great, but I'm pretty sure I tried the tape outs before with all cards out and that this is a new development.
 
I suspect that something is dying in the power supply. If it were me I'd. Be isolating the power supply (and you have to make up a set of extender cables to do this on a 388) or take it to a trusted tech and pay the price.

I bet you hear something bad at the buss out jacks too?

Too global and changing too fast to be considered a small problem anymore and diagnostics always start back at the power supply.

Sorry man...do another quick check that includes the buss out jacks if you want but you'd do good to leave it powered down until it can be setup proper to check the supply.

Other thoughts anybody?
 
Hmm, the extender cables are probably over my head--are they easily explained?

I'm happy to take it to a tech, but I'm out in the country, so it probably won't be easy to find someone.

Would it be worth me trying to recap the power supply card and hope that does it, or is that a foolish, shot-in-the-dark fix?

Should I continue testing with all the cards out, or re-install them?

THANKS for all the help, Sweetbeats.
 
I can point you to some links of thread posts on the extender cables and you can decide.

It wouldn't hurt to recap the supply...may take care of it, may not. My concern is that it could be a bad rectifier, bad regulator, WHO KNOWS. And until you isolate and test the supply you really ARE just shooting in the dark.

As far as cards in/out/further testing? Like I said, if you test the buss out jacks be brief, but if it was me I wouldn't be turning it on again before I could isolate the supply. If you DO decide to test further I'd do it with the cards out.
 
Yeah, man, if you can point me to those threads I'd appreciate it. I'll search some in the meantime.

I did test again briefly and the noise is at the bus outs as well. The tape outs are still making the nasty 60hz sound.

Is the concern that the power supply could hurt something else in the machine?

I've got a call into the only electronics repair I could find in the area, so we'll see.

Anyone know of a good audio electronics repair person in Ithaca, NY? If not, maybe Syracuse?

THANKS again, Sweetbeats.
 
Yeah, man, if you can point me to those threads I'd appreciate it
Click to expand...

Tascam 388 problems Part 2

The extender cables bit starts on about post #24.

Is the concern that the power supply could hurt something else in the machine?
Click to expand...

Sort of...power supplies essentially do three things: convert, filter and regulate power so that it is clean and appropriate to the circuits being powered. In the case of the audio power rails on the 388 the power supply and main transformer work together to convert your 120V AC household power to regulated +15V and -15V DC.

My concern is that some part of your power supply is unable to do its job and therefore everything downstream on that rail or rails is getting power that is not rectified (has AC components in part or outright), is not regulated (regulator is toast and 15V would now be in the 20-some-volt range), or is not filtered well (bad cap) but based on the symptoms you are describing my first suspect would be a bad rectifier.
 
Thanks, Sweetbeats. Hopefully this helps whatever tech I find.

If I understand your explanation right, any unrectified AC might explain the 60hz sound at the tape outs, yes?
 
Mmmmmmmayyyybe...i don't know for sure and I wouldn't get stuck on that. You've just got something global going on and checking the power supply is a good place to start imo.
 
I would watch out for fractured / broken carbon film resistors on the circuit cards, I had one that was cracked on my TSR8 playback channel causing various pops & hiss, replaced it & machine worked fine afterwards.

Also checking the resistors & other parts properly with a magnifiying glass or jewellers loupe will help find problems quicker.
 
Thanks for the advice, Trance. It's at the shop, now, so hopefully I'll get some good news soon.
 
Hi. I am getting pops and crackles, even a squelching sound when sweeping the EQ knobs. Is this just dirt or something else? Thanks!
 
I've experienced that squealing sound when sweeping eq knobs before. Iirc I was able to abate it by cleaning the pots well (jetting DeoxIT FaderLube into the pot body from underneath, sweeping the pot, letting it drain and then repeating the process one more time). I also recall never having this issue after a recap.
 
Well, I finally heard back from the tech. He never heard the problem I was having, which seems crazy, but that's the frustrating reality of intermittent issues, I suppose.

Upon visual inspecting inside the unit, he DID find bad solder joints on the buss pcb that connects all the mixer/channel cards. Does it make sense that this could be causing my problem? It seems to me that it would explain the intermittent and hard to track nature of the problem. He also said that some of the caps on the power supply pcb were "starting to get high ESR," whatever that means.

What do y'all think? Does it sound like he's got the problem targeted? Looks like I'm out $120 for repairs and parts, which ain't bad if it's working, but I'm worried the problem will persist.
 
The price is reasonable imo.

"ESR" is the abbreviation for Equivalent Series Resistance. It is really the true measure of the condition of a capacitor and you need an ESR meter to measure it. Every capacitor has a specified ESR value. If a cap is out-of-spec (if its ESR measures higher than spec), its time to replace it. As the ESR of a cap increases the high frequency performance decreases. Low ESR is better. For instance a filter cap with lower ESR will do a better job filtering out unwanted high frequency noise; an audio path coupling cap with low ESR will do a better job passing high frequency audio content. You can measure the capacitance of an old cap and it can measure in spec all day long but if you measure the ESR it might be way out of spec. Time to replace. In the case of your power supply caps measuring high that could arguably lead to a higher noise floor in the audio systems of the unit. Not crackles and pops though.

Am I correct that when you were doing your testing you had NOTHING else connected to the 388 except for your cable from (for instance) a SEND or OUT jack to your monitoring system?

Did the tech reflow the faulty solder joints?
 
Sweetbeats,

Yes, I followed your instructions before, disconnecting everything from the unit except for the send (or monitor, or whatever i was testing) to my monitors.

They did reflow a number of bad joints apparently. They said they never heard the problem, but did see, on internal inspection, potentially failing joints on the bus strip that the channel cards go into. Hopefully this takes care of my problem. If I remember correctly, you mentioned that a broken joint on a pcb could explain the intermittent nature of the problem, yes?

As long as they had it, they replaced the power transistor that had gotten hot, the caps with high ESR and all the filter caps on the power supply pcb, which ended up being $140 or so. Seems reasonable to me.

Well, I just gave it an initial test, and so far so good, but who knows. I'll say this: monitoring the channel send in MIC mode (which never really had a problem before) seems noticeably quieter than it used to be.

fingers crossed that they fixed it.
 
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