Tascam 388 VU meter

Zonderplan

New member
Hello,

First of all, lovely forum this is, hopefully I can contribute anything usefull myself in the near future.
I was for now hoping maybe someone knows something about the following issue.
I just got a Tascam 388 which is in good shape, as far as I can tell at the moment. I'm really excited about that! The only thing that's weird is the VU meter of channel 5, which when I switch on the power, moves all the way to the right and stays there. It does'nt respond to any input, also it doesn't blink the red light saying Peak. The unit is very quiet, and I checked the channel and it's output with a frequency analyser, to see if there's maybe something going on outside the audible spectrum. It doesn't show anything. All in all it seems nicely quiet, so I assume there is not really any signal going on. I am still awaiting my new tapes to arrive, so I haven't tried recording to tape yet. Does anyone maybe have an idea about why this VU meter straight away moves to the right when I turn my unit's power on? Could it be harmful if un-treated? Somehow it feels worrysome, even if all in all it works correctly furthermore.
Thanks for the time to read this, and I'd love to hear any thoughts,

Happy recording! And happy holidays!

Jan
 
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Hi Chris! Thank you so much for your reply! I'm not the most technical person, but what you say sounds very much like it's the problem..! I'm trying to locate where the specific part sits exactly, in the manual. I'm not sure if this would be super advanced to operate on myself, otherwise I'll get someone confident involved to do it for me.. Pointing out this specific part is a great help, so again thanks! Finding it in the manual is a bit of a challenge I notice, I'll post on how I progress once further action is taken!
 
You're welcome! It could also be the other components around the OP, like resistors and also bad solder joints, or loose cables.
 
The first thing I would do, if you haven’t done it already, is remove the meter amp plugin board, inspect the solder joints of the connectors on the board for damage or misalignment, and also look down at the pins on motherboard where it connects and look for bent pins. If all looks good then reseat the board and check again to see if that resolves your problem. Take care in reseating any of the plug-in boards...this is when pins get bent and connectors broken.

The meter amp board is in the cardbay in the far left bank, the front-most board in front of the dbx channel 1~4 board. You will need to remove the dbx board and maybe one or both of the rec/play amp boards behind it to access and remove the meter amp board. See pic attached below for reference.

If all the connections look good, then it may very well be a bad opamp driver on the board for that meter. The fact the board is a plug-in type makes it much easier to service, but if you don’t have a lot of experience removing opamps and soldering in new ones you may want to get some assistance or practice on a scrap board. The plug-in boards in the 388 use phenolic resin board material...perfectly common, but not as robust as glass fiber, so some care has to be taken with how much heat and for how long you apply heat when removing or installing components...traces and lands can lift off the board and then have to be repaired.

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Hi Sweetbeats! Thanks so much for the detailed information! Definately helpful that you pointed out the location! I was kind of lost in al the scematics of the manual and all the tiny numbers on there. I will try to work up the courage to open the machine and see how it feels removing boards 'n all, I'm a bit nervous haha..! At the same time, there has to be a first for everything.. My soldering skills are not impeccable, so practicing is not a bad idea, I might get someone involved if it looks neccesary! Again, thanks a lot! I'll post the results! Cheers!
 
Thanks wkrbee! That sounds intriguing! I'll look into the meaning of that! I'm not the most technically schooled person...
 
All he’s saying is it sounds like there is direct current (DC) voltage in the signal to that meter. Audio signal is comprised of alternating current (AC) voltage. A blown opamp could generate DC at its output. Typically, in a system design like the 388, there are capacitors in series at the outputs of opamps. Capacitors block DC and pass AC. There is typically some “DC offset” at the output of an opamp depending on the circuit design, but if the opamp is blown and outputting large amounts of DC this cap (a “coupling” capacitor) may not be able to filter it, or maybe the cap is also blown. So...we don’t know. I would still start with the steps I stated earlier.

As far as removing the cards in the cardbay, remember they are meant to removed and reinstalled. They may just present as a bit stubborn, and you want to take care when reinstalling to ensure they are aligned on their connectors before seating them.
 
Meters only get a single polarity Dc signal and it is often very small. I would have a look at the DC output level of the op amp to see if it is not near a rail voltage.
NJM4558 and other 4558 op amps are powered on pin 8 plus and pin 4 minus or may be ground. If the meter is getting pegged you do not know how much that is and could damage the coil. I would investigate this quickly and fix it or disconnect that meter while the problem is there.
 
Electrolytic capacitors tend to get blamed first for all sorts of faults but they are generally very reliable, especially in small signal 'cool' circuits (an exception is old Tantalums.ALWAYS have them out!)

Sweetbeat's idea of 'dry' joints is the best one IMHO because this could lead to the circuit going ape***t AKA 'instable' and cause full deflection of the meter. A clue would be if the needle is trembling, if dead steady on the end stop, likely DC.

Dave.
 
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