388 troubleshootin'

  • Thread starter Thread starter CAZ-PNX
  • Start date Start date
oh, and swapped the rec/play pcbs, too, with the same result.
 
...

Try bussing one test signal from one channel to all 8 busses, enable all 8 tracks and record a test tone to all 8 tracks simultaneously. Rewind and play back. Determine which tracks play back. Then carefully dismount the tape, mount it backward and play back again, noting which tracks seem to play back or not. This will give a solid determination about which tracks are recording AND/OR playing back properly.
:spank::eek:;)
 
that's the method i've used most frequently (1 signal to all busses). 1-4 do not print, but when the tape is flipped, the original recording on 5-8 will play through 1-4.
 
He already did that, Dave. We've narrowed down the issue to be a record issue with tracks 1-4. Playback is fine with the tape flipped.

CAZ, sit tight...the dbx card swap was just off the top of my head. I haven't pulled out the block diagram or schematics yet. I have to look and see where tracks 1-4 are grouped electronically and then go to the beginning of that chain and come up with some diagnostic tests that you can do. Like I said earlier the amp cards are NOT broken down into 1-4 and 5-8 groupings...they are odd and even pairs which is why I figured it wouldn't make a difference swapping them...i don't think your problem is there. So I need to see where in the signal flow the signal is grouped into 1-4 and 5-8 groups and, IIRC the dbx circuit is one such place so that's why I had you do the shotgun step of swapping the dbx cards.

Three more "shotgun" steps to try whilst you wait for me to look at the geek docs:

1. If you haven't done so already, try recording with dbx engaged (you said earlier dbx was out). I don't expect this will make any difference, but it may be handy for ruling something out when I look at the geek docs.
2. Confirm for me that when you record-enable tracks 1-4 in the master section that their red "ready" lights blink when in ready mode, and that they go steady ON when the transport is rolling in record mode.
3. Try swapping the bias cards.

Report back.

Do you have a multimeter?
 
-i've tried dbx on and off, it doesn't make a difference.
-the indicator light blinks when enabled, goes steady upon record. the 1-4 VUs also respond while recording (don't know if that tidbit is handy).
-i'll try swapping this bias cards right now.

yes, i've got a multimeter handy.
 
Okay. Thanks for the info. Sit tight. I'll try to peruse the geek docs tonight.
 
The next step is to make sure that power is geting to the dbx card for tracks 1-4. Remove the card and, with the machine powered, measure for DC volts between pins 2 and 3 (should be about +12V) and pins 3 and 4 (should be about -12V) of P112.

This may be difficult because its so tight in there...you need to be ABSOLUTELY certain you can safely probe those pins without touching anything else...I'd be making an extender cable so I could remove the card, plug the cable in and then be able to just probe the other end of the extender cable. In that way I'd avoid having to probe down amidst the other cards.
 
thanks for the extender tip, don't know if i woulda come up with that. as for the results (drum roll...) between pins 2 & 3, i'm reading +12.28. with 3 & 4, i'm getting nothing.
 
Okay. Check on the back-side of that mother PCB where the pins of P112 are soldered and see if the -12V is present there. Keep probing the traces until you find the -12V alive and well and that's where the problem is. Maybe cold solder joint or broken trace.
 
okay, the back side of the board (on the pins) reads roughly -12V. i read it from the top of the board again, though, and no dice. 0V.
 
I would then re-flow the solder joint for the pin and and add a little solder to the joint, then re-test.
 
i tested it one last time before i started to disassemble that motherboard, and i got pins 3 & 4 to read around -12V. argh! i'm also looking over the manual's schematics, and based on your diagnoses, am better to able to understand them (i've always wanted to, but have never dabbled in this arena). thanks for your patience, hah!
 
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Disassemble the motherboard??

There shouldn't be anything to disassemble...:eek:

Just put one probe on a ground point and then go from the solder land of pin 4 of P112 and go to the next land like a dot-to-dot until you get -12V.

So are you saying you probed pins 3 and 4 of P112 and and now got -12V? My hunch is that you've got a broken solder joint there and just need to re-flow the joint.

Do you have a soldering iron and/or experience with soldering?
 
i was going to remove the motherboard from the unit to get to the top side of the pin. that's what i meant by disassemble, but there shouldn't be any need for that, if you're saying to reflow at the joint. since i WAS getting a read from the bottom, and originally WASN'T getting a reading from the top, i thought that's where i needed to focus. i don't have any experience with this stuff, but am eager and know that one has to start somewhere.

yeah, i'm saying that i got a -12V read from the top on 3 & 4 when i checked today.

your hunch was correct before, so i'm out to pick up a soldering iron today.
 
Okay. Yes. Ask those questions before you dive in (like "I need to remove the motherboard?"). Always remember that ANY time you remove/disassemble something you are inviting the potential for component stress and failure, incorrect reinstallation or fault during the process. Always do just the minimum necessary. The question "what do I NEED to do?" should be the first step in the process.

Get an iron with a small tip. High wattage is not a sign of a better iron. The higher the wattage the faster the heat can replenish in the tip, but that is not critical and actually potentially a defecit with component work on a PCB. Wattage does not determine tip temp. 15W is fine. Get something with a small tip if you can.

Do a search on the forum for "soldering" with me as the user...there is a thread I put up with helpful tips for soldering.

Minimize the time your iron is on the work. Melt the joint, add just a little solder to the joint (touch your solder to the tip while its in the molten joint)...done. Takes just a few seconds.
 
alright, operation: solder was a smashing success. thanks to your video and handy-tips, it was easier than easy. those pins now read -12V all day long. unfortunately, the deck still only works for 5-8. any other avenues to pursue?
 
Well, getting power to those P112 pins was step 1. Next is to confirm that power is propagating to the card. I have to go back to the geek docs and figure out the best way to do that since I don't own a 388 anymore.

Hm...maybe I have a dbx card tho'.

Stay tuned.
 
Yep...got one.

So with the card plugged in and the unit powered up put your ground probe on the staple jumper W62, and put your + probe on the staple jumper W63.

W62 is located to the right of capacitor C310 right at the top of the card, and W63 is located between capacitors C310 and C410.

You should get -12V.
 
What about the dbx 1-4 switch? Is it possible that the problem lays somewhere within that switch having lost its ground or something silly like that?

Cheers! :)
 
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