Tascam 388 problems Part 2

sweetbeats said:
I'm nervous that if the bias cards were getting overloaded there may be other things as well, so I'm going to look at what else is getting carried over P32 and advise that Cobb hold off...what do you think?

Hm...I wonder...I did press the track arming switches while P32 was disconnected and they didn't all flash...they did until I pressed #8 with 1~7...then they all stopped flashing...then they (1~7) started again when I unlatched #8. urg...I REALLY hope I didn't cook my 388...:(
Click to expand...

Truely My wish as well. Back to basics.

-Ethan
 
I'm having horrible flashbacks of my whole Tascam 58 nightmare...I'm away from home so I can't check the 388's functions out but it is awfully hard to think about anything else...:mad::(
 
Well, no way to know completely since the headblock and half the tape path is off the 388 right now, but it doesn't seem to be totally dead...mixer passes audio and the transport functions work...I notice I can hear electrical noise in the headphone circuit when the capstan first spins up and I'm not sure if that's normal or not...I notice that meters #2 and #8 are NF now (lamps all work but the needles are dead). No way to know if is the result of the refurb process or something that I just now did to it...they worked back when I first got it so who knows...I won't know for sure if everything else works until I get it all back together and go through the calibration procedure...also probably a good idea to scope the bias oscillator and amp circuits to see if its still clean 100k tone coming out of them. *sigh*

BTW my DMM *does* have AC and DC current. My 388 at idle with everything plugged in pulls about 1.3A.

After studying the schematics tonight I can only make some vague suppositions in an effort to explain why the bias amps heated up with P32 disconnected...they run on the same +/-15V rails as the mixer audio circuitry...maybe when that became disconnected but I hit the record enable switches it allowed a gateway to open without a transistorized control mechanism powered and it was able to run away...just a thought. I think the best thing here might be to assume that we are okay to power up the system with cards out but maybe its not such a good idea to pull other power connections...the power to the mixer goes through the Monitor PCB which also has the record enable switches and there is, of course, all SORTS of intermingling there with the bias cards and record/play amp cards etc...the Monitor PCB also has +5V going to it...P32 would cut that source but because the record ready LED's were flashing there was power back-feeding to them and that's where I start thinking that there are limitless possibilities for what started running where. SO....

Any ideas where to safely go from here??
 
Well Cobb that's a tough spot for you then because you aren't responsible and yet you have assumed there is something for which forgiveness is needed. Forgiveness is an action of letting go of a burden of something for which you are responsible which usually takes the form of some sort of transgression, BUT...don't think for a SECOND that I didn't consiously ponder the reality that I was placing my 388 at risk by doing these tests...there is no way to really know all possible scenarios. You ALWAYS expose you and your gear to exponentially greater risk when you open it up and start tweakering and I've been doing this long enough to have experienced some extremes and so I have a visceral understanding of the risk.

I think its going to be fine, and no need to discuss it further even if its not. Tis thread is about YOUR 388 so back on topic, yes?

Please let the worry and concern go...I'D have a hard time knowing somebody else was beating themselves up over risks I knowingly took that were wholy and exclusively mine.

Ethan, you still in this one?
 
I'm still here but battling an Ampex Bias issue. In and out as always.

We just need to find out what is using up so much power that the heatsink gets hot.....

Divide and er succeed.

--Ethan
 
let me know

Let me know if there is any info i can compile or tests i can try and i will do the best i can to do them.

Do you think there is any chance the tech at Tascam might have an idea as to what might be the source of the problem based on the description?
 
That is certainly worth a shot...they may be able to point us in a direction...there is a Jimmy there as well as Jim Finch...you'd likely get Jimmy.

I'll spend some more time re-reading Ethan's thoughts and look some more at the schematics. I'll be back...
 
Nope.

If you send an email the response will be to send it in to the service department. They don't have time really to try and troubleshoot over email. Your best bet is to find a phone and call if you want to try it as you can get a quick real-time dialogue going that may bring up ideas for where to look. This IS a complicated issue though so don't expect a direct solution. We're looking for ideas and the reason I encourage contacting them is maybe they've seen this before.
 
Yes

Yes i feared that was what would happen, but i only suggest it out of lack of phone. Ill figure something out.

Is there any way to tell at this point what is definitely NOT the source of the problem based on what tests have been done? From the point the power cord enters the machine is there any way to go from there forward until the point where malfunction takes place?
 
The AC power from the wall gets converted to these DC voltages by the power supply:

+5V
+15V
-15V
+24V

As far as we can tell the problem resides on the +/-15V rails. Problem is a majority of the unit runs on those rails...they touch just about every system in some way.

If you could get ahold of an AC current meter that might be really helpful IMHO because then you don't need to wait for the heatsink to heat up...it would take the subjectivity out of things. We could see by numbers what happens with the current draw with different systems connected...
 
well

It looks like im going to have to go down and buy a multimeter that can read AC.

Im going to head out in an hour or so and try to get one.
 
Just to make sure, I am talking about AC current, not AC volts. Pretty much any digital multimeter reads both AC and DC volts, but we are wanting to measure the current drawn by the system prior to the power being converted to DC, hence the need for a way to measure AC current (in amps).

Let us know if you get one and if you do try the test again where you put the meter in series with fuse F3 and measure the AC current. There will be a setting the dial probably labeled 10A, and there will be a jack for the red probe labeled the same. If there isn't a jack labeled 10A then you would use the one labeled just A (as opposed to mA or uA). Hey...I know...read the manual. :D That is not a poke at you...I just as I was typing 'duh the guy can read the manual for his meter, sweetbeats...shut it!' :P

Anyway, recall that my 388 at idle draws about 1.3A of AC current with all the innards connected and dbx off. Check yours out and we'll compare notes. Then at some point in the near future I'll post a list of conditions and the current the unit draws (the conditions being what's plugged in and what's not). Make sense? You then setup up the same conditions on yours and post the current draw. We might catch the culprit this way.
 
Epic failure

Well i went down to radioshack... they didnt have any good multimeters. Not only that, they didnt really seem to care whether i lived or died.

Im going to have to order one online i guess, or maybe plan a trip out to a better electronics store.

Can you recommend a decently priced multimeter that does what you are asking as well as being true RMS?

I did get alligator clips though.

BTW i have another question about fuses. In the 388 what is that fuse type thats in the very bottom position marked "F1"? It doesnt look like a normal fuse even though it is rated 4A 250V. Can any slow blow 4A 250V fuse work as a replacement?
 
4A 250V slo-blo is what you want in there...as long as it meets those specs it is fine. There are different brands so thy are bound to look different.

If you are going to get a new meter (and I suggest that...I got a used meter once to save money...ended up needing repairs...it works now but nothing more frustrating than having to fix the tools you need to fix the gear) it will set you back about $70 +/-...Parts Express has a couple that'll work (like this) and you can find plenty on eBay...just do a search for true RMS (like this, or this or this one...) In that price-range they are all basically the same thing...some I think just have different skin on them. **Note that they are all handy devices with AC/DC current, capacitance, frequency counter, etc., but they aren't suitable for measuring signal over the audio spectrum. IOW you won't be able to use these reliably to measure signal strength over the audio bandwidth (20Hz ~ 20kHz) as they are typically rated to be accurate only over about a range of 40Hz ~ 400Hz...this is actually not a total loss as it will work for basics on the 388 which uses the 400Hz tone as an anchor fequency for calibration, but in general it is better to get something tha can handle audio bandwidth. Here is my top bargain choice for a DMM that is pretty accurate over the audio bandwidth and beyond...Fluke reliability to boot: Fluke 115.
 
Here is a list of some different AC current draw measurements on my 388. Compare to yours when able...these are all with an AC current meter connected in series with fuse F3 as described by evm1024 earlier.

  1. Everything connected (all cards plugged in), dbx OFF, REC FUNCTION switches off = 1.34A
  2. Same as #1 but with dbx 1~4 and 5~8 switched ON = 1.37A
  3. Same as #2 but REC FUNCTION switches 1~8 switched ON = 1.47A
  4. Everything connected except rec/play amp cards (all other cards plugged in), dbx 1~4 and 5~8 switched OFF, REC FUNCTION switches 1~8 OFF = 0.95A
  5. Same as #4 except dbx 1~4 and 5~8 switched ON = 0.98A
  6. Same as #4 except dbx 5~8 card removed = 0.82A
  7. Same as #6 except dbx 1~4 switched ON = 0.84A
  8. All cards removed except the PSU PCB (duh) and the Meter Amplifier PCB, dbx 1~4 and 5~8 switched OFF, REC FUNCTION switches 1~8 OFF = 0.62A
  9. Same as #8 except dbx 1~4 and 5~8 switched ON = 0.65A
  10. Same as #9 except REC FUNCTION switches 1~8 switched ON = 0.76A
  11. Same as #8 except Reel Servo and Balance Amp PCB's installed = 0.67A
  12. Same as #11 except Bias PCB's 1 & 2 installed = 0.69A
  13. Same as #12 except tracks 1 & 3 rec/play amp card installed = 0.80A
  14. Same as #13 except tracks 2 & 4 rec/play amp card installed = 0.90A
  15. Same as #14 except tracks 5 & 7 rec/play amp card installed = 1.00A
  16. Same as #15 except tracks 6 & 8 rec/play amp card installed = 1.10A
 
ok

Got the meter, thanks again Sweetbeats for your help.

I should say, i dont know if its normal or not, but as i did these tests, the measurements would slowly drop as i sat and watched. Like at first it would say .999, then after about 15 seconds it would say .998, 15 seconds, .997 etc. They seemed to continue to descend the longer it was left on in that same fashion.

1. 1.29
2. 1.32
3. 1.41
4. .93
5. .96
6. .81
7. .83
8. .62
9. .66
10. .76
11. .67
12. .69
13. .78
14. .88
15. .97
16. 1.06

I dont know if it matters to the readings or not, but VU meter 7 lamp is out on this machine.
 
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