Tascma 388 Reel Servo PCB issue

[Psychotron1976]

I'm currently restoring an old Tascam 388 which has been not love for a long time. It have a lot of positive things (heads in very good condition, mixer sound great except some noisy EQ (TL072CP to change I'm pretty sure) but the transport is unfortunately not working.

I think I solved the first issue with the control of the Solenoids due to a strange cut on one track - Servo Ground - of the Mother Board 2 (even if I don't understand how it happened):


I said "I Think" because I found another issue on the Reel Servo PCB when I unload the cards connected to the Mother board 2:


The transistor Q141 had burnt completely, taking with him the resistors R155 and R154 and the transistor Q142.



My guess is that one of the motors shorted and burned the Q141 transistors with him. Does it seems possible for you?
I will change the burned components but can I just reinstall the card and test the transport as it is or does I need to check if one of the motor is not dead before to avoid new burning? I already check the 35v and 15v power rail and they work correctly.
Thanks in advance for your help!

 

[sweetbeats]

Motors don’t typically just fail that way.

There is no way to know what is wrong with your 388 from the information you provided. We can make guesses all day but that won’t help.

Your “strange cut” is a melted solder joint…a failure super-heated the solder joint and melted the solder. Is it related to your Q141 failure? Not sure. If there was excessive current to the servo ground you may want to check the power transistors Q3 and Q4 on the TR PCB which is on the backplane…the transistors are mounted to the heatsink.

The thing that’s frustrating about the 388 is there is a lot of intertwining of circuits across multiple PCB assemblies, and it’s hard to troubleshoot at times because often times you can’t isolate assemblies without damaging others.

AFAIC it’s completely unknown the extent of the damage. It’s likely not just Q141 and R153 & R154.

The other thing that’s a challenge with the 388 is there are so many errors in the schematics. Look at the schematic for the MOTHER 2 PCB…it shows nothing connected to pin 3 of P13. But clearly on the schematic for the REEL SERVO PCB pin 3 of P13 (J104) is the servo ground.

Your “noisy EQ”…what leads you to believe the 072 opamps are the cause? What kind of noise? The 072 is a quiet enough part. I don’t think I’ve ever thought “that’s noisy…I’ll replace the opamp.” If you’re having noise problems, I’d start with checking the audio power supply rails for AC ripple and noise. It may be time for a recap.

 

[Psychotron1976]

Hi sweatbeats,

First, thanks for your detailed answer, I really appreciate!

Regarding the Reel motors, I will check them to be sure that they work (I feel like one is not but it is just a guess so far). But it is important to note that the capstan motor is working when tape is loaded and it uses the 35v power rail. I tested the transistor Q143 with my Peak DCA75 and is working correctly as the diode D141. Looking the schematics, Q141/Q142 (which like Q141 is KO according my DCA75) "merge" the +15v and +35 power rails depending the Low Speed state and send the result to the Reel motors + pin. I will check again the +15v NST to be sure but if it works, it means that before Q141/Q142 there is no clear issue. This is why I'm wandering about the motors.

For the weird cut on the Mother Board 2, I checked the concerned pin and I have seen not trace at all of heat on both cards. Following your advise, I checked Q3 ad Q4 (and almost all the transistors on the TR card) and they seems ok so far with no trace of heat either. The only thing puzzling me is that, if I understand correctly the schematic, they control also the Reel motors (at least the direction) and use Servo Groung (as Reel Motors -) linked to the concerned damaged pin. So far, I really suspect the left motor to be dead (so during a rewind) shorting Q142/Q143 with Servo Ground. But, I agree, this is a wild guess at this stage.

The issue related to (some) EQ seems not a power supply issue for me as I have only 3 (on different channels) of them which make a weird whining noise when the pots are not in the middle. I have seen in the past dying op-amp making whining noise but I'm maybe wrong. Nevertheless, the issue seems not severe. I will try to check one at the oscillo when I will work on these cards. And furthermore, I will recap the 388 completely (I just received most of the capacitors). Just I want to diagnose all the issue related to the transport before to burn anything else.

 

[Psychotron1976]

I decided to make a test with the Reel Servo card inserted without the Q141/142 as, from my understanding, they only play during Fast Forward/Rewind and the 15v is "working alone" in Play mode. Until now (I have this Tascam for only a week), the transport didn't move as the solenoids. Its seems I was (at least partially) right because now the solenoids are engaged and Play is working (due the the Servo Ground fix)! I try to Fast forward and it "works" at play speed (because the 35v in not "mixed")! But not the Rewind... I really think that the rewinds motors is dead but I will check it after I removed it.

The only thing is that I googled the 2SB507 transistor and found some people talking about shorted case. Initially I thought about a motor failure burning Q142/143 but maybe it's the other way around. To be safe, I will change with fresh one the two others (in TR and Control board). Obviously, I need to make more test, fix and check but I'm quite happy to see the transport moving!

 

[Psychotron1976]

I made a lot of progress with my 388. Now the transport is fully functional (I found several issue linked to the 2SB507 failure that I corrected). I have changed all the electrolytic capacitors and fix an issue with the meter 3 due to a dead 1K60 diode. It play recorded tape correctly (I have a previously recorded tape even if it make the heads dirty quite quickly. At least it help me to check the good play ability of my 388) but unfortunately recording is not working as expected.

I have issue with 3 channels:
-#1 erase correctly but records quite low and distorted.
-#3 erase correctly and records but with a lower volume and sound quality that expected but not as bad as the two others.
-#7 do not erase and records quite low (lower that #1) and more heavily distorted.

Looking the meters, signal at the PGM out seems ok. I switched the Rec/Play cards and the issue remains exactly on the same channels so it's elsewhere. I even play the tape backward and the result was the same (but with channel reversed). I will switch the DBX card to be sure but I think some of the issue (especially 1 and 7) could be linked to the Bias Amp. To be honest I'm not sure how to check Bias Amp.

But first, I would like to check the signal at the output of the Rec/Play (especially for #3) but bias make quite hard the reading at the Oscillo. I will need to kill the bias to do so but is removing the bias boards is safe?
Secondly, the test points on Bias is before the Bias Amp. Is there a way to check if the Bias Amps are working as expected?
Thanks in advance.

 

[Psychotron1976]

Ok, I switched the DBX card and no impact. I also looked the level at the PGM outs and all is ok for all the channel (as indicated by the VU Meters).

I tried to setup the bias level at 110mv (following a post of sweetbeats for the LPR35 tape I'm using, I hope to have understand it clearly) with a 12,5k signal send at 0db on the VU meters (I have calibrated them before). And I have two new issues:
- on channel #2, the trimmer screw seems gripped and I can't change it (it is around 220mv).
- on channel #7, the screw is working but has no impact and it remains above 270mv.

Globally, I found the sound quality of the recording quite bad with a lot of "modulation" noise on all the working channel after the bias change, except #3 which is still play lower, but also lower in noise... Maybe for this one the issue is related to the playback level?

Well, I'm keep looking.

 

[Psychotron1976]

Good news, I succeed to unblock the trimmer of the channel #2 and setup the bias.
I checked all the Rec/Play cards at the R143 trimmer with oscilloscope and a 400hz signal. And the result is:
- channel #1 seems correct at R143.
- channel #3 seems correct but with some additional overshoots compare to the other working channel. Listening the recording, it seems like it have a kind of high pass filter effect (so it is why it feels like it less low modulation noise and volume)
- channel #7 is not correct at R143.
- channel #8 was initially working but not anymore and like #7 it is not correct at R143. Also, like #7, I can't setup the bias whatever I change the bias trimmer and it remains at 130mv (170 for #7).

I think to purchase a new Bias Card (if I succeed to find one). Do you think that is a good solution? I would greatly appreciate any help. Thanks in advance.