Tascam 388 not Erasing or Recording

[sary]

Hello Guys,
Hope this finds you all well,
I've recently decided to recap my Tascam 388, after the process, all mixer function are working fine but unfortunately the unit no longer erases or records to tape.
I checked all connections, re-soldered some sketchy points.

All PCB's working fine but the REC/PLAY and BIAS PCBs are completely bypassed or completely out of the circuit. " Thats just a guess" as I don't get any readings when measuring between the specified TP's.

I'd really appreciate any ideas or help from you guys.

 

[ecc83]

Good morning Sary and welcome. I have never seen or heard of that mixer/recorder but you are on the right track (!) with "bias circuit" that is the function that has been lost I have the same problem with my Teac A3440 OR machine and hope to get to fix it when/if my health improves.

I strongly suspect that in both out cases the bias generator circuit is not getting power. That might be a faulty relay or a transistor or a physical switch. I assume from your mention of "test points" that you have the full service manual? If not that really must be your first step toward fixing the machine.

We are most fortunate at HR to have a chap with vast knowledge of all this 'legacy' gear by the handle "Sweetbeats" and I am sure he will be along to advise very soon.

Dave.

 

[sweetbeats]

Sary and I have been chatting offline via my YouTube channel. I think there is some “cookage” as a result of the device being powered with one or more plug-in cards not being installed. This is not necessarily a criticism of Sary, but rather the device in an age where users are doing what they can to service and maintain their own equipment. The 388 is complex and designed in such a way care must be taken to ensure all systems are interconnected when powering the device. There is no guidance in the maintenance manual to address this, and damage will occur if not all systems are terminated.

 

[ecc83]

I am obliged Sweetbeats and I am sure you will do all you can to get the chap's rig 'flying' again!
Re leaving circuit 'unloaded'? Can I dob in a caution about valve equipment. If the heater supply is lost or some valves, power valves especially are not fitted then HT can rise to a voltage above the filter capacitor's rating and the next thing you know is a bang and a bloody mess. If you must power up in such circumstances have a voltmeter crocce'd in and watch it like a hawk! If the gear has solid state rectifiers, don't do it at all unless you can calculate and fit a load.

Dave.

 

[sary]

Good morning Sary and welcome. I have never seen or heard of that mixer/recorder but you are on the right track (!) with "bias circuit" that is the function that has been lost I have the same problem with my Teac A3440 OR machine and hope to get to fix it when/if my health improves.

I strongly suspect that in both out cases the bias generator circuit is not getting power. That might be a faulty relay or a transistor or a physical switch. I assume from your mention of "test points" that you have the full service manual? If not that really must be your first step toward fixing the machine.

We are most fortunate at HR to have a chap with vast knowledge of all this 'legacy' gear by the handle "Sweetbeats" and I am sure he will be along to advise very soon.

Dave.

Thanks Dave for your reply, I really appreciate it, and best of luck with your Teac A3440
Yes I do have a service manual, but unfortunately on the Bias PCB there are no voltages to trace after the input voltage to the card.
I'm still fiddling around on the hope of finding a bad connection and fix everything
Yes Sweetbeats has been really helpful.
Thank you again, Ill keep you posted if I reach anything.

 

[sary]

Sary and I have been chatting offline via my YouTube channel. I think there is some “cookage” as a result of the device being powered with one or more plug-in cards not being installed. This is not necessarily a criticism of Sary, but rather the device in an age where users are doing what they can to service and maintain their own equipment. The 388 is complex and designed in such a way care must be taken to ensure all systems are interconnected when powering the device. There is no guidance in the maintenance manual to address this, and damage will occur if not all systems are terminated.

Thanks sweetbeats, I'm still a noob when it comes to electric circuits just built a couple of microphones and preamps and thought I could fix it. Hopefully I'll figure something out. checked the BIAS OSC today using an Oscilloscope and its working 100KHZ. I opened one of the BIAS amps, Desoldered every part and measured all parts and all is working including the inductors, the transistors measured a little less hFe than on the datasheets. connected everything again using extended cables " Your Awesome Idea" and then tried measuring voltages. All seemed fine at the pins 1_-15v, 2_GND, 3_+15, 4_ OSC but then got stuck of what more to measure, as I don't have an idea yet of how BIAS Amp modules works, I also got back to the schematics and have all the voltages right going to the pins of the BIAS amp U101, the only points that had no reading where pin 4 and 5. So I'm still at point zero, but still trying. I'm afraid if its a logic problem, for example the recording signal is not going from the record button, just saying maybe, LEDS are lighting and all mixer functions working but still no Erase / Record . Didn't have the signal going to the L/R master swapped U104 as you said in one of your replies and BOOM it worked. Right now trying to find a replacement board or part, but the internet market is still empty

 

[sary]

Good morning Sary and welcome. I have never seen or heard of that mixer/recorder but you are on the right track (!) with "bias circuit" that is the function that has been lost I have the same problem with my Teac A3440 OR machine and hope to get to fix it when/if my health improves.

I strongly suspect that in both out cases the bias generator circuit is not getting power. That might be a faulty relay or a transistor or a physical switch. I assume from your mention of "test points" that you have the full service manual? If not that really must be your first step toward fixing the machine.

We are most fortunate at HR to have a chap with vast knowledge of all this 'legacy' gear by the handle "Sweetbeats" and I am sure he will be along to advise very soon.

Dave.

Dave I think I got it to work

 

[RFR]

Dave I think I got it to work

How? Reseating the cards? I’ve heard they’re delicate.

 

[sweetbeats]

How? Reseating the cards? I’ve heard they’re delicate.

There’s more to it than that. He’s done quite a bit. I have it in email. Maybe he will copy and post it here for continuity.

 

[sary]

Hi Dave sorry for my late reply. that was my email to Cory

 

[sary]

Following up I was able to adjust the bias for all channels but 1 & 6, still have no clue why these, I'll check the connections between the Bias cards and the Rec/Play Cards.
I got the unit to erase and record but for just a few seconds and then I hear what was already recorded on the tape, I think this might be an IC problem or switching, I still have no clue. will trace the Rec Signal and check if there is anything here I can change.
Hope your all doing well

 

[sweetbeats]

So when you got it to erase and record for a few seconds was that across all tracks, or all tracks but 1 & 6?

All of the switching that engages the bias amps for record or erase comes from the rec/play amp cards. If the problem is across all tracks there are two different control signals, one for record and one for erase that tell the bias amps what to do, and if none of that works I would start be looking at the source of those control signals and those connections. If it is all tracks but 1 & 6 try swapping rec/play amp cards to see if the problem follows the card swap. That will confirm your issue is rec/play amp card related.

I did spend some time studying the schematics, but didn’t come to much of a conclusion as far as what powering the unit with the reel servo PCB uninstalled might do…there’s no obvious interruption with that PCB assembly out. What I *can* say is typically if stuff heats up and cooks it is related to one or more power rails. The power for the bias PCBs is a set of +/-15V rails, and the reel servo PCB taps the +15V rail for its servo amp. But there’s no interruption. There’s also a separate +15V supply for the reel motors, and that goes through the reel servo PCB which also contains a power boost circuit paired with the +35V supply…goes to a pair of power transistors on the heat sink. That’s all for the different modes of the reel motors and of course with the reel servo PCB out those pathways are interrupted, but I can’t see any way that would impact the bias PCBs.

 

[sary]

Thanks Cory for following up, I hope all is well with you and the fam.
Actually I got good news, I got all channels erasing and recording and every channel is working perfectly. Waiting on a tentelometer and MRL 4 freq tape to calibrate properly. The only thing I calibrated with the bias and bias trap.
The culprit was in the IC chip in the monitor board, for some reason it was sending a record off or stop signal, changed this all worked but channel 4 and 6. Channel 6 was a bad transistor in the bias board 1318 if I remember correctly, swapped this and channel 6 worked, channel 4 was in the buss B board it wasnt sending the signal for the tape & PGM outs nor the vu meters.
Changed and IC chip here and it worked. I got lucky I guess , but this thing was mind boggling, I’m an architect & music composer, and have very very little background in electronics, right now I think I can kind of find my way around schematics , I kept drawing maps and traces to know what is going from where to where. Anyways I’m so happy I got it to work and thank you guys so much for all the help.
And please feel free to ask me anything, I’m not an expert but been drowned in these drawings for the last couple of weeks, so maybe I know a thing or two.
Regards,

 

[sweetbeats]

@sary I’ve been away but intend on analyzing the information you shared. Excellent job getting it going and for sharing what you did. This will be useful to others and I’d like to do the analysis to better understand what failed in order to help others. And BTW this is not novice level work you’re doing. Well done.