Tascam 388 calibration questions.

[j.harv]

Okay. I started a thread like this about a year ago. I did start to do a set up on my second 388 but gave up and sent it off to a tech. Now I have decided to drag my first 388 out and try it for myself. Im not gonna go all out with the frequency response,T.H.D,cross talk,S/N ratio etc... I just want to set the meters,record playback levels and Im guessing Bias, which has to be done before recording levels.
I have a test tape, signal generator/scope software and True RMS multi-meter.
I have ran across a few things I don't understand. Im not expecting a tonne of time from anyone or a complete walkthrough,I have just ran into a couple snags. I know explaining can take time.
I have gone through the first steps in lining up the meters and checking the mixer with tones.So far so good.
Now Im at the recorder/reproducer section. Step 1-5-1 is telling me to load the test tape and produce 400Hz . My tape doesn't have that freq. There is only one tone on the tape for rec/repro and that's the last one on the tape. It's the 1khz if im correct.
I did use this tone for the repro level and it worked out. Just want to be sure thats correct.
Next step would be Bias. Now, I tried this with no luck. Im going with Sweetbeats bias level for RMG LPR53, which he came up with 110mv.
I connected the multi meter to TP2 and TPG and got a reading of something like .200
Im guessing this is high. So I tried to adjust the bias trim pots and they did nothing. The number didn't change at all no matter how far I turned it.

I'll leave it hear for now, I did skip over to the recording level adjustment and ran into problems there too which I'll explain a little later.
This is probably as far as I want to go with the set up.

Any thoughts on these issues.????

 

[The Ghost of FM]

Any thoughts on these issues.????

This won't be helpful but trying to bias 2 head machines sucks donkey balls because you don't have a dedicated repro head that you can meter to show the peak point and then additional turn to get to the desired 3 db drop thereafter. I wasn't aware of the voltage trick but perhaps its a question of putting your voltage meter on the correct test points to get that done. Does the service manual offer any guidance on that method? If so, I'd follow that to the T.

Good luck.

Cheers!

 

[j.harv]

The manual says to measure from specific points on the amp cards. I did a quick check and tried to adjust the pots as described but nothing. Come to think of it....im probably gonna skip the whole bias step and go to the record level check. The manual says to do bias before record level checks, but bias seems a little beyond me right now. I did do a quick record level check a couple days ago and had some issues which I will post here in a day or two. I'll go through the steps again and come back with results.

 

[sweetbeats]

Do the bias first.

Did you have your multi-meter set to DC or AC volts?

You say your test tape only has one tone for record/reproduce. I don't understand this. Tones are tones on a test tape. You're *okay* to use the 1K tone for reproduce level. 400Hz would be ideal and is more critical when lining up the frequency response.

 

[j.harv]

Do the bias first.

Did you have your multi-meter set to DC or AC volts?

You say your test tape only has one tone for record/reproduce. I don't understand this. Tones are tones on a test tape. You're *okay* to use the 1K tone for reproduce level. 400Hz would be ideal and is more critical when lining up the frequency response.

Thanks Cory.
I have the meter set for AC. I have left it set the same as when I was using it for the meter/ mixer tests.
Not sure if that's right. As for the test tape. What I meant was, there is the guy's voice on there telling me the tone for each calibration check. The rec/play tone is at the very end and it's only one tone which is the 1k if I'm correct. The manual states to use a 400hz. I'm not sure if that tone is on my tape. I'm not at home right now so I'll get back into it when I get there.
By the way, this test tone tape sounds like it was recorded in the 50's. I'm just going by the speech style of the man who is prompting the tones

 

[j.harv]

Okay. I went back to the bias check. I found all the test points and put the 388 into record mode with no tape. For the bias trap check, I had my multimeter set to AC and the range set at 2. I probed the test points and got readings ranging from lowest at .254 at track 8 and the highest reading was track 4 with a .534 reading. In the manual it says to adjust the pots to obtain a minimum reading. Does this mean the meter should be reading 0 for this check.? I didn't adjust anything yet.
After that, I checked the Recording bias adjustment. The same way with 388 in record mode, all tracks armed, no tape.
At the test points I was getting readings from .98 at the lowest to about .126 at the highest. I did get a couple at .120, which the manual calls for. So they're close. But before I try and adjust these numbers down to 110mv for LPR35 tape, I want to get the bias trap part cleared up. What would be the minimum reading on the multimeter that the manual calls for be????

 

[The Ghost of FM]

I would assume a minimum reading is simply that: the lowest reading obtainable via adjustment of the pot in question for that step. If there was a specific number to shoot for, the manual would have mentioned one as they're generally fairly well written.

Cheers!

 

[j.harv]

I just did the bias as per the manual. Then I attempted the recording level. Here's my problem.......
I recorded a 400hz tone at .316mv(-10db) to tape on track 1. The meter pinned while also wavering all over the place. I know that this has to do with it being an edge track. I tried adjusting the trim pot R143 and it did nothing. I went on to track 2 to get away from the crazy needle of track 1. Track 2 also pinned upon playback,but with no wavering of the meter. But... the trim pot did nothing to bring the level down. I tried with track 3, and the same thing happens. No adjustment is possible using these trim pots.They just turn with no results. I'm following the manual.Am I missing something????

 

[The Ghost of FM]

I wasn't aware you had two 388's. The one you're currently calibrating in this thread, was it known to be working correctly prior to right now? Were there any known service issues with it?

Also, is the dbx switched off for the calibration process? Levels would obviously be way out if you were doing this with the dbx still on.

Sorry for the rudimentary questions.

Cheers!

 

[j.harv]

I wasn't aware you had two 388's. The one you're currently calibrating in this thread, was it known to be working correctly prior to right now? Were there any known service issues with it?

Also, is the dbx switched off for the calibration process? Levels would obviously be way out if you were doing this with the dbx still on.

Sorry for the rudimentary questions.

Cheers!

Jeff, this is my first 388 that I have been using for the past 4-5 years. It's always served me well. I just figured that while Im not using this one right now, and because Im bored and have time, I would like to practice doing some calibrations on it as I've never had it done on this one. Now with the dbx.... it has to be off???? ooops
I didn't notice anything in the manual about having it off for calibration.
Would this affect the trim pots not changing levels in anyway upon playback????

 

[The Ghost of FM]

Jeff, this is my first 388 that I have been using for the past 4-5 years. It's always served me well. I just figured that while Im not using this one right now, and because Im bored and have time, I would like to practice doing some calibrations on it as I've never had it done on this one. Now with the dbx.... it has to be off???? ooops
I didn't notice anything in the manual about having it off for calibration.
Would this affect the trim pots not changing levels in anyway upon playback????

I don't know about the dbx being on not allowing the pots to have any effect, but it most certainly would affect all the level settings.

About what the manual says or doesn't say, every deck I've ever owned always talks about turning all noise reductions systems off during calibration. Maybe re-read it for that tip/warning.

On the bright side, you get to start all over again to gain more practice!

Cheers!

 

[j.harv]

Yeah I don't mind a redo. Just gotta try and figure out why these playback levels are pinning the meters.

 

[The Ghost of FM]

I took a look through the service manual out of curiosity to see if they mentioned anything about turning off the dbx during the calibration process and shockingly, not a word was mentioned about that!?

I'm still pretty sure that it should be off though. One of the test/steps in the process talked about s:n ratio and stated the spec which corresponded to the dbx being off number...so that would support my idea of it being necessary that the dbx be off.

Perhaps one of our other 388 experts out there could chime in this as it would help explain a few of the level/adjustment issues j.harv was experiencing.

Cheers!

 

[j.harv]

Thanks Ghost!
The thing I don't understand is, after recording the test tone, the manual says to play back and adjust the record level trim pot if the Vu meters aren't at 0db or if there isn't a .316mv reading coming out of the tape out jack.
I though the trim pot for the playback level would be the one to adjust for this being as Im playing back the tape. Still working on it

 

[The Ghost of FM]

Thanks Ghost!
The thing I don't understand is, after recording the test tone, the manual says to play back and adjust the record level trim pot if the Vu meters aren't at 0db or if there isn't a .316mv reading coming out of the tape out jack.
I though the trim pot for the playback level would be the one to adjust for this being as Im playing back the tape. Still working on it

No. Once the playback level is set via the calibration test tape, you then want the recording level to hit that same mark, not then readjust the playback level, which defeats the purpose of adjusting it in the first place...right?

Cheers!

 

[j.harv]

No. Once the playback level is set via the calibration test tape, you then want the recording level to hit that same mark, not then readjust the playback level, which defeats the purpose of adjusting it in the first place...right?

Cheers!

I see... Im just wondering then, why this adjustment wouldn't be made while the 388 is recording, not playing back. I have my playback levels set, which I did while listening to the test tape. Now after recording the test tone on the blank tape, Im making an adjustment while playing the tape back. This to me seems like the same step as a playback adjustment because there is no recording going on while adjusting the trim pot for recording level. Land of confusion!
Still, those trim pots don't seem to want to do anything while listening back to the tone.
I'll get it sooner or later.....

 

[The Ghost of FM]

This relates back to my original comment in this thread: Because the 388 is a two head system, actual live monitoring and adjustment can not be done real time because there is no dedicated repro head to hear what you are adjusting. So the idea is to record for a bit at one setting, then rewind it back to actually see what level got printed to tape. Too low? Bump it up a bit more on the re-try. Too high? Turn it back a bit and try again. Repeating this step until you get it within a db or less of where it should be.

Getting any clearer to you now?

Cheers!

 

[j.harv]

This relates back to my original comment in this thread: Because the 388 is a two head system, actual live monitoring and adjustment can not be done real time because there is no dedicated repro head to hear what you are adjusting. So the idea is to record for a bit at one setting, then rewind it back to actually see what level got printed to tape. Too low? Bump it up a bit more on the re-try. Too high? Turn it back a bit and try again. Repeating this step until you get it within a db or less of where it should be.

Getting any clearer to you now?

Cheers!

Thanks again Jeff.
Well that seems to have done it. I was kinda going by how I read the manual. I was trying to adjust the level while playing back. But making the adjustments before I record now makes sense. Duh!
Im just wondering why all of the track recorded super hot. All the VU's were maxed out on this part of the calibration. Just gotta even them all out and it should be all good.
THANKS!

 

[The Ghost of FM]

About the hot levels: if the dbx was on at the time, that would easily explain that as dbx uses a 2:1 and 1:2 compression expansion system on recording and playback. So a signal that appears to be -7 going in might actually be hitting the tape at +7 or somewhere around there. All of that to get the quieter signal well above the hiss floor during encoding/recording.

Anyway, glad to read you're making some progress in spite of my anti-service manual advice!

Cheers!

 

[j.harv]

Okay. Back to this real quick.
Should I go over the adjustments again with dbx off. I did them with it on, but now I'm not sure what truly
Is right or wrong. I wish the manual would have included this.