biasing a 388 for 0db tape

Great! Found it.

How can I be sure that my interface is outputting exactly 0db? Last time I measured my multi-meter accuracy I seemed to have no problems and I just turned the output of my sound card up to full. I then adjusted the output db in the 'test plugin' which has the signal generator. Is this the correct way of doing things?
 
Great! Found it.

How can I be sure that my interface is outputting exactly 0db? Last time I measured my multi-meter accuracy I seemed to have no problems and I just turned the output of my sound card up to full. I then adjusted the output db in the 'test plugin' which has the signal generator. Is this the correct way of doing things?

At low frequencies, 100Hz say, the meter should easily be accurate enough* A quick look at some meter speccs showed even a Fluke 114, likely $200+ in US still has a 'best' AC voltage accuracy of only 1% . If we double that for a cheaper meter, 2% only comes out at 0.17dB so, worse case your meter is unlikely to be more than 0.4dB out. But you MUST check it a 2,5, 10Khz and if it goes out of spec derive a correction chart. You can pretty much guarantee the soundcard will be flat to 20kHz.

*Really worth buying a second meter because, a) being brand spanking it will be at top accuracy. b) you can compare readings. If the old one is worse than say 4% ref the new, be suspicious. I would bet they would be within 2% of each other and so if you were REALLY picky you could take an average and then work out the decibabbles. But! I tell thee! MUCH easier to invest in (or build) a direct reading dBu AC analogue scale meter!

Dave.
 
I've calibrated the meters as per the '1-4-6 PGM BUS IN - PGM OUT' section of the manual. Curiously, every meter was at -1db on the VU when showing .3v on the meter and so I adjusted the internal VU adjustment trimmers until the meters read 0db as the manual directs.


Firstly, I tested the multimeter's calibration by giving it 1K @ -10db from my signal generator (protools test osc). The multimeter gave an ac voltage reading of .3068 on the multimeter.

When calibrating the VU meters, bringing the PGM master fader up to read the required 0.3068 on the multimeter would show as -1db on the PGM VU meters. I would then adjust the PGM VU meter (via the internal adjustment tabs) to read 0db.


Is this correct? I can always change them back but just thought it was curious that all PGM meters read about -1db when applying the specified 1k @ -10db through the PGM section.



Assuming what I've done so far is correct, would the next step be to test each channels rec/reprod levels for LPR35?

Should I test each individual channel separately? For example from channel 1 to PGM 1 to go to track 1 on tape? Then check repro?

Or should I just go into the LINE IN of channel 1 with my test tone and assign them to the various PGMs to go to all 8 tracks on the tape?
 
You need to study the manual. The manual lays it out step by step. Try and figure out from the manual what the next step is, post what you think it is and we'll go from here. I want you to learn how to use the manual to be your resource, but you kind of have to dig in and try. Okay?
 
haha thanks

Yeah I would move along to the next steps in the manual but I don't have a test tape. I'm more just thinking of workerbee's advice of:

Record 100HZ,1KHZ, and 10KHZ @0vu with the DBX off.Do they come back close to zero,(-1db), @ 10KHZ relative to 1KHZ? Yes,don't mess with it.No, means you will have to fine adjust the bias.

Now that the meters are aligned, I was wondering what might be the best method to do this? Do it on each channel or just run one signal to every track on the tape? Because if I put a test signal into channel 1 and then assign it through the 8 PGMs, not all PGM VUs read 0db like channel 1 does. PGM 3 and 4 are slightly out for instance while 1-2 and 7-8 are very close to 0db.
 
If your playback electronics are not aligned, you cannot align the record electronics. Think about it.
 
Hmm, I wasn't going to align anything though. I was under the impression workerbee's initial advise was to see if the machine is somewhat calibrated by comparing reprod at 1k relative to 10k and if so leave well enough alone. As I don't have a test tape yet I was just going to do that for the time being.
 
Well, unless someone messed with it it should be calibrated. You can have increased resistance as pots try to corrode - like your -1 reading ? Have you massaged and lubed the pots. The amp I'm sitting in front of commonly has power meters that hardly move from the years of high temps
 
Go ahead and see if it's "somewhat calibrated" then, but you're asking what the next step is, and the manual and logic tells you its calibrating the playback electronics. If the playback electronics aren't aligned, you are playing back your recorded tones through those misaligned playback electronics and all bets are off. If this doesn't make sense to you please ask and I can go into more detail. Maybe the playback electronics are still fairly well lined up...maybe not. You don't know until you put a cal tape in there and see. Period. It's a gamble. I think you need to just decide if you want to gamble, or calibrate the machine.
 
Yeah I hear what you're saying. I definitely need to get a test tape then if I want to move forward from here. As soon as I have the money i'll order one.

I did the simpler rec/reprod test and it's pretty incredible the difference between tapes. The lpr35 comes back strongly at 10k and the 388 seems to be reproducing what was recorded quite well. So it would seem the calibration hasn't been messed with. I have some older +1 tapes which I love the sound off and they reproduce 10k at less than -10db! The +1 tape always seemed very soft and pillowy when distorted and now I know why. Kind of funny that in some instances it would appear I prefer the sound of an incorrect and inferior tape. I just don't like the brown oxide it leaves on the path which means I have to clean it a lot. I'm also always wondering if I'm wearing the heads out faster than a new modern tape would be. My 35-90 at +3 seems to be a good middle ground between the two in terms of reprod too.

Anyway, I've learned a lot which looking back on it was the purpose of this thread more than anything. I feel like I can use the 388 and different tape types now depending on what I want achieve and know where I am and what I'm doing where as before I was working a lot more intuitively.

Thanks for the help everyone. Really do appreciate you all taking the time to school me on the basics here. I think my next 388 related project will be to recap the power supply. I've never done that before and think it would be a good learning experience also :)
 
The reason the +0 tape has much lower response at 10K is probably because the bias level is too high. That's one of the effects when there is too much bias...decreased HF response. I wish you could see what happens with the HF response when turning the bias level trimmer in real time. That's how it works when setting up a three head machine, because you can record on the record head and reproduce and monitor that while recording, you can inject your 10K tone and monitor the playback of the recorded 10K tone while recording, you can watch the level meter as you turn the bias level trimmer...as you increase the level the HF response rises, rises, rises, and then reaches a peak and then as you continue increasing the bias level you can watch the HF level (the reproduced 10K tone) start to fall.
 
The supply caps would be a perfect move, if you ask me.

" incorrect and inferior tape".

One of your secret weapons. I dunno, is there any tape switching on this deck ? With the mixer right there, you can experiment with your own EQ curves when mis-matching things.

If you plan on "some" red/brown tape, you can put a wiper on the supply side, somewhere
 
Whilst you are saving and waiting Mr B, (and this in NO WAY excludes the test tape!) might be good to make you own 'sample' tapes at a few frequencies and levels. Then SHOULD urge to tweak be irresistible you have a reference of sorts to fall back on.

When you DO get the TT it will be instructive perhaps to compare data? Oh! BTW: BEFORE you let a test tape within 300mm of the deck you MUST degauss the heads and tape path! (TOLD you 'tape' was expensive and gadget ridden!)

Dave.
 
Or, press the DEMAGNETIZER button : )

Really, though, you can take a snip of tape and pass it close to the tape path to see it goes magnetic anywhere along the line
 

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The reason the +0 tape has much lower response at 10K is probably because the bias level is too high. That's one of the effects when there is too much bias...decreased HF response. I wish you could see what happens with the HF response when turning the bias level trimmer in real time. That's how it works when setting up a three head machine, because you can record on the record head and reproduce and monitor that while recording, you can inject your 10K tone and monitor the playback of the recorded 10K tone while recording, you can watch the level meter as you turn the bias level trimmer...as you increase the level the HF response rises, rises, rises, and then reaches a peak and then as you continue increasing the bias level you can watch the HF level (the reproduced 10K tone) start to fall.


Yeah I figured this was the case. I have a tascam 122mkII that came biased for Denon Hd8. I see there are 4 screws on the front, two for bias, and another two for levels. It then has another button which gives an osc at 10k and 400hz. As this is a three head deck maybe I should play around on this one as a learning experiment? I know this machine is well calibrated though and I'm hesitant to mess it up. I'll check the manual and see if I need a TT to adjust levels. I'm sure the level reproduction would be okay as I had it serviced and biased when I purchased it maybe a year ago.

The supply caps would be a perfect move, if you ask me.

" incorrect and inferior tape".

One of your secret weapons. I dunno, is there any tape switching on this deck ? With the mixer right there, you can experiment with your own EQ curves when mis-matching things.

If you plan on "some" red/brown tape, you can put a wiper on the supply side, somewhere

Please tell me more about these wipers. The tape I'm using is red oxide...641. Was it common for this tape to shed a slightly larger amount of oxide than more modern back coated tapes? And is this tape rougher on the heads? My 388 has great heads and as they are impossible to find now I'll like them to last as long as possible. Is there anywhere I could read up on the standard maintenance practices when using tape in the 60s? I've found one reference online to a deck having felt tongs that the tape would run through before hitting the heads. This is literally the only info about this stuff I've been able to find.

Whilst you are saving and waiting Mr B, (and this in NO WAY excludes the test tape!) might be good to make you own 'sample' tapes at a few frequencies and levels. Then SHOULD urge to tweak be irresistible you have a reference of sorts to fall back on.

Yeah I had this thought but you know... playing with fire. I'd love to hear the bias adjusted to get more 10k on the +0 tape. Then again I like it biased where it is also because I can use +0 tape to get that old 'eq' filter over everything. Then use the +6 for a more contemporary sound. I also wonder if the +0 tape would lose its magic when opening up its 'eq filter' so to speak.


Really, though, you can take a snip of tape and pass it close to the tape path to see it goes magnetic anywhere along the line

Hey this is a great trick! Thanks. I'm saving for a handy-mag atm so will try this out!
 
".641"

I'm not gonna pull this one out to confirm, but the stubb by the supply guide is a felt roller that doesn't roll, I seem to think the was something under the old Tandberg head block, also
4669209719_df2529b564_b.jpg


EDIT oh, stubby is on a tension arm. seems I took another PIC where its above the guide
 
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a quick update!

the machine is going well. I've made some wipers for the red oxide and they seem to work okay. I rarely use that tape though because im scared it's wearing out the heads faster than modern tape would. I need to look for a good polished modern 0+ tape that's the right thickness for the 388. Anyone have an idea on what might work? Did they even make polished modern +0 1/4" tape? Or was it all +3 by then.

I really like the lpr35 also. very tempted to lower biases levels to the sweetbeats recommended voltage for the tape.

I'm going to recap the power supply soon. I'm just enjoying making music with it at the moment.

I also recently had to get an old roland synth repaired, and I asked the tech if he would let me watch... he said yes! He was actually very good at explaining things too. So now I know the basics of how to test a power supply etc. Though we didn't manage to find the bad part on the day I was there, it was interesting to see his repair process. He would first theorize and then rigorously test the theoretically suspect part until he knew for certain it was good. I also got to see how an oscilloscope works which was pretty cool too!
 
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