Tascam DX-2D DX-4D Calibration

evm1024

New member
Sweetbeats and I will be working together on making a Howto for calibrating these Tascam DBX units. This thread should be the running commentary on that effort.

Past modification is outlined in this thread: https://homerecording.com/bbs/showthread.php?p=3189653#post3189653


I've made a web page for mods, upgrades and calibration here: http://arafel.org/audio/dx4d/

I've got my DX-4D on the bench and have had a first pass at calibration. The plan is to have it done using software tools for measurements.

After the DX-4D I'll tackle the DX8 unit cal.

Regards, Ethan
 
I will also be looking to see about compatability of these instructions for calibrating the dbx n/r in the 388 and 234. I have both of those and from memory they are very similar. I'll have to look closer at the service manuals for them...they may or may not have calibration detail for the n/r and if they don't I'll have to look more closely at the schematics to see if there is a direct and usable correlation.

Ethan, thanks so much for your work on this so far...dunno if there is anybody else out there drooling right now, but I'm excited. I can only imagine that an out-of-whack DX-4D could certainly effect the recording process, maybe a little, maybe a lot. Its time to take the veil off of keeping these things in spec...
 
Quick perusal of the 234 and 388 service manuals indicate that instructions for calibrating the DX-2D/4D will translate pretty well to those other units. I'll likely put up detail here when the day comes that I get to doing that on my 234 and 388.
 
They are pretty hands-off but the proprietary dbx chips in them are doing complicated wizmo things with the audio and things need to be carefully setup for them to be able to properly track while encoding and decoding. Once that's setup it is indeed hands-off but components inside drift...components drift as soon as they come off the assembly line but it is so minimal for many years (depending on the conditions) but after 20, 25, 30, 35 years the drift can easily be to the point that it wouldn't hurt (or may be necessary) to bring things into spec again, or in my case having recapped one of my DX-4D's, or in Ethan's case with possible opamp changes.

Ethan? Anything to add to that?

BUT, you're point is awesome. It may, in general, be such a minimal issue for a DX-2D/4D that is stock (no mods/upgrades/new components). Once I get to calibrating my recapped unit and get comfy with the process I plan on applying the process to my untouched unit and seeing how far off it is. That may give others of you out there information that will help you decide whether it is worth messing with it.

Our intentions here are not to create a stir. If your system is working good and sounding good and you're happy with it and/or if you're not into the technical thing, then all's well. If you are having problems, or if you've had to or need to replace components inside a Teac n/r system OR you like to tinker and/or know that your system is in spec, then in that case this is for you. We wanted to be able to help people DIY the calibration for those that want to or need to.

I'm pretty sure you are in agreement with that, Ethan?

I do get the impression that the Teac n/r units are very stable and reliable. It greatly depends on the environment a unit has been subjected to and/or whether or not there has been some failure or problem that is known.

So I'll let you all know just how an untouched 20-some year old DX-4D is after all these years as soon as I can.
 
Blueprinting

Ditto.

There are 3 adjustments on encode and 3 adjustments on decode. That makes for 6 pots to be bounced out of calibration over time not to speak of component drift with age.

The DBX process uses VCAs (voltage controlled amplifier) to adjust the gain of the unit based on the spectral content and levels of the signal. What all this means is that the unit is non linear and thus any mis-calibration can be quite obvious. Breathing and pumping more pronounced and so on.

Tuning these things up and keeping them to spec will ensure that your DBX recorded tapes will sound the same 5 years from now or on someone elses DBX unit. Not to speak of how funny your tape sounds when encode and decode do not complement each other.

Imagine recording some really nice stuff for a few years through a mis-calibrated/drifted unit and then that unit smokes. Now you get a nice new DBX and your tapes don't quite sound the same.

For me the first thing to do is to get the hardware in spec and then keep it there.

Regards, Ethan

PS Plus DIY is fun
 
Calibrated !

Well, I got some bench time last night and was able to run through the cal procedure. This post will be an outline and some general info.

the first thing to note is that my DX-4D was not really out of calibration. If this is the rule rather than the exception then your DX-2/4D should be fine as it is. (this is what I expect is true)

I used 2 pieces of test gear for this and will use them for the (future howto).

First is a multimeter that can read AC mV in the full audio range. THis is something that we all should have. By using the same meter for all calibration needs you get a consistant -10 dBv reference across all your gear.

My meter is a tek DM501 but yours could be a fluke or just about anything else so long as it reads (true) RMS AC voltages at frequences up to 20 kHz without error. (cheap meters read OK at line level 60 Hz but have significant errors above that)

The second piece of test gear is a PC/Notebook with a sound card (don't they all now days?) running trueRTA software. TrueRTA functions as a signal generator, scope and spectrum analyser through the sound card. The 1 octave resolution version is free and will do fine for this project.

Download it here: http://www.trueaudio.com/rta_abt1.htm

TrueRTA has the ability to measure RMS voltages but would need some calibration first... This is why I use a dedicated hardware meter.

You will also need some mini jacks (for the PC) to rca plugs. I only use one channel on the PC so as to not have to worry about the differences in sensitivity on input and level on output between the channels (left vs Right).


Here is a quick step through of the calibration procedure.

First up is the output level adjustment

1) Using trueRTA turn the signal generator on and set for 1 kHz sine wave. Adjust trueRTA's output to get 0.316 mV (-10 dBv) into your meter (you may have to adjust the volume in windows as well.) I was not able to get exactly 0.316 mV so I got it as close as my PC would allow. Remember this value. Whenever I say -10 dBv it means this value.
2) Connect the 1kHz -10 dBv signal to the DBX input 1.
3) Connect your meter to ENCODE OUT 1
4) adjust VR13 until the meter reads -10 dBv (input equals output)

Next adjust for minimum distortion

1) connect your PC line in to ENCODE OUT 1
2) Set trueRTS to spectrum analysis mode
3) you should see a large signal in the 1 kHz band and much lower signals
elsewhere (see the photo)
4) adjust VR11 to minimize all other bands except for the 1 kHz Band.

Next adjust for Symmetry

1) change the signal to 100 Hz at -10 dBv
2) set trueRTA to scope mode
3) connect PC line in to TP-1
4) adjust sensitivty until you can see a trace on the scope (it is around 5mV)
5) adjust VR12 to give the most symmetrical looking waveform.

Return to level adjustment - Distortion and symmetry adjustments change te level settings. So you have to go back and do Output Level adjustments again.

Do this for each channel and then you move on to the decode section and do the same thing. It is outlined in section 6 of the DX2/4 manual.

There are some checks you can do for Frequency response, signal to noise ratio, linearity and Release rate. I'll write about them soon and get this all on the web page.

Regards, Ethan
 

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Ladies and Gentleman...

Give it up for ETHAN!!

This is cool.

Can't wait to try it.

Minor point of clarification...its inferred, but just to be totally clear, when you move to step two (the "minimum distortion" check), you are still pumping the 1kHz -10dB signal into input one. You are just unhooking your voltmeter from channel 1 output and connecting it to the PC input so the TrueRTA signal generator function is outputting the 1kHz tone to the DX-2D/4D and the spectrum analyzer function is inputting the output from the DX-2D/4D.
 
Give it up for ETHAN!!

This is cool.

Can't wait to try it.

Minor point of clarification...its inferred, but just to be totally clear, when you move to step two (the "minimum distortion" check), you are still pumping the 1kHz -10dB signal into input one. You are just unhooking your voltmeter from channel 1 output and connecting it to the PC input so the TrueRTA signal generator function is outputting the 1kHz tone to the DX-2D/4D and the spectrum analyzer function is inputting the output from the DX-2D/4D.

That is correct.
 
Hi,

You made a minor mistake. VR13 is not channel one. It is channel 3.

I am concerned about these Tascam units. I just used my Audio Precision to track the frequency response of both the Tascam and of the DBX 150 versus input level. I connected the encode out to decode out for both types of units. I then varied the input signal from -10 to -80 dBV.

The 150s produced flat audio responses down to -80, with a slight level problem. The output was more like -77 dB. When I did the same for the Tascam, it fell apart at -80. The output was more like 72 db on all channels. Worse yet, the frequency response was up 10 dB at 20 kHz.

I repeated the tests on another 150 and it was identical. Also, all 4 channels of the TASCAM were checked. They showed the same anomaly. I don't know for sure if this is going to be audible, but the reason for my checking was that some things sounded wrong with the Tascam, which I was using to archive some 30 year old tapes of Basie, James and Ellington. In particular. Loud blasting of mics by solo intruments were accompanied by a sound of hum, which varied with the intensity. This could have happened during encode back in the 60s. I did not have an opportunity to replay these tapes on the 150s, but for now, the Tascam will sit on my bench till I can find a complete calibration instruction.
 
DX-4D Calibration / Service Manual

Hello,
I just did the calibration as you have mentioned.
But I can't find the TP1 - is there still a service manual with the layout of the pcb's available ?
This would be great.

Thanks a lot

Regards
Dietmar
 
I have it and it does not represent a lot of pages. When we did these at Teac we found out that you need this special stair step generator that they tell you how to make one but it is with errors in their diagram. In my case and my years of working with them they hardly ever fail and if they do it is a power supply regulator or some easy to fix item. It has been my experience over the years as well as advice from people at Teac to NOT mess with anything other than encode or decode nominal levels. In keeping this rule you stay out of trouble- some guys have to jump in feet first and get into trouble no matter what you tell them. Then if you can do this with a spectrum analyzer or RTA which I do not have good luck to you. This kind of product falls under the "If it ain't broken, don't mess with it" category and I have followed that a lot.
OH, Teac only had one stair step generator and it was returned to CA but they may have thrown it out when Gibson came along. I would have to ask Jim if he has it. It seemed like a pro made device but then I only saw it one time. The schematic is pretty much all there is with a bunch of warning pages. The DX-2D and 4D are the same just more channels with the 4D. I do not think I have ever seen board diagrams for these products- they just we not offered to that degree. The schematic is on Hi Fi Engine.
 

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Yeah…generally any type of dynamics or companding processor, you want to avoid at all costs messing with components inside the internal processing loops. No component value changes…no opamp “upgrades”…leave it alone. Recapping is fine but don’t change values unless there is an obvious voltage tolerance issue.
 
Thanks.
I bought the used DX-4D and only channel 1 has 3 volts on the output and sent me the amp in protection.
I checked inside and I have found the 2sc2878B broken on the decoding line.
On the internet I have found the 2sc2878A but it has a lower hFE.
Once I mounted the new IC I thought of checking all the parameters and for this I was looking for the service.
For now I have removed the IC from channel 4 which I don't use and put it on channel 1 and it works fine.
I found this file and I think the procedure is the same.
 

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These two guys are gentlemen AND scholars!!
Can I ask, is the M-Audio Delta 2496 a suitable soundcard? I have one in a W7 tower and might have bash with that software sometime. I don't have the DBX machines quoted but do have two decent Dolby casestte decks and a Teac A3440 I could checkout.

On a practical note: How good is the quality of the preset trimmers on the DBX units? In my service experience they go 'gappy' with age and need replacing with Cermet components. Personally I would fit multi-turn trimmers, they can fitted to the standard PCB holes and are not costly.

Rock on chaps!

Dave.
 
Look at the schematic- the 2SC2878 are all muting transistors used for grounding out the audio and in a switch transistor mode. There is no need to worry about the hFE in these cases. What IC did you change? The IC's are easy to obtain and nothing special to get. It is strange that the IC went out and is usually caused by people fooling around in the unit.
I can agree that the carbon trimmers could be replaced with Cermet but then if the ones you do not need to adjust are still working you might ask for trouble touching them, The encode and Decode nominal levels are no problem but the others I would not touch. The laptop mess with all that cheap RCA cables is not the best way to go- thpose 50 cent RCA cables are filters in themselves- you want to use a better grade of RCA interconnection if doing testing. Low capacitance cable is a must on any test bench. I use some cables between equipment that is coax and good for 3GHz. It is also shielded.
 
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