Question about dx4d

[SteveM]

I was trying out a dx4d on a tape deck that isn't set up for it and it seems to work. It encodes and decodes at least. So what I'm wondering is if anyone here knows exactly what the function of the cascade cable that's used to plug into the back of a tape deck set up for thedx4d and why, if known, why the dbx unit wouldn't work right with out it.

 

[RRuskin]

I was trying out a dx4d on a tape deck that isn't set up for it and it seems to work. It encodes and decodes at least. So what I'm wondering is if anyone here knows exactly what the function of the cascade cable that's used to plug into the back of a tape deck set up for thedx4d and why, if known, why the dbx unit wouldn't work right with out it.

The licensing agreement Tascam had with dbx Corp. dictated that by design, no Teac/Tascam dbx unit could work with anything other than Teac/Tascam equipment. To comply, the DX4D needs to connect with a deck that tells it when to turn either the encode or decode circuit on. I cannot remember which one is tied to the control voltage. If your unit truly encodes & decodes without that voltage, it has been modded to keep all circuits "on."

 

[SteveM]

The licensing agreement Tascam had with dbx Corp. dictated that by design, no Teac/Tascam dbx unit could work with anything other than Teac/Tascam equipment. To comply, the DX4D needs to connect with a deck that tells it when to turn either the encode or decode circuit on. I cannot remember which one is tied to the control voltage. If your unit truly encodes & decodes without that voltage, it has been modded to keep all circuits "on."

Thanks RRuskin. so far it seems to encode and decode with out it. If I play a no encoded tape you get that crazy pumping but with the tracks recorded through it it plays back fine and when it's disengaged you get that thin sound that you get playing back encoded tapes un-decoded. ?? I also plugged the cascade in to the back of my 32 and played with the function buttons on the 32 to see if it would change anything while recording on the other deck but it didn't.

 

[RRuskin]

Thanks RRuskin. so far it seems to encode and decode with out it. If I play a no encoded tape you get that crazy pumping but with the tracks recorded through it it plays back fine and when it's disengaged you get that thin sound that you get playing back encoded tapes un-decoded. ?? I also plugged the cascade in to the back of my 32 and played with the function buttons on the 32 to see if it would change anything while recording on the other deck but it didn't.

Are you sure it says "cascade" and not "control" on that cable?

 

[evm1024]

I've been looking over the schematic for the DX4 and see that there is no switching back and forth between modes. It appears that if you plug it into a tascam unit it unmutes the audio path and does nothing else. There is also a cap that is switched in and out (when plugged in) that I have not thought out what it does.

The control cable supplies +5, gnd and a control signal that is at a fixed value. THese are on the first 3 pins. They are also replicated on the last 3 pine (4-6) as well.

This replication implied that you can cascade only 2 dx4 units - is this a way to get you not to put this on a 16 track machine?

Anyway the +5 powers an opto isolator driver with the control signal setting the opto state. This is transferred to the opto receiver which sends mute and cont signals to the encoder/decoders.

Mute goes to a mute transistor that shorts the output to ground (Q104 and Q109). The cont goes to another transistor (Q101 and Q106) which shorts the input to ground.

I would need to open up my dx4 to get a better feel for levels.

It appears that there is a default mode for when the dx4 is not plugged in. I cannot say offhand weather it is to work or not work. I'll test that at home. I'm guessing that some of this is left over from earlier units (RX series) that might have needed a control signal to switch between encode and decode.

In any case one could make a DX4 work without using the control cable quite easily.

--Ethan

 

[SteveM]

Are you sure it says "cascade" and not "control" on that cable?

Yeah, I mean the "control" cable with the 4 pins that plugs in the back of the recorder. Wasn't sure what it was called.

 

[SteveM]

I've been looking over the schematic for the DX4 and see that there is no switching back and forth between modes. It appears that if you plug it into a tascam unit it unmutes the audio path and does nothing else. There is also a cap that is switched in and out (when plugged in) that I have not thought out what it does.

The control cable supplies +5, gnd and a control signal that is at a fixed value. THese are on the first 3 pins. They are also replicated on the last 3 pine (4-6) as well.

This replication implied that you can cascade only 2 dx4 units - is this a way to get you not to put this on a 16 track machine?

Anyway the +5 powers an opto isolator driver with the control signal setting the opto state. This is transferred to the opto receiver which sends mute and cont signals to the encoder/decoders.

Mute goes to a mute transistor that shorts the output to ground (Q104 and Q109). The cont goes to another transistor (Q101 and Q106) which shorts the input to ground.

I would need to open up my dx4 to get a better feel for levels.

It appears that there is a default mode for when the dx4 is not plugged in. I cannot say offhand weather it is to work or not work. I'll test that at home. I'm guessing that some of this is left over from earlier units (RX series) that might have needed a control signal to switch between encode and decode.

In any case one could make a DX4 work without using the control cable quite easily.

--Ethan

Cool. Thanks for that info Ethan.
Off hand, is there something else I ought to do to use it this way? I don't know if it's optimum performance but it seems to work without it.

 

[evm1024]

I opened up my DX4 this evening. I poked and prodded and measured here and there. Clever if it worked. OK here goes.

Decks that use DX4 (and presumably dxN) generate a 1 Hz square wave 0 to 5 volts. This is sent to the DX4 along with ground and +5. The ground and +5 powers the opamp that drives the opto isolator.

The idea being that if you don't have the voltages and 1 Hz then the DX does not un mute. Thus it only works with Tascam.

If it only worked that way.... My DX4 works correctly regardless of the state of the control line. Plug it into the deck and no change, turn the deck off and no change, manually trigger the opto isolator and no change.

OK, so I think that it must have worked at one time. But what has happened?Oh, I guess I don't care. Something in my DX4 went bad or someone defeated it or Tascam defeated it. Who knows. I would need to pull the mute board and the control board out to get to test points. The end results for me is that the DX4 I have works with any deck, it works without the control cable plugged.

The mute circuit also does a power on mute. This works correctly. FOr the first few seconds after power on you can see (er, measure) and hear (er, not hear) the power mute in action. This lets the VCAs get stable before passing who knows what noise.

The DX4 is set for 0 gain at -10 dBv line level (0.316 mV) of your line level is calibrated to 0 VU = -10 dBv (as is most all teac/tascam unbalanced and most other such decks) then you are running optimum!

Regards, Ethan

 

[sweetbeats]

Nice work, Ethan, and great thread SteveM! I've been curious about this as well and I imagine I'll be referring to this thread when I get my DX-4D's hooked up to my 48.

I agree with Rick that there is (somehow) an easy mod...not that it is the most solid reference, but I saw an eBay listing for a DX-4D once where the Seller indicated that it would work with any deck after a simple mod (should have contacted to get the info for future reference). Anyway, I bet if we really wanted to know we may actually be able to Tascam about it...my guess is the license has expired, but...whatever.

SteveM I second Ethan's advisement that you don't need to do anything else; that you are running optimum...you could always check with a level meter to make sure that -10 going in is -10 coming out bi-directionally...I'd probably do that but I tend to chase those rabbit trails anyway, and sometimes I get lost. In the memorable (to me) words of Pianodano, 'how does it sound?' Good? Then it is.

 

[SteveM]

Thanks for the feedback Ethan and Sweetbeats. This is good news. I don't need to get another NR unit if I want to use one.

 

[Herm]

Well last week I bought 2 dx-4d,s off ebay and when I got them I pluged them in and I have power lights but no channel lights.
They pass audio in the bypass mode but nothing in the dbx mode.
Its kinda odd that both would have the same issue.

Any ideas?

 

[evm1024]

Well last week I bought 2 dx-4d,s off ebay and when I got them I pluged them in and I have power lights but no channel lights.
They pass audio in the bypass mode but nothing in the dbx mode.
Its kinda odd that both would have the same issue.

Any ideas?

Are the control cables plugged into a Teac/Tascam deck?

these may be an "Un-modded" set.

 

[Herm]

Yea they are plugged in. This isnt my first units I have had 8 of these. But the first time I have come across a pair bad like this. I even hooked up one of my good ones and made sure everything was right and then pluged the new units in wire for wire. still No go

I was wondering has any of you had the bottom off and are there fuses in the power supply box? I would just take it off and look but its not a small deal to do so. So I would like to know before going to that stage.

 

[evm1024]

I assumed that they were plugged in but wanted to be sure.

To open them up just remove the 4 side screws and pull top back and off. It can need a bit of pulling.

Actually, I don't recall if the fuses are accessable fromthe top or bottem....

There are 3 fuses - one each for the audio rails (+-12v) and one for the power/enable LEDs.

If you open it up you should be able to find the + and - 12 volt jumpers on the boards to get a voltage check on.

I'm going to presume that you have good rails and that the mute/control circuit has failed.

This is hellping me understand what they were doing in the circuits.

How much help do you want in getting this figured out? I'm not sure of your comfort level at the component level.

Regards, Ethan

 

[Herm]

Well I just went down and did some more work. And this is a suprise to me.
I hooked up the dbx units to another tascam (22-4) and they both work fine and the lights come on as soon as I turn the machine on.

But they wont work on my 34. Now my 3rd dx 4d works just fine with the 34 but when I turn the 34 off the lights stay on on the dbx unit. Could it be I have a modded dbx unit and a bad control signal on my 34?
This is weird I bought these along time ago so I dont know if they came together or not. I have been through a few of these in the last few years so I lost track of what came with what.
I guess its no big deal as long as I keep them seperated.

Thanks Ethan

 

[evm1024]

\ NO problem


You know the DX4D has a hard power switch. you press it and the unit should go off!

--Ethan

 

[Herm]

I dont mean the power on the dx but when the recorder is shut off the channel lights stay on. They are suppose to go out when the recorder is shut off.

 

[evm1024]

I dont mean the power on the dx but when the recorder is shut off the channel lights stay on. They are suppose to go out when the recorder is shut off.

More info - cool.

Mine stay on all the time. In a "normal" unit (as if I've seen one) the 1 Hz signal from the deck is decoded and enables the dx4d. I'll have to scope that circuit out some more when I get my scope back. When the 1 Hz is there then the channel LED will light.

On the moded or broken units the LED are on even without the 1 Hz.

-Ethan

PS I only have the schematic, no operations or service manuals.

 

[RRuskin]

More info - cool.

Mine stay on all the time. In a "normal" unit (as if I've seen one) the 1 Hz signal from the deck is decoded and enables the dx4d. I'll have to scope that circuit out some more when I get my scope back. When the 1 Hz is there then the channel LED will light.

On the moded or broken units the LED are on even without the 1 Hz.

FYI - A dx4d that is modded to eliminate the need for the the control cable and signals theoretically let you punch in/out slightly quicker than a stock unit.

 

[sweetbeats]

Goodness...you guys make me wanna hook up my DX-D's to my 48 and check 'em out. One of them used to be yours, Herm!