Tascam 38

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SteveM said:
EVM, how exactly would you do that?
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The 30 series channel cards have a set of links that allow the channel to be switched between NAB and IEC depending on which links are there. Kind of like a jumper, but with soldering. It's in the service manual.

Modifying it to 7.5 IPS would be interesting, though - AFAIK the capstan speed control board is attached to the motor itself, but I guess you could swap in a '32 controller and see what happens - or just switch it into 7.5 mode, since it may even be the same board, just without the speed switch...
 
jpmorris said:
The 30 series channel cards have a set of links that allow the channel to be switched between NAB and IEC depending on which links are there. Kind of like a jumper, but with soldering. It's in the service manual.

Modifying it to 7.5 IPS would be interesting, though - AFAIK the capstan speed control board is attached to the motor itself, but I guess you could swap in a '32 controller and see what happens - or just switch it into 7.5 mode, since it may even be the same board, just without the speed switch...
Click to expand...


It is the same motor and controller as the 32 and 34. Just need to put a switch in the place of the jumper and take a look at what other switching is done. It would be the same as the 34 or 32.
 
jpmorris said:
The 30 series channel cards have a set of links that allow the channel to be switched between NAB and IEC depending on which links are there. Kind of like a jumper, but with soldering. It's in the service manual.

Modifying it to 7.5 IPS would be interesting, though - AFAIK the capstan speed control board is attached to the motor itself, but I guess you could swap in a '32 controller and see what happens - or just switch it into 7.5 mode, since it may even be the same board, just without the speed switch...
Click to expand...


Interesting. I'll have to look it up in the service manual.
 
THe plot thickens

I do need to get a service manual for this 38. Looking over the schematics for the 32 and 34 B versions of the rec/play board gives different values for the eq components (and perhaps others). The board layout is the same, just the values have changed.

Also looking at the actual 38 board gives a 3rd set of values. This means that the boards are not interchangeable between the 32, 34 and 38. The 32 schematic shows the NAB and IEC values and the jumpers to move to switch between them.

My 34 manual shows only NAB values and omits the IEC compoments from the schematic and component layout. But the pads are still there. (I'll have to pull a board out of my34)

The 38 board omits the all of the components for NAB eq and for 7.5 ips including the selection transistors. Values are different from the 32 and 34 values. I guess this is to be expected. I am guessing that the differing values for the same eq are to compensate for the differences in the heads....

So I need to find a manual with schematics for the 38. Till then a scan of the 38's rec/play board layout, schematic and parts list would be useful.

Here is a photo of the 38 rec/play board. As you can see a number of components are missing. A more interesting problem than I thought it would be.
 

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Those scoundrels!

......................... :eek: ;)
 
Ugh. I'll have to dig out my spare channel cards - I bought them as spares for the '32, but they came from a dropped Tascam 38. IIRC my '32 is IEC anyway, but all the same...
 
After a little research (Techno-babble warning)

I got a copy of the 38 schematics yesterday so I was able to compare all 3 machines (32,34,38). I should note that the 32 schematic I have is drawn incorrectly for the playback EQ section. This threw me for a while)

I would offer that 32 cards could be used in a 34 or 38. Jumper as needed

34 cards could go into a 38 (and it would become a NAB machine) But they are not populated with all the 32 card components.

38 cards are only good for a 38. Missing too many components.

I should note that the PCB is the same and by adding/subtracting parts and changing a few values you can convert one to the other.

Also, the adjustment ranges are so great that you most likely would be able to get it to calibrate in an other machine. (38 card in a 32 for example) with only minor problems. The 32 cards have some minor NAB adjustments that are missing on the 34 and 38 cards.

The differences between NAB and IEC are 2 fold. First, NAB has a low freq compensation where IEC is flat. The second is that the high freq EQ compensation has a different frequency.

Recording circuitry is the easier of the two. The high freq EQ has enough adjustment that it can be set for NAB or IEC without changing components. Ant the Low Freq EQ is adjusted by the inclusion/exclusion of a simple cap.

Playback circuitry is more complex. (Major technobabble warning)

Looking at the schematic of the 38 (others are similar) you will see a network of resistors and caps in the feedback loop. (Perhaps I'll scan a hand drawing)

Here is what they do:

C102 and R103 form a low frequency cutoff filter. At DC the loop gain is unity and as you get above the cutoff freq that AC gain increases. This filters freq is 4.8 Hz.

R104 and C103 form the low freq EQ. In IEC configurations R104 would be 470K ohms (and with C103 at 0.012 uF this gives a low eq freq of 28 Hz which is below the heads low end response and thus appears as a flat EQ. In NAB mode R104 (in essence) becomes 235k so the EQ freq becomes 56 Hz or close enough to 3180 uS for government work. (should be 50 Hz)

R106,R107,R108 and C103 form the high freq EQ. R108 is the adjustment. Rhe 3 resistors can sum together to form a minimum of 1.44k and a max of 5.4k. This gives a high freq EQ minimum freq of 2.45 kHz and a max of 9.21 kHz. Plenty of range for NAB and IEC.

The other 2 networks that form have less significance and can be disregarded. But there they are anyway....

R106,R107,R108 and C104 change the phase lead compensation.
R104 and C104 tune to 1.25 kHz.

Whew!

About the only thing really usefull it that it would be quite easy to convert a 34 to IEC.

Regards
 

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Fascinating!

Thanx for the in depth analysis and pointers! All potentially useful, I'm sure! :eek: ;)
 
evm1024 said:
R104 and C103 form the low freq EQ. In IEC configurations R104 would be 470K ohms (and with C103 at 0.012 uF this gives a low eq freq of 28 Hz which is below the heads low end response and thus appears as a flat EQ. In NAB mode R104 (in essence) becomes 235k so the EQ freq becomes 56 Hz or close enough to 3180 uS for government work. (should be 50 Hz)



Regards
Click to expand...

EVM, a little over my head but are you saying the caps and resistors working together create the frequency? Just trying to get a little idea how that works.
 
Filter math

SteveM said:
EVM, a little over my head but are you saying the caps and resistors working together create the frequency? Just trying to get a little idea how that works.
Click to expand...

Steve, do a quick google on RC filter and you will see how RC filters work. Some good write-up but no need to go beyond a photo of the filters curves. RC filters selectivly pass based of freq.

Not sure what yo mean by create the freq but the photos of filter curvs should be yyour answer.

How it works in the tascam EQ....

The caps and resistors form a newtwork of low and hi pass filters which are in the feedback loop of the EQ opamp.

An opamp tries to combine the input signal and the output (of the opamp) so that it equals 0. The feedback network takes the output signal and returns it to the input.

If the feedback network has 0 resistance (actually the same resistance as the input side resistance but I am simplifing) then the opamp has a gain of 1. 1mv in is cancled by 1 mV out.

A low pass or high pass filter has constant resistance (up to or down to respectivly) the filters frequency then an increasing resistance beyond that.

As the resistance (impedence actually) of the filter increases (based on change signal freq) the gain of the opamp increases.
 
I'll reread that a couple more times. :D

evm1024 said:
Steve, do a quick google on RC filter and you will see how RC filters work. Some good write-up but no need to go beyond a photo of the filters curves. RC filters selectivly pass based of freq.

Not sure what yo mean by create the freq but the photos of filter curvs should be yyour answer.

How it works in the tascam EQ....

The caps and resistors form a newtwork of low and hi pass filters which are in the feedback loop of the EQ opamp.

An opamp tries to combine the input signal and the output (of the opamp) so that it equals 0. The feedback network takes the output signal and returns it to the input.

If the feedback network has 0 resistance (actually the same resistance as the input side resistance but I am simplifing) then the opamp has a gain of 1. 1mv in is cancled by 1 mV out.

A low pass or high pass filter has constant resistance (up to or down to respectivly) the filters frequency then an increasing resistance beyond that.

As the resistance (impedence actually) of the filter increases (based on change signal freq) the gain of the opamp increases.
Click to expand...
 
SteveM said:
Actually Mike, good info there.
Click to expand...

Lots of good info, I'll second that. Filters aren't hard at the level we need to know about them. Toss 2 of them together and it does take a little thinking about it.
 
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