7400 series TTL peculiarity in transport control for Sony JH

D

Dan Charette

Guest
Hi All...

I've got a rather odd problem with a Sony JH-24 24 track tape deck
transport. The control circuitry is quite simple and based on
straight 7400 series TTL. Each of the functions for the unit, i.e.
play, stop, rec, etc. are operated from a respective momentary switch
that is grounded at it's pole and then ties to the control board
through a 4700ohm resistor. Each of the functions are the same switch
circuit up to the control board, so I'll just describe only one. When
the switch signal enters the control board, it is tied to it's section
of logic inputs be it a 7408, 7420, 7400 gates. Also on these inputs,
there is a pair of 1N4004 diodes tied together on one end to the
signal input to the gate and are tied on the other ends, reverse
biased, one to the power supply and the other to ground. See the
following URL for a schematic:

http://www.thesonicfrog.com/logic1.jpg

Now, what's happening is that when I just take a measurement of the
input to the gate when I push the control switch, I see a +5V turn to
+1.5V. Now, that's certainly not enough to be considered a logic 0 as
the margin on the low side is 0.8V, right? This characteristic is
showing up all over this control board. And I believe it to be the
reason why this machine is completely inoperable. But, my gut is
telling me that there is something else going on here because the
chances of a system wide failure of this many gates is astronomical.
Also, I have removed all of them and tested them individually out of
circuit. I've installed sockets and have changed the different gates
to 74F logic and I find that some of the logic functions now somewhat
work, but still this isn't curing the problem. I suspect that the 74F
is a little more forgiving of the sort of dead zone between .8 and 2V
that the control switches are generating. So, I guess my question is
this... do the inputs to straight 7400 series when tied through diodes
as I said and unterminated on the switch ends when one is not
depressed, normally float like I'm describing? What would be causing
the 1.5V levels that I'm reading? One could guess the diodes leaking,
but there are 4004s and I highly doubt so many of them would be
experiencing the same problem. Any ideas?

Thanks for any suggestion.
Dan


Dan Charette {dan@thesonicfrogFUZZ.com}
Remove the "FUZZ"
from my e-mail address
to contact me.

"I may not always be right, but I'm never wrong."
 
Dan Charette wrote:

Hi All...

I've got a rather odd problem with a Sony JH-24 24 track tape deck
transport. The control circuitry is quite simple and based on
straight 7400 series TTL. Each of the functions for the unit, i.e.
play, stop, rec, etc. are operated from a respective momentary switch
that is grounded at it's pole and then ties to the control board
through a 4700ohm resistor. Each of the functions are the same switch
circuit up to the control board, so I'll just describe only one. When
the switch signal enters the control board, it is tied to it's section
of logic inputs be it a 7408, 7420, 7400 gates. Also on these inputs,
there is a pair of 1N4004 diodes tied together on one end to the
signal input to the gate and are tied on the other ends, reverse
biased, one to the power supply and the other to ground. See the
following URL for a schematic:

http://www.thesonicfrog.com/logic1.jpg

Now, what's happening is that when I just take a measurement of the
input to the gate when I push the control switch, I see a +5V turn to
+1.5V. Now, that's certainly not enough to be considered a logic 0 as
the margin on the low side is 0.8V, right? This characteristic is
showing up all over this control board. And I believe it to be the
reason why this machine is completely inoperable. But, my gut is
telling me that there is something else going on here because the
chances of a system wide failure of this many gates is astronomical.
Also, I have removed all of them and tested them individually out of
circuit. I've installed sockets and have changed the different gates
to 74F logic and I find that some of the logic functions now somewhat
work, but still this isn't curing the problem. I suspect that the 74F
is a little more forgiving of the sort of dead zone between .8 and 2V
that the control switches are generating. So, I guess my question is
this... do the inputs to straight 7400 series when tied through diodes
as I said and unterminated on the switch ends when one is not
depressed, normally float like I'm describing? What would be causing
the 1.5V levels that I'm reading? One could guess the diodes leaking,
but there are 4004s and I highly doubt so many of them would be
experiencing the same problem. Any ideas?

Thanks for any suggestion.
Dan


Dan Charette {dan@thesonicfrogFUZZ.com}
Remove the "FUZZ"
from my e-mail address
to contact me.

"I may not always be right, but I'm never wrong."


The inputs to a straight 7400-series gate are the emitters of BJTs.
Click to expand...
They source a fair amount of current, specified to be 1.6 mA or less,
and will float to about 1.4-1.6V when left open, a marginal logic high.

It sounds as though whatever the other ends of the switches connect to
has gone open-circuit.

Cheers,

Phil Hobbs
 
Dan Charette wrote:
Hi All...

I've got a rather odd problem with a Sony JH-24 24 track tape deck
transport. The control circuitry is quite simple and based on
straight 7400 series TTL. Each of the functions for the unit, i.e.
play, stop, rec, etc. are operated from a respective momentary switch
that is grounded at it's pole and then ties to the control board
through a 4700ohm resistor. ...

http://www.thesonicfrog.com/logic1.jpg

Now, what's happening is that when I just take a measurement of the
input to the gate when I push the control switch, I see a +5V turn to
+1.5V. Now, that's certainly not enough to be considered a logic 0 as
the margin on the low side is 0.8V, right?...
Click to expand...
The schematic shows the switches connected to "-V". The diodes
are connected to ground. 4.7K ground would probably not work,
but 4.7 k to a sufficient negative supply would. If you see
ground on the switches instead of a negative voltage, maybe the
negative supply is dead.
 
Dan,

What is the value of the -V that the switches are connected to? My guess is
that it should be -8V to -15V. Check out the negative supply. The diodes to
ground are there to keep the inputs from going negative.

Tam
"Dan Charette" <dan@thesonicfrogFUZZ.com> wrote in message
news:u9lr60hs4gquf7hk2keh7rsp6u11fh2aht@4ax.com...
Hi All...

I've got a rather odd problem with a Sony JH-24 24 track tape deck
transport. The control circuitry is quite simple and based on
straight 7400 series TTL. Each of the functions for the unit, i.e.
play, stop, rec, etc. are operated from a respective momentary switch
that is grounded at it's pole and then ties to the control board
through a 4700ohm resistor. Each of the functions are the same switch
circuit up to the control board, so I'll just describe only one. When
the switch signal enters the control board, it is tied to it's section
of logic inputs be it a 7408, 7420, 7400 gates. Also on these inputs,
there is a pair of 1N4004 diodes tied together on one end to the
signal input to the gate and are tied on the other ends, reverse
biased, one to the power supply and the other to ground. See the
following URL for a schematic:

http://www.thesonicfrog.com/logic1.jpg

Now, what's happening is that when I just take a measurement of the
input to the gate when I push the control switch, I see a +5V turn to
+1.5V. Now, that's certainly not enough to be considered a logic 0 as
the margin on the low side is 0.8V, right? This characteristic is
showing up all over this control board. And I believe it to be the
reason why this machine is completely inoperable. But, my gut is
telling me that there is something else going on here because the
chances of a system wide failure of this many gates is astronomical.
Also, I have removed all of them and tested them individually out of
circuit. I've installed sockets and have changed the different gates
to 74F logic and I find that some of the logic functions now somewhat
work, but still this isn't curing the problem. I suspect that the 74F
is a little more forgiving of the sort of dead zone between .8 and 2V
that the control switches are generating. So, I guess my question is
this... do the inputs to straight 7400 series when tied through diodes
as I said and unterminated on the switch ends when one is not
depressed, normally float like I'm describing? What would be causing
the 1.5V levels that I'm reading? One could guess the diodes leaking,
but there are 4004s and I highly doubt so many of them would be
experiencing the same problem. Any ideas?

Thanks for any suggestion.
Dan


Dan Charette {dan@thesonicfrogFUZZ.com}
Remove the "FUZZ"
from my e-mail address
to contact me.

"I may not always be right, but I'm never wrong."
Click to expand...
 
Thanks fellows! Sometimes, I'll get caught up in analyzing something
so deep that I completly miss such a simple solution right in front of
my eyes. Now that someone else's eyes saw it, it makes perfect sense!
Of course... the switches are tied to the negative supply and so,
that's why I'm reading voltages as such. I've been real skeptical of
these schematics. I've had to correct a number of things on them due
to what I was finding on the actual circuit boards. Believe it or
not, some of the switches on the other pages are actually are drawn
the same, but instead of being tied to the v- they're tied to ground.
So, first order of business when I get in front of it again is to
verify that negative supply is working properly and it is in fact tied
to those switches..
Thanks again!

Dan Charette {dan@thesonicfrogFUZZ.com}
Remove the "FUZZ"
from my e-mail address
to contact me.

"I may not always be right, but I'm never wrong."
 
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