[[email protected]]

Hello,

I am an owner of a 3M M56 8-track and recently got around to setting
up the playback after having purchased an MRL tape.

I am now ready to check the bias frequency and am having a problem.
Manual says to put the machine in record and measure at one of the
test points for 120kHz, +/- 500Hz. According to the manual, there are
test points on the front of the machine:

http://img150.imageshack.us/img150/2...6adjlocdk9.jpg

I am not sure if it's my machine or the frequency counter I am using,
but I am having a hard time getting anything to register on the
counter (using an HP 5345A). I hooked up my HP204C to see if I get a
reading at all and I do see that the 5345A reads the oscillator,
though at 1k it shows ~1.00xx without the k designation, although
changing the display setting gets it to display the proper ~1000.xx.
It seems weird that I would have to do this instead of the counter
auto designating the "k"...

I have a feeling it's with the way I am measuring or setting up the
frequency counter.

Anyone have any ideas?

[Mark]

On Jan 21, 7:29*pm, wrote:
Hello,

I am an owner of a 3M M56 8-track and recently got around to setting
up the playback after having purchased an MRL tape.

I am now ready to check the bias frequency and am having a problem.
Manual says to put the machine in record and measure at one of the
test points for 120kHz, +/- 500Hz. According to the manual, there are
test points on the front of the machine:

http://img150.imageshack.us/img150/2...6adjlocdk9.jpg

I am not sure if it's my machine or the frequency counter I am using,
but I am having a hard time getting anything to register on the
counter (using an HP 5345A). I hooked up my HP204C to see if I get a
reading at all and I do see that the 5345A reads the oscillator,
though at 1k it shows ~1.00xx without the k designation, although
changing the display setting gets it to display the proper ~1000.xx.
It seems weird that I would have to do this instead of the counter
auto designating the "k"...

I have a feeling it's with the way I am measuring or setting up the
frequency counter.

Anyone have any ideas?

counters can sometimes be tricky to trigger correctly.

Did you try adjusitng the various trigger level/ termination and
filter settings on the counter. I also suggest using a scope to
actually SEE the signal and verify it is close to the correct
frequency and then work with the counter to get the exact frequency.
Some scopes have a verical OUTPUT (usually on the back). Once you get
a good display on the scope you might be able to feed the vertical
output to the counter.
Mark

[Scott Dorsey]

wrote:

I am not sure if it's my machine or the frequency counter I am using,
but I am having a hard time getting anything to register on the
counter (using an HP 5345A). I hooked up my HP204C to see if I get a
reading at all and I do see that the 5345A reads the oscillator,
though at 1k it shows ~1.00xx without the k designation, although
changing the display setting gets it to display the proper ~1000.xx.
It seems weird that I would have to do this instead of the counter
auto designating the "k"...

Put a scope on the signal and see what is there. If you have a nice
clean sine wave with five or six volts on it, the counter should
key on it nicely. If the voltage is too low, it may not be high enough
for whatever the threshold is on your counter. And if the waveform is
bizarre, all kinds of things will happen.

The waveform should be very clean; if it's not you have trouble. If the
waveform is clean I'd measure the peak-to-peak voltage and check it
with the manual on the frequency counter.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

On Jan 21, 4:54 pm, (Scott Dorsey) wrote:
wrote:

I am not sure if it's my machine or the frequency counter I am using,
but I am having a hard time getting anything to register on the
counter (using an HP 5345A). I hooked up my HP204C to see if I get a
reading at all and I do see that the 5345A reads the oscillator,
though at 1k it shows ~1.00xx without the k designation, although
changing the display setting gets it to display the proper ~1000.xx.
It seems weird that I would have to do this instead of the counter
auto designating the "k"...

Put a scope on the signal and see what is there. If you have a nice
clean sine wave with five or six volts on it, the counter should
key on it nicely. If the voltage is too low, it may not be high enough
for whatever the threshold is on your counter. And if the waveform is
bizarre, all kinds of things will happen.

The waveform should be very clean; if it's not you have trouble. If the
waveform is clean I'd measure the peak-to-peak voltage and check it
with the manual on the frequency counter.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Hi Scott and Mark,

OK, still got nothing on the scope, so I actually put tape on the
machine, set all tracks to ready and put the machine in record and
then made measurements. Sure enough, I got about 119.2 kHz. Now I need
to adjust some cap on the logic board up a couple of hundred hertz.

It's weird because I'm following the 3M manual and it said nothing
about loading tape on the machine, arming the tracks and actually
recording on machine. It just said to put in the tape sensor mask
(basically defeats the opto cell) and put the machine in record mode
before making the measurement.

Thanks for your help.

[[email protected]]

Next in the manual is the bias trap adjustment.

I started to futz around with the adjustment and noticed shortly
thereafter that I could not get any reading that made sense no matter
where I adjusted the bias trap variable cap.

However, I did some research and have read that some would say I
should be making the measurement with an RF probe. But now I've
already tweaked the variable cap around to where I'm sure it needs
adjustment now, with no immediate accurate method of measuring.

Short of buying an RF probe, could I just use my scope to detemine the
lowest signal? Or would that not be accurate enough?

On Jan 21, 7:16*pm, wrote:
On Jan 21, 4:54 pm, (Scott Dorsey) wrote:
wrote:
Thanks for your help.

[[email protected]]

Sorry, forgot to mention, I am using an HP 400LR VTVM to attempt to
measure and adjust the bias trap.

On Jan 22, 12:15 am, wrote:
Next in the manual is the bias trap adjustment.

I started to futz around with the adjustment and noticed shortly
thereafter that I could not get any reading that made sense no matter
where I adjusted the bias trap variable cap.

However, I did some research and have read that some would say I
should be making the measurement with an RF probe. But now I've
already tweaked the variable cap around to where I'm sure it needs
adjustment now, with no immediate accurate method of measuring.

Short of buying an RF probe, could I just use my scope to detemine the
lowest signal? Or would that not be accurate enough?

On Jan 21, 7:16 pm, wrote:

On Jan 21, 4:54 pm, (Scott Dorsey) wrote:
wrote:
Thanks for your help.

[John Williamson]

wrote:
Sorry, forgot to mention, I am using an HP 400LR VTVM to attempt to
measure and adjust the bias trap.

As you've got a scope already, use that on a low scan speed to check the
bias component, as the width of the line on screen. The thicker the
line, the more HF you've got. It works for me....

--
Tciao for Now!

John.

[Scott Dorsey]

wrote:
Next in the manual is the bias trap adjustment.

I started to futz around with the adjustment and noticed shortly
thereafter that I could not get any reading that made sense no matter
where I adjusted the bias trap variable cap.

You'll hardly ever need to adjust the bias trap. It's something you might
want to check once a decade. I normally wouldn't recommend touching it,
but now that you did, it's too late.

However, I did some research and have read that some would say I
should be making the measurement with an RF probe. But now I've
already tweaked the variable cap around to where I'm sure it needs
adjustment now, with no immediate accurate method of measuring.

Short of buying an RF probe, could I just use my scope to detemine the
lowest signal? Or would that not be accurate enough?

Sure, use the scope. The right tool is a VTVM with an RF probe, and you
can make your own RF probe with a schottky diode and a ceramic cap mounted
into an old scope probe. But if you already have the scope out, it'll do
the job well enough. You don't need a precise value, you just need to find
the null.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."