Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

"Todd"

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only.


** So you cannot centre the trace with the position control - OR - is there
no trace, just a dot ?

Are both traces so affected ?


.... Phil

On Jun 14, 12:03*am, "Phil Allison" wrote:
"Todd"



Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only.

** So you cannot centre the trace with the position control - OR - is there
no trace, just a dot ?

* Are both traces so affected ?

... * Phil

There is a trace not a dot and yes both traces are affected.

Todd

"Todd"
"Phil Allison"

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only.

** So you cannot centre the trace with the position control - OR - is
there
no trace, just a dot ?

Are both traces so affected ?


There is a trace not a dot and yes both traces are affected.

** Think it is something very simple.

Maybe an open resistor or a bad zener diode.

Check everything you can around that position pot.


..... Phil

On Jun 14, 7:31*am, "Phil Allison" wrote:
"Todd"
*"Phil Allison"



Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only.

** So you cannot centre the trace with the position control - OR - is
there
no trace, just a dot ?

Are both traces so affected ?

There is a trace not a dot and yes both traces are affected.

** Think it is something very simple.

Maybe an open resistor or a bad zener diode.

Check everything you can around that position pot.

.... * Phil

Ok thanks will look at it today

Todd

Todd wrote in message
...
Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd


With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

On Jun 14, 12:31*am, "N_Cook" wrote:
Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok, thanks that will give me a idea where to look on the board.


Todd

On Jun 14, 12:31*am, "N_Cook" wrote:
Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok messed with it today some. DC wise one of the transistors has a cap
that slowly charges with the ohm meter hooked up..
Checked the 2 power resistors on each curcuit and they measured ok.

With power turned on on of the transistors will vary its voltage
between 171 and 36 with the horz pos knob.
The other one will only change between 18.7 and 20.4 volts.
pulled the transistor that was no varying much and checked it with a
ohm meter it looks good.

Is there a generic schmetic I could look at to see how this curcuit
should work?

thanks for the help.

Todd

On Jun 14, 12:31*am, "N_Cook" wrote:
Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok Tenma came thru with a schematic :-) Looks like I have a open 820uh
inductor that explans the low voltage on the one side. Now to try and
find one.

Thanks again for the help

Todd

Todd wrote in message
...
On Jun 14, 12:31 am, "N_Cook" wrote:
Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok Tenma came thru with a schematic :-) Looks like I have a open 820uh
inductor that explans the low voltage on the one side. Now to try and
find one.

Thanks again for the help

Todd

+++++

You have to find why the inductor failed, maybe has a secondary function as
a fuse.

On Jun 15, 12:28*am, "N_Cook" wrote:
Todd wrote in message

...
On Jun 14, 12:31 am, "N_Cook" wrote:

Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok Tenma came thru with a schematic :-) Looks like I have a open 820uh
inductor that explans the low voltage on the one side. Now to try and
find one.

Thanks again for the help

Todd

+++++

You have to find why the inductor failed, maybe has a secondary function as
a fuse.



Ok I guess I should check the current on the 2 transistors to make
sure they are close to the same then? One of the inductors has a
resistor across it for some reason, is this to prevent a oscilattion
like on my vacuum tube rf amp?

Todd

On Jun 15, 12:48*pm, Todd wrote:
On Jun 15, 12:28*am, "N_Cook" wrote:



Todd wrote in message

....
On Jun 14, 12:31 am, "N_Cook" wrote:

Todd wrote in message

....

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok Tenma came thru with a schematic :-) Looks like I have a open 820uh
inductor that explans the low voltage on the one side. Now to try and
find one.

Thanks again for the help

Todd

+++++

You have to find why the inductor failed, maybe has a secondary function as
a fuse.

Ok I guess I should check the current on the 2 transistors to make
sure they are close to the same then? One of the inductors has a
resistor across it for some reason, is this to prevent a oscilattion
like on my vacuum tube rf amp?

Todd

I was given a Tek 453 some years ago with this problem. It turned out
to be as a previous poster suggested one of the X drive transistors. I
wonder if you have a similar problem that may have caused the choke to
open as well. Lenny

Todd wrote in message
...
On Jun 15, 12:28 am, "N_Cook" wrote:
Todd wrote in message

...
On Jun 14, 12:31 am, "N_Cook" wrote:

Todd wrote in message

...

Anyone have a source for a service manual for a Tenma 72-905 O'scope?
I have one that the horizontal trace stays on the left side of the
tube only. Figure it's a problem with the sweep circuit. Anyone seen
this?

Todd

With the power off check cold, initially, for differences between the 2
final X-drive transistors, usually long-tail paired.

Ok Tenma came thru with a schematic :-) Looks like I have a open 820uh
inductor that explans the low voltage on the one side. Now to try and
find one.

Thanks again for the help

Todd

+++++

You have to find why the inductor failed, maybe has a secondary function
as
a fuse.



Ok I guess I should check the current on the 2 transistors to make
sure they are close to the same then? One of the inductors has a
resistor across it for some reason, is this to prevent a oscilattion
like on my vacuum tube rf amp?

Todd

+++++


It was often a fudge/kludge to use a resistor as a former for winding the
coil on ,and nothing electrical in purpose