Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

"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

Tenma 72-905 O'scope
 

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.

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

"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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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.

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

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

Tenma 72-905 O'scope
 

On Jun 16, 12:13*am, "N_Cook" wrote:
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- Hide quoted text -

- Show quoted text -

Ok will be a few days till I get back to it. Still not sure why there
is inductors there. Just for a temp repair I replaced with a jumper
and everything seemed to operate correctly then. Need to find a
replacement for the inductor Tenma doesn't have them any longer.

Todd

Tenma 72-905 O'scope
 

Todd wrote in message
...
On Jun 16, 12:13 am, "N_Cook" wrote:
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- Hide quoted text -

- Show quoted text -

Ok will be a few days till I get back to it. Still not sure why there
is inductors there. Just for a temp repair I replaced with a jumper
and everything seemed to operate correctly then. Need to find a
replacement for the inductor Tenma doesn't have them any longer.

Todd

+++++


scrape off the lacquer/coating to inspect why it failed mainly. If you
cannot find a replacement, count turns and rewind it yourself

Tenma 72-905 O'scope
 

The question is do you really need that coil. Mny coils in scopes are
to tailor the frequency response of the amplifiers, but some are not.
If the coil is bypassed with caps on both sides, likely it is just
there to reduce HF AC currents in the PS busses and/or ground paths.
If so, you can probably just throw a hundrd onhm resistor in it and
forget about it.

Now if it is not bypassed on both ends, it has something to do with
frequency response, and being the horizontal amp you should see some
non-linearity in the sweep at thigher uS/DIV rates. Most likely it
would be a compression at one end or the other, but if it's severe it
could even be a foldover, but that's not all that likely. It really
does depend on the circuit.

Inductors are not generally meant to be fuses, but there are
exceptions. And no matter what, you should determine why it is open.
Did a connection fail, the wire break or whatever ? Or is it burnt to
a crisp ? If it's burnt of course something is shorted. Fix that and
then deal with the coil.

Thinking engineeringwise, 820uH is almost a mH. Pretty big. Now to
tailor the response of an amp stage, they have to deal with high
frequencies. The IE capacitance of the devices is not all tht high
[these days] so at that inductance, it is most likely part of a pi
filter supplying bias to the output stage. If that's true and you just
throw a resistor in ther (after fixing the short) it might not meet
FCC guidlines for RF or EMI. Oh please, whip me to death over that.
This is the most heinous crime of the century. Makes Hitler look like
Ghandi. We must repent.

But if the scope works, we can work another day. Eat, drink, be merry
or whatever.

Wjhat people fail to do is to understand the FUNCTION of the
components they replace. Some are redundant, some oare not critical,
some seem to just be there for looks. At one time I thought with
corporate greed and prifoits above all being the prevailing mindset,
that there would be nothing in a circuit that is no absolutely
essential, but I have learned different. What throws people off is the
don't realize just how cheap it is. That coil could have literaslly
been cheaper than a 47 ohm resistor for all we know.

J