Tenma 72-905 O'scope

"Todd"

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

Are both traces so affected ?

... Phil

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

re

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

Todd

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

Todd

"Todd" "Phil Allison"

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

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

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

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

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

as

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

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n as

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

as

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

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

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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

O'scope?

2

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

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