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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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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 |
#2
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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 |
#3
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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. |
#4
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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 |
#5
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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 |
#6
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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 |
#7
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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 |
#8
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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 |
#9
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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 |
#10
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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. |
#11
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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 |
#12
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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 |
#13
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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 |
#14
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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 |
#15
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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 |
#16
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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 |
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