Help fixing Oscilloscope

[jfls44]

I have a Tenma Model 72-320 Oscilloscope given to me that doesn't work. It won't sweep. The screen lights up and the strobe is present but doesn't sweep anymore. It did when I first started it up but after awhile it just stopped. Does anyone in here have knowledge on where I can start looking to determine if I should fix this or chuck it. I don't have a schematic or owners manual for it and have looked for a free one online but no luck. I brought home a Fluke 123 Industrial Scopemeter from work to try and troubleshoot it but with no schematics I'm not sure where to start. Do you think it could be the sweep generator bad? or something else. any hints or tips is much appreciated.

Jeff

 

[Sceadwian]

I'm asuming it has an XY mode? If it does you can test it using the stereo output from a sound card to modulate the X and Y lines to see if you get some kind of result. If the XY works then your timebase is probably shot. If you can't figure out how to repair it many scopes have an external horizontal input, you could use a crystal run micro controller and a decent DAC to generate an external timebase, or if you're better with analog just use discretes. I'll have to do this myself oneday as the timebase on my scope is just about done for. I think a micro controlled timebase would be better that the one originally in it anyways.

 

[ljcox]

I agree with Sceadwain, try it in the XY mode. This will prove whether the X amp is working. If you don't have a signal generator, take the signal from the secondary of a mains transformer. eg. 6 V AC.

I have had to repair my HP scope many times as it is old and resistors tend to go high resistance.

However, I have the service manual, which makes it easier.

Since you don't have this, I suggest that you do a visual inspection and look for any dry joints, burnt components, etc. You could also measure the resistance of the resistors with a meter. Remember that there may be parallel paths that will affect the reading. However, if it is higher than what the colour code shows, then it must be faulty.

If the reading is lower than expected, reverse the leads as there may be a parallel path through a semiconductor.

Also measure the voltage at various points including the supply rails.

You can also trace out the circuit in the area where the fault is. It is tedious and time consuming.

But the fault is likely to be some thing simple such as a dry joint or a faulty component. So it would be a pity throw the scope out if you can spare the time.

 

[Nigel Goodwin]

As suggested, a manual would be really useful - well worth paying for to be honest!. When I got my Hitachi scope the first thing I did was buy a service manual for it.

 

[kchriste]

Do you get any vertical deflection at all?
If XY mode works then try it with a free running sweep setting as the triggered sweep circuitry may be defective. Does your scope have a yoke or does it use electrostatic deflection? (Most older scopes use electrostatic deflection)
BTW, this is one of the better equipment manual sites I've found but your scope isn't listed. I've included it anyway for general interests sake:
**broken link removed**

 

[Nigel Goodwin]

Does your scope have a yoke or does it use electrostatic deflection? (Most older scopes use electrostatic deflection)

I though all scopes used electrostatic deflection?.

The only ones which wouldn't would be where it's a computer, and the display is just a monitor - and most of those use LCD's.

 

[kchriste]

That's what I was thinking of when I mentioned a yoke: some early CRT digital scopes had yokes. Haven't seen an analog scope with a yoke myself.

 

[Super_voip]

try **broken link removed** or http://www.w7fg.com/search.php?find= &page=77