Techtronix 454 oscilloscope

[windozeuser]

Hello board,

I recently got a hold of a Techtronic 454 oscilloscope for free off of someone, shes a beauty but is misbehaving. Tried measuring a simple square wave, but it seems even with auto trigger the waveform will fade off the screen and randomly come back. I even tried to use manual triggering and for some reason it will display the waveform correctly as a square wave, but perodically will connect the spaces between the rising edges into a solid line? What is that?


Also, on the gradient screen the waveform will sometimes "drift" downward on the screen, say I had it centered on the zero line of the screen, maybe a minute later the waveform will drift and move downward?

I'm looking for someone to give advice on what could be wrong with the scope it seems to be in general working condition, especially for a 40 year old scope.

 

[KeepItSimpleStupid]

The manual is here: https://bama.edebris.com/manuals/tek/454/ There is an A revision too.

One problem with old equipment is the necessity to reform the electrolytic capacitors. If you do a search on "reforming capacitors", you should get some results. generally, it means bringing up equipment that has been unused for a long time slowly up on a Variac while monitoring the current.

You may have a power supply issue or at least that's where I'd start. If it has plug-ins, I'd re-seat the connectors and exercise the switches.

The addition of the connecting lines really says that the intensity or blanking circuits have a problem. Loosing the trace altogether could also indicate power supply troubles.

I have seen a CRT screen have no control of brightness. Essentially full on all of the time and it was a short in the CRT.

You could try to shake it with the face of the tube facing up with the unit off.

 

[bountyhunter]

The waveform looks like the trigger is "rolling" which is to say not triggering at the same point each cycle. It looks like vertical and horizontal deflection are working but not the trigger circuits.

 

[bountyhunter]

Also, on the gradient screen the waveform will sometimes "drift" downward on the screen, say I had it centered on the zero line of the screen, maybe a minute later the waveform will drift and move downward?

That sounds more like a power supply problem. The vertical amplifiers are not staying balanced and are wandering. If one of the power supply rails is drifting, it could cause that.

 

[johansen]

One problem with old equipment is the necessity to reform the electrolytic capacitors. If you do a search on "reforming capacitors", you should get some results. generally, it means bringing up equipment that has been unused for a long time slowly up on a Variac while monitoring the current.

well he already turned it on and it doesn't sound like any of them exploded, so i'm not sure why you would suggest that would be a problem, or a solution.

I would say go ahead and replace ALL of the non critical electrolytic capacitors, that alone will probably fix most everything, including the periodic vertical drift.