Telequipment S54A scope problem

[aharchenko]

Hi,

I'm fairly new to electronics and I've been given a free Telequipment S54A scope with no guarantee that it works.

Here is the issue I'm having with it. The scope turns on, but instead of a sweep I get a dot in the middle of the display. And no matter what I do, nothing else happens. X and Y shift knobs do not move the dot. I tried to follow the manual for the "first time operation" procedure which tells how to set all the knobs and buttons for base operation, but no change in the display.

Is it broken?

I Googled it, but did not find any information on the particular symptoms I am seeing.

Thanks in advance.

[IMGDEAD]https://s33.postimg.org/aqmpj6g33/IMG_20160605_140108.jpg[/IMGDEAD]

 

[Mooly]

I'm afraid it does sound kaput tbh.

If you can come up with the service manuals then it might not be a lost cause. You should begin by checking the basics and confirming that the internal power supplies are producing the correct voltages but be careful because there are high voltages floating around.

 

[aharchenko]

Thanks for the reply.
Actually I managed to repair it. Opened it up and first thing I saw that one of the resistors had one of it's legs hanging in the air.
Soldered it in place and now it works like a charm, get a nice solid trace.

However I saw something interesting inside that I've never heard of before.
Two of the transistors on the board had coins slipped over. The coins had wholes drilled into them just the right size for the transistors.
Can anyone enlighten me on why in the world would someone do it????
Here is the pic of two Canadian pennies over the transistors.

[IMGHTTPSDEAD]https://s33.postimg.org/um9yqn1vj/IMG_20160605_210620.jpg[/IMGHTTPSDEAD]

 

[Mooly]

Well done, that's great

The coins look like some attempt to add heatsinks to those transistors. Seeing as they are a pair, and also seeing a pair of high wattage (so hot running) 5k6 metal oxide resistors below leads me to believe these are either the X or Y output stages that drive the plates of the tube.

The big question is whether the original design used heatsinks on these transistors (you can get 'T05' heatsinks) or whether it was some misplaced attempt to supposedly improve reliability. Adding a large chunk of metal to the transistor case (which electrically is also the collector) can in some cases alter the circuits high frequency behaviour due to the effect of stray capacitance.

 

[Tube_Dude]

Adding a large chunk of metal to the transistor case (which electrically is also the collector) can in some cases alter the circuits high frequency behaviour due to the effect of stray capacitance.

Right, Mooly! And it also increase inertia, because coins are heavy, making the scope more sensitive to mechanical shocks and vibrations.

 

[Mooly]

Good point, it could certainly do that, and particularly so in an instrument that got carried around a fair bit.