Do These Look Like Bad Caps...

Hi gang,

I just got a Tenma Oscilloscope 72-6805 off ebay for a great price; it has a function generator built in!

After about 20 minutes being on, the Vertical scaling starts drifting upwards - you can see it moving! It goes towards infinity. Tapping the outside of the box makes the vertical scaling jump to smaller values. Sounds like a combination of bad caps and perhaps a loose connection?

I opened the thing and can't see any loose connections. I am posting a pic here of 4 gigantic eletrolytics. There's a suspicious yellow goo along the bottom you'll see, but it has the consistency of some kind of glue. Why glue would be there is beyond me, and it looks like a spill. So what kind of goo do bad caps leak, and does this look like it? It would be nice because these bad boys should be relatively easy to replace being so accessible.

Thanks in advance,

Mike

glue is common, it holds the caps in place. After a long time it turns from yellow to a rust color like on all the spx90 and spx90II psu's.  If you suspect a leaky cap, test them. If you suspect a solder connection re flow the solder. Either or will not hurt.

Thanks for the wisdom. I guess I have no idea how to go about fixing this thing then, since no caps appear obviously bad

I think I can still use it as long as I use it for only 20 mins at a time, since it drifts after that. If anyone has any suggestions to a newbie how to go about fixing something like this, I would appreciate it. Short of replacing every cap - you have to remove caps to even test them, right? Sorry to be so ignorant. It only cost $70 on ebay plus shipping, so kind of a little toy almost but nevertheless it could be really useful if I could get it to work reliably.

Thanks all,

Mike

here this will give you some idea

http://www.epanorama.net/blog/2010/03/23/electrolytic-capacitor-failures/

That was a terrific article! Thanks Pucho, you gave me some ideas! I'm thinking about building that cap tester just for fun! After all, I can still use the scope as is for the moment, and possibly even restore it completely!

Thank-you! I'll post progress if I build the meter, which I'm thinking about doing now...

Scopes run hot, so it is a good idea to look for dry solder joints on the PCB, especially it is an older unit.

Make sure the HT supply is DISCHARGED!!!!!

Peter

Thanks Peter; will do. I get the concept that fooling around with vacuum tubes can get you dead. Or at least "mostly dead" if you saw The Princess Bride!

Anyway seriously thanks; I'll watch out for the HT circuit. It's a beautiful little scope looks like from around 2000 or 2002; shouldn't really have much to do on it. I decided to just buy a capacitor tester with an ESR meter; I looked at a bunch of plans for building them but my DIY muscle is tuckered out from building the green! But I'm having so much fun! I'm 97% complete; the proud father will post pix when done.

Thanks again for making such a great preamp and I'm really excited to debug everything and get it working!

Take care,

Mike

pucho812 said:

glue is common, it holds the caps in place. After a long time it turns from yellow to a rust color like on all the spx90 and spx90II psu's.  If you suspect a leaky cap, test them. If you suspect a solder connection re flow the solder. Either or will not hurt.

Click to expand...

Is the glue added because the cap is big and the unit is susceptible to regular movement?

What sort of glue do they use? Can it widthstand high temperatures?

canidoit said:

pucho812 said:

glue is common, it holds the caps in place. After a long time it turns from yellow to a rust color like on all the spx90 and spx90II psu's.  If you suspect a leaky cap, test them. If you suspect a solder connection re flow the solder. Either or will not hurt.

Click to expand...

Is the glue added because the cap is big and the unit is susceptible to regular movement?

What sort of glue do they use? Can it widthstand high temperatures?

Click to expand...

Some use a variant of neoprene glue that is cold applied, but the majority today is heated glue guns. Two common types of glue, one at 105°C and one at 165°C. Electronic mfg uses the 105°C.

Interesting development. Now that I've powered the unit on/off quite a few times and left it running 20-30 minutes at a stretch, the issue seems to be going away. I'm wondering if the capacitors are self-healing somehow or reforming just by being at operating voltages; perhaps the scope just needed to be used more to rejuvenate them?

Any thoughts on that? My capacitor tester (ESR) meter has not yet arrived!

Mike

since this seems to be a problem that appears after heating up (i have the same problem on one of my scopes) would it be a good idea to go looking for an area with a coldspray ?
something like this http://at.farnell.com/chemtronics/es1550e/kaeltespray-400ml/dp/860440  ?

I've never heard of a "cold spray"! And the blurb in farnell is in another language and doesn't really explain it...so...what is a cold spray?

it´s a spray that produces cold air up to -55 degree Celcius .
on my can it states : " for quick localisation of thermic failures in electronic circuits . for cooling of transistors , resistors , silicon diodes , etc."

i´m sorry for the german site . in english they´re called freeze sprays
http://www.alliedelec.com/chemicals/freeze-sprays/

Thanks for explaining...now...how do you use it?

I'm assuming you have the scope OPEN and then you spray the freeze spray on the parts you "think" are the problems, and if the problems go away then you have found the source of the problems?

Am I getting you right?

Thanks, and Merry Christmas!!

Mike

i really don´t know .
i bought mine with the intention to just give it a try as you described , but never have until now cause i don´t wanna damage the scope .
i thought someone with some experience using this sprays would chime in .

Phrazemaster said:

I'm assuming you have the scope OPEN and then you spray the freeze spray on the parts you "think" are the problems, and if the problems go away then you have found the source of the problems?

Click to expand...

nearly right.
the problem doesn't need to go away, but there will be a reaction

Thanks for letting me know.