Should I buy this Oscilloscope?

[Wittgenstein]

I saw this Kikusui Model COS 5021 20 Mhz dual trace scope being sold locally for about $80. It includes one 100Mhz probe that does not have a ground clip, and is full working condition. Think I should jump on it? I have seen similar models sell for $200 more on ebay, but I"m not exactly sure what to look for in an oscilloscope. I plan on using it mostly for troubleshooting and furthering my understanding of guitar amps. Here are the pics:

[Chuck H]

It's a good price only if it's in good working order. If you don't know, or can't yet tell, and the seller is unaware of it's condition, I wouldn't buy it unless the seller is willing to take it back in the event that it's broken or needs expensive service. I'm not sure what's running on that upper trace in the pic? I've never had to have my scope serviced/calibrated so I don't know what the cost is for this work but I'd bet it can easily outwiegh your savings if the unit is broken or needs extensive service.

[Wittgenstein]

The second photo shows both channels being displyed. The second channel on bottom has nothing hooked to it (flat line), and the first channel has a signal generator (also for sale $30) running through it. All the description says is, "fully working", but I will have a change to look at this weekend.

[GregS]

At that kind of price it may be common for the seller to not guarantee it. I bought a scope from a local electronic parts surplus place for $100 with a couple of new probes thrown in. We turned it on and connected one of the probes to the calibration lug but that was the extent of testing it. He would not guarantee it at that price. It was buy at my own risk. Turned out to be the right decision as the scope works fine.

The worst case scenario is you might lose $80. If that isn't the end of the world then you might consider it. If the seller does guarantee it then you have nothing to lose. Might as well get it unless you find something you like better.

[Chuck H]

A signal generator for $30 is probably worth doing too if you don't have one now and it's not a toy. The scope isn't worth much in practice without a signal generator of some kind. You'll also need to build a dummy load. Here's a thread with a bunch of good info and a schem for what I built and use:

http://music-electronics-forum.com/t17289/

[Wittgenstein]

Thanks for the suggestions, I feel more confident now in my gut decision to get it. I will definitely give that thread a look. I'm hoping that I could talk him down to to $100 for both (or maybe less =) ). BTW, it is a Eico Model 377 sine/squre wave audio generator.

[loudthud]

It's easy to get burned buying a scope. In the pic in the first post, the channel with the signal isn't triggered. That might indicate a triggering problem or just that the seller didn't have a clue how to operate the scope. If you don't know how to operate the scope either, you might be buying something that needs repair. Getting documentation and parts can be a challenge when dealing with some test equipment.

One thing you should do is operate all the switches and knobs to make sure they operate mechanically. The next thing is look at the display. Check the Focus control seems to give you a nice sharp spot. Check that the sweep will go all the way from one side of the screen to the other. Check that as you turn up the Intensity (brightness) control that things don't change size. They might de-focus a little at bright intensity, but the overall size shouldn't change (high voltage problem). Check that the position controls can move the trace vertically above and below the edge of the screen.

When you buy a probe with a scope, usually most of the accessories are missing like the ground clip. Don't worry, you can get probes pretty cheap.

[Chuck H]

That's a perfectly respectable signal generator. It's one of those models that was sold all over. Lot's of them around. In other words, it's a work horse. $30 is a good price too. Pretty sure that's a tube unit, so it may need some service (filter caps and cleaning sockets, etc.)

[Wittgenstein]

Good to know. I found the manual for the O-scope and printed it out. I will give it a read so I can properly test it out before purchase. Now the seller informs me he has a Precision model 912 tube checker that quote, "works great, but a recap wouldn’t hurt it" that he is selling as well for $50.... what have I gotten myself into, lol. Perhaps if I bought the o-scope for $80, the tester for $50, he would throw in the sine wave generator for free. $130 for the lot does not seem to bad to me, and they are all components I have wanted to have, but seemed too expensive.

[Chuck H]

If you can do it that way, sure, do it. I don't have a tube tester. If a tube works, doesn't squeal and sounds good, That's a good tube for me. There's some debate about the suitability of tube testers for this application. ie: They don't test at guitar amp voltages, don't detect microphonics, etc. I'd own one if it fell in my lap though.

[JHow]

...Now the seller informs me he has a Precision model 912 tube checker ...

Ha! The guy knows how to up-sell! Watch that you don't leave with the "Slap-chop" or the "Sham-wow" (Works great for cleaning tolex!)

[loudthud]

A quick note on probes. Most of the low cost probes you can find don't like high voltage. The ones I've seen (X10) were rated at 400V. Connect it to the plate of a power tube, apply a signal and POOF, the probe is dead. Tektronix made an X100 probe that is rated at 2000V IIRC. You can find them on ebay, the price varies.

[km6xz]

I would suggest getting all three, and sell the tube tester to pay for everything plus new probes.
Tube testers are in demand, primarily among those who think they they tell them something useful. I have not had one since I was 12 years old. If you want to tell what a tube is doing, measure the operating parameters in a circuit or use a curve tracer, both would tell you 100% more and not require a separate box.

The generator is decent, not terribly low distortion but quite usable. The scope, if working, is a good deal and will pay for itself many times over in faster more accurate repairs and adjustments to your own gear, or even some income repairing gear for others.
The photo seems to be a slow sweep speed and probably is triggered, the trace is bright. It has some higher end scope features such as delayed alternate time base. For tube circuits, a 100:1 probe would be good to find.

[Wittgenstein]

I ended up buying all three for $135. I think I will do as you suggested km6xz, and sell the tube tester for what I can to recoup some of my cost. Everything works perfectly btw. Both channels of the scope are fully working, it was not triggered in the picture. The tester is in really good condition as well. It even has the original manual. Now I will have to build a load box and try this thing out.

[J M Fahey]

Speaking of Kikusui and old scopes in general, my own old faithful 5501 "Stereo Triggerscope" died 3v days ago.
I bought it new in the 80's and it has never left my bench since.
Murphy's law being the one trat runs the Universe, I bumped into its Owner/Service manual a couple months ago, obviously when searching for something else, and lost contact since.
I'm sure the thing has legs and runs away from me when I get close.
Oh well.
Symptom? No trace or bright spot anywhere ¿¿No HV??
I've been repairing things, no manual involved, for ages, so, let's open it.
To begin with, found the 10 stage voltage multiplier full of 4.7uFx350V leaking electrolytics, output voltage didn't go beyond 550V (it should reach that already at the 2nd stage).
After replacing all them , the first stage had around 250V, the second one over 500, whan I foolishly tried to measure the last stage (which by logic should be over 2000/2500V a spark flew into my multimeter probe, causing an audible snap.
What was I thinking? The highest scale on it states 500V !!!!
No, i can't find my HV probe, last used around 30 years ago.
The Fool's God took pity on me, and didn't burn my cheap multimeter. Thanks.
Now I had a 1K 1/2W resistor smoking badly, in the middle of a very high voltage Pi filter, made out of two .1uFx1700V "dynamite stick" Rubycon capacitors.
The first one had already been replaced ages ago by two .1x1000 Mallory capacitors in series., now found the second one shorted.
Went cruising electronics shops, *had* yo repair it yesterday same day or buy some working klunker as a stop gap, I use my Scope *a lot*.
Everybody politely laughed at me, at last found a few .022x1600V , 8 of them replaced the two original ones, still somewhat fitting in the (very limited) original space.
Oh well, I should be grateful.
End of the story: my old workhorse is riding again, as good as new.
I mention this to show you what can easily happen on any old piece of equipment, with the added problem that its circuits have very little to do with our everyday practice, there are no "generic" models to use as a reference, parts are way beyond what's usually available, etc.
I just got lucky.
Next time I find that yellowing manual, I'll handcuff it to some furniture's leg.