Hi guys! Brand new here, and so glad i found this forum. you can't find jack for info on solid state amps. lol. Well i'm still a noob with electronics, but i'm learning. the hard way it seems. lol. i have a kinda recent crate gfx 65. it sounds fine, nice clean signal and all effects work and sound like they should. except there's only half the volume until it get warmed up after about 20 minutes then the sound slowly fades away to nothing. everything visually looks fine. i was thinking maybe it's the big caps in the power stage. which i can't seem to get loose since they glued the bajesus out of it. lol. any suggestions on how to get them out, or maybe i should be checking other areas as well?
thanks,
mike
Leave them alone. Why do you think they are the problem?
If main filter caps fail, you get loud hum from the unfiltered power that results. You don't get fading output.
First suspects are always the jacks with cutout contacts. You have a jack on the panel for extension speaker? It has a cutout contact, and if that gets dirty, you can lose level or sound altogether. Easy test is to just plug so0me extension cab into that jack. If all the speakers now sound, the jack is the issue.
You have an insert jack that can cause the same issues. Poke a plug in and out of that jack a couple times and see if it wakes up the amp.
Right next to the extension speaker jack is a large 5 watt power resistor, R77. Check it for opens. It is a 0.1 ohm or 0.2 ohm, so it will read like a short on your meter if it is good. Also check its solder or wiggle it to see if it is loose. Does wiggling it restore sound?
Those are things to check first.
Quote from: DiverDownafter about 20 minutes then the sound slowly fades away to nothing.
Not the power supply caps.
Don't have a circuit for this one, but it sounds like it could be some chip with internal thermal protection shutting itself down - wild guess.
Need to a) know if it's still alive from the Fx Return (if it has one) and b) confirm that you still have main and preamp supply voltages under fault conditions.
Thanks guys.
QuoteLeave them alone. Why do you think they are the problem?Quote
I had thought that maybe once the amp warmed up the caps would become leaky and drain out. But thinking about that now that would drain the power from the whole amp, not just the output.
QuoteYou have a jack on the panel for extension speaker?
yes it does. as well as a effects loop.
Quoteb) confirm that you still have main and preamp supply voltages under fault conditions.
gonna check up on all these asap. I had thought about a chip overheating. Since it has been to a tech before it was given to me to learn on. He never found the problem. I thought that might have been easy to overlook? As I said though i know jack by comparison. Will return with results. thanks again.
Quote from: DiverDownit has been to a tech before ... He never found the problem.
While we are waiting, pardon a tiny rant about people who call themselves "tech's" who seem unable to find a blown fuse. It's not like this is a
subtle or
intermittent fault in a seriously strange and complex item of gear. Signal tracing should be able to quickly locate the offending stage, and that should confine it to perhaps half a dozen possible components (most likely an active device of some sort). Even then, using the dumb "blunderbuss" method and replacing everything in that stage should have got him
somewhere. {Sheeeee...} :grr
Well so far I haven't been able to get the amp to fail, besides the quarter volume problem. i had the chassis out of the cabinet at first, so i put everything back together, and still no failure. so i never got the voltages under failure. the 5w resistor tested ok, but was super fricken hot. wow. 5w of dissipation must be alot. lol. i rigged up a cable to hook up an extension speaker, and yes the speaker in the combo did not cut out. also found that there is no signal from the fx return. so went to the jacks. they look good. i sprayed them all with deoxit. no change. hit it with some fresh solder. nothing. i guess i'm gonna have to get under the board to check where the ribbon cable connects. i did have to re solder the power caps because i was being hastey and not methodical. bad moves on my part. won't do that again, i promise. lol. and as far as the tech guy goes. i don't think he took any real time with this amp. there is noone within 50 miles of where i'm at that works on amps. so he gets flooded. why i want to be a amp tech.
*note* i found that with a vibro pedal in the loop, and the amp fx set to delay. even though the return isn't working when i pulled the plug out to where only the sleeve is making contact in the jack. it makes this awesomely wild noise.
*note2* the amp is not a gfx65, it is a glx 65. sorry about that. i just realized i wrote the wrong amp model. :duh
Sorry can't help with the problem but note that Your efx loop explanation is rather confusing.
EFX loop has an Output (Send) and a Return (Input).
So highly unlikely you will get a signal *Out* of Return socket.
If that is what you are trying to achieve?
Phil.
sorry i've never used the effects loop on any amp. so i don't know alot about it. well with a pedal hooked up in the loop. the sound does not come through. the mention of the crazy noise was created by the delay and hum from the plug, not the actual vibro sound. actually when the fx return jack has a plug in it, it kills all sound. i guess signal was the wrong term to use, lol. though i'm not real sure how to test a jack with a multimeter. what i did was while the amp was on i plugged into that jack, and set my meter to 200v dc since that is the next setting beyond 20v (which i tried as well). and put my leads on the 2 tabs on the jack and got nothing, 0v. i figured there should be some voltage going across the jack when plugged in.
OK then get a guitar cord (That is known to be good) and plug one end into the *Send* and the other end into the *Return*.
This bypasses the internal switching setup and of course should guarantee signal passes from preamp to power amp unaltered.
It is possible the internal switch may not be connecting properly and display the symptoms you describe. (unlikely but certainly possible)
The idea being lets get the easy stuff out of the way first before resorting to open heart surgery of the gizzards. 8|
I personally would love a dollar for every efx loop that eF's up,, it is a common cause of heart ache for the novice.
If I could shoot the idiot that ever invented such a daffed idea I would :-X
Am I being too harsh? :-[
Phil.
Ah geez, um... it is kinda important to use the right model number, if nothing else, any reference to resistor number so and so will be meaningless.
Nonetheless, any intermittant level problems or loss of sound is still most likely a jack problem. The FX return may work fine as a return jack, but the problem is when the jack is not in use, there are contacts inside it that carry the signal on past. If those get dirty, they don;t always work. Same with extension speaker jqack. It may work with a speaker plugged into it, but when that jack is not in use, it has contacts to complete the circuit to the main speaker. Those contacts get dirty, you lose level or get silence.
i tried the loop bypass trick. no change, but a faint hiss. i also tried plugging everything in, and nothing. i took some pics of the amp and schematic i have, but they all take too much memory to post. xP
hey i figured it out. lol. the 1st pic is of original fx loop jacks. 2nd is of pcb with replaced jacks, the jack without the nut is the speaker extension. 3rd is the power supply side of amp and 4th and 5th pics are a continuing of the 3rd. the 5th has the effects stage with the blue and white braids. the schematics are of amplifier stages. if you guys want the fx stage pages i'll get those up. on the fx board in the 3rd pic there is a grey ribbon cable with a white connector. can someone please tell me how to disconnect those without damaging them? lol
Push down on the top of the white connector, it feels springy doesn't it?, hold it down and pull the wires out of the holes. PLEASE mark the wires and connector somehow so you do not install the wires backwards later.
SOo what exactly did you figure out? What solution is underway?
oh, lol, i was talking about figuring out how to get the pics posted. i had to resize them to reduce file size. the amp has me thrown for a loop. lol ah geez i can't believe i didn't think of that. :loco
thanks
Quote from: DiverDownwhy i want to be a amp tech.
In the land of the blind the one-eyed man is king.
"Why do you rob banks?"
"Coz that's where the money is."
Quote from: phattI personally would love a dollar for every efx loop that eF's up,, it is a common cause of heart ache for the novice.
If I could shoot the idiot that ever invented such a daffed idea I would :-X
Am I being too harsh? :-[
Me too.
Just a tad; it's a basically reasonable idea but the typical problem is lack of use so the switch contacts don't get scrubbed, oxidise, and there we have yet another Fx loop socket fault.
To be specific, Fx Send is the signal from the preamp to the outboard Fx, Fx Return is the signal coming back in to the power amp, so the Fx Return is often effectively the input to the main power amp and therefore a handy point to inject a signal (from guitar, radio, cassette, CD, iPlod, &c) to determine if the power amp section is healthy. This is normally after the amp volume control, so when injecting a signal you need to have a source (like an MP3 player) that has its own output level control, and start with it set zero/very low if you don't want to scare ten years growth out of the cat.
The huge blurt you got, for whatever reason,
tends to suggest that the power amp in this one is okay and that the signal is getting strangled somewhere upstream, and as
@phatt has observed, the bridging contacts that are supposed to carry the signal across from the Fx Send to the Fx Return when nothing is plugged in, are a very common cause. By plugging a (known good) guitar lead into both you provide an alternate signal path that bypasses the jack switch contacts.
Another thing to try is to see if you can get a signal
out of Fx Send to drive another amp (e.g. its Fx Return). If this is healthy it
tends to point to the Fx jacks themselves, but if it's still dodgy then it
suggests the problem is in the preamp before the Fx jacks. (you can also do this in reverse, other amp Fx Send to faulty amp Fx Return)
The only voltage you should see on the Fx Send should be about 1 volt of audio (fully driven), no DC. Signal tests across to/from another amp with Fx jacks are generally more reliable than using a multimeter, but then the ideal is to have an oscilloscope (CRO), and IMO
any CRO is better than no CRO; however something like an old cassette deck with VU meters (in record mode) can be pressed into service as a poor mans audio millivoltmeter, and if it has a headphone socket you can also monitor what it is indicating for noise and distortion.
Circuits.
"Doctor, my eye-ess..."
Okay, well those are (somewhat) better than nothing.
Locate Test Point 9 (TP9) near the Fx Return. This is the main power amp input and a volt or so of signal there should produce pretty much full output (and blow down a wall).
An early check to make are the two main DC voltages +/-40-(something) volts at TP14 and TP15, and the preamp supplies which are ... er ... um ... (blink blink) ... on the two zeners just to the right, I dunno, whatever it says, but normally around +/-15 volts. Making a habit of checking that these supplies are about right early can save a lot of wasted time.
{Nothing ever goes according to plan, and if it does, you've missed something -
Murphy's Law Rules, okay? :lmao: }
HTH
Quote from: Roly on March 05, 2013, 09:24:11 AM{Nothing ever goes according to plan, and if it does, you've missed something - Murphy's Law Rules, okay? :lmao: }
HTH
Dude middle name is murphy i swear. lol. :lmao:
i don't think any of the other amps i have, have a fx loop. but i'll check and definately try pairing them up through the loop on the crate. i want a scope bad. i would like the top dog high speed digital scope, but there's no way i can afford one of those, or even a crt scope right now. not much going in the piano moving business. but no use fusing. gotta get this amp working. lol yeah i know the pics are small even expanded. i guess i can try without hd next time. maybe use less space for a sd pic to keep the image size big. yes sir, a couple of diode tests and test points 9, 14, 15. a hunting i will go. 8|
well the marshall mg30fx i have doesn't have a fx loop, but it has a cd in and a emulated line out. which from the idea i'm getting from what you guys have been telling me is that those 2 jacks should be relatively the same. so i tried plugging into the crate's fx return with the marshall in the line out. no difference in volume, but the sound was distorted. i tested the 2 diodes both had 15-16v on them. tested the bridge while i was at it, and they had about 4.5v on them. i'm having trouble locating the test points though. i don't see any marked pads on the top side, and on the bottom i only see 1 or 2 empty pads.
I'd suggest that you avoid digital scopes initially, shop around for an analogue/CRT scope, maybe 2nd hand, more knobs the better. You can get a new Chinese one for under $100 and people throw out their old ones when they upgrade, tap local Ham sources &c - you might get lucky and find an old Ham with a CRO they don't want who is willing to give you a flying start. Old analogue CRO's are a biot like a stereo that feeds a tube instead of a speaker, and provided you don't poke a finger at the high voltage, pretty straightforward to repair. I'm a bit of a CRO fanatic and have a fair collection, so if you find an old valve one that needs repair...
Finding the original circuit (PDF &c) is best, scanning is 2nd best, but if you can only photograph
do not reduce the resolution, they are hard enough to read as it is (but at least we can see the general signal flow if not all the text) Lots of light helps digital cameras.
You seem to be generally on the right track.
Quote from: DiverDowntested the bridge while i was at it, and they had about 4.5v on them
Not sure what you mean by that.
Just to be explicit; you found 15V across each of the zeners, yes? If so that looks like the supply is okay.
ok, i had to go back. i'll just say i will remember zeners are the little glass looking diodes from now on. *facepalm i just meant that i tested the bridge rectifiers also. i didn't know if it would matter, but hey there it is, lol. ahem, well the zeners did not have 15v on them, more a up and down range from 0.00 to 0.065 volts? ??? whaaaaaa? D17, D14, D13, and D23 all did this. D28 and D29 actually that time more like 16.5v.
D28, D29 are 16v zeners and ought to have 16v across them with power on in the amp.D17, D14, D13, and D23 are not zeners, and so will only show a forward drop of about half a volt at max. In this schematic, they will probably not have any voltage across them at idle.
Any kind of diode can be glass. Zeners could be glass or they could be plastic. That is not a reliable way to tell them apart.
Oh Enzo, my eyes thank you. <3) :dbtu:
Okay, so it seems like the power supply rails are good.
Now you need to inject some music from your MP3/CD/Cassette player into Fx Return and see if you can get the power amp and speakers to make a really loud noise.
Post results.
{Measuring components in-circuit has many traps simply because other components are connected to them which may give confusing/misleading results. If you really need to know then you have to lift one end, but we don't just do this all over the board because it can do more harm than good by damaging components; you really need a suspect target first, then it's worth the risk}
aww geez the diodes got me again. i plugged the ipod into fx return and no noticeable change in volume.
Personally I would want to measure TP14 and TP15 for the (+/-40vDC). I would measure this once immediately after turning on the amp and then play the amp until it starts to cut out completely. Then I would test the voltage on the power side of the amp. To me it would seem something is heating up and likely could be a bad solder joint/component. I mean the power amp is lost already to some degree (half of or quarter the volume) and then after it heats up, it cuts out completely. If you are doing your tests right we can't blame the preamp for this occurrence so far.
What I think we all would like to see here all the TP9-TP13 voltages measured, recorded and reported. For another I would like to you to check all the IC(1-5) chips pin 4 and 8 for the -/+16v to see what those are measuring. Describe how you are measuring these voltages as much as what numbers you actually measured. Got to know you are doing this correct, since reading this I am a bit confused as to where we are going at this stage of troubleshooting. Take is slow and don't rush, as I don't mean to come off as rushing. Let's get down to numbers and measuring them the right way.
For example, I would test all the TP9-TP15 at the starting of the amp... Write those numbers down, and then play mp3 signal into the amp till it cuts out... Then test TP9-TP15 and lets see what we get... Are they the same or different?? My gut tells me it is something very simple and the voltages might show us better where to look for a bad solder joint or component. Personally, after doing this I would solder the entire power amp portion again. Then try the same test again.
P.S. When you run the iPod into the FX return for while does the amp eventually cut out too?? All of these questions need answers. Good luck :tu:
P.P.S. Also here is a site that you can create a free login to get 1kHz sine wave sample that is 30 seconds long... Just create mp3 and loop that track on your iPod. That will suffice on testing the power amp circuit.
http://www.freesound.org/people/klangfabrik/sounds/28636/#
TP14, (which i'm guessing is C67 + lead to C69?) meter set to 200v DC reading is 37.2 volts. TP15, (C70 - lead to C66?) reading is 37 volts. TP9 is giving me some trouble in locating the points. i've figured out that the fx jack tip is connect to RC2 wire 5, but i cannot determine which cap is C68 due to all the glue. i'm gonna go back and try to trace that down. i'm still having trouble getting the amp to fade out like it was doing. i don't know if i should be happy or mad about that. it's been cooler than normal in the house. maybe that's not helping? the only thing i've done to the amp beside dissamble it and reassemble it 5 times, lol, is reapply solder to the power caps and touch up a grounding pad that had some weak dirty looking solder on it. gonna get back to it. just wanted to update with that while i spend a couple hours deciphering the layout of the pcb. :lmao:
oh yeah thanks enzo for the pdf. that will be great to add to my project log. :dbtu:
and dr.gonz thanks for sharing the freesound.org link. i've actually been using that site for some time now for samples to add into music ideas. haven't ever used it as a tone generator, but now i have the perfect application for that. 8|
ok i tested TP9 as wire 5 in RC2 to C68 (finally found it) tested for continuitity, got the tone, turned on the amp 0 volts. which i have now figured out is correct. i don't on the, other hand, know how to measure AC Vpp? i tried just switching my meter over to ac, but that gave me nothing. do i need to have the sine wave running throught the amp while i'm testing for that? got the same results on TP10,12, and 13. TP11 shows being coming from the base of Q5. i looked up the pinout and datasheet for the TIP-142 and 147, and went to find the correct lead to trace and discovered there is no Q5 in the board. just an unused slot for one.
OK, then you will like this too...
oh wow! dude thanks! :dbtu:
{Just a quicky 'coz I gotta make a trip to help push out an old school chum in his burning longboat.}
Quote from: DiverDowni'm still having trouble getting the amp to fade out like it was doing. i don't know if i should be happy or mad about that.
Not so much mad and annoyed. This goes with the turf. Faults may be intermittent, but
they don't repair themselves, and this one is now hiding under the sofa.
Two points flow from this; stage gear is mission critical so it's very important to
correctly identify the fault so you can be certain you have actually
repaired it, not just "fixed" it. When you do this for money these are called "bouncers", they bounce back into the workshop and your lap, and they (sometimes unfairly) shred your reputation. Nobody cares about the other 99 that didn't, it's 100% of
this case, and it really pisses clients off.
From this flows the reason we don't use the "bunderbuss method" and simply rip into a faulty item of gear randomly replacing parts in the hope. This hardly ever (never?) works out, you muddy your own waters, can just as easily paper over a fault as cure it, and there is a very real risk of introducing
fresh faults in the process and getting yourself into a real pickle (because faults don't
add, they
multiply difficulty - which explains why it is sometimes harder to bring up a fresh build that may have several mistakes, than find just one fault in previously working gear).
Faultfinding can be like trying to catch a skittish cat, you need to sneak up on it a bit, trying not to disturb things too much initially until you get a handle on what is going on.
I've just built a box for a well-known guitarist and it's playing up, so another part of this trip is turning up on his doorstep tomorrow armed with tools, CRO and some parts. I want to see it
in situ, as he is actually using it, because it worked perfectly on test here and his problems may be situational. It's called "service", and when it's fixed he will tell people that I drove 200 miles to see him right, not that I'm a crap tech. It's going to cost me more in fuel than I'll make on the job, but sometimes you just have to do these things to keep your rep.
Quote from: DiverDownhow to measure AC Vpp? i tried just switching my meter over to ac, but that gave me nothing. do i need to have the sine wave running throught the amp while i'm testing for that?
Multimeters actually measure the
average AC voltage but are calibrated to read RMS on a
sinewave (i.e. it's not true for other waveforms such as clipped, square, &c).
The
peak voltage is strictly;
V
RMS * root(2), (root(2)= 1.414)
...or roughly half as much again (x1.5).
The
peak to peak is
twice that, so strictly;
2(V
RMS * root(2))
...or roughly
three times the RMS reading. Peak-to-peak generally implies it has been measured with a CRO since that is the easiest thing to determine on the screen.
These levels are by convention with the output just fully driven (unless marked otherwise, say "typical"). In any case they are intended to give you a strong indication rather than be highly accurate. So yes, AC signal measurements require a driving signal, however keep in mind that little DVM's are rather deaf when it comes to small signal voltages - a CRO is the ideal.
Don't worry about making mistakes, mistakes are what we learn from.
www.youtube.com/watch?v=xhQ7d3BK3KQ (http://www.youtube.com/watch?v=xhQ7d3BK3KQ)
i'm having some troubles with my ipod, and i keep forgetting to grab the wife's before she leaves. been looking out for a scope too. i found a $60 diy scope kit on amazon. seems dodgy from the reviews. what is a suitable scope's specs for this kind of work? do i really need to go up to or past 100MHz? a book i read suggested 100MHz. so, i've been working on a digital delay pedal project in the mean time. lol hope to have it debugged and some pics up soon.
next time i'll have some results to report.
Experienced oldtimers such as Enzo, often suggest getting a good, working old scope, for peanuts, from places like Ham Fests, garage sales from old Techs now retired, but specially School Labs which have no more use for them.
Only precaution is to check them to work perfectly.
After all, you buy them to repair *other* stuff, not themselves ;)
Quote from: J M FaheyAfter all, you buy them to repair *other* stuff, not themselves
Awww ... where the fun in that? ;)
I assume you mean this one;
http://www.amazon.com/CanaKit-Digital-Oscilloscope-DIY-Kit/dp/B004G2KQT8 (http://www.amazon.com/CanaKit-Digital-Oscilloscope-DIY-Kit/dp/B004G2KQT8)
Spec;
9VDC power supply voltage
<280mA current draw 8 bit resolution
256 sample memory depth
1MHz analog bandwidth
100mV/Div-5V/Div sensitivity
1M? impedance
50Vpeak-to-peak max input voltage
DC/AC coupling
Save and display up to 6 captures to memory
Transfer screen capture to PC as a bitmap file (serial adapter not included)
Backlit LCD display
Now while this is pretty cheap it is also pretty basic. A while back I bought an ex-ADF CRT CRO and probes for about three times this price that is very full featured (Philips PM3055) with dual trace, A delayed by B timebase, LCD setting readout, computer controlled auto-set button, and a number of other features. I'm not suggesting you need a CRO like this, but it gives you an idea of how much you can get on the 2nd hand market for similar to double the price.
Looking at the specs above, it draws over a quarter of an amp at 9 volts, so you won't be running it on a battery, a suitable mains supply is a must, and ideally should be well regulated.
It also requires a case, and a proper probe kit like this;
http://www.amazon.com/150-MHz-Oscilloscope-Probe-Kit/dp/B0002KR9FW (http://www.amazon.com/150-MHz-Oscilloscope-Probe-Kit/dp/B0002KR9FW)
1MHz bandwidth is fine for audio work but there are a couple of gotchas with digital CRO's; "8 bit" means it has a vertical resolution of 256 pixels, +/-1 pixel; it has a 256 sample depth which means its horizontal resolution is also only 256 pixels. While these will give you a good indication they will also appear a bit "steppy" and could be potentially misleading in diagnostic work which, apart from school labs, is mainly what you use a CRO for.
The sensitivity range on the Y axis is also not very impressive and 100mV/div wouldn't be enough for a lot of what I use a CRO for, so it would need a protected preamp.
At the other extreme 50Vp-p max input is about a tenth of a typical CRT CRO, and since I do a lot of work on valve amps would be a major liability.
It can save and restore waveforms to/from PC storage which a CRT CRO generally can't; BUT, you can't leave it connected to your PC because PC's are earthed and this will at least lead to earth loops in your test setup. It also ties you to making ground referenced measurements when you sometimes need to float the CRO "ground" to some other reference. And you need an "adapter" not supplied. But how useful is this storage facility? When I've had it available I've never actually had cause to use it, and with modern and cheap digital cameras I can take very good screen shots when I need to (which isn't often except to illustrate a point in an article).
Then there are the customer reviews;
Reckless and negligent business practice
"The seller is well aware of design error"
CanaKit DSO 062 Oscilloscope
"I would not recommend this little oscilloscope ... it does NOT work ... There is NO customer service ... I would not recommend CanaKits"
Good kit if you're advanced
"Persons with no experience in electronics should not get a kit. ... your directions for building it were not that clear."
Miss Led
"the kit did not include a power supply, case, probe or instructions."
You need to be in the right frame of mind
"This kit is definitely not for everyone. Do not even attempt it unless you are well experienced putting electronic kits together."
So all in all I'd consider this a gadget, toy, or demonstrator, not a serious oscilloscope.
I have reviewed a number of freeware oscilloscope programs for Windows that employ the sound card input. These have most of the limitations of this kit, plus they are AC only. They particularly suffer from the common grounding problem, only one is actually voltage calibratable, and unless you build a protective preamp/attenuator to go in front of your sound card you place your computer at risk. I get around this by using old junkers like 386's to P-1's running W95 to XP as my workshop computer, of which I have a ready supply free, so if I blow one up by doing something stupid then it's no big deal - but they only have a limited role anyway (e.g. spectrum analyser, data recorder) since I have a range of analogue test gear available which are safe to 500 or more volts with ground isolated signal commons.
http://www.zen22142.zen.co.uk/Prac/winscope.htm (http://www.zen22142.zen.co.uk/Prac/winscope.htm)
Overall I agree with
JM, shop around 2nd hand sources - there are good CRT CRO's going for cheap prices thanks to the infatuation with digital CRO's - take advantage of it. {Jaycar here in Australia are currently clearing full-featured $600 CRT CRO's for half price.}
My thoughts:
ANY scope will work fine for audio in terms of specs. They do not make a scope with too low a bandwidth or freq response. When was the last time you saw a scope with LESS than 1MHz bandwidth? I recall some fine old Tek scopes with qa 500kHz response. 1/2 of a megahertz. Even that is way more bandwidth than you need.
Are some scopes more desirable than others? Sure. Are some guitars more desirable than others? Yes. But you don;t have to buy a $4000 guitar to play, a useful old beater will work just fine and set you back $100. If a scope puts a trace on the screen and actually responds to the input, it will be useful. My point is that it is MUCH more important that a scope be working than it is what its specs are.
I much prefer triggered sweep. It will stabilize the image better. But older scopes, especially entry level ones, could be "recurrent sweep." That means the horizontal sweep is not linked to the signal, it just sits there sweeping at whatever speed you set. SO your sine wave might slowly creep along the screen instead of sitting there rock steady. well even if that picture slowly sweeps along - like the heart monitor scope in a hostpital - you STILL can see the ripple on a power supply or the clipping on an audio signal. And that is what the scope is FOR. Most scopes are triggered these days, but don;t pass up a cheap or free one because it lacks triggering.
Like a cheap guitar, a cheap scope will teach you how scopes work and what they do and what they are good for. Someday you may be ready for a fancier scope amd by then you wilol know what you are looking for.
Just my opinion, but I don;t think digital scopes are for beginners, and frankly maybe not the best choice for audio anyway. We associate "digital" with new and up to date and high tech, but digital scopes have their own characteristics, and if you do not understand those, the results may be confusing or less than helpful.
Now if you are doing serious work, and especially work in the digital comain, then maybe to consider a more powerful scope, and/or a digital one. But for straight amplifiers, not needed.
I agree with JM agreeing with me, look at old used scopes for cheap.
Quote from: EnzoJust my opinion, but I don;t think digital scopes are for beginners, and frankly maybe not the best choice for audio anyway. We associate "digital" with new and up to date and high tech, but digital scopes have their own characteristics, and if you do not understand those, the results may be confusing or less than helpful.
Well I kinda skirted around saying that, but that is very much my opinion too.
A basic digital CRO can be built around something like an Arduino very cheaply. The problem is that the performance is normally so compromised (like in the one above) that it is virtually reduced to a practically useless toy.
By the time you get to a digital CRO that can equal a CRT CRO you are generally starting to pay some serious money.
When looking at any CRO one of the basic telling features of sufficient quality is that you can adjust the trace for a fine line right across the screen. The correct setting for the brightness/intensity is
just bright enough to clearly see, not "blooming" (smudging). The focus control is obvious, but a control marked "Astig" is to balance the focus between the ends of the trace and the middle. A fine sharp trace means that the EHT supplies are healthy.
(http://us.123rf.com/400wm/400/400/tihis/tihis0804/tihis080400389/2981531-oscilloscope-screen.jpg)
This is actually set a bit too bright. Note that the traces don't quite align with the graticule, which is what the "trace rotation" control is for (if it has one).
CRO's may have an external graticule in front of the tube, or in better CRO's it is etched into the tube face.
It is possible that an old CRO may have suffered a screen "burn" due to excessive intensity and/or a stationary trace or dot for a long time. This shows as a darker shadow of the old trace crossing whatever you are displaying, but is generally only a problem where a CRO has been used as something like a modulation monitor of years, and a
bit of burn-in isn't really a problem.
Most CRO's these days will have pretty full-on timebase and sync/trigger controls, but really old ones had timebase course and fine, and a sync control. One of these is still very useful for audio work but the timebase isn't really calibrated; so look for one with a timebase control that is actually marked in "time/div" like 50mS/cm or such. Similarly really old CRO's just had course and fine vertical attenuator, and the same goes here, useful, but one calibrated in "volts/div" is moreso.
An important feature that is sometimes missing on
really old CRO's (and a few later real cheapies, and some Heathkits) is the ability to display DC signals, so look for an AC/DC coupling switch. More modern CRO's will almost certainly be able to display DC signals.
As
Enzo said, just being able to
see the signal puts you streets ahead, and all the extra nick-knackery really only serves to make things more comfortable. So as long as you can see a trace on the screen and make it jump up and down and show hum by putting your finger on the probe, it is fairly hard to go wrong with a first CRO. I actually built my first CRO, and believe me it was
really basic, but even so got used a lot.
Switches and pots in CRO's are like switches and pots everywhere, they get dirty and noisy and cause the trace to jump up and down or the timebase to freeze until the switch is worked a bit, but just like an amp a good clean and squirt of metho or contact cleaner can work wonders.
Some examples;
(http://www.hamsterking.com/temp/pm3055-fullsize.jpg)
This is the one that I currently have on my bench. It is very full-on, 65MHz, and just about all the CRO one could ever want, and certainly for audio work. I picked this one up for $185AU and I notice they are currently offering on eBay at that price. This is to give you an idea of how much CRO you can get for under $200.
(http://i188.photobucket.com/albums/z265/kersanov/Misc/Tek465.jpg)
Another up-market CRO.
(http://notinteractive.com/stuff/scope/front.jpg)
Very basic homebrew, uncalibrated and very basic, but still useful.
(http://i01.i.aliimg.com/photo/v0/285726265/CQ5002_student_oscilloscope.jpg)
Typical cheap Chinese "student" CRO, semi-calibrated, ~$100AU new.
(http://img.2dehands.be/f/preview/71686234-oscilloscoop.jpg)
Typical basic service workshop CRO (note terminals rather than BNC connectors)
(http://thumbs4.picclick.com/pict/1208101777274040_/Telequipment-Oscilloscope-Model-S51B.jpg)
Another fairly basic workshop CRO, a bit more up-market (BNC connectors).
http://hacknmod.com/hack/how-to-build-your-own-oscilloscope/ (http://hacknmod.com/hack/how-to-build-your-own-oscilloscope/)
You could build your own, but by the time you pay for a tube and properly mount it this isn't the cheapest way to go.