Swapping Op-Amps... you have checked to see it's stable haven't you ?

As an additional word of caution, I have been bitten by the oscillations more than once replacing OPAMPS with 'identical' parts. Obviously not exactly identical but with enough variation between different manufactures to cause marginal instability. This seems even more common with 78/79xx regulator IC's. As mooly pointed out the effects can be slight - in one case I recall the actual effect on audio was at about -80db relative but could easily be seen on the power rail (OPAMP based regulator).

If you have a scope - USE IT !!

dc
 
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BTW, if you do not have a signal generator, just feed your amp with a CD source (which samples at 44KHz, and should have no signal above 20KHz). This is probably an even better source for the oscillation sniffer than a 1KHz signal because with a CD cource you will be able to excite any potential supersonic oscillation problems.

Just for the fun of it, then over drive your amp and see what the 'sniffer' gives you.
 
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Tektronics are good. Even a 20Mhz scope will open a whole new world up to you, I started with a 6 Mhz. Remember that it doesn't suddenly stop displaying at its specified frequency, the response just rolls away (like an amp). Many a 20Mhz scope will display 30 or 40Mhz with reduced accuracy and many hign bandwidth scopes (such as my 100Mhz) have a 20Mhz "limiter" switch which to be honest I often use as it sharpens the trace (narrower bandwidth less noise).
 
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I always prefer,

CRT I dont like the digital stuff it seems to miss the information and the resolution just isn't the same...Even the old stuff as long as its stable and the caps are good. I like the D83 Telequipment...but still prefer any CRT to digital...Then again I suppose its what your used to using.. :)

Regards
M. Gregg
 
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I guess,

I should also say that to say everyone MUST have a scope, I don't agree...when starting out in this hobby its just great to get something working and if you have a volt meter you are doing well..

It is a good idea to take the project to someone or for someone to check you don't have any HF thats going to damage your hearing...That you cant hear...

How does anyone start this hobby...you arn't going to have a full tool kit are you..

Regards
M. Gregg
 
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I guess,

I should also say that to say everyone MUST have a scope, I don't agree...when starting out in this hobby its just great to get something working and if you have a volt meter you are doing well..

It is a good idea to take the project to someone or for someone to check you don't have any HF thats going to damage your hearing...That you cant hear...

How does anyone start this hobby...you arn't going to have a full tool kit are you..

Regards
M. Gregg

Depends how keen and how seriously you are going to get involved. I knew a scope was absolutely essential for all audio electronic work and so one of my first projects was to build one, and I've found a web picture too. I drew and etched all the boards by hand but used the same case as this one. It served well for many years too and then I passed it to a friend.

Oscilloscope | Flickr - Photo Sharing!

I agree though, that there is nothing like getting that first project to work, even if it's just an LED flashing.
 
I used to use a 20Mhz scope in the past when I studied electronics. If a 20Mhz scope is fine, I'd prefer not to spend more.

Thank you again for your support!

Regards,

Ricardo

Tektronics are good. Even a 20Mhz scope will open a whole new world up to you, I started with a 6 Mhz. Remember that it doesn't suddenly stop displaying at its specified frequency, the response just rolls away (like an amp). Many a 20Mhz scope will display 30 or 40Mhz with reduced accuracy and many hign bandwidth scopes (such as my 100Mhz) have a 20Mhz "limiter" switch which to be honest I often use as it sharpens the trace (narrower bandwidth less noise).
 
A 20MHz scope is better than nothing. You can see if there is clipping or any other type of electronic misbehavior.

For the longest time, I used a Hitachi 30 Mhz scope - not too large, rugged, simple user interface, sharp trace (the V-302B - still worth picking up used). It was useful to some extent for detecting oscillation, but it wouldn't tell you the whole picture if the oscillation was really high frequency. A low bandwidth scope with a sharp trace will at least be able to indicate high frequency oscillation. Stay away from scopes with blurry traces.

As an example, I was designing a cascoded emitter follower buffer, and got the characteristic "fuzzy trace" indicative of problems. On the Hitachi scope the fuzz was about 50mV in amplitude. I brought the offending circuit in to work and checked it out on a higher bandwidth scope, and the nastiness was a full half-volt in amplitude at a couple of hundred MHz ( it's not just a buffer, it's a wireless mike, too...).

BTW, these days, you can pick up a pair of 10X probes on E-pay for about ~$25. They aren't Tek quailty, but they're most certainly better than nothing.
 
Speaking of swaps and possible stability issues.....

Yesterday I helped a buddy put new supply caps in a SWR SM400 bass amp (circa ~1990). The preamp section uses 9x TL072 op-amps and has NO supply decoupling at all. I happened to have exactly 9pcs of OPA1642 and some DIP adapters and wanted to see if they would make any appreciable difference.

Now knowing there wasn't any supply decoupling I figured the least I could do for a plug-n-play scenario was put 100nf arcoss the supply pins on each op-amp. The power supply is a basic zener setup dropping about 40V off the main rails to get +/-15V. I knew I couldn't draw much more current than it was designed for, but after looking at the datasheets I figured the 1.8mA per channel of the OPA1642 was a pretty close match to the 1.4mA of the TL072.

After powering the amp up there was a horrible noise and I started swapping TL072 back in until it went away; I had to replace two of them before it did. Then I decided to check the supply rails and noticed that it was under +/-8V.

So my question is, am I overlooking something basic here? Is the 1.8mA per channel of the OPA1642 too much for the supply, or were they oscillating and drawing excessive current because of the lack of proper decoupling?
 
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Depends how keen and how seriously you are going to get involved. I knew a scope was absolutely essential for all audio electronic work and so one of my first projects was to build one, and I've found a web picture too. I drew and etched all the boards by hand but used the same case as this one. It served well for many years too and then I passed it to a friend.

Oscilloscope | Flickr - Photo Sharing!

I agree though, that there is nothing like getting that first project to work, even if it's just an LED flashing.

Great...

I built the one in Practical Wireless in the 70's...never got the flyback suppression quite right...never took any photos it was tooo long ago Kodak cube flash..:) School days..

I guess quite young for the HT at the time..Its interesting that I already knew how to discharge the caps after a few first hand encounters with electrons...learn by doing...er I can think of better ways...LOL

But the sound of your first radio and putting it through the 78 record player tube amp (short wave Dxing)...sorry I digress....

Regards
M. Gregg
 
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Great...

I built the one in Practical Wireless in the 70's...never got the flyback suppression quite right...never took any photos it was tooo long ago Kodak cube flash..:) School days..

I guess quite young for the HT at the time..Its interesting that I already knew how to discharge the caps after a few first hand encounters with electrons...learn by doing...er I can think of better ways...LOL

But the sound of your first radio and putting it through the 78 record player tube amp (short wave Dxing)...sorry I digress....

Regards
M. Gregg

I never had any success whatsoever with any radios as such although I had more than one amplifier that seemd to work quite well as a radio :)

I tried the ZN414 (remember them!) and various other contraptions. Yep, I was keen. Walkie talkies made with 555 timers as an osc and using the control pin to modulate the carrier. Must have wiped out half the neighbourhood ;)

The Purbeck scope was great, 800 volt HT zener regulated although the brightness seemed to dip right down if anyone turned the shower on and pulled the mains down a bit. The flyback used a 0.1 ??? 1000 volt cap from memory from CRT to a small transistor. It worked but was the one component that failed for me giving a "dead" screen. The cap went short.

But yes, to anyone reading I say this you can't beat a real analogue scope that traces out in real time on a screen what is going on.

wrenchone... I had a 30 Mhz Hitachi for several years with the day job. Forget the number but it had user positional cursors on screen. Used for servicing video equipment. Trio scopes were good too.