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

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M Gregg

M Gregg

Disabled Account
Joined 2010
Ahh the ZN414,

Looked like a BC109 made by Ferranti quite sensitive but the selectivity was rubbish..:D Just remembered it had a stupid 1.5V supply...try making one of those without any experience...LOL but if I put a diode in I loose .6V... LOL... Took ages to think of an LED with a series diode...though I had discovered the theory of relativity..


I remember building an AM transmitter to pipe cassette music arround the house..my friend who went to work for the BBC had a full home made SW radio rig and an aerial that took us two weeks to get erected. The buzz at the time was to hear Ecuador radio (don't ask I have no idea why, it was a weak station..)

I had a few amps that semed to work as relaxation oscillators...sometimes took weeks to work out why the music sounded great then "tick" thump brrrrrr...power down and music again.. time out seemed to vary with how close your hand was to the circuit....worked well as a theremin.:D

after trying the move wires arround out came the dreaded books on how to build amps...and a large stock of old 1960's tube magazines donated by various people...help from the school physics department and use of a scope.
RIAA was the in thing for record....perhaps an everthing else thread "memories of early days electronics"..would be good...:D

Regards
M. Gregg
 
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Mooly

Mooly

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Joined 2007
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Audio Maniac said:
Hi Mooly!

I purchased a second hand 20Mhz Hitachi scope. Is it good enough to check op-amp stability?

Will this scope show high frequency oscilations working at 20Mhz?

I know a 100Mhz osciloscope is more apropriate but I can't stand it.
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The Hitachi scopes I have used have been good so get to know it and how it all works :)

A good 20Mhz "analogue" scope will display frequencies significantly higher than that although with reduced accuracy of course.

Make sure you have some decent probes (a 10-1 type) as these reduce the capacitive loading on the circuit as it is being tested.
 
Mooly

Mooly

Administrator
Joined 2007
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Something like this which is a switchable 10-1 or 1-1 probe. Good probes can be expensive and it depends on what is available locally.

60mhz Oscilloscope Probe

Any "divider" probe has a small trimmer cap built into it to exactly match the probe to the oscilloscope it is used with. This is adjusted by connecting the probe (when set to divide) to the CAL output on the scope which is normally a 1Khz squarewave. The trimmer is adjusted to give no over or undershoot on the waveform.

Of interest,
How to Build Your Own Oscilloscope Probes
 
RCruz

RCruz

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Joined 2006
Paid Member
Audio Maniac said:
HI there!

Sorry, but I am not a native english speaker and sometimes I don't understand figurative language. What did you mean? :eek:
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wrenchone said:
You're lucky you got any probes at all... The ones that came with my orig. Hitachi Scope (new!) were nothing to write home about.
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Traducao: Mesmo assim tens sorte de ter "Probes".. As que vieram com o meu osciloscopio Hitachi (novo), nao eram nada de especial.

Maybe he means you are lucky that your rig came with probes.... it could be worst and you could have none at all.
 
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A

AndrewT

R.I.P.
Joined 2004
Buy fast probes.
There's no point in buying cheap 10MHz nor 20MHz probes and then find they are not much use when you upgrade to 60MHz or 100MHz scope.
A switchable probe is nice, but a fixed 1X and a fixed 10X are likely to be just as useful. Maybe even a 100x, for very special HV use.
 
wrenchone

wrenchone

Member
Joined 2004
For a rough understanding of what I meant (a common English (at least, American English) saying), think - WOW! these probes are so fantastic I must write my parents and tell them all about them! Now consider the opposite of that. The translators got the basic meaning, I'm just providing the context. A pair of 60-100 MHz 10X probes are cheap these days, if you don't mind that they come from China. I just bought some Chinese 100X probes (nice for tube voltages) that were very reasonable. Most of the time your scope will be used for "indication" rather than ultra-precise measurements, so cheap probes are still useful.
 
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A

Audio Maniac

Member
Joined 2011
Hi! Thank you for being pacient!
And yes, I agree with you. There's no need of ultra precision, I just need an osciloscope to detect oscilation on my audio modifications.

wrenchone said:
For a rough understanding of what I meant (a common English (at least, American English) saying), think - WOW! these probes are so fantastic I must write my parents and tell them all about them! Now consider the opposite of that. The translators got the basic meaning, I'm just providing the context. A pair of 60-100 MHz 10X probes are cheap these days, if you don't mind that they come from China. I just bought some Chinese 100X probes (nice for tube voltages) that were very reasonable. Most of the time your scope will be used for "indication" rather than ultra-precise measurements, so cheap probes are still useful.
Click to expand...
 
D

dmccoach

Member
Joined 2012
Hi all,

Question perhaps related -- I have the EE Minimax and have placed (through brown dog adapters and yes they are correctly oriented) dual LME49990 and single LME49990 in U1 and U2, and U6 and U7 respectively...

Here is my question:
1) U1 and U6 (well I assume they are U1 and U6 -- they are closest to the back of the unit when facing it from the front) - U1 and U6 (again opamps closest to back) run hotter than U2 and U7 in both tube and SS mode. Why the difference? U1 runs about 130F and U6 runs about 105F, U2 and U7 run "cool" in the 80'sF
 
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Mooly

Mooly

Administrator
Joined 2007
Paid Member
That is impossible to answer without looking at both the circuit and the unit with the replaced IC's. It may be normal and just due to different loading on the IC's or different rail voltages to different opamps. Or instability ! Have you put a scope on the opamps to check ?
 
qusp

qusp

Member
Joined 2009
the 49990 does run pretty hot, like worryingly hot and thats normal and every condition ive tried it, but without the schematic and pics I cant tell you if that mismatch between them is what you should expect, you may be asking too much of them, or 2 may indeed be oscillating.
 
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