Date   

Re: 2221 100Mhz non A

Roger Evans
 

I have one of these 100MHz non-A versions, it is really nice to use and has the GPIB option. I found my way around the analogue circuitry with the aid of the 2230 manual but found nothing that resembles the digital board, You only have to look at the front panels to see that the 'A' is completely different with buttons below the CRT to select digital functions.

To lift up the digital board you have to release each of the push buttons that run along the top right of the front panel, I worried that the old plastic would have become brittle but they surviced.

Roger


Re: Slightly OT. What is the grime I always see in HV areas?

Craig Thibodeaux
 

Since I have an actual cognitive diagnosis I never fret the quality of the question nor the answer….I am grateful for both…
One mans dumb question or answer is another man paycheck jewel…..take that to the bank.

When I said that the accumulation was indicative of the environment and not an equipment problem in origination but maybe other problems…..I was in a rush….and was hesitant to tell someone to clean that area, as its been since 97’ since I have….

The arcs and shorts and snap, crackles and pops, mentioned are self explanatory, as normally you can either locate by the charcoal or watch the light show…..if this is an area of electronics that you are in doubt as to how to proceed…..then don’t.

Please find out how to do it correctly…..let me tell you that 20+K volts of stored charge in a CRT can be real painful, and secondary damage to stuff or yourself can be serious….

So, actually, for me, this was a great general interest question, really smart, just none of the answers you got are anywhere near as complete as they should be if one assumes what you are likely to do next….

As I am mostly a spectator these days, I do have a lot of Tek stuff I picked up a few years ago, and this stuff is for me play with as the next years figure themselves out, I have still, a 465, 466, 468, 7603, 2445, and HP181T……so lets say that I spotted an arc trail and had to remove a HV connection on a CRT in one those……how would that compare to one of those TV crts, and in the case of the original question poster…..how should he proceed? I have my HV probe in hand

Craig
KM4YEC

On May 10, 2018, at 9:06 PM, Harvey White <madyn@dragonworks.info> wrote:

On Thu, 10 May 2018 14:16:09 -0700, you wrote:

Honest question to things one does not know are not or never dumb.

Sadly that can not be said sometimes for the answers or advice one gets.

Have a problem, ask a question.

Get the answer, feel dumb.

This is a good thing, you already knew the answer, didn't think of it.

Easy to fix.

If you don't feel dumb, then you have either just learned something,
or have something to learn.

Harvey




-pete

On Thu, May 10, 2018 at 9:17 AM, lop pol via Groups.Io <
the_infinite_penguin=yahoo.com@groups.io> wrote:

Sorry for the dumb question.






Re: 2246A--Voltage Self Cal Failure

@DC912
 

The voltage reference signals were active and the relays were audible, so I decided to see if going through the internal vertical adjustment procedure might make a difference.

Apparently, it did. I’m no longer getting the failure message. I guess the internal adjustments were sufficiently out of line to prevent the auto routine from being able to do its job?

I’m looking forward to using the scope. Seems like a nice blend of analog display and basic measurements, without too much clutter on the screen.

Thanks.


Re: 2221 100Mhz non A

Brendan
 

It says 100 Mhz on the front bezel. I ran it through the paces today. https://groups.io/g/TekScopes/photo/49036/0?p=Name,,,20,1,0,0 Seems to work really well. Picture is fast rise from pg506.


Re: WTB - Power Supply For 7704A

JJ
 

Do you get the classic "ticking" sound from the power supply that indicates
that it's trying to start? Did you check to see that you inserted the
cables correctly- i.e., it's easy to offset a connector by one pin. Been
there.

JJ

On Fri, May 11, 2018 at 10:33 PM, Mike Harmon <mharmon@att.net> wrote:

Hi Folks,

I am the owner of a 7704A of dubious origin that I would like to get
running. I already have a nice 7904A, but I rescued this scope from the
dust and dirt and cleaned it up the best I could. One of the first things
I did was do a thorough inspection and cleaning followed by a careful
power-up. The +5 and +15 power supplies were dead, and it looked like
someone had been in there before with a a screwdriver and (BIG) soldering
iron in several places. I went all the way through the scope and replaced
all the dipped radial tantalums with aluminum electrolytics. After
replacing a couple of the NE-23 neons with broken leads on the HV board, I
tried the smoke test again. Success! The scope came up, the positioning
controls worked, and I got a nice calibrator trace. It even triggered!

I shut it down and put the covers back on, and turned it on again.
Absolutely nothing! The neon tell-tale light (DS3019) on the inverter
board flickers, so I know that there is power to the inverter. The
inverter fuse (F3003) is OK.

At this point, I'm starting to get lost. I have no signs of life outside
the power supply "cage". My brain keeps telling me that it has to be
something really dumb and simple, but for the life of me, I'm just not
finding it!

The only thing I can think of to try next is to get my hands on a working
7704A power supply "cage", swap out the one in the scope and try it again.
If I get life, it has to be the power supply. As I said before, someone
has been in the power supply (probably several times), based upon the
scorched insulation on some of the wires that get soldered on the component
side of the board. The big 3 uF inverter filter cap has some pretty deep
burns on it from someone trying to solder the transformer wires coming
through the hole next to it.

If anyone has a "parts" 7704A they would be willing to sacrifice a power
supply "cage" from, I'd be glad to pay a reasonable price and/or shipping
so I could try a swap-out.

Please let me know!
Thanks!
Mike Harmon, WB0LDJ
mharmon at att dot net .

.




Re: 1A4 added signal

 

I think it should be there at J 124. (? too long to look it back up)

If you want it no matter what other settings there are then you need a couple high value resistors, like 82 K or so from J 101 & 102. The junction of those should produce it, and of course should be buffed with a common drain FET stage. The 82 Ks should not upset the circuit given the other values around there.


WTB - Power Supply For 7704A

Mike Harmon
 

Hi Folks,

I am the owner of a 7704A of dubious origin that I would like to get running. I already have a nice 7904A, but I rescued this scope from the dust and dirt and cleaned it up the best I could. One of the first things I did was do a thorough inspection and cleaning followed by a careful power-up. The +5 and +15 power supplies were dead, and it looked like someone had been in there before with a a screwdriver and (BIG) soldering iron in several places. I went all the way through the scope and replaced all the dipped radial tantalums with aluminum electrolytics. After replacing a couple of the NE-23 neons with broken leads on the HV board, I tried the smoke test again. Success! The scope came up, the positioning controls worked, and I got a nice calibrator trace. It even triggered!

I shut it down and put the covers back on, and turned it on again. Absolutely nothing! The neon tell-tale light (DS3019) on the inverter board flickers, so I know that there is power to the inverter. The inverter fuse (F3003) is OK.

At this point, I'm starting to get lost. I have no signs of life outside the power supply "cage". My brain keeps telling me that it has to be something really dumb and simple, but for the life of me, I'm just not finding it!

The only thing I can think of to try next is to get my hands on a working 7704A power supply "cage", swap out the one in the scope and try it again. If I get life, it has to be the power supply. As I said before, someone has been in the power supply (probably several times), based upon the scorched insulation on some of the wires that get soldered on the component side of the board. The big 3 uF inverter filter cap has some pretty deep burns on it from someone trying to solder the transformer wires coming through the hole next to it.

If anyone has a "parts" 7704A they would be willing to sacrifice a power supply "cage" from, I'd be glad to pay a reasonable price and/or shipping so I could try a swap-out.

Please let me know!
Thanks!
Mike Harmon, WB0LDJ
mharmon at att dot net .

.


Re: 2246A--Voltage Self Cal Failure

@DC912
 

Thanks Tom. I’ll try to see if there’s any life in the reference IC before I replace relays.


Re: 2221 100Mhz non A

 

I come up with a blank on that. What I found out there is that it is a 60 MHz scope. How do you know it is 100 MHz ?

And, have you downloaded the A manual ad compared it to what you have ? I know that sometimes the A and no suffix versions look like they were built on different planets, but that is not always the case. Even if they use the same ICs that is better than nothing.

Personally I would give it a good evaluation. Test drive it. Throw it some square waves, then triangle if you got them. Some sine waves and check the bandwidth. See how high it wants to trigger well. See the focus, how does it look at higher sweep speeds ?

It is totally possible you simply have a working unit. If so, we'll alert the media.


Re: 453 calibrator

 

On Fri, May 11, 2018 at 02:58 pm, Fabio Trevisan wrote:


I tend to concur with you, that being solid state-devices, there should be at
least some devices that can handle some high frequency.
"At least from TI, the fastest ones are 30kHz and those are the digital ones... the faster Linear ones (analog) are 20kHz."

That's about what I found. (I decided to look just now and go to your links) I still wonder why. The question might be better put to a physicist or chemist, or a metallurgist ??? If the semiconductor is controlled by magnetism rather than light, the difference is not the speed of propagation obviously. I had assumed and have confirmed on the net that the speed of magnetism is C, just like light or any EM wave. (or whatever) Then if light can control it up to 4 MHz, why not magnetism which is simply a different form (but not wavelength) of EM radiation ? It must be a characteristic of the semiconductor then.

Of course a photosensitive device does not react equally to all wavelengths of light, whether visible or not. Somehow they customize them like LEDs. If they act like LEDs in any way this is probably a problem for them. LEDs put out generally a narrow frequency range, i.e. if you see green you might be looking at cyan and magenta mixed and never know it, just like the red and blue phosphors on a CRT make an apparent purple. But by spectography can tell, and now they can find one pot plant in an acre of corn or other plant that looks identical to the human eye. LEDs have such a narrow output band that they couldn't use them for natural color displays (TVs) until they took a different approach and started actually using phosphors. Before that it was impossible to attain the proper colorimetry.

But we were talking about magnetism. The speed of magnetism being C, it should be fast as anything. Even if not, in many rotational sensors like the one I mentioned in the engine, the field is not being modulated per se, it is being interrupted by a material that is relatively opaque to it. So it must be the semiconductor material itself, and not a normal limitation as a transistor would have. I also would bet that if a magnetic field is modulated or masked intermittently or whatever, anything magnetic in close enough proximity would act as a capacitor does in an electronic circuit.

Those pole pieces you noticed were so finely polished, on some units they are not. They are simply laminations like a transformer and some of them interlock in a way, wit alternating lengths at the end and grouped into segment about 3/16" thick. I personally have seen the readings go low when the jaws are not fully engaged during the optimization phase of an central A/C unit installation. the HVAC specialist (friend of mine did it for beer) uses the current draw f the compressor to tweak the refrigerant charge. So there's another use...

I find such subjects fascinating, but it would have been hard to pursue them all with formal education unless I wanted to spend alot of time and money at it. I was young and had a good job (and a musclecar) so guess what happened. So do, always, point out when I make mistakes because I make them.


Re: OT but could be very helpful to someone: Parasitic Battery Drain

Brad Thompson <brad.thompson@...>
 

On 5/11/2018 8:04 AM, David Slipper wrote:
Had a couple of these - a faulty trunk/boot switch was one occasion and
a faulty electric window switch/controller was another.
Hello--

1972 Volvo, with an intermittently-actuated glove-compartment switch
that was loosely mounted through the cardboard side of the compartment.

I chased this one through an alternator and battery before the light dawned (well,
seeped) one evening through cracks in the glove compartment.

73--

Brad AA1IP


Re: OT but could be very helpful to someone: Parasitic Battery Drain

Tam Hanna
 

Menachem,

how good is your C#? If it is decent, I send you the code. It technically is owned by a bankrupt firm, but who cares.


Basically, you have one channel which is CONTROL, and one which is SNIFF. You can sniff for voltage or current if you use a Solartron 7150.


Tam


Re: 2246A--Voltage Self Cal Failure

tom jobe <tomjobe@...>
 

Dave,
I have some of those nice 224x family oscilloscopes, and those six relays are not really a chronic problem with that family of scopes in my experience, plus I don't think I have never changed one.
You might possibly have to change one or two relays... but I would make sure you have not missed something else that makes the problem look like a bad relay.
Playing with the scope to exercise everything a bit might help those 'iffy' relay contacts, and everything else too.
I have two work areas to use, and one has a very nice 2247A on it that I enjoy using.
tom jobe...
PS The 2252 is also in that same family of oscilloscopes, and it seems to just be a 2247A with GPIB capability added (I think?).
Thinking back on the problems I have seen on 224x scopes, I would say a dead main transformer would be the number one problem I've had on eBay purchases of 224x scopes... and you obviously don't have that problem. Of course the replacement of the electrolytic capacitors in the power supply is mandatory on any scope of this vintage.

On 5/11/2018 3:04 PM, dcane4@gmail.com wrote:
Thanks Bob.

I see on the vertical input schematic that both channel 1 and channel 2 each have three relays.

I might just go ahead and replace all of them while the unit is out of its case, but Is the culprit most likely the first relay that receives the volt cal signal from the Voltage Reference Generator?

Dave.



Re: 2246A--Voltage Self Cal Failure

@DC912
 

Thanks Bob.

I see on the vertical input schematic that both channel 1 and channel 2 each have three relays.

I might just go ahead and replace all of them while the unit is out of its case, but Is the culprit most likely the first relay that receives the volt cal signal from the Voltage Reference Generator?

Dave.


Re: 453 calibrator

Fabio Trevisan
 

Hey Jeff,

Thanks for the heads up on my hunch about the possible construction of the actual current probes...
I tend to concur with you, that being solid state-devices, there should be at least some devices that can handle some high frequency.
But, as a a matter of fact, my knowledge in this area is an absolute zero.
The closest I was to a current scope probe was some 30 years ago in my first job, but it belonged to the switching power supply folks (and I was on the personal computer R&D dept) so, I had it in my hands and found it beautiful how the clamp would slide open / close and how precise and smooth the movement was, and how polished the matching surfaces of the clamp were.
I suppose that one really need to make sure the gap of the clamp is consistent day in day out, for it not to interfere with the picked signal.
But as I read your message it got me curious. I googled a little bit on it and got to these pages from TI:
http://www.ti.com/sensing-products/magnetic-sensors/hall-effect/digital/products.html
http://www.ti.com/sensing-products/magnetic-sensors/hall-effect/analog/products.html
At least from TI, the fastest ones are 30kHz and those are the digital ones... the faster Linear ones (analog) are 20kHz... so it seems they're not that fast after all (Granted... it may be only the TI range that is narrow).
Anyway, I think it's conceivable to implement a hybrid design, mixing Hall-Effect for DC response and DC stability, and magnetic pickup for frequencies - say - above 10kHz, but I`m sure this is already invented wheel and it can be probably seen on the service manuals of those probes.
By the way, since you mentioned Opto-Couplers... Not to counter your thinking, but coincidentally supporting mine, I remember of having seen once on eBay an Galvanically Isolated Tek probe (one that would allow up to some 1200V isolation IIRC), that would split the signal in two paths... the DC isolation path using optos up to some frequency and the AC isolation path using a capacitive coupling, to allow the probe's claimed bandwidth (I think it was 20Mhz).
I can't recall what probe that was...

Rgrds,
Fabio


Re: 2246A--Voltage Self Cal Failure

Bob Koller <testtech@...>
 

This is most likely caused by relay problems. I have had to replace relays on this and similar models several times, as they develop contact resistance problems.
This can be done with the motherboard installed, after removal of all the interfering bits. But, you need a GOOD vacuum desoldering system with sufficient reach.
The motherboard is a bit complicated to remove, but this may be the only way.

On Friday, May 11, 2018, 11:25:57 AM PDT, dcane4@gmail.com <dcane4@gmail.com> wrote:

Hello.

I recently acquired a 2246A.

On startup, the voltage self cal failure message comes up.  Changing the battery hasn't correct whatever the fault is.  The same message appears when I attempt to do a voltage measurement from the front panel buttons.

External signals seem to be displaying properly at both Channel 1 and Channel 2 inputs.  I don't get any error message when introducing an external signal on either channel.

I've reviewed the service manual, and note that the "Exercise Volt Ref" menu item in the service menu will step through all of the settings of the Voltage Reference Generator (U931) that are used to calibrate the Volts Measurement system, but it isn't clear to me where I should be checking the voltage select lines for activity, or where the generated calibration voltage values may be checked.  Should I be checking for these at U931, and if so at which pin(s)?  I'm presuming that running this exerciser might help me locate the issue, but if I'm way off base, please let me know!

I read on another post that the relays on the inputs of the vertical amplifiers (K101 and K108) are common failure points.  Might a relay failure cause the voltage self cal failure, and if so would both of these relays have to fail to generate the message?

Thanks much.  Dave.


Re: 453 calibrator

Fabio Trevisan
 

Hello Mike,

Never need to excuse yourself, the gold in exchanging with civilized people like it's usually the case in this group is that not knowing or misunderstanding is no sin (although I`m myself a caveman).
Be careful indeed not to getting shocked when playing with those toys, but no care needed when expressing yourself.
We learn from our mistakes and from the others too!

The current loop of the 453's calibrator is not meant to be "voltage-probed"... the voltage there is essentially zero, because the current is so small and the loop is a bare ground wire (barring possible defects, like false contacts to ground or loose tightening nuts). The voltage output is at J579 only.
Now, while seeing the extents you took into trying to measure it at the ground loop, it just occurred to me that this calibrator is unlike the one from the 475 itself, and from others of the same family (465, 464, 466 etc...). So there's maybe a possible misunderstanding here.
On your 475, the current sensing loop is, AT THE SAME TIME, the calibrator's output voltage too. (there's no Calibrator output connector like on the 453).
On the 475 the bottom resistor of the voltage divider (a 10 ohm resistor, across which the 300mV voltage develops) is between the current loop and ground so you can either surround the current loop with a current sensing clamp, or probe the loop wire directly for calibrator voltage. It's a more economic approach.
This is not the case for the 453, in which the calibrator's current loop is in the bottom of the voltage divider and it's a ground wire, not meant to be probed for voltage.
You shouldn't see even a 2.5mV there like you managed and, if you do see something there's a chance there's false contacts at play. You may be needing to tighten the nuts that hold the current loop and connect it to ground.

On your background... Wow! It's not that common to see someone from ecology / biology / botany to have interest in electronics and as such, you can consider yourself already a champion.
I for myself, apart from "Avatar" pseudo-science, didn't think there was electrical activity on plants (apart from the electrical exchange that is intrinsic to the chemical reactions). So maybe Avatar is not so SyFy as it seems (BTW, I loved the base concept... It's indeed conceivable that a world covered by forests could actually develop a "mind" of its own, from the possible interconnections there could exist).

Rgrds,
Fabio

On Fri, May 11, 2018 at 10:22 am, Mike Merigliano wrote:


Hello Fabio,

Thanks for the response!  It's clear now that I need to be more clear in
my messages.

Despite contrary evidence, I know what a current probe is, and I have a
bigger clamp-type one for dealing with house wiring etc., but I really
appreciate your succinct explanation of the ones involved with scopes; I
mentioned the probe model number so readers would know which voltage
probe I was using.  Also, I don't mind if people assume I don't know
something.

You are correct about my mistake about the voltage divider -- I mis-read
it on the schematic.  I need more practice reading these things quickly
without checking with actual measurements. That's the main reason I have
been responding to posts lately. I'll be more careful next time.

After sending my previous message to you, I was a little curious and
looked for the 1kHz signal in all of the noise on the current probe
loop. I know this is weird place to measure the 1kHz voltage, but wanted
to check what I could see anyway.  I could see it much more clearly via
cascading the inputs on a Tek 475, and cutting the bandwidth to 20 MHz.
Still quite a bit of high frequency noise but the 1kHz square wave
shows, and my best guess is p-p voltage of 2.5 mV at the current probe
using the 1x voltage probe (P6028).  I did the same cascading on the 453
-- not much worse than what I can see on the 475.

My background is in ecology -- especially plants. They have some common
aspects to electrical currents and potentials in their vascular systems,
and plant ecologists use various sensors to measure activity from the
leaf scale to much broader ones via satellites.  I read the Tekscopes
forum quite often, just to expand my horizons and think out of the plant
box a little bit. I also fix up audio gear, mainly to save money, but
find it an interesting hobby. Of the many electronics forums out there,
I think the Tekscopes one has mostly experienced people, and some of the
wide-range of problems are quite difficult.


2246A--Voltage Self Cal Failure

@DC912
 

Hello.

I recently acquired a 2246A.

On startup, the voltage self cal failure message comes up. Changing the battery hasn't correct whatever the fault is. The same message appears when I attempt to do a voltage measurement from the front panel buttons.

External signals seem to be displaying properly at both Channel 1 and Channel 2 inputs. I don't get any error message when introducing an external signal on either channel.

I've reviewed the service manual, and note that the "Exercise Volt Ref" menu item in the service menu will step through all of the settings of the Voltage Reference Generator (U931) that are used to calibrate the Volts Measurement system, but it isn't clear to me where I should be checking the voltage select lines for activity, or where the generated calibration voltage values may be checked. Should I be checking for these at U931, and if so at which pin(s)? I'm presuming that running this exerciser might help me locate the issue, but if I'm way off base, please let me know!

I read on another post that the relays on the inputs of the vertical amplifiers (K101 and K108) are common failure points. Might a relay failure cause the voltage self cal failure, and if so would both of these relays have to fail to generate the message?

Thanks much. Dave.


Re: 453 calibrator

 

On Fri, May 11, 2018 at 08:36 am, Fabio Trevisan wrote:


There are current probes that uses a coil around the magnetic core, and
therefore can only sense AC current, and there are ones that uses hall-effect
transistors to sense the magnetic field, which can sense DC current but is
limited in frequency to the hall transistor's frequency response.
I never got my hands on either
I had an Amprobe AC version of one made for electrical work, another one of the trades of this jack. But it got stolen on the job. It was digital, the one I have now was given to me by a friend and is analog, and includes a 10X multiplier in the form of a round coil with an outlet into which you plug in your appliance and plug the "coil" into the wall outlet. The appliance is plugged into one of two outlets, one 10X the other 1X. Not sure exactly how it works, in any case the probe goes through the middle. I guess it's ten turns ? Anyway..

Hall effect devices must have some sort of decent frequency response. I was working on a 1990 3800 GM engine, vin C (IIRC). It has two hall effect sensors on the crankshaft, one of them is 18 pulses per revolution. Assuming the engine will do approximately 5,000 RPM that 5,000 per minute times 18 = 90,000 per minute / 60 = 1,500 Hz and it looked like a pretty good square wave so you can guess it is reproducing the 9th harmonic at least. Any closer figures than that we'll have to find a part number and look up the datasheet.

I don't really see why a Hall effect device should have less bandwidth than an optocoupler for example. Both are transistors that are controlled by something other than base current. Optocouplers have been used at composite video frequencies which span to about 4 MHz, so their response must extend somewhat beyond that. I think a coil large enough to respond to 60 Hz would suffer from its own inductance at MHz frequencies. Maybe. That would give Hall effect devices a decisive edge, however I am not sure of the linearity of either the optical of magnetic device(s). A coil might have that distinct advantage.


Re: Diagnosing faulty Transistors with a Curve Tracer

Ed Breya
 

On revisiting this thread, I think there may be some confusion right from the beginning. First, the title is "Diagnosing faulty Transistors with a Curve Tracer," so one would assume a CT is involved. But, in the discussion it appears that the OP'er doesn't have one. The best and most general setup for testing a wide range of devices is to just get or make a CT. For testing only a small assortment of device types commonly encountered in these audio amplifiers, one could instead build a simple DC bias tester that especially for power transistors, could operate and test with conditions closer to those in the application.

Second, it appears that the motivation for the testing is to determine whether a power transistor pulled from a "failed" output stage is actually good or bad, and to save or reinstall good ones, since new replacements are becoming harder to find. Questions: What conditions cause the failure? Is it random, spontaneous occurrence, or from shorting or overloading or overdriving the amplifier, or overheating from bad ventilation, etc, or unknown? What constitutes failure here, and is it always the same kind? There is another possibility that can cause a failure of the amplifier, even with "good" devices, while replacement apparently "fixes" it. I'll address that below.

There are a number of possible, subtle issues with the parts, as mentioned throughout the discussion, but I still think exceeding the SOA (or conversely, having the SOA durability of the part gradually degrade) is the most likely cause, especially if external events like overloading are initiating failure. The way to get more confidence or eliminate this possibility, is to test the parts near the SOA limits, whether it's done with a curve tracer or a special test setup.

Finally, another possibility is that an output stage device is oscillating at a high frequency - all by itself - regardless of the overall feedback loop. This can occur with amplifier stages of any size, and especially whenever a transistor is set up in the common-collector (emitter follower) configuration. This is almost always the setup with output transistors in common totem-pole arrangement - they are emitter followers, and always biased for linear operation, and can have all sorts of circuitry hanging off their base nodes. Sufficiently high capacitance from the base can cause instability that is very dependent on the device characteristics and the associated circuitry, and for output amplifiers, the external load.

Small, high-ft devices may oscillate way up into the VHF or UHF region, while big, slow power ones may be in MHz to VHF. If this happens, the devices and circuit conditions will essentially be out of control, wreaking havoc with the operation. The oscillations may be marginal too, so it could appear stable at some point, but different load characteristics, signal levels, temperature, line voltage, etc may trigger them. And, the frequency, size, and location (say superimposed on an audio signal) may be different. All kinds of possibilities can arise in this situation. So, if you find that changing out one or more of the parts apparently fixes it, while testing the original parts indicates they are OK, all you may have done is changed the conditions and device characteristics so that it's "stable" again. But how stable? That's the big question.

The end result of these oscillations may range from excessive audio distortion, to destruction of the output stage.

Fortunately, you can easily spot any HF oscillation in something that is at least functioning, by looking at the signals through the amplifier with a fairly fast scope. This is a case where a very low BW scope that easily has enough to see audio, won't do. About 100 MHz or better should do for these kinds of things.

Ed

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