Tek 475a vs Tek 2445

[Feyman_Follower]

Hello, I am a newbie who wants to buy an oscilloscope for a project, it should be able to measure frequencies as low as 20-30 khz up to more than 100Mhz.
I don't have a very high budget and I found two used oscilloscopes at about the same price: Tektronix 475a, older but with a bandwidth of 250Mhz and a Tektronix 2445, with a bandwidth of 150Mhz. I am not an expert so I ask to the users of this forum: in your opinion which is better? They are both perfectly working.

[ledtester]

I would get the 2445. You'll appreciate having the four channels and 150 Mhz is perfectly adequate for a first scope.

[bdunham7]

The 2445 is a very nice scope for basic and intermediate work.  They can have cooling fan issues and assorted other problems--not unusual for an instrument of that age but potentially annoying if you aren't intending to fix up your oscilloscope as your first project.  So either make sure it works properly or, if cheap enough, get ready to do some oscilloscope repairs.

The 475A will be a decade older and looks the part.  Other than that, it is also quite nice, but probably even more likely to have issues.  The CRT on this does have a very bright and sharp image, IIRC.  However, the 475A will lack some of the features of the 2445, like separated B-sweeps, 50-ohm inputs, Auto-PP triggering, cursors/readouts etc.

How expensive are the scopes you are looking at.  Can you try them before you buy?

[tggzzz]

Scopea are not good at "measuring frequencies"; a frequency counter or spectrum analyser are better. Scopes are good at displaying waveforms.

If you are going to be making some other form of measurements, say what they are - we can't guess. (The XY problem).

Make sure you:

• have budgeted for the correct class of probe. FFI see the refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
• understand the limitations of the scopes, in particular the inaccuracy of the vertical and horizontal measurements
• check each scope works fully before buying it

From the reliability point of view....

I would consider getting the 475a. Main failures are replaceable tantalum capacitors, plus cleaning the "finger" switches in the attenuators and AC/GND/DC switch.

The 2445 almost certainly needs its RIFA delayed action smoke generators replaced, and while you are "there" you might as well replace the PSU electrolytics. Be careful with the ventilation fan and motor; trying to extract them "incorrectly" will probably break the fragile plastic. If it is a 2445A or 2445B, you've got the added problem of the battery failing and it losing the calibration.

[Feyman_Follower]

Ok, maybe I used the wrong word: I need to see the amplitude of a sine wave signal and see the waveform, I already have a nice National frequencimeter up to 100 Mhz.
I can't try them because the sellers are in my country far from my home but they sent me some picture of the instruments connected to a signal generator (the 475a connected to the internal square wave generator) and they look ok. I also have the assurance of the website (the payment is only forwarded if I receive the same item as the description). The 475a has only a minor mechanical problem because the time base switch sometimes doesn't work and you have to turn it again. the price is about 120$ for both

[tggzzz]

I need to see the amplitude of a sine wave signal and see the waveform

Are you aware the amplitude accuracy is ~2%? Is that sufficient for your unspecified purpose?

If it is a sinewave, why do you need to see the waveform? If you are trying to measure harmonic distortion, neither scope is suitable.

[bdunham7]

Are you aware the amplitude accuracy is ~2%? Is that sufficient for your unspecified purpose?

If it is a sinewave, why do you need to see the waveform? If you are trying to measure harmonic distortion, neither scope is suitable.

I realize that he hasn't gone into great detail about what he is hoping for, but a scope is almost always a pretty good starting point--especially over that large of a frequency range.  Do you have a good, inexpensive option that is better than 2% from say 0 to 50MHz?  In any case, I'd almost always want a scope handy even if I had more precise instruments for specific parameters. 

[Feyman_Follower]

I need to do more than one thing: I need to compare the amplitude of a sinewave signal with one other at the same frequency (30Khz), use the xy mode to display the Chua's circuit signal, and see a high frequency signal from a tunnel diode oscillator (I will try to lower it to about 250 Mhz). I am a tinkerer and a Physics lover so I will certainly find more uses for it. 2% of accuracy is more than enough.

[bdunham7]

I need to do more than one thing: I need to compare the amplitude of a sinewave signal with one other at the same frequency (30Khz), use the xy mode to display the Chua's circuit signal, and see a high frequency signal from a tunnel diode oscillator (I will try to lower it to about 250 Mhz). I am a tinkerer and a Physics lover so I will certainly find more uses for it. 2% of accuracy is more than enough.

A few things to keep in mind:

The XY bandwidth of most analog scopes (these included) will be much less than the vertical bandwidth, perhaps 1-5MHz.

Tunnel diodes are pretty low power devices and usually the oscillators operate at pretty high frequencies.  I suppose you can get them to operate at lower frequencies with larger tank components, but I'd wonder about the power issue.

IIRC, the 475A sacrifices a bit of vertical sensitivity to get its bandwidth and only goes down to 5mV/div as opposed to the 2445 at 2mV/div (and reliable Auto-PP triggering at that level).

[tggzzz]

Tunnel diodes are useful for more than oscillators.

Tektronix 4x5 scopes used them in the trigger circuit. My 350MHz Tek 485 triggers perfectly well when presented with a >1GHz signal; the only limitation is the reduced vertical amplitude.

50 years ago the Tektronix 1502 TDRs used them to generate a 50ps edge. See https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1097048/#msg1097048

[mawyatt]

I need to do more than one thing: I need to compare the amplitude of a sinewave signal with one other at the same frequency (30Khz), use the xy mode to display the Chua's circuit signal, and see a high frequency signal from a tunnel diode oscillator (I will try to lower it to about 250 Mhz). I am a tinkerer and a Physics lover so I will certainly find more uses for it. 2% of accuracy is more than enough.

Around ~1990 or we did some work with a pair of Chua diodes arranged in the chaotic LC oscillator circuit as originally described by Dr M Kennedy. The inductor was synthesized with a Gyrator and the concept was to use the Lorenz attractors as "Analog Encryption Keys" for a specialized communication system. We couldn't secure funding so the project never got off the ground, altho MIT much later patented a very similar concept. However, we did build a prototype oscillator and used an old Tek Scope (probably one of those under discussion) and remember using a Polaroid Camera Head to take snapshots of the chaotic trajectories with the scope XY mode. Can try and dig up more information if the OP is interested, but don't want to clutter up the topic.

Best,

[mawyatt]

Per request by OP, here's some information on a Chaotic circuit by Dr. Michael Kennedy.

https://people.eecs.berkeley.edu/~chua/papers/Kennedy93.pdf

https://people.eecs.berkeley.edu/~chua/papers/Kennedy93b.pdf

LTspice simulation. Wish we could find our old article published internally on this from way back, but recall the old Tek scope XY screen images looked just like the simulation shown.

Best,

[David Hess]

Hello, I am a newbie who wants to buy an oscilloscope for a project, it should be able to measure frequencies as low as 20-30 khz up to more than 100Mhz.
I don't have a very high budget and I found two used oscilloscopes at about the same price: Tektronix 475a, older but with a bandwidth of 250Mhz and a Tektronix 2445, with a bandwidth of 150Mhz. I am not an expert so I ask to the users of this forum: in your opinion which is better? They are both perfectly working.

The higher bandwidth 475A will give better amplitude accuracy up to 100 MHz.  The 2445 will not make its rated accuracy at 100 MHz.  Oscilloscopes are not really intended for amplitude measurement, with the exception of sampling oscilloscopes.

A calibrated measurement could be made of the oscilloscope's response at your frequency of interest to get reasonable accuracy.

Otherwise it is pick one and take your chances.  The 475A is older and more likely to have mechanical problems, and the 2445 is newer but more complicated.  Both use integrated circuit and hybrid construction which is difficult to repair, but the custom ICs and hybrids rarely fail.

[bd139]

2445 problems: power supply recap will be required, exploding RIFAs and mains filter already mentioned need dealing with immediately, the stack of hybrids which do unfortunately fail, computer side of it is complex and may have a Dallas NVRAM in it depending on revision.

475A problems: CRT bloom, dodgy switches, HT issues, duff filter caps.

Choose your funeral!

Whichever old scope you buy, get two and a service manual.

Edit: note that the 475A amplifier chain is considerably nosier than most scopes with 250MHz bandwidth. This leads to thick fuzzy traces. This was a design cock up.

[Wallace Gasiewicz]

A general ballbark statement:

Analog scopes are rated at 80% of amplitude at advertised high freq limit.  They can go higher but the amplitude value is not predictable.

Digital, scopes fall off just above their rated freq but maintain better amplitude representation.
There are a lot of people to help with maintenance of Tek Scopes. There are few groups on Groups.io that are dedicated to Tek Scopes.

As already said, also consider the cost and availability of probes. Probes should be matched to the input capacitance of the particular scope.
If your scopes come with probes, so much the better.

$120  for either scope is a very good price, if they are working OK.

[jonpaul]

best to avoid the unknown of ebay.

Most sellers cannot test or pack, shipping costs are crazy.

We see good scopes at ham fleas, and street sales, garage sales, just need patience.

Bought Hameg French made 1, 2 ch 10, 20 MHz for €5 and €25 works fine.

All the decades old 465,475, 465B get defective HV multiplier or transformer and die, hard to get spare parts or repair.

Would get 2445 or 2465, better 2465B if it's 100%

Jon

[Feyman_Follower]

thank you all for the help, when I'll have another question I'll ask it again on this Forum. I am not buying the oscilloscope on Ebay but on a Craiglist similar website.

[mawyatt]

If you like the old analog scopes, we can second the Tek 2465 types mentioned by Jon. We have 2 that were fixed. Schematics with voltages are available, and a fun project for someone to restore!! We were able to repair the channel input hybrids, a faulty probe calibration output (defective zener diode), the usual leaky electrolytic caps and the infamous RIFA Firecracker Caps!!

A word of caution tho, be very careful while working on these old CRT type scopes, there are some high voltages around the CRT.

Best,