Recommendation for used oscilloscope

[itsikhefez]

(first post here)

Hey all,
I'm looking to buy a used analog oscilloscope.
I am an electronics beginner and build SS and tube audio amplifiers and am looking for a tool that can help debug and inspect the circuit.
The most immediate task will be to inspect the power supply input/output of a tube amp I am building.
I have a different setup with sound card for FFT and such.

I've been looking at Craiglist and Offerup for a while and nothing has come up... so am willing to pay a bit more.
On eBay, I have found Tektronix 2225, 2235 and Hitachi V-422 and V-660 for about ~150 shipped (some a bit above some below).

Any of these recommended for the above use case?
Thanks!

[Grandchuck]

Used Tektronix oscilloscopes can be a good buy.  However, be sure you can return it if it does not work properly.  Don't forget about probes.  If the one you buy does not come with them, you will have to buy them.  Some of the inexpensive eBay probes are fine for hobby use.

Good luck!

[james_s]

Those are all good options, I would also include the older Tek 465B in the running, I have one I bought about 20 years ago and it's a great scope. The Hitachi scopes you mention are also quite nice, I had one for a while that someone gave me, they're more compact than the Tek and the one I used had a very nice sharp tube. They tend to be a bit cheaper than Tek as well since the Tek name usually carries a premium.

[David Hess]

For Tektronix in order of age from oldest to newest I recommend:

455/465M   Military 465
475A
465B

2335/2336/2337

2235/2236
2213A/2215A   60MHz 2235

2225

2245/2246
2245A/2246A/2247A
2252

Be warned that any oscilloscope of this age may require maintenance and electrolytic capacitor replacement simply because they are old.  So stick with instruments which have full service documentation available.

Craigslist is a great resource for old oscilloscopes in urban areas which had aerospace companies in the 1970s and 1980s.

If you are interested in tube instruments, then an even older 500 series Tektronix oscilloscope is another possibility; the 547 is nice.

[itsikhefez]

Thanks all for the suggestions.
I was looking on craigslist for a few weeks but all listings are from a local reseller and are quite pricey.

I was in a bit of a hurry since my tube amp project has been stalled on measuring the power supply ripple before and after the voltage regulator I was using before I could place an order on a custom power transformer.

I ended up pulling the trigger on this 2235A for $125+shipping.
https://www.ebay.com/itm/Tektronix-2235A-2-CH-100-MHz-Oscilloscope/383083495753?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

The photos show 2 traces so I have reason to suspect it should be in relatively good condition.
Seller also offers 14 day returns.

Hoping that if I ever need to replace caps, I have enough experience based on the several amps I have already built

BTW David, curious why you recommend 2235 and not 2235A ?

[David Hess]

BTW David, curious why you recommend 2235 and not 2235A ?

Because I forgot them.  So include the 2235A and 2236A there at the end.

A multimeter in AC mode works fine for measuring power supply ripple.

[MadTux]

Tek 7000 series, especially 7904/7904A.
Still my favorite scope for everyday use, because it's modular and there are lots of different plugins available. CRT readout is also nice, because no need to look at the knobs.

[David Hess]

Tek 7000 series, especially 7904/7904A.
Still my favorite scope for everyday use, because it's modular and there are lots of different plugins available. CRT readout is also nice, because no need to look at the knobs.

I did not recommend any 7000 mainframes because they are more difficult to maintain and they are mostly older.

The combination analog and digital storage 22xx models have a full readout in analog mode which is nice but they are also more difficult to maintain.

The 4 channel 22xx models (2245, 2246, 2247, 2252) all have a readout and being newer, are less likely to have problems so I included them.

[itsikhefez]

A multimeter in AC mode works fine for measuring power supply ripple.

This is how I've been generally doing this but have been having issues in this specific case.
Originally I was using CLC filtering and had noticeable hum in the amp output.
I replaced the standard inductor with a HV regulator circuit that promises 20uV ripple and no hum.
With the Fluke 87-V I have the AC reading on the HV regulator output is jumping around 3-10mV.

The suspicions are:
1) HV regulator is faulty or not receiving required voltage to regulate
2) Issue with DMM: AC reading is not as accurate in the presence of high DC (which is 285V), DMM not accurate in that range (based on the spec, the Fluke has up to 4mV tolerance) or it is not calibrated

With the oscilloscope, I intend to see the input voltage to reg (which should have a nice ripple as it is only after the first filter cap), and on the output.
On that note, since I am new to scopes, which options should be used in order to measure below 500mV (and  potentially even lower if the regulator works as claimed)

[David Hess]

A multimeter in AC mode works fine for measuring power supply ripple.

This is how I've been generally doing this but have been having issues in this specific case.
Originally I was using CLC filtering and had noticeable hum in the amp output.
I replaced the standard inductor with a HV regulator circuit that promises 20uV ripple and no hum.
With the Fluke 87-V I have the AC reading on the HV regulator output is jumping around 3-10mV.

The suspicions are:
1) HV regulator is faulty or not receiving required voltage to regulate
2) Issue with DMM: AC reading is not as accurate in the presence of high DC (which is 285V), DMM not accurate in that range (based on the spec, the Fluke has up to 4mV tolerance) or it is not calibrated

With the oscilloscope, I intend to see the input voltage to reg (which should have a nice ripple as it is only after the first filter cap), and on the output.

That makes sense to me.  I have found myself in the same situation more than once but was always fortunate to have an oscilloscope available.

Before you go wild however, is it a hum or a buzz that you hear?  It is possible that the rectifier diodes are acting like step recovery diodes and producing high frequency impulse noise during reverse recovery which will blow through or around almost any filter.  To test this, try adding like a 1000 picofarad high voltage capacitor directly across each diode.  This is a common solution to this problem but sometimes ferrite beads in series or special diodes which do not suffer from this problem are used instead.

On that note, since I am new to scopes, which options should be used in order to measure below 500mV (and  potentially even lower if the regulator works as claimed)

AC coupling with a x10 probe will give you a frequency response below 60 Hz and at least a sensitivity of 20mV/div.  If you need better than that, then a x1 probe can be used to get 2mV/div but the low frequency cutoff will move up.

WARNING: AC coupled mode on an oscilloscope will only support about 400 volts DC unless a special probe is used.  If you want to measure ripple on a DC supply above 400 volts, then other methods must be used.

MadTux's suggestion of a 7000 mainframe starts to make sense if it means getting a 7A22 differential amplifier for it but I do not think you need to go that far.  The 7A22 is especially useful for audio work with its 10uV/div sensitivity and differential inputs although its bandwidth is only 1 MHz.  The same thing can be added to any oscilloscope with the Tektronix AM502 external differential amplifier.  Note that these do not solve the AC coupled measurement problem mentioned above; you still have to be careful.

[0culus]

Tek 7000 series, especially 7904/7904A.
Still my favorite scope for everyday use, because it's modular and there are lots of different plugins available. CRT readout is also nice, because no need to look at the knobs.

I did not recommend any 7000 mainframes because they are more difficult to maintain and they are mostly older.
*snip*

It's a trade off though...it's something the OP may still want to consider when they have more experience under their belt. The plug-in architecture is super versatile and well worth it if you know what you're signing up for. And the 7904A is a lot less fiddly to work with because they made the case a lot easier to get into than the 7904. Personally, I view making repairs as a valuable learning experience. Apply sound troubleshooting techniques, common sense safety principles, and never have just one instrument you rely on. That's the TEA spirit!  I'm personally wanting to find a nice 7104 to go with my 7904A. 

OP, if you're interested in getting a Tektronix scope, be sure to check out the TekScopes list on groups.io. Lots of helpful folks there. I purchased my 7904A from a fellow on that list, and am purchasing some more plugins from him soon as well.

[james_s]

The old Tek mainframes are super cool, but I don't see a lot of value in recommending them to someone new to scopes who is just looking for an instrument to use rather than another hobby. They're big and bulky, many of the advantages are esoteric today, if space were unlimited I certainly wouldn't mind having one but I can't see it replacing my DSO for day to day use.

[0culus]

Tek 7000 series, especially 7904/7904A.
Still my favorite scope for everyday use, because it's modular and there are lots of different plugins available. CRT readout is also nice, because no need to look at the knobs.

I did not recommend any 7000 mainframes because they are more difficult to maintain and they are mostly older.
*snip*

It's a trade off though...it's something the OP may still want to consider when they have more experience under their belt. *snip*

The old Tek mainframes are super cool, but I don't see a lot of value in recommending them to someone new to scopes who is just looking for an instrument to use rather than another hobby. They're big and bulky, many of the advantages are esoteric today, if space were unlimited I certainly wouldn't mind having one but I can't see it replacing my DSO for day to day use.

It might be something the OP wants to keep in mind for later. I didn't say they should run out and get one right now.

[rsjsouza]

I know the OP already made the decision, but if you are looking for something cheaper and fully repairable (or perhaps a second scope), the regular 20MHz oscilloscopes that use off the shelf parts can also be a lifesaver - they can be used to repair a more complex one and can be had for cheap. Just be sure to see if they have a full service manual on elektrotanya.com, eserviceinfo.com, k04bb.com, etc.

[itsikhefez]

Thanks again for the recommendations and information.
I'll keep this in mind as my requirements change and whether the Tek I ordered does not meet them.

Before you go wild however, is it a hum or a buzz that you hear?  It is possible that the rectifier diodes are acting like step recovery diodes and producing high frequency impulse noise during reverse recovery which will blow through or around almost any filter.  To test this, try adding like a 1000 picofarad high voltage capacitor directly across each diode.  This is a common solution to this problem but sometimes ferrite beads in series or special diodes which do not suffer from this problem are used instead.

On that note, since I am new to scopes, which options should be used in order to measure below 500mV (and  potentially even lower if the regulator works as claimed)

AC coupling with a x10 probe will give you a frequency response below 60 Hz and at least a sensitivity of 20mV/div.  If you need better than that, then a x1 probe can be used to get 2mV/div but the low frequency cutoff will move up.
WARNING: AC coupled mode on an oscilloscope will only support about 400 volts DC unless a special probe is used.  If you want to measure ripple on a DC supply above 400 volts, then other methods must be used.

With the CLC filtering, it am pretty certain it was a hum. I also measured more than 1mV AC at the output transformer when idle and generally that is audible with high sensitivity speakers/headphone.
After I switched to the HV regulator, the noise changed. There was less hum but more of an unspecified noise.
I couldn't know if the regulator circuit is faulty or not, and that is actually what triggered my pursuit for a scope. The designer of that circuit was not certain if the reg is "browning out" and we wanted to confirm the input voltage was above the required drop-out voltage through the entire cycle.
I have since tidied up the wiring and also found that the power transformer was causing most the noise. Moving its position relative to the circuit and output transformers made the amp pretty silent.
I still want to measure the power supply... for learning purposes.

The rectifier for positive HV is actually a vacuum tube so can't try that solution. The negative bias uses SS diodes but I dont think the issue is there.
I do still have questions regarding the operation of the scope, but I think I'll start a new thread in `Beginners` specific to that.

[David Hess]

The rectifier for positive HV is actually a vacuum tube so can't try that solution. The negative bias uses SS diodes but I dont think the issue is there.

Do vacuum tube rectifiers even have reverse recovery?

The solid state diodes used for the negative bias could still be doing it.  The high frequency noise couples through the transformer so moving it around would affect it.