Topics

Advice about buying a 7904


Reginald Beardsley
 

I have a 485 and a 7104. The fragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg


Dave Daniel
 

The 7904A is easier to wotk pne because it has the “kickstand” feature where one can split the display section up off of the lower section to make repairs. I’ve never compared the specs of a 7904 and 7904A. I imagine they are quite similar. I’d probably buy a 7904A if I had to choose. As far as I know there are well-known problems with either ‘scope.

If it were me, I’d also consider a 7854.

Selecting plug-ins is a pretty deep subject. 7A26s with 7B92s is a good choice for general purpose work. IIRC, 7A19s are 50 ohm input only.

Once you get a 7xxx ‘scope you will probably want to get a differential amp, pulse and counter plug-ins as well. It’s a rabbit hole.

DaveD

On May 28, 2020, at 18:50, Reginald Beardsley via groups.io <pulaskite=yahoo.com@groups.io> wrote:

I have a 485 and a 7104. The onlyfragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg



@0culus
 

Reg,

As you may know from our prior conversations, I have both a 7104 and a 7904A. The 7904A is definitely my workhorse because, as you mention, I want to preserve and protect the mcp tube in the 7104. As Dave mentioned, the chassis is different between the 7904 and 7904A. The 7904A is going to be easier to work on from an access perspective. One thing the 7904 has that I wish the 7904A carried on is a current loop calibrator for current probes. Oh well. Another thing to keep in mind is that the 7904A uses the hypcon packaged hybrids like the 7104 in the vertical amp[1], while the 7904 does not.

As far as reliability, I would imagine both have tantalum capacitors. Since these scopes have a switch mode power supply, it's easy to tell when you have a short because you get the infamous clicking as the supply tries to turn off then shuts down. Finding the short is another matter, though fortunately for me the short my 7104 developed when I got it home from the auction house turned out to be super simple and not tant related at all.

As far as plugins go, the 7A26 is a perfect vertical option for general purpose work. There's a couple different revisions hanging about. Some have a 20 MHz bandwidth switch and some do not. I have examples of both. Honestly, the one that doesn't have it has a sharper trace all around in my experience. Use two of them and have a full on 4 trace scope. The 7A19 is a good match for the mainframe if you want a vertical with a dedicated 50 ohm input. Keep in mind that unlike the 7A29's circuit breaker arrangement, the 7A19 uses fuses for input overload protection. Still useful for specialty probes that expect 50 ohms, such as current probes. Another vertical plugin in find indispensable for low frequency work is the 7A22 differential amplifier. Even single ended with a 10X probe, you can get down into microvolts/div ranges and the adjustable bandpass filter is very useful for isolating signals in noise. I have a 7B15 and a 7B92A as timebases in mine; no complaints.

[1]: http://w140.com/tekwiki/wiki/7904


 

On Fri, May 29, 2020 at 12:50 AM, Reginald Beardsley wrote:


which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???
Hi Reginald,

Advantages of the 7904A over the 7904:
- Newer, so possibly less failure-prone
- Easier to service because of split construction
- Front panel layout very much like the 7104 (or is that a disadvantage?)
- Internal construction and components very much like the 7104
- More modern look (to some at least)


Advantages of the 7904 over the 7904A:
- Fewer hard-to-get components
- No fan (a huge advantage according to some)
- Backlit Vertical Mode and Horizontal Mode buttons
- More versatile calibrator, includes current loop
- Can be had for very little money

Make sure that in either case, you buy one with the Alphanumeric screen-readout, which is standard.

It's very difficult to determine the better choice on long term support aspects: Do you want a younger and therefore possibly slightly more reliable instrument with more expensive, hard-to-get components or do you want an older, slightly less reliable instrument with fewer and cheaper hard-to get components? If you have room to spare, have 3 7904's for the price of one (or 2) 7904A's.

As regards plugins: For general-purpose work, put in a 7A26 (Hi-Z, 200 MHz) and a 7A19 (Lo-Z, 50 Ohm only) or, preferably, a 7A29 (resettable overvoltage protection, continuously variable attenuation), a 7D15 counter in the A Horizontal slot and a 7B92A into the B horizontal slot. Make sure you have all the other plugins nearby (7A13, 7A22, a sampler system for > 1 GHz).

Both have a switching supply. Apart from the well-known lifetime-driven weaknesses like electrolytic (mains voltage buffer) caps and tantalum (bead) caps, I'm not aware of any structural vulnerabilities.

I concur with Dave D. re. the 7854. It's more complicated and therefore more vulnerable but it's also one great piece of equipment, especially if you use sampling plugins.

Raymond


Dave Daniel
 

My email should have read “no well-known problems”.

I had both a 7904 and a 7904A; I gave the 7904 away to a good home when I moved across the country, reducing my collection to five 7xxx ‘scopes.

If you get a 7854 you will also want the keyboard that goes with it. Like I wrote, rabbit hole.

DaveD

On May 28, 2020, at 19:05, Dave Daniel via groups.io <kc0wjn=gmail.com@groups.io> wrote:

The 7904A is easier to wotk pne because it has the “kickstand” feature where one can split the display section up off of the lower section to make repairs. I’ve never compared the specs of a 7904 and 7904A. I imagine they are quite similar. I’d probably buy a 7904A if I had to choose. As far as I know there are well-known problems with either ‘scope.

If it were me, I’d also consider a 7854.

Selecting plug-ins is a pretty deep subject. 7A26s with 7B92s is a good choice for general purpose work. IIRC, 7A19s are 50 ohm input only.

Once you get a 7xxx ‘scope you will probably want to get a differential amp, pulse and counter plug-ins as well. It’s a rabbit hole.

DaveD

On May 28, 2020, at 18:50, Reginald Beardsley via groups.io <pulaskite=yahoo.com@groups.io> wrote:

I have a 485 and a 7104. The onlyfragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg




Chuck Harris
 

Issues and features that I recall:

7904 has lots of tantalum caps, and a couple are bound to blow.
They are easy to find and fix.

7904 has a weak power supply that can't handle many of the digital
plugins, such as the logic analyzer and spectrum analyzer plugins.

7904 always ticks 3 times when it is turned on after being off for
a while.

7904 has at least one "blue" harmonica connector that will have
ossified by now. If you touch it, it will crumble to dust.

Fortunately, the so-called Dupont connectors are compatible.

7904 has a united power supply transformer that has everything
from the low voltage windings through the EHT windings on the
same core. It is reliable, but repair isn't likely.

7904A Has few tantalum capacitors.
7904A has a separate EHT transformer... but because that was too
reliable, they drove it from an AC tap on the main transformer.

Why do I care? If the main inverter goes full tilt on, the HV
and filament voltage for the CRT doubles.

Specs that matter are the same for 7904 and 7904A.

7904A has a noisy fan. 7904 has none.

7904A uses hypacon connectors on its many, many hybrids.

All this said, I prefer the 7854 as my go-to scope.

-Chuck Harris

Reginald Beardsley via groups.io wrote:

I have a 485 and a 7104. The fragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg




Harvey White
 

I can speak of this from the 7904 perspective, since I have two that work, and some backup parts sources,

I had 7904's available, and no 7904As.

the 7A19 are the "appropriate" plugin for a 500 mhz single channel plugin.  However, they're protected by fuses which may be (impossible) hard to find.  The 7A29 has a higher bandwidth and is apparently not protected by fuses (and by something that can be reset), so I prefer them.

My general setup is a 7A26, 7D12 (with sampling plugin), 7D15, and 7B92A.

I've swapped the vertical plugins as needed, 7A22, 7A13, 7A24, 7A29.

I've used the 7B10 and 7B15, but only rarely.

Don't have the 7B8x or 7b7X to speak of, at least, I may have some of them, but don't use them.

I've had (generally) few issues, a hybrid perhaps bad, a PS bad, but nothing that I thought was system wide.

Harvey

On 5/28/2020 6:50 PM, Reginald Beardsley via groups.io wrote:
I have a 485 and a 7104. The fragile CRT of the latter discourages regular use unless needed. Reed refurbished my 485 and obviously it's a really fine instrument.

I'm mulling over getting a 7904 for more routine work because I find the overshoot on DSOs very annoying. My 1.5 GHz LeCroy has a spec of 17% overshoot. I don't consider anything above 2-3% acceptable. I've asked Tek for a step response for a 3 or 4 series 1 GHz DSO using one of Leo's pulsers or similar. After my adventures with Keysight's MSOX3104T and the Rohde & Schwarz RTM3104 I don't want to get a demo unit unless it has an acceptable step response. It will be interesting to see if I get one.

I've never touched a 7904 and have no knowledge of them other than what I've read on TekWiki.

So some questions:

which is the best choice from a long term support basis, 7904 or 7904A?

what plugins? 2x 7A19 and 7B92 + ???

common issues (e.g things like the 576 transformer problem)?

anything else?

Thanks,
Reg



saipan59 (Pete)
 

I had a 7904A recently for a short time.
The fan was one reason I sold it - the noise is obnoxious to me. I don't think there was anything wrong with the fan, but I'm sensitive to that white noise - it bugs me.

Pete


Reginald Beardsley
 

Thanks for all the responses. I also hate fan noise. That and more readily available parts make the 7904 preferable. However, I'll also look into the 7854.

As I have a Tek 11801 and HP 16702B and 16500A LAs and 8560A and 8566B SAs I'd probably use those instead anyway. I'm one of those "Tek for scopes and HP for everything else" guys.

Have Fun!
Reg


Bob Koller <testtech@...>
 

The fan on my 7904A is very quiet, a non issue on mine at least. I suggest that the most important thing will be finding the best instrument available around your location. Packing properly is difficult and shipping is expensive.


Roger Evans
 

I have a 7904 and a 7934 (obviously different but I believe the PSU is very similar to the 7904A). I find the fan noise of the 7934 becomes a distraction after a while when you are trying to concentrate on something else, but I agree, if you find a nice scope of either the 7904 or 4A then you should be happy with it. If you are concerned about the pulse response for very fast risetimes I would recommend the 7A29 over the 7A19. I have two 7A19s and they are very difficult to set up for clean pulse response, you need an extender (which I don't have) and decent non metallic tools to adjust the hairpin inductors which work harden slightly after several attempts to adjust them. The 7A29 has a much cleaner step response but may be getting hard to find.

Regards,

Roger


Dick
 

If anyone in Southern Arizona is interested, I have a 7904A
with two 7A26, 7B92A and 7B70 Plug-Ins.

Contact me off list to discuss.

Motivated Seller.

73, Dick, W1KSZ
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Roger Evans via groups.io <very_fuzzy_logic=yahoo.com@groups.io>
Sent: Friday, May 29, 2020 8:13 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] Advice about buying a 7904

I have a 7904 and a 7934 (obviously different but I believe the PSU is very similar to the 7904A). I find the fan noise of the 7934 becomes a distraction after a while when you are trying to concentrate on something else, but I agree, if you find a nice scope of either the 7904 or 4A then you should be happy with it. If you are concerned about the pulse response for very fast risetimes I would recommend the 7A29 over the 7A19. I have two 7A19s and they are very difficult to set up for clean pulse response, you need an extender (which I don't have) and decent non metallic tools to adjust the hairpin inductors which work harden slightly after several attempts to adjust them. The 7A29 has a much cleaner step response but may be getting hard to find.

Regards,

Roger


 

On Fri, May 29, 2020 at 05:13 PM, Roger Evans wrote:


If you are concerned about the pulse response for very fast risetimes I would
recommend the 7A29 over the 7A19. I have two 7A19s and they are very difficult
to set up for clean pulse response, you need an extender (which I don't have)
and decent non metallic tools to adjust the hairpin inductors which work
harden slightly after several attempts to adjust them. The 7A29 has a much
cleaner step response but may be getting hard to find.
The 7A29 was a much better (and newer) amplifier than the 7A19 altogether. Much has been written about that and can be found on TekWiki and other sources.

Raymond


fiftythreebuick
 

Hi Reg-

What y'all need to solve the fan noise problem on the 7904 is a Type 555 mainframe! Just turn it on for a little while and when you turn it back off you won't even realize the 7904 is on! <laughing>

The 555 is my favorite scope, but it does have two very serious fans!

73

Tom AE5I


Jim Ford
 

Yeah, the fans in my HP 8566A spectrum analyzer and 8350B sweeper are pretty darn loud, too.  Don't have a 7904A to compare, just a 7904.  FWIW. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: fiftythreebuick <ae5i@...> Date: 5/29/20 10:09 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Advice about buying a 7904 Hi Reg-What y'all need to solve the fan noise problem on the 7904 is a Type 555 mainframe!  Just turn it on for a little while and when you turn it back off you won't even realize the 7904 is on!  <laughing>The 555 is my favorite scope, but it does have two very serious fans!73Tom AE5I


Chuck Harris
 

I find all of the fans get quieter as I get older.

I wonder why that is?

-Chuck Harris

Bob Koller via groups.io wrote:

The fan on my 7904A is very quiet, a non issue on mine at least. I suggest that the most important thing will be finding the best instrument available around your location. Packing properly is difficult and shipping is expensive.




Reginald Beardsley
 

I'm in central Arkansas, so T&M gear "near me" is in Dallas. Except for my 577 everything I've bought has been shipped. Picking up the 577 was a 6 hr drive each way. But I had bought $1000 of the wrong LED 4 ft fluorescent replacement lamps and return shipping would have cost as much as the 577. So the 577 was effectively free. Paid for by the LED lamp shipping charges.

I had one casualty with a poorly packed DDA-120. I now give very specific instructions about packing. But I've gotten 2 dozen major instruments shipped without damage including a 7104.

My LeCroy DDA-125 is almost jet engine level.

Have Fun!
Reg


Reginald Beardsley
 

Please forgive me my ignorance, but for most of my life these things were pure unobtainium for budgetary reasons. And as I was not an EE and not allowed to touch the ones at work, there was no incentive to torture myself by looking at catalogs of things I could not afford. Getting a 465 was a huge event for me. And I don't mean all the work it took to repair it. I am acutely aware of how fiddly adjusting the vertical amplifiers is. But when you get it right, wow! Though in all honesty, I like my Dumont 1062 better as it does not have a fan. It's only 60 MHz, but otherwise matches the 465 in features.

Can I use my 7104 plugins in a 7904 mainframe, albeit with reduced BW and sweep speeds? That would greatly ease my concerns about using the 7104 on a regular basis. And make getting a 7904 mainframe pretty much a no-brainer. Just a question of getting a good deal.

Is there a matrix which shows which plugins will work in which mainframes? I'm really only interested in the vertical and horizontal plugins. For the other stuff I have HP gear. I'm not aware of anything Tek made that can compete with an 8566B or 8510C.

Thanks,
Reg


Harvey White
 

Please see interleaved comments:

On 5/29/2020 8:53 PM, Reginald Beardsley via groups.io wrote:
Please forgive me my ignorance, but for most of my life these things were pure unobtainium for budgetary reasons.
Tell me about that one.
And as I was not an EE and not allowed to touch the ones at work, there was no incentive to torture myself by looking at catalogs of things I could not afford.
I was allowed to touch them.  It's worse....
Getting a 465 was a huge event for me. And I don't mean all the work it took to repair it. I am acutely aware of how fiddly adjusting the vertical amplifiers is. But when you get it right, wow! Though in all honesty, I like my Dumont 1062 better as it does not have a fan. It's only 60 MHz, but otherwise matches the 465 in features.
Much depends on how many channels you need, (most need 4 or less), what the bandwidth needs to be (most need 150 Mhz or less)
Can I use my 7104 plugins in a 7904 mainframe, albeit with reduced BW and sweep speeds? That would greatly ease my concerns about using the 7104 on a regular basis. And make getting a 7904 mainframe pretty much a no-brainer. Just a question of getting a good deal.
In a sense, there are no such things as 7104 plugins.  If you mean the  7B10, 7B15, 7A19, 7A24, and 7A29.  The overall answer is yes, you can with reduced performance at the high end.  The bandwidth of a plugin and scope frame work a bit like parallel resistors.  With a plugin of infinite bandwidth, the scope frame is the limiting factor.  A 7A29 will give  you closer to the frame's bandwidth than a 7A19, but not more.


Is there a matrix which shows which plugins will work in which mainframes? I'm really only interested in the vertical and horizontal plugins. For the other stuff I have HP gear. I'm not aware of anything Tek made that can compete with an 8566B or 8510C.
Practically any vertical plugin can be used in any frame.  The bandwidth is effectively the lower of the frame or the plugin. Yes, you could use a 7A22 with all the bandwidth limitations in a 7104.  I tend to be careful of the CRT, though, so the 7904 is the workhorse.

Not sure where there is a chart, but there is one.  The scope horizontal sweeps are recommended for the various series.  Tek made 100 Mhz, 200, 400, 500, and 1Ghz frames.  Each one has a recommended horizontal sweep pair.  7B10 and 7B15 for the 7104/3 for example.

IIRC, and this is only what I remember...

7B92 is for 500 Mhz as is 7B8x

 7B7X is for 200 and perhaps 400, not sure on the 400

7B5x is for 100 Mhz.  7B50/7B55, 7B53 work in the 100 Mhz frames.

What I have is mostly 7B92, 7B8x, 7B5x, 7B10, and 7B15 depending on the scopes, IIRC.

Harvey


Thanks,
Reg



ChrisBeee
 

:-DDD
Know what you are talkin' about!
Chris

On Fri, May 29, 2020 at 08:12 PM, Chuck Harris wrote:


I find all of the fans get quieter as I get older.

I wonder why that is?

-Chuck Harris

Bob Koller via groups.io wrote:
The fan on my 7904A is very quiet, a non issue on mine at least. I suggest
that the most important thing will be finding the best instrument available
around your location. Packing properly is difficult and shipping is expensive.