Re: Advice about buying a 7904
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
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On May 28, 2020, at 19:05, Dave Daniel via groups.io <kc0wjn@...> 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@...> 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
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Re: Advice about buying a 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
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Re: Tek 4041 GPIB Controller
We can get the 4041 service manual scanned from the vintagetek.org microfiche library of all Tektronix documentation - after COVID-19 is over. Right now Oregon is still in lockdown. They only charge $25 an hour for scans.
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Re: Advice about buying a 7904
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]:
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Re: Advice about buying a 7904
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
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On May 28, 2020, at 18:50, Reginald Beardsley via groups.io <pulaskite@...> 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
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Advice about buying a 7904
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
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IIRC, only the 432, 434, and 485 had switchers.
The 485 was much like a 7K with the Tek-made controller chip. The 43X's
were discrete.
-ls-
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Hey guys! Look at me with my newly acquired acronyms! ;-)
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On Thu, May 28, 2020 at 03:39 PM, <ciclista41@...> wrote:
Still great advice for later when I'm working on a DUT with a SMPS. :-)
On Thu, May 28, 2020 at 03:34 PM, <ciclista41@...> wrote:
Excellent advice, Eric!
Now I'm glad I just put that incandescent bulb in series when I turned it on
the first time. Not that I have a variac (yet).
Bruce
On Thu, May 28, 2020 at 12:53 PM, Eric wrote:
I would test the rectifiers the silicon is usually pretty resilient and
normally grossly fails "open or short". I have only run in to one
transistor
so far that has "kind of" failed that one took a 576 to find it, was out
of
a
fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac.
The 400 series have switching power supplies they are not the linear supplies
They can over current when the line voltage drops to low to regulate and
can
be damaged. Slow up on a varic is ONLY good for linear power supplies.
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Still great advice for later when I'm working on a DUT with a SMPS. :-)
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On Thu, May 28, 2020 at 03:34 PM, <ciclista41@...> wrote:
Excellent advice, Eric!
Now I'm glad I just put that incandescent bulb in series when I turned it on
the first time. Not that I have a variac (yet).
Bruce
On Thu, May 28, 2020 at 12:53 PM, Eric wrote:
I would test the rectifiers the silicon is usually pretty resilient and
normally grossly fails "open or short". I have only run in to one transistor
so far that has "kind of" failed that one took a 576 to find it, was out of
a
fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac.
The 400 series have switching power supplies they are not the linear supplies
They can over current when the line voltage drops to low to regulate and can
be damaged. Slow up on a varic is ONLY good for linear power supplies.
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Hi Eric,
Must they be removed from the circuit to get decent readings? If so, I'll do it. This thing seems to have been designed to make it easy to remove them once the large caps are removed.
Bruce
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On Thu, May 28, 2020 at 12:53 PM, Eric wrote:
I would test the rectifiers the silicon is usually pretty resilient and
normally grossly fails "open or short". I have only run in to one transistor
so far that has "kind of" failed that one took a 576 to find it, was out of a
fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac.
The 400 series have switching power supplies they are not the linear supplies
They can over current when the line voltage drops to low to regulate and can
be damaged. Slow up on a varic is ONLY good for linear power supplies.
|
Excellent advice, Eric!
Now I'm glad I just put that incandescent bulb in series when I turned it on the first time. Not that I have a variac (yet).
Bruce
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Show quoted text
On Thu, May 28, 2020 at 12:53 PM, Eric wrote:
I would test the rectifiers the silicon is usually pretty resilient and
normally grossly fails "open or short". I have only run in to one transistor
so far that has "kind of" failed that one took a 576 to find it, was out of a
fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac.
The 400 series have switching power supplies they are not the linear supplies
They can over current when the line voltage drops to low to regulate and can
be damaged. Slow up on a varic is ONLY good for linear power supplies.
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184 Time-Mark Generator lamp
Hi all,
I recently acquired a rather nice shape 184 for the princely sum of $70. After some good burn in and exercising the controls following by a few adjustments, it ticks along perfectly except for the HF oscillator (but that's beyond the scope of this topic) and the green power lamp (above the oven lamp, which is working), which is burned out. I was surprised to find the bulb is one assembly that plugs into the socket. I assume these are probably unobtainium, but would be interested in being contacted off list if someone has one they are willing to part with.
Thanks!
Sean
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One of the things being added to my workshop is a vacuum system that would be suitable to pump down a CRT, but I wouldn't entrust myself to a CRT as rare as the one in the 576 as a test candidate! I can live with the burned spot for now, but thanks for the offer.
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From what I've read...Tek took a systems approach with this series of 2400 scopes... so it's fair to consider a 2465B scope plus the Tek recommended P6137 as what Tek intended... when interpreting the Tek specs about them.
Taking that way of looking at it... the DUT is on one end of the system, and you are on the other... and so what you see is what you get.
I've a feeling that Tek enthusiasts might buy a Tek scope just based on the nominal bandwidth, and then look for probes with a matching nominal bandwidth.
Given just the above, and assuming too, that the front end, filter response, of a 2465B is Gaussian, then:
(1) BW(system) = 1 / sqrt[ 1/(probe bandwith ^2) + 1/(scope bandwidth ^2) ]
So from (1) it is easy to see... if the bandwidth of the scope and the probe match... the nominal bandwidth of the system is scaled by 1/sqrt(2)
Thus, for a 400 MHz nominal bandwidth... (1) gives a system bandwidth of approximately 282 MHz. (Check the calculation!)
For a 2465B... that's a "sweet spot" in the sense that reducing the nominal bandwidth of the probe is only going to reduce the system bandwidth... but given the high non-linearity of (1), its not a proportional reduction.
So consider using a probe with a nominal bandwidth of 100 MHz. Then (1) gives a system bandwidth of approximately 97 MHz. That is pretty close to the nominal bandwidth of the probe.
The takeaway from (1) is that just considering bandwidth specs... if you are working on 10 MHz timebase, with a 2465,B... say just for the features that Tek offers.. then using a P6105 probe is probably okay. (I say probably...because Tek specs a P6105 for 15 pf, on the lower end of the compensation... and Tek specs the input of some? 2465Bs at 15 pF + - 2 pF.)
Best regards and wishes.
Roy
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Raymond,
Thanks for verifying. I thought that I knew these scopes pretty well. But I have been wrong before and it would not have been the first time that I stuck my foot in my mouth.
I have a TDS360 and TDS460A and those both use the SMPS design, but totally unrelated to the 4xx Series.
--
Michael Lynch
Dardanelle, Arkansas
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On Thu, May 28, 2020 at 10:36 PM, Michael W. Lynch wrote:
I am pretty certain that the 455/465/475 use a linear power supply.
That is correct. Raymond
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However do NOT bring up a 400 series scope slowly on a variac. The 400 series have switching power supplies they are not the linear supplies They can over current when the line voltage drops to low to regulate and can be damaged. >Slow up on a varic is ONLY good for linear power supplies. Eric, I am pretty certain that the 455/465/475 use a linear power supply. That is the purpose of the big heavy transformer in the back of the scope. If it was a SMPS then there would be no need for that heavy beast. -- Michael Lynch Dardanelle, Arkansas
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On Thu, May 28, 2020 at 08:36 PM, Jim Ford wrote:
I was able to adjust the probe to get a nice flat response when connected to
the calibrator on the 54504A. Fortunately it also looked flat with the
calibrator on my Tek 7904.
Hi Jim, The artifacts that I was referring to don't show up on standard 'scope calibrators. Those only allow you to adjust "LF" behavior with a 1 kHz square wave (flat top). I meant fast-edge behavior, like that visible with <=1 ns steps in the first few ns or in the edge itself. That's where the differences are. You won't just see a flat or single-curved top but all kinds of ripples, resonance-like behavior etc. I know about only one "classic" Tek 'scope that offers that kind of fast step: the 485 in addition to a nice 1 kHz square wave. It allows LF adjustment *and* HF adjustment. The latter is done with separate adjustment regulators in the compensation box of the probe, underneath the plastic cover. *And* you need a well-matched input to the probe, like a probe-shaft-to-BNC adapter The calibrator's edges in the 7904 are no faster than 250ns so they are useless for showing the differences between e.g. the P6139A and P6137, much less perform the HF adjustment I was referring to.
And I'd check the input capacitance of the scope (7 pF in the case of the 54504A) versus the output capacitance of the probe (8 pF in the case of the P6139A), and
if they are close, you will probably get good results when you connect the probe to the calibrator and adjust it for flatness.
That's not correct: 8 pF is the nominal *input* capacitance of the P6139A and they don't have to be nor will be equal. Otherwise, you wouldn't be able to adjust an 8pF or 1.5 pF FET input capacitance probe to several different 'scopes, like the 2465B with its 15pF nominal input capacitance or a 7A26 with 22pF nominal. It's all about RC times, probe input, cable, probe output, 'scope input. The publications I referred to show it all. Nothing to worry about for general 'scoping in the sub-100MHz domain though. Who said that 100 MHz actually is DC? Raymond
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I would test the rectifiers the silicon is usually pretty resilient and normally grossly fails "open or short". I have only run in to one transistor so far that has "kind of" failed that one took a 576 to find it, was out of a fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac. The 400 series have switching power supplies they are not the linear supplies They can over current when the line voltage drops to low to regulate and can be damaged. Slow up on a varic is ONLY good for linear power supplies.
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Bruce,
Another critical peace of advice. Before you tare something down take a TON of picture from ALL angles. This will aid you in reassembly. A digital camera in the lab can save you hours in a schematic when you have a 50/50 shot of getting a power plug backwards and popping something. Also however many picture you THINK you need take double invariably there will be an angle you are missing that you need.
Eric
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Raymond Domp Frank
Michael W. Lynch