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Re: Encouraging beginners: What are we accomplishing?
On Wed, May 27, 2020 at 10:52 AM, LarryS wrote:
I thought I had read somewhere the actual figure was 83%. You must be right! larry
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Re: Advice about buying a 7904
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 |
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Mystery part Type 576 Curve tracer.
All:
Now that I have got my 576 working reliably, I was going to start calibration and I found a pair of what appear to be silicone washers laying in the bottom of the unit. I have no idea where these are supposed to go. The color is a translucent and slightly off white. The parts are very soft and pliable. The appearance is like that of silicone or a similar rubber like material. The dimensions are 22mm OD x 8mm ID x 2.25mm H (or thick). There is no appearance of any adhesive residue on these parts. These were laying at the bottom of the unit and had a layer of dust on top. Photo is here: /g/TekScopes/album?id=247763 This unit is a very Low Serial number unit B0100169 and this is the one that I used as my "Test Mule" for the HV transformer project, it is in pristine condition inside. The unit seems to work just fine (other than needing a calibration) and I have not done anything else to disturb the internal parts of the unit, other than the HV Supplu cover and board. I do not believe that I dislodged these, since they appear to have been laying in the bottom of the unit for quite some time. Any ideas what these might be or where they need to be installed? Thanks in advance. . -- Michael Lynch Dardanelle, Arkansas |
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Re: 475 questions
Bruce,
There are 2 fundamental differences that need to be taken in to account when replacing “vintage” caps. And I use the term vintage loosely. One is tolerance. In the worst case I can remember a capacitors value was +150% -%100 tolerance. The means that for a 1 uF the measured value of anything between 2.5 uF to .1 uF would be considered “in spec” old radios still amaze me they ever worked. Now a days +-20% is the normal. The service manual will tell you the tolerance of the filters. From memory I am guessing it is going to be +100% to -20 % so pretty wide design tolerance on the filters. The other important spec is the voltage rating of the cap. And here vintage caps and modern caps differ greatly. Vintage caps were very tolerant of over voltage especially given how tube gear warms up before the tube’s comes in to operation the B+ will spike some times as high as 200 to 250 volts higher then when the device is operating and can hang there for about 15-30 seconds. This is not an issue in your 475 as it is solid state. However modern caps are completely intolerant of over voltage so if you have the physical space it is always good to bump up the voltage rating of the cap it wont effect anything to replace a 63V cap with a 400V cap excepta little cost in $ maybe one or 2 and physical space it will be slightly bigger then it’s modern lower voltage counterpart. However both are usually smaller then their vintage counter part even doubling the voltage the modern can will be smaller physically. Eric On 5/29/2020 12:50 AM, ciclista41 via groups.io wrote: It might help if I posted the values of the capacitors that were originally in the scope. I'll do this below in the quoted text. I have used Kemet in projects but mainly their surface mount stuff I tend to lean to Nichicon Rubycon - Actual? (frequently found counter fit parts) Veshay and Illinois Capasitor
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Re: 475 questions
It might help if I posted the values of the capacitors that were originally in the scope. I'll do this below in the quoted text.
On Thu, May 28, 2020 at 05:59 PM, <ciclista41@...> wrote: To replace the tantalum I broke (C1091), and the other one I removed (C1093). Originals were 2.2μF 20V. 478-1870-ND?To replace C1414 1000μF 75VDC ?493-17708-ND?To replace C1412 350μF 75VDC ?493-17783-1-ND?To replace C1442 and C1462 5500μF 30VDC ?399-19618-ND?To replace C1452 5000μF 25VDC ?493-14637-ND?To replace C1472 3000μF 35VDC ?493-14660-1-ND? |
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Re: Advice about buying a 7904
I can speak of this from the 7904 perspective, since I have two that work, and some backup parts sources,
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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. |
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Re: 475 questions
As a precaution, some switching supplies don't work well with a voltage below the operating voltage.
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For a linear supply, as the voltage to the supply rises, the regulator generally (if it has a decent reference) will turn on full until the input voltage to the regulator reaches a threshold where the output voltage would exceed the setting, then the regulator stops being a short circuit and starts regulating. Depending on the circuit, for example a 5 volt regulator may need anything from 5.25 volts to 8 volts to start regulating. On the other hand, a switching regulator is not so well behaved. Remember that a switching regulator has a transistor that, instead of being a variable resistor, is a switch.? It's either on or off.? The switching regulator works by pulsing the transistor on with a limited duty cycle.? That complete (on) dead short lasts for enough time to build up the output voltage, then it turns off.? There are several geometries of switching regulator, some of which, if the switching element is turned on, will draw a lot of current. The problem in a switching regulator (or one that has two transistors driving a transformer as an inverter), is the question:? What happens when the input voltage is too low? Some turn on and try to build a proper output voltage.? Some may not operate until they get enough input voltage to work properly. The problem with some of them is that they draw a LOT of current before they start to operate properly (and switch on/off). Tektronix uses both switching and linear supplies, and the switching supplies may or may not "like" low input voltages. I have brought up scopes using a variac, but I have monitored the input current.? I've seen it go over the proper rating and then go back to a lower voltage when the supply starts to switch. I've seen them work into a short (and the tick mode/overcurrent mode) not work well.? You find that out by monitoring the current the scope draws as the voltage goes up. Each supply/scope may be different.? You'll need to watch what each one does. I have a sencore unit that has an isolation transformer and a variac.? It measures both leakage current from the supply lines to ground (not neutral) and the input voltage, output to the UUT, and current. It's been useful.? You could make one if you wanted, with the exception of the leakage current measurements.? Not sure how they did that. I'd seriously recommend simultaneous output voltage/output current monitoring, but on this one, I know where the variac will put out full voltage, and can still monitor the current. Harvey On 5/28/2020 6:39 PM, ciclista41 via groups.io wrote:
Still great advice for later when I'm working on a DUT with a SMPS. :-) |
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Re: probe.
I stand corrected. Sometimes you can teach an old dog new tricks! ;)
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Thanks, Raymond! Jim ------ Original Message ------
From: "Raymond Domp Frank" <hewpatek@...> To: [email protected] Sent: 5/28/2020 1:31:23 PM Subject: Re: [TekScopes] probe. On Thu, May 28, 2020 at 08:36 PM, Jim Ford wrote:Hi Jim, |
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Re: Advice about buying a 7904
Chuck Harris
Issues and features that I recall:
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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. |
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Re: 475 questions
?Hi folks,
Here's my proposed parts order on Digi-Key, except that I haven't decided between the first or second set of tantalums. The caps below those are to replace the large C1412, 14, 42, 52, 62, and 72 capacitors. I tried to go up a bit on the rating of both the capacitance and the voltage, but was hampered somewhat by a lack of choice when only purchasing one or two of each. I'd had Nichicon recommended to me, so I stuck with them where I could. Apparently, Nichicon sold their tantalum capacitor manufacturing to AVX. I don't know about Kemet, but couldn't find Nichicon near those specs. Also, I haven't yet received the circular PCB's, so I assumed from the photos of them that they can accommodate a range of lead spacings. Just thought I'd run it by you all before I place the order in case I'd be advised to change anything. 478-1870-ND? ?TAP225K025SCS? AVX Corporation CAP TANT 2.2UF 10% 25V RADIAL Immediate 0.46400 $9.28 or 478-8972-1-ND? ?TAP225M035SRW? AVX Corporation CAP TANT 2.2UF 20% 35V RADIAL Immediate 0.63500 $12.70 ?493-17708-ND? ?UBY1K102MHL? Nichicon CAP ALUM 1000UF 20% 80V T/H Immediate 3.34000 $3.34 ?493-17783-1-ND? ?UBY2A361MHL6TN? Nichicon CAP ALUM 360UF 20% 100V T/H Immediate 3.18000 $3.18 ?399-19618-ND? ?ALC70A562BB040? KEMET SNAP-IN HIGH CV 85C 5600UF 40V Immediate 5.10000 $10.20 ?493-14637-ND? ?UBY1V512MHL? Nichicon CAP ALUM 5100UF 20% 35V RADIAL Immediate 5.44000 $5.44 ?493-14660-1-ND? ?UBY1V302MHL1TN? Nichicon CAP ALUM 3000UF 20% 35V RADIAL Immediate 3.02000 $3.02 Thanks for your input! Bruce |
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Re: Advice about buying a 7904
My email should have read “no well-known problems”.
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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@...> wrote: |
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Re: Advice about buying a 7904
On Fri, May 29, 2020 at 12:50 AM, Reginald Beardsley wrote:
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: 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.
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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: |
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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 |
Michael W. Lynch
saipan59 (Pete)
Raymond Domp Frank
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