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Resistor in series
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Milan,
Thanks for this explanation, it makes sense to me. I've noticed that component values that were common 50 years ago (or more) are not common today, so the reverse is also understandable. It was also suggested that this was an attempt to make a higher wattage part than was either available, or than would fit in the space provided. The resistors are bigger than 1/4 W ones, but the manual only specs them as 1/2 W. Regardless, they've drifted pretty far out of spec (more than 20%) and will need to be replaced. I've ordered a selection of 1/2 W resistors that should include 7.5 K Ohm parts. -- Jeff Dutky |
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I think Roger¡¯s explanation of the reason for two resistors in series makes sense (better heat dissipation in a small space) as 2 x 7.5 k? = 15 k?, which is a common value. It looks as though they are there to alleviate Q1354 and reduce its C-E current. Before you change these resistors, are you sure they are out of spec.Without desoldering the junction and testing each one individually you can¡¯t be sure. You cannot accurately measure resistance in circuit. Roger has a keen eye and I would do as he suggests.
Simon |
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Bob Albert
I think there is a way.? If nothing else is connected at the junction of the resistors, short the ends of the combination together and measure from the shorted ends to the junction of the two resistors.? Make sure your Ohmmeter is isolated.? You should measure 3750 Ohms.? No soldering required.
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Bob On Tuesday, November 24, 2020, 09:45:52 PM PST, Simon <tenareze32@...> wrote:
I think Roger¡¯s explanation of the reason for two resistors in series makes sense (better heat dissipation in a small space) as 2 x 7.5 k? = 15 k?, which is a common value. It looks as though they are there to alleviate Q1354 and reduce its C-E current. Before you change these resistors, are you sure they are out of spec.Without desoldering the junction and testing each one individually you can¡¯t be sure. You cannot accurately measure resistance in circuit. Roger has a keen eye and I would do as he suggests. Simon |
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Simon,
my reading of the schematic says that once I have pulled Q1352, Q1354, and Q1358 from their sockets the resistors are only connected to the rest of the circuit through two diodes, one of which is reversed biased (assuming that it's not shorted, which is not a given in this case). If the diode across the collector and base of Q1354 were shorted, however, I would expect that the resistance I read across the two resistors would be less than 15 Ohms, since some other resistance would be in parallel with them. The resistance I'm reading is higher than the marked value of the resistors (and the value indicated in the schematic), not lower. That said, I really should check the diode CR1354 to make sure it's not also shorted, because, with my luck, it probably is. (I was originally planning to shelve this project until the replacement transistors arrived, but then I got to thinking about why a couple of transistors might be blown, and started worrying about other components that must have failed in the same event, and realized I could test a bunch of the components if I just removed all the socketed transistors from the circuit, leaving a bunch of disconnected legs. I'm really glad I succumbed to my worry on this, because I'm finding, and learning, all kinds of things along the way) -- Jeff Dutky |
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If the measured resistance is higher than that in the circuit diagram you may have a problem as it may not bypass enough current across Q1354. If you are getting replacements it would be worth putting them in, I would have tried it out and checked the temperature of Q1354 with a calibrated finger.
Simon |
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Guys,
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I know this confusion can happen from time to time, but someone coming to these posts on "Resistor in Series" will have no idea what item of Tek gear is being referred to. I am assuming that it is the 475A. This confusion can be avoided if prior text is appended to the post and it will save people a lot of time, especially if they have some Tek test-gear which is the same or similar and is exhibiting similar features/symptoms. Thanks in advance, Colin. -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Milan Trcka Sent: 25 November 2020 05:09 To: [email protected] Subject: Re: [TekScopes] Resistor in series Jeff, the resistors connected end to end are an attempt to make a required resistance that was not available as a standard resistor value. Shortage of parts? Engineering change? Select in test? Who knows. I have a few of those in my 453 scope. |
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Colin,
Yes, this thread is a response to my thread "Fix or Part Out a 475A" and specifically is a discussion of the perplexing arrangement of resistors R1354 and R1356 across the collector-emitter of the Q1354 transistor in the beam intensity amplifier circuit. In the actual circuit we have two fat (1/2 W) resistors mounted vertically with their upward pointing leads soldered together. In the schematic this looks exactly as you would expect a voltage divider to look, except that there isn't anything tapping the voltage between the two resistors, so the question was why Tek put them in rather than using a single resistor of twice the resistance, and why they mounted them in this odd way rather than flat to the PCB. The two theories on offer (from Roger Evans and Milan Trcka) are 1) that the two parts were used to get better thermal properties in a small space, and 2) that the two resistors were put together to get a total resistance that was not available in a single part. -- Jeff Dutky |
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Chuck Harris
Resistors have voltage ratings, and power ratings in addition to
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their resistance values. It is probably not the voltage rating that is at issue here, but possibly the power rating. More likely, however is sometimes a value is needed that cannot be bought due to temporary shortages, in those cases, you can sit and wait for the part, or you can make your own out of two parts. How do you know tektronix did this? Most of the stuff we have hasn't been at the factory for 20 or 30 years. -Chuck Harris Jeff Dutky wrote: Colin, |
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Chuck Harris wrote:
How do you know tektronix did this? Most of the stuff we have hasn'tThe main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier. -- Jeff Dutky |
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In my >60 years experience with this type of equipment I have seen these two series resistors (modification?) in many different situations and I agree with Jeff 100%.? He has it right.? ? ?Terry
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On Wednesday, November 25, 2020, 02:44:56 PM CST, Jeff Dutky <jeff.dutky@...> wrote:
? Chuck Harris wrote: How do you know tektronix did this?? Most of the stuff we have hasn'tThe main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier. -- Jeff Dutky |
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To get a high-wattage resistance, there are a couple of options: Get a bigger resistor; or make it out of a series or parallel combination of lower power resistors.
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Tek chose a series combination, and chose not to implement that combination in the pcb artwork, but did show it in the schematic as a series combination. To me, that suggests a very deliberate choice not done in haste. The voltages are too low for flashover concerns to have been the reason here. I think that something else besides that or a parts shortage drove the decision. To first order, the shunt capacitance in pF of a resistor equals the power rating in watts. A 1W resistor, or two half-watt resistors in parallel, has about 1pF in parasitic capacitance. But a series combination creates a 1W resistor with only 0.25pF capacitance. That's a consideration in a high-swing, high slew-rate blanking circuit, where you can easily burn ~10mA driving each pF of parasitic load cap. To reap the full benefits of the series connection, you would want to float the common point above the pcb, which is exactly what they did. I regularly use this method in rf circuits. The trade off is an increase in series inductance, but in this circuit, we're looking at 15 kilohms, so parasitic inductance is a non-issue. -- Cheers, Tom Sent from an iThing, so please pardon the typos and brevity On Nov 25, 2020, at 1:12 PM, "Terry Gray via groups.io" <tlgray42@...> wrote:
In my >60 years experience with this type of equipment I have seen these two series resistors (modification?) in many different situations and I agree with Jeff 100%. He has it right. Terry |
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Chuck Harris
I've done this for many, many years, and usually my reason is
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I cannot fit a higher wattage resistor in the same space as two smaller resistors sitting upright like this. So, I would guess that a smaller resistor was specified when the board was designed, and it was found to get too hot, so two resistors of the same wattage, in series were fitted... with no changes in the circuit board. -Chuck Harris. Jeff Dutky wrote: Chuck Harris wrote:How do you know tektronix did this? Most of the stuff we have hasn'tThe main reasons that I think Tek did this are 1) that the two resistors appear in the schematic in the service manual, and 2) the resistors look like matches for resistors found elsewhere on the unit. Also, the method of vertically mounting the resistors and soldering their upward pointing leads together is found elsewhere on the unit, specifically on the vertical preamp board in the 3rd stage amplifier. |
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In a word, no.
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Anything is possible, I suppose, but asking us to believe that Tek's design team carelessly forgot to check power dissipation is a lot to swallow. This wasn't their first rodeo -- they'd already done the 465, 465B (it was pretty much the same team) and the 475. If you look at the 465, there is a similar series combo in the same general part of the blanking circuit. It is obviously done to reduce capacitance. There is a tiny trimmer of about 1pF across the combo, so clearly they were carefully accounting for fractions of a pF. I'm sticking with parasitic capacitance concerns as the most likely reason for the choice in the 475A. The team did make some mistakes (in the 465 in particular; they fixed those in the B version), but forgetting to calculate power dissipation here was not one of them. Cheers, Tom Sent from my iThing, so please forgive the terseness and typos. On Nov 25, 2020, at 14:54, "Chuck Harris" <cfharris@...> wrote:
I've done this for many, many years, and usually my reason is |
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Chuck Harris wrote:
but this resistor pair is present in the schematics, not just for the 475A, but also in the early 475 service manual schematics. I know that's not really a refutation of your point, but it sure looks like they meant to do this from really early on. The on-end resistors are clearly visible in the PCB images. I haven't opened up my oldest 475 to check the physical board, but I was planning to do that, and will report back what I find. Also, I know that schematics do not necessarily precede the physical objects they represent, so they may not accurately reflect original intent. I once worked at an engineering company where, as we were packing a large machine to be shipped to the client the lead engineer was taking each part and comparing it to the existing drawings, in order to find parts that had been modified (or completely fabricated) during testing and development. When he would find a part that didn't have a drawing he would quickly gin one up in AutoCAD before the part was packed and shipped. My impression, at the time, was that this was part of our contractual obligations to the client, but it occurs to me now that it may have been entirely internal; so that our people would be able to correctly reassemble the machine at the client site based on the engineering drawings. |
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sadly they did make that very mistake quite a few times! Witness the 7904
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vertical board ... R724 and R725 (499 Ohm 1/2W) always run way too hot (burnt fingers hot) and are often well off value from overcooking ... the PCB gets well toasted too! I normally replace them with two 1kOhm 2W metal film in parallel and they still run pretty warm. David -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Tom Lee Sent: 25 November 2020 23:20 To: [email protected] Subject: Re: [TekScopes] Resistor in series In a word, no. Anything is possible, I suppose, but asking us to believe that Tek's design team carelessly forgot to check power dissipation is a lot to swallow. |
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David Partridge wrote:
So, while I'm in here, and planning to replace these two resistors, should I increase the wattage? I expect that these are carbon composite resistors (but certainly don't know that for sure) and what I have ordered are supposed to be metal film 1/2 W resistors. Maybe I should order a kit of 1 W metal film resistors as well and use them instead? Part of me thinks that it won't make much difference: these are underneath the metal shield over the HV section, so they have restricted air-flow under any circumstance. Will larger components really make much difference in their heat dissipation if they are trapped in a hot box? Also, these resistors have likely been cooking for over 30 years, and they've only drifted by about 20%, so either they're not getting too badly cooked, or they're made of sterner stuff. -- Jeff Dutky |
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Ah, not so fast, David! You happened to have chosen an example that illustrates a larger point, but not that there was an oversight in a power calculation.
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First, in my sm, R724 and R725 are not 499 ohms, but 332, but no matter. The designer of the main vert. amp, Thor Hallen, had a tough decision: Use a bigger resistor and suffer the parasitics, or use a resistor on the edge of the power spec? He went for the latter. The resistor runs hot, but it is, on paper, within spec. The scope met the bandwidth and operational lifetime targets. This was one of several instances of compromises being forced on them because they were pushing technology to the edge. The 7904 was an extremely important product for Tek; HP was winning the bandwidth wars and mocking Tek for bells and whistles (e.g., on-screen display). The 7904 was given the goal of "bandwidth uber alles". Trading off other parameters was ok, but not risking 500MHz bandwidths. So the 7A19 is a delicate beast, the 7B92 is a bad design, and you couldn't swap plug ins freely with the mainframes without going through a cal as an ensemble. That was permissible, but risking a failure to hit 500MHz was not. If you elect to put in different resistors in the vertical gain path, you will see their effects. The scope will still "work" but you might find it challenging to keep the aberrations within spec, or you might find that the bandwidth doesn't quite hit 500MHz with every plug in. I'm not saying that Tek never made a mistake. But I am saying that one shouldn't be so quick to indict them, especially in the specific case that Jeff's post is about. Again, the team had already had ample experience with the 475 (and the 465B and 465 before that), so if there had been a dissipation problem, it would have been caught and fixed long before the 475A came along. And, as I've explained, this particular circuit is extremely sensitive to parasitic capacitance. While we're on this subject, anyone who cares about this more deeply, take a look at the corresponding circuit in the 465. The two series-connected resistors there are not equal in value. A clue as to why is that these resistors are in a negative feedback path. Consider the effect of parasitic capacitance from the common point to ground. By ratioing the resistors in the way they have, that common point turns out not to vary in voltage, making any capacitance there irrelevant. These engineers were thoughtful. Not perfect, but thoughtful. Any time I think I've spotted a mistake, I have to be very careful, because additional thinking has almost always proven that I was too quick on the draw. We can discuss their legitimate mistakes in the 465, 7B92 and some other circuits/products in some other thread. I've devoted a good portion of an upcoming book chapter to them. --Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 11/25/2020 16:59, David C. Partridge wrote:
sadly they did make that very mistake quite a few times! Witness the 7904 |
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Chuck Harris
Perhaps Tom, but I have seen many instances of them doing
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just that. U800 as installed on the 2465 was built to handle much higher wattage, and a heat sink it didn't need. When they decided not to spend the money on a heat sink it didn't need, they used 4 star washers to make a kludge fix that created a problem with U800 case cracking... Bad epoxy in the 547 transformers... something I know a little bit about, as I have built hundreds to replace their mistake. They weren't perfect; they were engineers. They fixed their mistakes, often with what they called "tents" made of suspended parts and black wires. The rest of us called it gumball construction... best left for prototypes. If you haven't seen any tektronix mistakes, you haven't been looking very hard. -Chuck Harris Tom Lee wrote: In a word, no. |
David C. Partridge
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