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Re: Help required finding a Tek 466 power rail short
Hi again, and thanks for the heads-up regarding the 151-0367-00's. I'll look into those.
Just been having another look and removed the 5v bridge rectifier, which unfortunately tested fine off of the board. I'm just gonna have to bite the bullet and remove the horizonatal boards and main filter caps to access the underside ofthe A6 board in order to remove, test and possibly replace any other bridge rectifiers. It is something I have been putting off, but seem to be getting nowhere, so I'll just disconnect everything back to the transformer and take it from there. If I'm honest, I am thinking toward a semi conductor failure, as I have tested every single tantalum (and others) on both the A6 and A10 boards, and have run out of ideas. James |
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Re: Timebase Question : Auto vs Norm Mode
Here's how you know that you've misinterpreted how the trigger mode works. In your description, there's no way for a stable signal to be displayed in AUTO mode. In every scope I'm familiar with, AUTO merely generates a baseline if no valid triggering event is seen after some interval (and in most scope, that interval is a function of the timebase setting). But once a valid trigger event happens, synchronization happens, too. If it didn't, AUTO mode would be sort of useless, and one couldn't properly call it a trigger mode.
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-- Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 12/12/2021 16:33, n4buq wrote:
Hi Jeff, |
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Re: Timebase Question : Auto vs Norm Mode
On Sun, Dec 12, 2021 at 04:33 PM, n4buq wrote:
as long asC535 is discharged even with auto-generated sweeps. Ozan |
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Re: OT: microwave oven blows fuse
Hi Jim,
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I haven't fixed nearly as many microwaves as scopes, but more than a handful. For ovens using a traditional power supply with line-frequency magnetics, the most common failure mode is a shorted/leaky cap, and a close second is a shorted/leaky HV rectifier. It can be hard to determine the exact failure using only low-voltage gear, so a 577 curve tracer or equivalent is almost obligatory if a DMM, e.g., doesn't identify the bad component. Substitution is often the way to go if you don't have HV test gear, unless the replacement parts are ridiculously expensive. One note on the cap: Its value matters, as it is chosen to resonate with the leakage inductance of the HV transformer. -- Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 12/12/2021 15:05, Jim Ford wrote:
Hi, everybody.? Has anybody else seen this issue: one day recently, out of the blue, our Kitchen Aid microwave oven stopped working. One day it was fine, and the next day the display was blank, and the keypad wouldn't respond to anything.? So I attempted to open it up, thinking a catastrophic fault like this might be easy to fix.? I had to go to Home Depot to get a set of those Torx drivers with the hole in the center because the screws holding the cover on are Torx with the pin in the center.? Not that I mind buying a new tool, of course!? So I opened it up yesterday, and sure enough, the 20 amp fuse was blown.? Really, really blown - the metal inside was splattered all over the glass! |
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Re: OT: microwave oven blows fuse
Bob Albert
This is a common problem.? Chances are one of three things.? Bad capacitor, bad rectifier, bad transformer.? The transformer may be arcing due to high voltage.? The same for the capacitor.? You could test the capacitor and the diode with a power supply.? Testing the transformer requires energizing it with no load to see if it withstands the voltage and has no shorted turns.
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In extreme cases it could be the magnetron. Bob On Sunday, December 12, 2021, 05:05:19 PM PST, Jim Ford <james.ford@...> wrote:
Hi, everybody.? Has anybody else seen this issue: one day recently, out of the blue, our Kitchen Aid microwave oven stopped working.? One day it was fine, and the next day the display was blank, and the keypad wouldn't respond to anything.? So I attempted to open it up, thinking a catastrophic fault like this might be easy to fix.? I had to go to Home Depot to get a set of those Torx drivers with the hole in the center because the screws holding the cover on are Torx with the pin in the center.? Not that I mind buying a new tool, of course!? So I opened it up yesterday, and sure enough, the 20 amp fuse was blown.? Really, really blown - the metal inside was splattered all over the glass! I checked YouTube, watched a few videos, and decided to discharge and measure the high voltage capacitor, a known weak point.? So I grabbed some alligator clips, some banana leads, and a 2 ohm, 25 W wirewound resistor I had lying around.? I didn't think discharging the cap, a 0.95 uF job, with a dead short was a good idea.? The cap measured about 130 ohms and 0.00 V, but the ohms reading was very unstable.? I realized the HV transformer was still in the circuit, so I disconnected the cap leads (very hard to get the clips off, BTW) and measured again.? Megohms this time and about 0.2 V (I chalked that up to dielectric absorption). I figured it wasn't the cap, and maybe the original fuse was faulty or a power surge had fried it.? I hoped for the best and made another trip to Home Depot and bought a ceramic fuse.? Actually 2 in a pack.? Put one in the microwave oven and was pleased to hear it beep and get all zeros on the display.? Short lived, though, because the kitchen lights dimmed, and the display went blank.? Sure enough, the fuse was open now. So, anybody have an idea what to check next?? Or is the cap indeed blown and in need of replacement?? I'd sure hate to have to trash this appliance after only about 4 years (1 year warranty, of course!).? And I'd hate even more to have to spring for another one.? IIRC, it was about $750 new. Thanks. Jim Ford in Southern California, USA |
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OT: microwave oven blows fuse
Hi, everybody. Has anybody else seen this issue: one day recently, out of the blue, our Kitchen Aid microwave oven stopped working. One day it was fine, and the next day the display was blank, and the keypad wouldn't respond to anything. So I attempted to open it up, thinking a catastrophic fault like this might be easy to fix. I had to go to Home Depot to get a set of those Torx drivers with the hole in the center because the screws holding the cover on are Torx with the pin in the center. Not that I mind buying a new tool, of course! So I opened it up yesterday, and sure enough, the 20 amp fuse was blown. Really, really blown - the metal inside was splattered all over the glass!
I checked YouTube, watched a few videos, and decided to discharge and measure the high voltage capacitor, a known weak point. So I grabbed some alligator clips, some banana leads, and a 2 ohm, 25 W wirewound resistor I had lying around. I didn't think discharging the cap, a 0.95 uF job, with a dead short was a good idea. The cap measured about 130 ohms and 0.00 V, but the ohms reading was very unstable. I realized the HV transformer was still in the circuit, so I disconnected the cap leads (very hard to get the clips off, BTW) and measured again. Megohms this time and about 0.2 V (I chalked that up to dielectric absorption). I figured it wasn't the cap, and maybe the original fuse was faulty or a power surge had fried it. I hoped for the best and made another trip to Home Depot and bought a ceramic fuse. Actually 2 in a pack. Put one in the microwave oven and was pleased to hear it beep and get all zeros on the display. Short lived, though, because the kitchen lights dimmed, and the display went blank. Sure enough, the fuse was open now. So, anybody have an idea what to check next? Or is the cap indeed blown and in need of replacement? I'd sure hate to have to trash this appliance after only about 4 years (1 year warranty, of course!). And I'd hate even more to have to spring for another one. IIRC, it was about $750 new. Thanks. Jim Ford in Southern California, USA |
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Re: Tk 2445 Mains switch jammed 'on'
Tnx Bob
I changed the EHT unit years back, I had an idea , there was a rod , found the manual is on the Tek site , feels like the latch on the switch has jammed, as the button , depresses and releases , may not fully travel, Hopefully its on view when the cover is off , Tnx-Graham |
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Re: Timebase Question : Auto vs Norm Mode
Hi Jeff,
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I'm not trying to argue and I know I'm probably the least likely one in this group to fully understand the circuit, but I way I understand it, as long as C535 is charged, the sweep starts immediately after the previous sweep finishes and, C535 is discharged only when a trigger is present; therefore, with no trigger, there's virtually no delay from end-of-sweep to beginning-of-sweep. Is that incorrect? Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...> Barry, |
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Re: Tk 2445 Mains switch jammed 'on'
Bob Albert
I believe there is a long plastic actuator with angles that goes to the rear and moves the switch.? The switch itself is nowhere near the button.
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The problem is either the switch itself is jammed or the actuator isn't doing its job.? You have to open the scope to see. Bob On Sunday, December 12, 2021, 12:43:21 PM PST, Graham <g0nbd@...> wrote:
Tek 2445 Mains switch jammed 'on' Hi Before start looking? inside,? Just came to power the? scope off , and find the? power switch, although it? travels inwards, the mechanism appears jammed? .. fails to? power off? .. Q where is the? actual switch located, how is it accessed ? switch the? mains? or soft start ? Tnx-Graham |
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Re: Fixing up a 5S14N (for search: 7S14)
On Sun, Dec 12, 2021 at 05:21 PM, Paul McClay wrote:
And if that works.... maybe connect the sampler horizontal and vertical outputs to the respective plugin inputs - then the mainframe could selectively show either the sampler display or normal Yt operation. Assuming a 2-channel vertical amp. Or maybe send the vertical output back thru the power Y extender and add a "DISPLAY" switch for that slot. With attention to routing vert plugin/channel select logic thru the plugin connector. ??. Anyone know what traps I'm not considering? |
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Wanted: Feet for TDS6xx scope..
I'm looking for a set of 4 feet for a TDS5xx, TDS6xx or TDS 7xx scope. The part
number is: 348¨C1110¨C04 If you have them available, please let me know (off list) your asking price including shipping. Thanks! Cheers, Lyle -- 73 NM6Y Bickley Consulting West "Black holes are where God is dividing by zero" |
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Fixing up a 5S14N (for search: 7S14)
As already mentioned, I have a new-to-me 5S14N. Yay.
So far it seems that the horizontal section and triggering work as expected but it has a few problems in the vertical section. I certainly haven't done all my troubleshooting homework yet, but I thought I'd post up some early findings to see if anyone already knows what they indicate. As received, it had good bias cells in channel 2 and dead cells in channel 1. Not 0.0V dead but sub-0.1V, iirc. Pretty sure they were under 0.5V. I didn't record the numbers. Channel 2 looked imperfect but basically functional -- square wave tops not flat but low noise, about expected amplitude allowing for feeding both ch2 & ext trigger, and <500ps rise/fall from a source that's spec'd not quite that fast. Using a 1x probe to snag the ext trigger input probably contributed to distortion. Channel 1 was a flat line, which made sense for dead bias cells, but responsive to DC offset control, and close to display center with the control centered. Intermittently the inputs appear to self-oscillate, even with nothing connected. The last time that happened a power cycle fixed it. About 0.5-1V amplitude iirc - similar but not the same amplitude between channels - looks like it might be a sine but not stable enough to distinguish a clear waveform vs aliasing - suggesting a frequency close enough to some harmonic of the scan rate to drift in and out of looking confusingly almost like signal. While typing this, I suppose that indicates that it's happening after the trigger pick-off -- which I suppose couldn't be otherwise because it's all passive until after the trigger pick-off. ?. Anyhow, I mention it because it's there, but seems like a secondary issue at this point because most of the time it doesn't do that. I tried swapping the sampler boards to put the good cells on channel 1. Internal triggering worked fine. Square waves looked more distorted than before, even without the 1x probe in play, but I figured that could be due to mismatch between the ch2 sampler and the rest of the ch1 circuit until re-adjusted. ?. I've since un-swapped the samplers. On to more disabling faults: As described elsewhere[1], I replaced the bias cells in ch1 with silver oxide cells. Nominally 1.55V but measuring 1.59V fresh from the blister pack. That did not magically fix ch1, and now ch1 behaves differently. With the cells removed, which I'd expect to be like it was with dead cells, the ch1 trace is pinned high off screen and unresponsive to the offset control -- as revealed by the beam-finder button. With the cells in place, the trace shifts off the bottom of the display, but responds to the upper range of DC offset and can be shifted up to about 1/3 up the display at maximum DC offset and 0.5V/div. At higher sensitivity (0.2V, higher?) the beam-finder shows the trace responding to the upper range of DC offset but not enough to bring it into the display area. When the trace is visible (0.5V/div, max offset), the unit can trigger and show a stable but noisy and attenuated signal. I've read that vertical trace shift can indicate fault with the sampling diode pair. If indeed operation with no cells is like operation with dead cells, then this wasn't a fault before I replaced the cells. I'll be a little disappointed if I killed the diode(s) myself, after trying to not do that. And then a couple things seem kind of strange and maybe specific enough to suggest specific faults. When ch1 trace is visible (0.5V/div, max offset) and no input connected, the trace shows ~50mV (1/2 minor division) "noise". But not really noise. It appears to alternate between two distinct levels about 50mV apart, at irregular intervals that look like a few 10s of transitions/sec i.e. a random-ish distribution of intervals clustering around a few 10s of msec. Noise on ch2, visible at high sensitivities, looks like ordinary noise. The next strange-seeming thing is that the "LO NOISE" function shifts the ch1 trace up the display and amplifies it about 2x, in addition to reducing noise. The up-shift looks like less than doubling the large DC offset needed to shift the trace into view. The ch2 trace does not get shifted or amplified. (for completeness: at high sensitivity such that the no-input trace shows noise, the ch2 trace shifts up a fraction of the width of the noisy trace, i.e. noise-reduced trace runs above the center of the normal trace blur, but I assume that's within normal adjustment) The ch1 up-shift brings the top of the offset range for a no-input trace at 0.2V/div into the bottom half of the screen and the 0.1V/div trace into the bottom division at max offset. Shifting the higher sensitivities into view, together with the slow sweep and mysterious ~2x amplification the LO NOISE mode, gives a better look at the mysterious bi-level "noise" that isn't noise. Single-sweep traces clearly show irregular alternation between two distinct levels that appear about 100mV apart (about 2x the apparent difference in non-lo-noise view). In LO NOISE operation, it looks like sweeps run about 3/sec, making each horizontal division represent about 1sec/30. The distribution of irregular intervals of alternation appear to cluster around 1 div (i.e. shortest intervals probably not visible and longest intervals <10div), which would be consistent with the appearance of ~tens of level-switches/second. I've posted a photo of three single-sweep LO NOISE traces at 0.2V/div separated by different DC offsets: /g/TekScopes/photo/270747/3351116. It looks like level-switches happen at ~random intervals with a distribution around ~1/div. Assuming the pictured sweeps show about ~1s/30 per horizontal division, that would be consistent with the subjective impression of a few 10s of alternations/second. At the same time, e.g. dual trace display, ch2 at high sensitivity shows what looks like actual random noise. Having only one scope complicates troubleshooting. I have access to other scopes but I have to take myself and stuff to them with some inconvenience. I'm thinking of making up a partial Y extender (I have edge connectors) to power to the 5S14N from a single vertical slot while using the scope in normal Yt operation with two plugins. Assuming it won't be hard to persuade the unit to operate like it had normal signal connections also. But I don't know how much power the plugins draw vs what the mainframe or a single slot connector can deliver. Seeing that the unit uses the HV supply also ups the ante for build quality if I attempt that :-/ I've written this from memory, so I might end up correcting details later. Any insights appreciated! [1] /g/TekScopes/topic/7s14_5s14n_bias_supplies/87551264 ; /g/TekScopes/album?id=270597 keyword: 7S14 |
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Tk 2445 Mains switch jammed 'on'
Tek 2445 Mains switch jammed 'on'
Hi Before start looking inside, Just came to power the scope off , and find the power switch, although it travels inwards, the mechanism appears jammed .. fails to power off .. Q where is the actual switch located, how is it accessed ? switch the mains or soft start ? Tnx-Graham |
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Re: Timebase Question : Auto vs Norm Mode
Barry,
I'm pretty sure that you have misinterpreted how AUTO mode works, the 7B53A service manual (May 1988 revision) clearly says on page 3-9 "When the AUTO button is pressed, a weep is produced as in NORM except that a free-running trace is displayed when a trigger pulse is not present." Then on page 3-11 the Auto Trigger Mode section reads: "Operation of the Main Sweep Generator circuit in the AUTO position of the MAIN TRIGGERING MODE switch is the same as for the NORM position just described when a trigger pulse is applied. However, when a trigger pulse is not present, a free running reference trace is produced in the AUTO position. This occurs as follows: "The Auto Triggering circuit consists of pins 1, 3, 6, and 19 of U520. When the AUTO button of the MAIN TRIGGERING MODE switch is pressed, a low at pin 19 of U520 enables the Auto Circuit. When a repetitive trigger signal above 30 Hz, and of adequate amplitude, is applied to the Main Sweep Start Comparator and pin 1 of U520, the internal Auto Multi at pin 6 of U520 charges towards five volts through C535 and R535, but is discharged by each incoming trigger pulse. "In the absense of a trigger pulse, C535 charges toward +5 V, switching pin 6 to its high state and pin 3 to its low state. Q547 turns off, its collector rises and a high is coupled through emitter follower Q551 to pin 1 of U580, causing the sweep to run." This basically identical to the Auto Sweep circuit in the 7B92, which also uses a sweep process IC, and an RC etwork off of pin 6 (the AUTO TIMING pin). The sweep process IC in both the 7B53A and the 7B92 is the same Tek custom IC, 155-0049 (-00 in the 7B92 and -01 in the 7B53A). The upshot is, as I said, that the sweep occurs either a) when you get an appropriate trigger condition, or b) after some short delay determined by the RC network. The RC network off of pin 6 in the 7B53A is 120k ¦¸ and 1 uF, giving a TC of 120 ms, but the voltage charging C535 is +15 V, so it will get to +5 V after only a couple of TCs, meaning that the actual repetition rate of the sweep in AUTO mode will only be in the range of ten Hertz, independent of the selected horizontal sweep speed (maybe I did the math wrong, because the sweep rep rate I observe in AUTO mode seems much faster than the TC of the RC network would imply. I'd guess that the sweep rep rate around at least 30 Hz, if not 60 Hz. Obviously I could observe the sawtooth directly to confirm my observation, but I haven't done that because I'm lazy). -- Jeff Dutky |
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Re: Horizontal preamplifier
#chat-notice
Hi,
I once repaired a 2215 with similar problems. The description of that repair is here: /g/TekScopes/message/164497 Good luck with your repair, Leo |
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Re: Horizontal preamplifier
#chat-notice
Hello,
Sorry to answer you so late but I got a little confused in this site that I recently discovered. Yes, I have tested these connections and even the horizontal amp which is working properly. I always come back to the horizontal preamp, the input signals are good and the output signals bad. All DC voltages are OK. Raymond put me in touch with someone who has a 155-0124. I await the delivery. |
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Internal Drive Mechanism for 7B53N (et., al.) Sweep Speed Cam
The rear cam in my 7B53N does not turn when the sweep speed knob is rotated. I have disassembled the entire cam assembly but whatever mechanism causes the shaft to be able to rotate the cam is virtually inaccessible. From what I can tell, the shaft and drive mechanism were assembled and then the end caps were glued onto the cam.
I presume that in order to allow the shaft to both rotate the cam and be pulled so that the rear cam can be rotated independently of the front cam, there may be a spline inside the cam but I can't see that. From what I can tell, the cam in the 7B53N is different from the other 7B53 variants so finding a replacement may be a challenge. Does anyone know if this might be repairable? I'm not sure I can get the end cap off without destroying the cam but maybe that's possible but, even if I can get it apart, I'm not sure what this looks like inside and whether it could be repaired. Thanks, Barry - N4BUQ |
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Re: Timebase Question : Auto vs Norm Mode
Jeff,
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I don't know if the 7B92 is very much different from the 7B53 series, but from what I read, when in AUTO, if there's no trigger signal, a capacitor (C786) stays charged that keeps the MILLER RUNUP section enabled which causes a continuous sweep. In other words, a sweep in AUTO mode doesn't care whether the trigger is armed. Someone please correct me, though, if that's wrong or if it's an over-simplification. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...> Barry, |
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Re: Timebase Question : Auto vs Norm Mode
Barry,
though you did say to disregard this question, I will answer simply because it will make the archived messages more useful to other people at a later time (and because I have been looking at this in a 7B92, so I think I understand how the free-running feature is supposed to work). In AUTO mode the sweep will trigger when either of two things happens: 1. when the trigger condition is met (and the trigger circuit is in the ARMED state), or 2. when a timer (an RC circuit in the 7B92) finishes (and the trigger circuit is in the ARMED state). This means that you get a short pause between free-run sweeps if the trigger conditions are not met, so they will not necessarily be happening at the specified sweep frequency (in other words, if you set a sweep speed of 1 ms/div, so that the sweep takes something like 10 ms, the actual free-run rate may be something like 20 ms, or even slower depending on the auto-sweep timing). -- Jeff Dutky |
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Re: 7S14/5S14N bias supplies: something to build on
#photo-notice
Ok - got parts, made a thing, added pics to album. In this case: a pair of SR44 button cell "retainers".
(apparently a "holder" is a complete unit while the plain metal clips that capture the pos. cell body over an assumed neg. contact pad on the PCB are called "retainers". things I didn't know I didn't know...) For simplicity, I wanted to do something like the cell holders shown in the picture from Stefan at the bottom of Ralf Ohmberger's 7S14 page: Cell holders I found that looked like that there were sized for relatively thin 12mm cells like 1225, and the photo shows 389 cells which are 11.6 x 3.05mm rather than the common 11.6 x 5.4mm SR44/"LR44"/1154/357 cells. The "LR44" retainers were simple to obtain quickly and retain the thicker cells without adding thickness for a plastic "holder" body when attached to a PCB. So I went that way instead. If I had a 1225 holder in hand I probably would have tried a CR1225 lithium cell + resistive divider (and caps). But while years of capacity and ubiquitous availability make that sound like a good idea with a bigger CR2032 cell, the smaller capacity of a less common 1225 (or two) diminished my motivation to find parts for that. And I didn't find a way to fit a CR2032 holder/retainer in the available space. (~10.2mm/0.40in between sampler board and cover, by the way) The album is open for anyone to add photos, so please feel free to add other ideas for taking advantage of the 0.1" grid alignment. |
James55
Lyle Bickley
satbeginner
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