Re: Tek TDS500/600 series scopes capacitor replacement best practice
I had an old Marantz that had the exact same issue. The Glue decomposed and attacked the PCB.
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Jerome,
The bouncing is completely normal. As the unkown capacitance approeches zero, the beat frequency decreases to a value where the meter indication is not smooth and the meter needle bounces up and down at the beat frequency. If I recollect properly, this frequency is around 62 Hz/pF.
I suspect that, in "dialing in" the variable oscillator feequency, you adjusted the tuning slug on T1. This is not the correct procedure. For reference, note that the 2 zeroing controls allow the user to compensate for fixture/cable/stray/etc capacitances at the unknown port. On my instrument (which, now, has completely solid state electronics and not quie the same as yours), I try to find an operating point where the zeroing capacitors are set to a fairly high value with nothing connected to the unknown port (after all, we usually don't try to measure negative capacitances with this type of instrument). This allows one to use the zeroing capacitors to bring the frequency back to the "zero" value when cables/fixtures/etc. are attached. The 1.6 kHz which you got rid of corresponds to about 26 pF capacitance at the unknown port. Things might look better if you take, as a starting point, the T1 setting which zeros the meter with the fine zeroing capacitor centered and the coarse one at about 85 or 90% of full value. Note, also, that the inductor range "zero point" does not coincide with the capacitance range "zero point". There is a small trimmer (on the lower right hand side) which allow some further adjustment which can probably be ignored until later in a proper calibration.
Stephen
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300MHz is likely the equivalent time bandwidth for repetitive signals, whereas the figure below is for single shot
David
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-----Original Message----- From: [email protected] < [email protected]> On Behalf Of David Templeton Sent: 21 March 2022 12:09 To: [email protected]Subject: Re: [TekScopes] 2440 for sale It¡¯s a bit odd that quote a 300MHz bandwidth on tekwiki David On 21 Mar 2022, at 09:22, Roger Evans via groups.io <very_fuzzy_logic@...> wrote:
?The 2440 does 500Msamples/s on each channel simultaneously, so 2nsec between samples. You can use your favourite formula to turn this into a single shot bandwidth, the numbers seem to have changed over time. My answer would be somewhere around 100MHz.
Roger
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Hi,
That is the analog bandwidth, you can display repetitive signals in
sampling mode. In my practice, I was able to display and trigger up to
700MHz, obviously with much reduced amplitude.
So the 300MHz specification is quite conservative, as usual for such
vintage test equipment.
Szabolcs
David Templeton <vexorg.calibra@...> ezt ¨ªrta (id?pont: 2022. m¨¢rc.
21., H, 13:09):
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It¡¯s a bit odd that quote a 300MHz bandwidth on tekwiki
David
On 21 Mar 2022, at 09:22, Roger Evans via groups.io <very_fuzzy_logic= [email protected]> wrote:
?The 2440 does 500Msamples/s on each channel simultaneously, so 2nsec
between samples. You can use your favourite formula to turn this into a
single shot bandwidth, the numbers seem to have changed over time. My
answer would be somewhere around 100MHz.
Roger
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Re: Tek TDS500/600 series scopes capacitor replacement best practice
On Mon, Mar 21, 2022 at 12:07 PM, saipan59 (Pete) wrote:
A related cap story from just last week:
I was checking out a Yamaha A-700 hi-fi amp (circa 1984) which had not been
used in many years. Opened it up to clean out dust and such, and noticed 3 or
4 vertical electrolytics had some hard, brownish 'stuff' apparently leaking
out of them. This seemed odd, so I unsoldered one from the PCB and examined it
closely. Nothing wrong with the cap. Turns out it was a 'glue' put there in
manufacturing - it was only seen under the somewhat larger vertical caps, so I
suspect it was for mechanical support. But it had actually caused some
corrosion on the leads of a couple of nearby resistors, so I scraped those
clean.
Pete
That brown stuff was probably white to start. I remember there was a rash of problems across a lot of gear from some type of hard white adhesive that would 'cook' and age with time and heat and actually end up going conductive. I most notably remember it in a brand of internet wifi router (I can't recall the brand now) that used this glue to prevent the wifi module from popping out in shipping (It used a plug-in laptop style wifi module). After only a couple years use or less, the aging glue would interfere with the operation of the module enough that it would just stop working. The fix, if not claiming warranty for a new unit that would just do the same in time, was to simply scrape off the glue away and go on with your life with a now happy wifi router.
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As you might have guessed, it is rare to find one that is truly NOS, and even rarer to find such a beast at a cost that doesn't have to be expressed in dBdollars to avoid sticker shock.
To increase your odds slightly, you might consider also looking for a crt from a 7704A. It's the same jug as that in the 7904.
Good luck!
-- Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 3/20/2022 22:30, K7XYM wrote: So I know its a ludicrous question, but, I want to ask anyway. Are there any NOS CRTs that ever come up for the 7904 ?
I am asking because I am in the middle of a _major_ restore to new, complete with tons of mods to go beyond original performance, on my 7904 and I am now aware that these CRT age and have issues with beam spot size not related to electronics. I have 3 7904's and I have swapped electronics, as a whole, between the chassis only one tube ends with with a super sharp spot size. The other 2 are larger spot size and one has kinda odd behavior with brightness adjustments. Again that is with swapping ALL the electronics. Its a tube thing.
So no hope of getting a NOS CRT for these right ?
I am going to refurb and mod 2 and I might have to just buy 7904's looking for a good tube.
Anyone have any full parted 7904 chassis with just the tube ?
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The 2440 does 500Msamples/s on each channel simultaneously, so 2nsec between samples. You can use your favourite formula to turn this into a single shot bandwidth, the numbers seem to have changed over time. My answer would be somewhere around 100MHz.
Roger
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I'd like one too but down here in Australia the prospects are even bleaker. I have one perfect 7904 and another that's got a dud CRT that will end up as a teaching exercise and parts mule unless a CRT turns up. I'm not holding my breath!
Morris
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I changed my mind. I think I'll keep the scope.
Bob
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So I know its a ludicrous question, but, I want to ask anyway. Are there any NOS CRTs that ever come up for the 7904 ?
I am asking because I am in the middle of a _major_ restore to new, complete with tons of mods to go beyond original performance, on my 7904 and I am now aware that these CRT age and have issues with beam spot size not related to electronics. I have 3 7904's and I have swapped electronics, as a whole, between the chassis only one tube ends with with a super sharp spot size. The other 2 are larger spot size and one has kinda odd behavior with brightness adjustments. Again that is with swapping ALL the electronics. Its a tube thing.
So no hope of getting a NOS CRT for these right ?
I am going to refurb and mod 2 and I might have to just buy 7904's looking for a good tube.
Anyone have any full parted 7904 chassis with just the tube ?
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Thanks for contributing your data, Michael. Yours lines up well with mine. I have little doubt that Bill Turner and his gang had fits trying to meet spec -- it's worse when you're almost there, but you hit a huge wall just shy of the goal. Sorting 2N3866s (the vertical output driver) in search of that small fraction possessing 10%-lower-than-typical output capacitance could not have been fun. The ones I own mainly skew above typical...
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 3/20/2022 12:13, Michael W. Lynch via groups.io wrote: Tom,
You beat me to posting this observation. I have several 465¡¯s and only one gets close to 100MHz. Generally they can be depended on for 85-90 at best, without fiddling about with optimized components.
Michael Lynch
From My I-Phone
mlynch003@...
479-477-1115
On Mar 20, 2022, at 2:03 PM, Tom Lee <tomlee@...> wrote:
?The 465 (as opposed to the 465B) is notorious for barely making the 100MHz spec. I've long used the 465's vertical amplifier chain as a tutorial example in one of my classes. Using typical values for transistor parameters and equalizer settings, the expected bandwidth is closer to 90MHz. You have to make lots of optimistic assumptions to get the spec'd bandwidth without trashing the transient response. It seemed to be a marginal design, but given the approximations I used, I couldn't declare confidence to 10% limits. Then I came across the recollections of Tektronix calibration tech Bill Turner (see tekwiki). He confirmed that getting to 100MHz was indeed challenging. Much cherry-picking of transistors to find the golden parts was routinely necessary, so it all fits. I have a collection of 465s, and they all arrived with bandwidths very, very close to 100MHz or below. It's a statistically insignificant sample size, but you can still feel the cal techs' desperation.
It may also be of relevance that tek ever offered the 463 for sale. It is basically a 465, but without some of the bandwidth-peaking networks in place. The 463 would've easily made its 75MHz spec without all that costly, labor-intensive tweaking and transistor-hunting.
My advice would be to leave things well enough alone. If you insist on going ahead, you'd first have to validate your setup (e.g., characterize the flatness of your 191+cable+termination combination, as well as the cleanliness of the pulse generator you'd use to evaluate time-domain response). Then it gets challenging, as you will have to adjust many interacting tweaks to fight your MHz-by-MHz battle. If none of that fazes you, please post again. I can send you instructions on how to upgrade the bandwidth to near-465B levels. But it requires significant additional effort. (Short version: A bridged T-coil as an interface to the vertical deflection plates does a world of good; that interface is the main bandwdith bottleneck in this scope. But retrofitting one is not a simple drop-in operation.)
-- Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 3/20/2022 10:23, david via groups.io wrote:
When I check the bandwidth of my 465 scope Channel 1 and Channel 2, as per the manual, with a Type191 sine generator, it checks a little low. Manual says 3.5 division minimum display @ 100 MHz, getting 3.5 div. at 92.2 MHz and 3.35 div. at 100 MHz. is this considered close enough? Or can it be corrected?
David
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Started Album on Tek 549 Serial #101
Uploaded a few photos on the new 549 'scope I recently acquired.
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The following photos have been
uploaded to
the 2712 VR album of the [email protected] group.
By:
Gerald <vk3gjm@...>
Gerald <vk3gjm@...> updated the album 2712 VR: Late model 2712 with top VR lid removed. Replaced Q350, MMBT3904, low HFE. The device gets quite hot. Measured at 72 degrees C, 2712 cover partially on and lid on VR after 1 hour.
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Not 1.6 Hz, 1.6 kHz. Fingers not working.
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On 3/20/22 23:17, Jerome D Leach via groups.io wrote: Took the great advice I received from you gents, and have finally gotten the 130 to register. Have not got it straightened out completely, but I can use the S-30 to cause the meter to deflect. After giving myself a bit of time away from tinkering with it, I took a more methodical approach. First checking voltages, which were fine. I then went through the checks within the calibration section of the manual. Checking the fixed oscillator found it to be spot-on 140 kHz. I next checked the variable osc., and it was off by 1.6 Hz. Dialed it in, and found that the needle came up to close to 0 on the scale. Unfortunately, when I connected the S-30, I found that the meter would not zero. I can get the needle close to zero, but it begins to bounce at a steady rate. Perhaps I am wrong, but I am thinking that the variable osc. is still off somewhat.
Anyway, will figure it out. Thank you folks, I have hope now.
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Re: Tek 2715 Adjustments Procedure
Hi Alfredo,
I had a hard time re-aligning my late model 2712 after battery failure and a systemic gain loss after a friend accidently put 50 watts at the input. I thought the input attenuator had failed, but as it turned out 50 watts in and 3.3dB loss were not related and the attenuator is fine.
Best to use latest 2715 manual if your unit has late model PCB. I would start alignment your digital screen setup right first and then use log input. But would suggest checking it first and only adjust as a last resort.
C204 will need raising from PCB, alignment is tricky and fine tuning makes the coupled wires de-shape. However, and in addition while you have the unit open, check your VR transistor Q350. I lost approx 3.3 dB of gain and the device HFE was about one quarter of the stated value, typically a min. HFE of about 100, my device was 11. I replaced the MMBT3904 on A13 assembly. Another member Jaap Rusticus told me He found this issue during his discoveries and suggested I investigate it. This device also gets very hot, I ended up creating a brass standoff on the lid and used a small amount of thermal conductive pad between device and brass post.
I ended up replacing the SMD device with a new 3904 HFE 186 and from that point forward alignment was much easier, I had gained back 4 dB that the normalisation process could do and using the input ref.
Best of luck.
Regards
Gerald
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Took the great advice I received from you gents, and have finally gotten the 130 to register. Have not got it straightened out completely, but I can use the S-30 to cause the meter to deflect. After giving myself a bit of time away from tinkering with it, I took a more methodical approach. First checking voltages, which were fine. I then went through the checks within the calibration section of the manual. Checking the fixed oscillator found it to be spot-on 140 kHz. I next checked the variable osc., and it was off by 1.6 Hz. Dialed it in, and found that the needle came up to close to 0 on the scale. Unfortunately, when I connected the S-30, I found that the meter would not zero. I can get the needle close to zero, but it begins to bounce at a steady rate. Perhaps I am wrong, but I am thinking that the variable osc. is still off somewhat.
Anyway, will figure it out. Thank you folks, I have hope now.
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Re: Tek TDS500/600 series scopes capacitor replacement best practice
A related cap story from just last week:
I was checking out a Yamaha A-700 hi-fi amp (circa 1984) which had not been used in many years. Opened it up to clean out dust and such, and noticed 3 or 4 vertical electrolytics had some hard, brownish 'stuff' apparently leaking out of them. This seemed odd, so I unsoldered one from the PCB and examined it closely. Nothing wrong with the cap. Turns out it was a 'glue' put there in manufacturing - it was only seen under the somewhat larger vertical caps, so I suspect it was for mechanical support. But it had actually caused some corrosion on the leads of a couple of nearby resistors, so I scraped those clean.
Re-installed the cap. The amp works perfect. Very nicely built, rated 100W/ch, 0.005% THD. Has an "Auto Class-A" button which puts the output stage in class-A when the signal level is less than a few watts. Of course, the amp then generates a lot of heat while doing nothing... I'm not clear on who has ears (or speakers) good enough to hear the difference.
Pete
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Re: Tek TDS500/600 series scopes capacitor replacement best practice
Rogerio,
From working on or restoring test equipment, audio, TVs and radios (receiving/transceiving/transmitting), I will replace the electrolytics with new. Some types can be replaced with film, e.g. 1mfd. Films have very low ESR and can improve operation. I use Nichicon ULD/UHE/UCY/LGR types as the first choices for electrolytics because they are high temp, low ESR and long life types. Surface mount types usually have a high ESR after enough years and should be replaced. Using radials with the leads bent will work fine. I have done this before. You say you have a TDS which uses a crt. The electrolytics that filter the B+ for voltages from the flyback really need to be a low ESR type and may need to be upped in voltage to the next range. The original 85C types are not for high frequency circuits. I have found that the better types make the flyback run cooler because of the new type being able to handle the frequency better. Because it "works" does not mean replacing parts will not improve things, most likely it will. Raising the capacitance is fine and sometimes advised. I have not regretted replacing condensers and resistors with new types that have better specs. I found that the extra time and expense was well worth it. Customers who paid for a restoration were happy with all new parts and the item(s) worked as expected if not better. For my items, I am pleased. New designs, I get all new parts.
I have found ESR meters are not that accurate on voltages 150V and higher unless the condenser is bad enough to show a higher resistance. I go by the age. If old enough, I put in new. Some off-brand electrolytics I have seen bad when they were a couple to several years old.
If it is a RIFA, replace it now. This one many on this group have done and will be a strong advocate for doing this to any that ask.
You likely have resistors in the TDS that should be raised in wattage. I do not have one or the schematic to confirm this.
The ultimate test is you do what you want with what you have. If you are happy with the results, that is the main thing. I spoke from my experience personally and an EE. You can take none to all of my post you want. As already said, parts that are visually bad, replace.
Mark
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Re: Tek TDS500/600 series scopes capacitor replacement best practice
I have performed a couple recaps of these units. The TDS600 and 700 series haven't been too bad in my experience, I believe it was mainly the 500 series that had leaking capacitor issues?
There are a few things to note though.
Replace C17 (47uf) in the power supply if present (There are two versions of the PSU, C17 is on the version with the metal box that fully encloses the mains input plug/fuse area).
It is critical for the standby power and as such works hard over the years as it is always in use whenever the unit is connected to mains.
When it wears out and the ESR rises, it puts stress on some TVS diodes that have to take spikes and junk that the cap should be filtering. They then die and take out a BU508A transistor that ends up killing your standby power and then your scope goes dark.
The cap is a low-ESR Nichicon (should be about 0.39ohms), and I've measured them at up to 2 ohms, so out with it and in with a new low-ESR cap.
Also, check for the four 1000pF Rifa caps in the PSU too...
On the front panel board, there are two 10V caps (C22 and C23) that are subject to 15V, they should be replaced accordingly too.
Besides that, it's your call. See if any are leaking and replace them. I've seen caps randomly leaking here and there in the CRT board, PSU board, etc. Just luck of the draw.
I did a full recap on my TDS784C, but only because it's my 'good' scope so I wanted it working at its best 'forever'.
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It will be the horizontal analyzer trace that sweeps across the screen that should not shift.
The idea is to not have the horizontal trace move up or down when changing the vertical scale between 10/5/1 db.
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David C. Partridge
Jared Cabot
saipan59 (Pete)