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Re: 7904 Triggering problem with Horizontal A slot
I had a problem in my 7704A when my 7D15 wouldn't count in one slot. I was able to verify with another scope that the custom Tektronix multiplexor IC, 155-0022-00, used in the trigger circuit of the scope was bad. The voltages on the output of the multiplexor were way off compared to the other slot. Since there are one of these ICs for each slot I swapped them and the problem moved to the other slot. Through Tekwiki I found that part was also used in many other Tek scopes and was able to buy a parts board from a 7603 with a working IC. At the time I don't think I could find the IC by itself or possibly find it at a reasonable price.
Looks like the 7904 is very different, 3 ICS per slot vs. 1 IC per slot, but a failure in any of the three could take down the trigger function. There are also the supporting circuits to worry about but swapping will help isolate IC vs board failure. It seems to me you have 2 independent problems, triggering and compressed waveform. I would start with the trigger problem by swapping the trigger ICs between trigger selection boards assuming the 7904 has socketed parts like the 7704A since only one of the plugins has the compressed problem. Another possibility is bad IC to to socket connection. I have several of these issues in my 40 year old Tek scopes. Of course I jumped to the ICs since that was my problem but you should check voltages first. |
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Re: 7623A HV dead - how to troubleshoot it?
Hi Max,
[I accidentally hit reply in previous post.] Hi Miguel,Legend says: XY display on winding pin 1 and 7. Excited from FG502 with with 1K ohm in series (Ry), applied sinusoidal signal @10KHz 10Vpp.Something seems to go wrong here. When winding V is vertical you get about 33 mH, when horizontal you get your value 8.3 mH. Both values make no sense. The 22 mH for N = 222 reported by Miguel is consistent with the specs of the core halves, 500 +/- 70 mH when N = 1000. In a early message you found the correct turns ratio between windings. Also a far too low inductance due to a short in another winding would immediately show up in the shape of the ellipse. but the ellipse shows no sign of it all. The axis are nicely horizontally and vertically. Albert |
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Re: 7623A HV dead - how to troubleshoot it?
On Fri, Dec 9, 2022 at 04:46 PM, unclebanjoman wrote:
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Re: Help (warmly) requested of 7844/7904 power supply
Thank you very much for putting this together (a labor of love, no doubt), Gianni! I'm looking forward to going through this new ebook, as I have a 7904 that I would like to keep running as long as it (and I) can.
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Jim Ford Laguna Hills, California, USA ------ Original Message ------
From "Gianni Becattini via groups.io" <Giovanni.becattini@...> To [email protected] Date 12/10/2022 3:46:10 PM Subject Re: [TekScopes] Help (warmly) requested of 7844/7904 power supply Hi Friends, |
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Re: 7904 Triggering problem with Horizontal A slot
IIRC, there's a buffer amplifier for each horizontal slot, which then feeds the switch controlling what happens and then that feeds the main horizontal amplifier.
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That the plugins work fine in one slot and not another narrows it down to that one slot.? That you get half deflection (compressed, not twice the frequency) suggests that one differential channel is not working properly.? That may be a part of U825 on my schematic, the horizontal switch.? I'd be looking at that part of the circuitry to start with. Harvey On 12/10/2022 12:56 PM, JJ via groups.io wrote:
Hi Folks, |
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7904 Triggering problem with Horizontal A slot
Hi Folks,
I've had my beautiful 7904 for over 25yrs now and it's been a treat. The Horizontal A compartment now appears to have become problematic with a triggering issue. Horizontal B compartment works well. I plugged in three different 7B92A's that work in the B compartment into the A compartment. Two of them wouldn't trigger at all in that compartment. The third triggers but I get a compressed waveform centered in the middle of the screen taking up about half the screen. The same number of pulses but the period is half what it should be. The digital read out reads correctly. Has anyone come across this issue? Does the one 7B92A that triggers but gives twice the frequency with a compressed display point to something specific in the mainframe? Best, John Justin |
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Re: THS7XX LCD Screen Rapair
Aaron,
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The polarization does matter. If the front and back polarizers are in parallel directions you get black traces on a white background (or is it white-on-black?). Polarizers at 90 degrees to one another give the opposite scheme. --John On Fri, Dec 9, 2022 at 07:50 AM, Aaron wrote:
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Re: Model 485 traces do not synch to triggering sources
Chuck,
If your S/N is below B15590, modify the circuit in the front end that drives the Z input relay. This change is to protect the IC from spikes from the relay turning on or off. There is a 100mfd condenser in each attenuator as a decoupling. The condenser is axial tantalum and is run almost at its rated voltage. I put in a 16V type to replace the 10V type that sees 9V. A ULD or UHE type would work. Mine has the same option as Tom G.'s. That is why I like mine even more. Mark |
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Re: Model 485 traces do not synch to triggering sources
Good advice for Tantalums and Aluminum Electrolytics as well.
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One of the things I like best about my 485 is the clean, bright, well focused trace it produces. My 2465BDV is the best it can be but is no match for the thin, bright beam on my 485. One other thing that I found decades ago with my 485 that had a trigger issue was Potentiometer R755 (Trigger Sensitivity) associated with Tunnel Diode CR751 was dirty. As with servicing any equipment with calibration pots, I always mark the position of all Pots on a Board I am working on. Moving the wiper through the travel cleared whatever debris was there and I never have had a problem since. By the way, triggering and displaying a trace out to 670 MHz with the 50 Ohm Input Attenuator was no issue for my 485. Very good Analog Oscilloscope. Ross -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom Lee Sent: Friday, December 09, 2022 10:28 AM To: [email protected] Subject: Re: [TekScopes] Model 485 traces do not synch to triggering sources Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never. When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement. --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 On 12/9/2022 07:55, Chuck Moore via groups.io wrote: Mark & Tom |
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Re: 2465B Recap
On Fri, Dec 9, 2022 at 09:55 AM, Matt Balmer wrote:
One of the maintenance routines display calibration coefficients on the screen. You could take pictures of the screen to make a backup of coefficients before replacing the battery in case something goes wrong. As Zen also said FRAM is not a drop in replacement, although it works in many scopes. Dallas chip isolates the memory when voltage starts going down, FRAM chips I know of do not have that feature. There is also a slight mismatch between Voh/Vih levels if you follow datasheet numbers although in general there is more margin. Since you don't have the Dallas chip but a battery backed RAM, there is no compelling reason for an FRAM replacement. Ozan |
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Re: 2465B Recap
Hi,
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If you do lose the Calibration Constants, the calibration can be difficult and as Zen says, requires specialized equipment to perform it. With experience, it takes time - 2 to 4 hours of sequenced steps using that equipment. If you haven't done it before, you may want to consider having someone else do it. The 2465 series oscilloscopes after refurbishment will last a long time. They also hold their calibration for a long time after battery replacement and calibration. I have a rebuilt 2465BDV as my main Analog Oscilloscope on my bench. Ross -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Zentronics42@... Sent: Friday, December 09, 2022 8:46 AM To: [email protected] Subject: Re: [TekScopes] 2465B Recap Matt, In terms of reading the values of the SRAM. It is critical that voltage stay always applied to the chip. Even a momentary loss of voltage for example shorting the power pins the data will be lost to the either and a recal will need to happen. This is what the battery is for to hold over the data when the scope is off. For replacing the sram for a flash chip. This will be a larger challenge as the different types of memory are accessed written too and red from differently. There might not be the correct drive voltages present for the flash chip. Lots of data sheet diving will need to happen for a suitable replacement if any. One other issue here is timings. During boot up the chip will need to respond within the current time as well as with the correct speeds. So this will need to be taken in to account as well if looking for a replacement memory chip. Given all the hurtles the "normal" path walked is refresh the power source (battery) data can be preserved with a bench supply when switching out the battery. If caution is to the wind wiping the data and running the scope through a recal would not be a bad idea given its age. I find the calibration routine will bring any areas that need attention to light. But before going in to the deep end on the calibration make sure ALL sources are available to you the 2xxx scopes are picky when it comes to alignment/calibration tolerances especially in the high frequency compensation section. The edge speed needed here can get in to an "exotic" source depending on the speed of the scope. Zen -----Original Message----- From: [email protected] <[email protected]> On Behalf Of Matt Balmer Sent: Friday, December 9, 2022 10:30 AM To: [email protected] Subject: Re: [TekScopes] 2465B Recap Those that messaged me privately, I appreciate it. I am learning a lot as I go. I can't tell you how many times I've asked a question on an electronics forum and been eaten alive by folks because I didn't "google enough" or I'm doing something that they personally view as frivolous or unnecessary. Suffice it to say that I did not expect to get read the riot act when I've decided to undertake replacing worn-out parts that are now almost 40 years old, especially with some of the stories I've read about some parts in these units failing in rather spectacular fashion due to age, stress, or both. I am a public school teacher, and I often get frustrated at how my students fear asking questions. This experience clarifies why that happens. In case my point isn't clear, when someone asks a question about something, if your response is to immediately jump down their throat and tell them that what they're doing is stupid, then that's probably not going to have the effect you wanted. What it's going to do is make the person do one of a few things: -- They are going to proceed with their plan (ill-advised or not) without hearing any advice that may have been embedded in the message -- They are going to stop everything and abandon the project -- They might take your advice, but they will stop asking further questions out of fear of being attacked again I hope this clarifies my hesitance to post a further response. Additionally, it would seem that the spreadsheet that's hosted here may either be out-of-date, incomplete, or (oddly enough) over-complete. I (now) know Menahem does recap sets for these, and have opted to use one of his kits to redo my scope. Most of the caps from DigiKey were returned (I did keep a couple of the smaller values for my parts bin). I've prepared the system for its recapping and am just waiting on Menahem's cap kit to arrive. The thing I'm now concerned about is retaining the calibration values, and potentially replacing the SRAM chip and battery combination with a FRAM chip instead. Is it possible to get the calibration values out of the SRAM chip without powering the system on? I have a DIP chip test clip that would allow me to attach header pins to the SRAM in-circuit and then run those to my EPROM programmer, but I'm not sure that would do the job if the system isn't powered. It's not a huge deal (if I have to do a re-cal on the thing after replacing the chip, oh well) but if it's possible to extract the cal values during servicing, that would be ideal. |
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Re: 2465B Recap
So is the calibration memory more of an "if it ain't broke, don't fix it" kind of thing in my case? My A5 board doesn't have the Dallas NVRAM chip, so my guess is that the urgency to replace it is probably somewhat lower.
Considering that, I almost wonder if the better question would be to try and grab the cal values once the board is recapped and reassembled, and then attach the DIP-clip to it and try and read them with the system powered on. |
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Re: Model 485 traces do not synch to triggering sources
Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never.
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When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement. --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 On 12/9/2022 07:55, Chuck Moore via groups.io wrote:
Mark & Tom |
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Re: Model 485 traces do not synch to triggering sources
On Fri, 9 Dec 2022 at 15:55, Chuck Moore via groups.io <wd4hxg=
[email protected]> wrote: Mark & TomThe 485 is the scope on my bench. I've had it stably triggering on a >1GHz signal, the limit being the greatly reduced signal amplitude rather than the trigger circuit. The 485 has two rare features: - the cal out has a risetime of <1ns, i.e. suitable for testing the scope's risetime - it has a real 50ohm input attenuator, i.e. not a crappy 50ohm resistor slapped across a 20pF//1Mohm input. It also has a separate traditional 1Mohm attenuator. Plus I like the blue trace :) |
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