However, I wanted to take the time to understand the circuit at little better, and get a feel for what the components in that list (or elsewhere on the board, if needed) are supposed to do in that circuit. I figure if I can understand a component's role, then why Tek selected the parts they did during the original design should become apparent, and therefore inform what currently-available parts would best serve as replacements in that role. For all I know, that list is still current. But, I'd like to understand why, as opposed to just blindly throwing a bunch of parts on a board.
Today there are better components available than the designers had available however as always the cost of the BOM always guided component choice.
But there are some traps where existing component characteristics must be carefully considered before any substitution can be made.
In the case of these Tek 24xx series power supplies, are you aware of gotchas in component substitution? Anything that if substituted with a part with supposedly better specs would actually work against the goal of a stable, clean, supply voltage with sufficient amp capacity?
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Another thing I note is that I don't have the famous Dallas battery-backed SRAM, as seen on the later, SMD-style A5 boards. This thing looks like a plain battery, not a RAM-chip or ROM chip of any kind, especially given it's only 4 pins on the board. Apologies for the blurry photo there, but it looks like there's some dried flux residue on those solder joints. I don't know if that's normally there, or if that means that this board has been worked on before, and the battery has already been replaced once. It's hard to tell, given the lack of date codes on the label. I'll have to compare the A5 boards in my other pre-B5xxxx scopes.
I'm not sure what this board is, or what it does. I do believe it is a standard board, as when I opened up B0120xx for comparison, it also had this board in it. Whatever it is/does, it fits in the central cavity of the scope, between the option boards and metal wall, and has a ribbon cable that goes through the floor of the central cavity and connects to the A1 board. I was going to leave it in place, initially, but I found it easier to get at the power supply assembly with both hands with it out of the cavity.
(Attachment Link) (Attachment Link)
That's the A4 readout board, responsible for the on-screen text and cursor display generation. The labeled chip in the middle is the character ROM (actually an EPROM). There's a giant section (10 pages) in the theory part of the service manual on how this board works, if you're curious.
If you follow the procedure as posted here and use the parts listed you will have no issues when re-capping the PSU:
https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/msg1658102/#msg1658102
In the case of these Tek 24xx series power supplies, are you aware of gotchas in component substitution? Anything that if substituted with a part with supposedly better specs would actually work against the goal of a stable, clean, supply voltage with sufficient amp capacity?Not 24xx in particular but study these posts where a member battled with FET Millar capacitance after selecting a replacement.
Start here:
https://www.eevblog.com/forum/repair/tektronix-2215-scope-repair/msg1647551/#msg1647551
Then jump to here:
https://www.eevblog.com/forum/repair/tektronix-2215-scope-repair/msg1824401/#msg1824401
Then read a few more posts.
I found it quite interesting and it just proves there are hidden traps for the unwary especially when just dropping a substituted component in and not understanding or modifying the design to account for it.
There is a pair of 15Ohm, 1/2W carbon film resistors parallel with the NTCs (R1010 & R1019) as part of the inrush protection mechanism. In the work I've read on this so far, I've seen people up-rating them and replacing them with 3W resistors or the like. Clearly, these are subject to some degree of thermal stress, being inline with with the mains before it enters the first rectifier. Is this necessary, or is it a bit of an insurance policy to make sure they don't fail and thus cause a cascade of failure in more expensive parts? Or is upgrading these resistors risking the failure of more expensive parts by causing what was intended as a sacrificial component to not fail when it should?
Additionally, I note that in both of the Pre-B05xxxx supplies I've seen so far, one or both of resistors R1016 and R1018, have been burnt by failures of C1015 and C1018. I find myself wondering if perhaps it wouldn't be a good idea to increase those from 1/2 watt thermal ratings to something higher. My understanding from reading about X and Y ratings for caps is that an X-rated capacitor is supposed to fail shorted, with the idea being that in doing so it would cause a breaker to trip for a fuse to short. By my reading, resistors typically fail open. So of C1015 or C1018 fails, and draws the full mains current across R1016 or R1018, forcing them to release their magic smoke and become open in short order. Do we want to preserve that short so fuses start blowing? Or are these resistors intended as sacrificial components, and thus supposed to self destruct in the presence C1015 or C1018 shorting? In which case, would it be a good idea to go for a lower thermal rating so the resistor blows even sooner? Or is 1/2W the smallest thermal rating that could fit in that role?
Looking at this parts list, I find myself questioning the the changes in voltage or capacitance.
For example let's take the filtering capacitors hanging off the output of the inverter transformer, C1110, C1111, C1113, C1114, C1115, and C1116. The originals are a blend of 180 and 250 uF. 330 seems like a bit of a jump, moving the poles of the LC filters the form from 2.2 kHz and 1.9kHz to 1.6kHz. That, I'm less worried about, since I don't see knocking an extra few dB off the ~41Khz switching noise as a particularly bad thing, and I don't think the increase in charging current from the increase in capacitance would load down the inverter too much more. However, is there not a consideration on undercharging electrolytic capacitors shortening their life? Per the voltages in the diagram, the stock 180uF caps on the +/- 15V lines, at 19.2V at this point in the circuit are being charge up to about 48% of their max voltage, and the 250uF caps on the unregulated +/- 5V lines (at 7.1V) are being charged up to about 35%. However, with the 50V rating on the proposed parts list, those unregulated lines are only being charged up to 14% of their rating. Is this too low of a charge for a healthy cap life?
I'm not saying these are unreasonable choices, given the choice of the Panasonic EB series. Looking on the datasheet, 330uF was the smallest value that wasn't lower than the original value 250uF caps. However, 220 uF is available to replace the 180 uF 40V, and a 220uF 50V is cheaper than 330uf 50V. Was this part selection driven somewhat by a desire to keep the parts list small and simple?
That's the A4 readout board, responsible for the on-screen text and cursor display generation. The labeled chip in the middle is the character ROM (actually an EPROM). There's a giant section (10 pages) in the theory part of the service manual on how this board works, if you're curious.
Interesting. I remember seeing an easter egg of Tetris, or maybe it was Centipede hidden on an HP scope, so perhaps I shall have to dump the ROM and see what secrets lurk therein. At a later date, though.
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Hello all, fine notes. But the recent posts refer only to 2465 and 2465A power supply recap. The 2465B PSU must be different.
At the moment, I have a very late SN 2465B opt 06,09 that needs recapping.
Is there a recent BOM for the 2565/7B PSU recap, the old boms have many broken links/ stock numbers from DK, Mouser.
Many thanks
Bon Soirée
Jon
I have a 2465B that I've come to (lovingly?) address as a red headed child. All voltages present at the test point and in spec, ripple is in spec, Nothing wrong with it except that it has no cal data (new chip) and won't boot. Anybody have any ideas here?
I have a 2465B that I've come to (lovingly?) address as a red headed child. All voltages present at the test point and in spec, ripple is in spec, Nothing wrong with it except that it has no cal data (new chip) and won't boot. Anybody have any ideas here?What's the history with this unit? Did it work and now it doesn't or did you receive it broken?
Are there any LEDs lit on the front panel? Does it go through part of the boot sequence and get stuck? Are any relays clicking as it boots? Any flashes of anything on the CRT? Or does it just sit there like a lump? Need more info.
One thing you could do is try working through the troubleshooting flowcharts in the back of the service manual (immediately after the schematics). They don't cover every possibility, but can at least lead you to the right area to start digging.
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Worked when I got it, then i tried restoring it.
Right now, what it does when powered on is exactly this;
Brief flash on the CRT, fan comes on, seemingly all front panel LEDS light, then it just sits there like a lump. Doing nothing except running the fan and staring at me with a brightly lit front panel.
Completely unresponsive beyond this and it will sit there for half an hour at least without change, does this every single time I try turning it on.
What "restoring" things did you do? Maybe posting decent resolution photos of those areas you worked on would allow someone to spot something.
One of the first things the CPU does is take control of the front panel LEDs. So, it's not even getting off the ground. Very few components need to be working for this initial phase. Not even the SRAM is needed.
I would focus on the CPU and EPROM functions. Make sure nRESET is going high and that the CPU has a working clock and supply. Make sure J503 NORM/DIAG jumper is in the NORM position.
Have you tried the troubleshooting flowchart? It will take you through all these checks, plus if the NOP diagnostic loop on the CPU should be run.
I bought this programmer:
http://www.ebay.com/itm/321085130796
it arrived safely and quickly.
for the chip, this is the one I ordered but it seems to be out of stock right now:
DS1225AD-200IND+-ND
AD is a wider temperate range (I think) but nothing critical to the scope. the 200 is the speed of the device and I used the same speed as the original chip. not sure if going faster or slower would matter but I just matched the same chip speed.
mouser has one that is not AD but very close and that's probably also ok:
DS1225AB-200+
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Maybe some one can answer this for me and others, maybe it has already been done.
Can we use the:
DS1225AD-70+ chip here? is 70ns vs 200ns.
Looking at this parts list, I find myself questioning the the changes in voltage or capacitance.
For example let's take the filtering capacitors hanging off the output of the inverter transformer, C1110, C1111, C1113, C1114, C1115, and C1116. The originals are a blend of 180 and 250 uF. 330 seems like a bit of a jump, moving the poles of the LC filters the form from 2.2 kHz and 1.9kHz to 1.6kHz. That, I'm less worried about, since I don't see knocking an extra few dB off the ~41Khz switching noise as a particularly bad thing, and I don't think the increase in charging current from the increase in capacitance would load down the inverter too much more. However, is there not a consideration on undercharging electrolytic capacitors shortening their life? Per the voltages in the diagram, the stock 180uF caps on the +/- 15V lines, at 19.2V at this point in the circuit are being charge up to about 48% of their max voltage, and the 250uF caps on the unregulated +/- 5V lines (at 7.1V) are being charged up to about 35%. However, with the 50V rating on the proposed parts list, those unregulated lines are only being charged up to 14% of their rating. Is this too low of a charge for a healthy cap life?
I'm not saying these are unreasonable choices, given the choice of the Panasonic EB series. Looking on the datasheet, 330uF was the smallest value that wasn't lower than the original value 250uF caps. However, 220 uF is available to replace the 180 uF 40V, and a 220uF 50V is cheaper than 330uf 50V. Was this part selection driven somewhat by a desire to keep the parts list small and simple?