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Hi -
I know the DC508 display is a common point of failure, and that Paul Read even offered replacement display boards at one point (Paul, do you still have any?).
I'm wondering if anyone has had a failure mode that wasn't the display itself, but the 7447 instead? In my case, I'm not missing any elements, but certain numbers are malformed, and the same number is malformed in each digit. Does a bum 7447 sound plausible? How do these displays usually fail?
thanks!
Paul
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It is certainly possible for a 7447 or any TTL IC that old to fail. On
my SG503, one of the 7475 latches feeding the 7447 failed. I would
expect 7447s to actually fail more often since they have to operate as
a driver and often run hot.
I would actually suspect bad Texas Instruments sockets first but I
know some of the LED displays in this modules had poor reliability
which is apparently not the issue you are having.
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On 26 Mar 2016 19:16:59 -0700, you wrote: Hi -
I know the DC508 display is a common point of failure, and that Paul Read even offered replacement display boards at one point (Paul, do you still have any?).
I'm wondering if anyone has had a failure mode that wasn't the display itself, but the 7447 instead? In my case, I'm not missing any elements, but certain numbers are malformed, and the same number is malformed in each digit. Does a bum 7447 sound plausible? How do these displays usually fail?
thanks!
Paul
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Hello,
Yes, it may be the 7447, or more rarely a bad contact between the 7447 and the segments.
A suggestion, if like me you prefer well formed 6 and 9, you can replace the 7447 by a 74ls247.
I change it on all my tek plugin and the display is better.
You can see it on my website (sorry in French)
You can find 74ls247 on ebay
TM500
TM500 Ce site utilise des frames. Si vous ne voyez pas les menus ¨¤ gauche, cliquez Ici Pr¨¦sentation J'ai commenc¨¦ l'electronique vers 10 ans ¨¤ l'aide...
View on philippe.demerliac.fr...
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Thank you David & Philippe - I'll try swapping out the 7447, I've got plenty lying around. Unfortunately it is not socketed, and it's partially buried under the RF section.
Philippe - that's a great page, thanks for sharing! Google translate seems to be handling it OK.
cheers,
Paul
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Hi Paul,
Your welcome !
Yes I agree, it's sometimes very hard to change these IC. I love the items where Tektronix had the good idea to use sockets for quite all the semiconductors...
Good luck !
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Just a shame the quality of some sockets they used was poor. Malcolm On 27 March 2016 15:01:56 "nixie@... [TekScopes]" <TekScopes@...> wrote: Hi Paul,
Your welcome !
Yes I agree, it's sometimes very hard to change these IC. I love the items
where Tektronix had the good idea to use sockets for quite all the
semiconductors...
Good luck !
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20+ years of various TTL work and I never learned about the 74247.
Thanks for bringing it up.
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On 27 Mar 2016 00:33:06 -0700, you wrote: Hello,
Yes, it may be the 7447, or more rarely a bad contact between the 7447 and the segments.
A suggestion, if like me you prefer well formed 6 and 9, you can replace the 7447 by a 74ls247.
I change it on all my tek plugin and the display is better.
You can see it on my website (sorry in French)
You can find 74ls247 on ebay
TM500
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I think there may be a simple way to get nice 6 and 9 digits including the upper and lower bars of the display, even using a regular old 7447. I did that years ago to make the display the way I wanted it. I can't remember exactly how, but it involved looking at the various decoded segment drive lines, and adding a little circuitry to light the bars. I vaguely recall that it only needed some extra resistors, and was simple because of some "X" states of the decoding. Could be wrong - it was long ago - maybe there was more to it.
Ed
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I think some of it came back to my brain. I think I used a couple of transistors gated from the input code or other segment codes, to add the extra segment pull-down states on the open-collector outputs. So, not as simple as I was imagining.
For example, for digits 8 and 9, an NPN driven from the BCD input MSB will be on for both, and can light the lower bar in both states. For all other decimal digits it doesn't matter. So, a base resistor and an NPN take care of the 9. I think the 6 fixer was similar.
Ed
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More came back. Another NPN with base driven from the BCD "2" input, and the collector tied to the upper segment output, will force it on in all states that include 2. Except for decimal 1 and 4 output, the others have it on anyway, so are don't-cares. When decimal 6 is called for, the BCD 2 will light the upper bar. So, another NPN and base resistor takes care of it.
Ed
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Welp. Not my proudest moment. don't de-solider angry. The frustration of de-soldering DIPs on multi-layer boards has probably chipped away months off my life, which is why there's a Hakko FR-300 now en-route to my office.
...
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On the plus side, there's one more good DC508A display out there, and one less functioning unit that needs it.
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Hi Ed,
Yes I agree, it's possible with 2 transistors to do the job, I done it too in the past, but anyway, I find simplest to just change The IC.
My challenge now is to do it on my 2 SG503 witch use strange segment decoders.
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On 03/27/2016 08:40 PM, devpool0@... [TekScopes] wrote: Welp. Not my proudest moment. don't de-solider angry. The frustration of de-soldering DIPs on multi-layer boards has probably chipped away months off my life, which is why there's a Hakko FR-300 now en-route to my office.
...
View on paulcarbone.com
Preview by Yahoo
On the plus side, there's one more good DC508A display out there, and one less functioning unit that needs it. Oooops. ;-) Mark
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So what's the point of the characters for 10 - 15 on the 74xx47,48,247? I'd always wondered why they didn't support a - f instead of those weird glyphs.
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Thanks for asking that. I wondered the same thing.
Barry - N4BUQ
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----- Original Message ----- From: "devpool0@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Monday, March 28, 2016 9:19:44 AM
Subject: Re: [TekScopes] Re: DC508A Display
So what's the point of the characters for 10 - 15 on the 74xx47,48,247? I'd
always wondered why they didn't support a - f instead of those weird glyphs.
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Ah yes, those Signetics 8T06s. When I saw those I worried that my
SG503 would never be repaired by luckily the problem was in the
preceding 7475 latches.
The same 2 transistor modification should work; except for 10 through
15, they have the same BCD to output decoding.
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On 28 Mar 2016 01:46:51 -0700, you wrote: Hi Ed,
Yes I agree, it's possible with 2 transistors to do the job, I done it too in the past, but anyway, I find simplest to just change The IC.
My challenge now is to do it on my 2 SG503 witch use strange segment decoders.
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That is a result of incomplete decoding which requires fewer logic
gates making the IC less expensive.
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On 28 Mar 2016 07:19:44 -0700, you wrote: So what's the point of the characters for 10 - 15 on the 74xx47,48,247? I'd always wondered why they didn't support a - f instead of those weird glyphs.
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Yes, David called it right - that was the minimum circuit that got the job done, so minimal die area and cost. I wouldn't call the decoding "incomplete" though. It was complete enough to decode all the desired decimal digits, while the rest are invalid in BCD, so are don't-cares. A HEX decoder would have more stuff, and cost more.
Ed
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Hi,
Since this decoder works by shorting the segments switched off, modification will be more difficult because it will need to cut some trace. I plan to do it with a little Picaxe microcontroler in replacement of the decoder.
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I am not sure if you are referring to the DC508A which uses the 7447
or the SG503 which uses the Signetics 8T06 but both decoders use
positive logic BCD inputs and open collector outputs so there should
not be any problem adding a pair of transistors to add the tails to
the 6 and 9.
Maybe I do not understand what want to accomplish?
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On 28 Mar 2016 13:22:07 -0700, you wrote: Hi,
Since this decoder works by shorting the segments switched off, modification will be more difficult because it will need to cut some trace. I plan to do it with a little Picaxe microcontroler in replacement of the decoder.
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David Hess