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Generating a "DC OK" signal from replacement / modern PSU

Bob-O-Rama

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Jan 11, 2022
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Allentown, PA, USA, Earth, ...
Hi,

I have a AT&T 4425 Teletype Terminal I am trying to restore for daily use by some students. I am having to give up on restoring the original Astec AA12433 PSU ( -12v, +5v, +12 ). It has a TTL "DC OK" signal which goes high or low depending on all three outputs reaching the proper voltage. The new PSU does not. The terminal requires this DC OK signal to boot, and it seems to inhibit much of the internals. If I wait a second after turning on the new PSU and pull DC OK high to 5v with a pullup resistor, it boots perfectly.

I was thinking "brute force" with a set of relays ( I have some very small 12v NO relays, and the 5v pull up passed through the relays is its own "proof" of 5v being OK. ) Other suggestions or flaws with this plan welcome.

-- Bob
 
There are specialized ICs made for this purpose (e.g. TI TPS35), but you can probably get away with a simple RC network. Follow with a Schmitt trigger or op-amp if it's super-critical.
 
I farted around for a little while trying to do it with RC circuits and schmitt triggers, just for luls. But it was never quite right and I ended up just using an esp8266 module.
 
An NE555V timer IC is probably all you require on the 5V supply.

Look at the Commodore PET schematics for an example of what you require. You will have to get the output sense correct though...

See http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-1.gif and look for the 555 in the lower left. You will probably require the 74LS04 inverter as well to give you a LOW signal on power-up that will go high after a delay time.

This won't monitor the other voltage rails though. But should give you what you want...

Dave
 
I ended up just using an esp8266 module
Talk about overkill. :) But I didn't think the ESP8266 was 5V tolerant. But yeah, a 555 would be fine--this is a cat that can be skinned quite a few ways. A comparator, such as an LM311 and a few discretes could do the job, for example. You could even use an old time octal-plug thermal time delay relay if wanted to go really retro. :)
 
Could you take some better resolution pictures of the 4425 unit and pcbs? The one you put of the PCB on IA is pretty blurry
 
Talk about overkill. :) But I didn't think the ESP8266 was 5V tolerant. But yeah, a 555 would be fine--this is a cat that can be skinned quite a few ways. A comparator, such as an LM311 and a few discretes could do the job, for example. You could even use an old time octal-plug thermal time delay relay if wanted to go really retro. :)

Yeah, the IO pins are 5v tolerant, but not the ADC. Overkill, I know, but it was like $5 and I was tired of fooling around, heh.
 
Yeah, the IO pins are 5v tolerant, but not the ADC. Overkill, I know, but it was like $5 and I was tired of fooling around, heh.
I'll confirm, I had issues using the IO pins with 3.3v signals, 5v has been fine for a long time.
 
Hmm, I've still got a bunch of those little modules. What kind of ftp/http speed can you get out of them? Someone commented that it wasn't great.
 
Hmm, I've still got a bunch of those little modules. What kind of ftp/http speed can you get out of them? Someone commented that it wasn't great.
Not to hijack the thread too much, but my main use for the ESP8266 has been on my RC2014. Since I'm primarily running zimodem at hayes modem speeds, or a telnet console, the wifi speed isn't of much concern to me. It has been very reliable though.
 
I like to hang them off serial ports as a giant wireless serial multiplexer, so I don't have to fool with switches and moving cables around and all that.

But yes, I digress. :p
 
Could you take some better resolution pictures of the 4425 unit and pcbs? The one you put of the PCB on IA is pretty blurry

Most of the project photos here:

https://bob-o-rama.smugmug.com/Outbound/ATT-4425-Teletype-Terminal/i-4spPxWF

You are welcome to root through them. That is probably the best one of the main board before any work on it. I was refurbing two of them - which was fortunate as one has bad character ROMs and I was able to re-burn them. But the photos would be mixed from either unit.
 
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Could you take some better resolution pictures of the 4425 unit and pcbs? The one you put of the PCB on IA is pretty blurry
Hi Al,

Before packing the guts back in, I took a bunch of the photos of both sides of the board and many tiles of small sections.

https://bob-o-rama.smugmug.com/Outbound/ATT-4425-Teletype-Terminal/

I did not get any images of the analog video board. Unfortunately each version of the board has different crystals, some marked, some only with a ATT part number. So the next time I have another one of these open, I'll get the crystals specs. So half the loaf:

i-pvTHK4w-X3.jpg


-- Bob
 
Hopefully this will help...this is an At&t 513 but Ill bet the difference in models is really just firmware.

IMG_7460.jpeg
 
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