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PDP-8/M on eBay

AteEm said:
@NF6X:
Congratulations on your purchase!
I'm in Chino Hills, so if you want to troubleshoot by board swapping, just let me know.
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Hi! I'm happy to know that there's another DEC collector nearby. I'm near Lake Mathews. Do you have any use for a Wyse 350 color dumb terminal? I got it as part of a "free for pickup" deal, and it's not the piece of the pile that I wanted. I'm up to my eyeballs in the junk I keep buying, so I'd like this terminal to move to a new home.
 
The seller sent me this scanned image to show me what my PDP-8/M came out of. It was part of an Automatic Controlled Stitcher Model A by USM Corporation, United Machinery Group. COOL!!

ACS-A.jpg
 
I can't quite make it out. My guess is that it's the cassette tape drive that this machine apparently talked to. The seller called the drive a Sykes TT100, and described it as "large".

Did I remember to mention that the seller is hoping to get around $500 + shipping for the ASR 33? If anybody is interested, I'll be happy to help you get in touch with him. I still call dibs on the 8M spare boards. I wonder if somebody here bought the 8A that he also listed?
 
NF6X said:
Hi! I'm happy to know that there's another DEC collector nearby. I'm near Lake Mathews. Do you have any use for a Wyse 350 color dumb terminal? I got it as part of a "free for pickup" deal, and it's not the piece of the pile that I wanted. I'm up to my eyeballs in the junk I keep buying, so I'd like this terminal to move to a new home.
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Thanks for the offer, but I don't really need a color monitor for my stuff. Hopefully someone here can make good use of it.
 
NF6X said:
I can't quite make it out. My guess is that it's the cassette tape drive that this machine apparently talked to. The seller called the drive a Sykes TT100, and described it as "large".

Did I remember to mention that the seller is hoping to get around $500 + shipping for the ASR 33? If anybody is interested, I'll be happy to help you get in touch with him. I still call dibs on the 8M spare boards. I wonder if somebody here bought the 8A that he also listed?
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Okay, the ASR33 is tempting...
 
I wonder whether any of y'all have recommendations for cleaning up the front panel switches. I found that some of the switch bodies can wiggle a bit from left to right despite the support rails. Their contacts are a bit loose in the phenolic bases.

The deposit switch was the worst offender. I found that its contacts were intermittent as I wiggled it, so I removed it for examination and cleaning. Unlike the most common slide switches I've seen which are open on both ends, these ones are fully enclosed by their plastic bodies, so I can't just blow them out and drip some DeOxIt inside of them. I disassembled the switch and found a bit of black crud inside. I cleaned the contacts with DeOxIt, scrubbed out the black crud with a toothbrush, and reassembled the switch. It seems to be working better now, despite the loose contacts. Even though it wiggles, the spring plunger seems to keep the contacts shorted now that the crud is out of there.

I'm a bit hesitant to remove and disassemble the rest of the switches if there's a better way, though. One of the metal tabs holding the switch together broke off. I can probably avoid breaking off more tabs with more care, but bending stamped metal tabs back and forth is still a bit risky. It was also a bit tricky to get the switch soldered back in straight and level.

So, before I disassemble more switches, has anybody found a better way to clean them? Maybe there are some current production rocker switches that have close enough dimensions that they can be used as replacements after swapping the rocker with a paddle from an original switch?

IMG_2873.jpg IMG_2887.jpg IMG_2874.jpg IMG_2875.jpg IMG_2879.jpg
 
The part number printed on the momentary deposit switch is Airpax 028-420-0002. The non-momentary ones are marked Airpax 028-420-0003. I didn't get any good search hits on the non-momentary one other than an old discussion form somebody else repairing a front panel many years ago, but the momentary one got me a hit: an eBay listing for four of them, minus the paddles and looking new-ish, on sale for $5 each. I've grabbed those, and if I don't end up using them, then maybe another PDP-8 collector will be able to use them.
 
NF6X said:
The part number printed on the momentary deposit switch is Airpax 028-420-0002. The non-momentary ones are marked Airpax 028-420-0003. I didn't get any good search hits on the non-momentary one other than an old discussion form somebody else repairing a front panel many years ago, but the momentary one got me a hit: an eBay listing for four of them, minus the paddles and looking new-ish, on sale for $5 each. I've grabbed those, and if I don't end up using them, then maybe another PDP-8 collector will be able to use them.
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Are these any help? http://www.elcomcomponents.com/index.php?main_page=product_info&products_id=614
 
Agent Orange said:
Are these any help? http://www.elcomcomponents.com/index.php?main_page=product_info&products_id=614
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They might be if A) the seller had any in stock and B) I hadn't just bought 4 of them on eBay. ;)

I searched on the non-momentary part number there, too. The results showed no details about the part, but said "We have stock on hand or inbound! Have a representative call you!". I suspect that the answer would have been the same for any random number I plugged in.
 
AK6DN said:
Well I would call 'elegant' being very kind. In my mind it is an unjustified hack. But I'm biased, I had to debug the darn thing.

In repairing my panel I had several of those 74175 devices that failed. I replaced them (in sockets) and plugged in NOS devices of the same type.

The panel switch function did not work at all. Switches that were nonfunctional remained so. Plus other switches stopped working too.

Turns out only some vendor's devices will work in that circuit. Many do not respond correctly to that circuit hack at all, or had a power supply voltage dependency.


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Arrr, in view of the above NOT elegant, interesting possibly though BL**DY useless would probably be more accurate, thinking about it operating a device outside its design parameters ( and using an output as an input would count as this ) is not a good idea for a production design. maybe they had 10,000 74175 they wanted to use up.
Dave H
 
2-10-2014 2-32-48 PM.jpg

For those of you that have not looked at this circuit, here it is. Basically the 74175 is used in RS latch mode. The clock and data inputs are grounded. The CLR~ input has a pullup to +5V, and a daisy chain connection to ground thru all the momentary console switches. When a switch is depressed (like EXAM) the chain to ground on the CLR~ input is broken, so the CLR~ input is pulled high. Once the momentary switch makes, it forces the Q~ output of the 74175 to ground. This is expected to set the corresponding Q output high, which then is inverted into the priority encoder and decoded.

Problem is that this circuit does not work with all 74175 chip designs as was previously noted in an earlier post. It requires an unbuffered output (so that it is also an input). It also then requires an explicit circuit behavior within the 74175 internal logic, acting as an RS flip flop.

My analysis is that DEC could have replaced the 7404 hex buffer and two 74175 devices with three 7400 quad nands using them as crosscoupled RS flipflops. And then the circuit would have worked as expected using only documented behavior.

Of course the only worse design would have been what was done in earlier versions of the console schematic. There they did not do any debouncing of these switches at all, but just routed the switch output with a pullup into the priority encoder.

So it makes me think this console design was given to some noob engineer just out of tech school ...

Don
 
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Hi All;
When I get some time, I think I will build this circuit, and then try and change it still using the 74175's and see if I can make it work like it should.. Just for the Fun of it.. On a Proto-type BreadBoard..
THANK YOU marty
 
When I consider how long ICs were in common use at the time this was designed, it really doesn't surprise me. I'm not defending the design, but I'm inclined to filter it with that understanding.

After all - how could he have known we'd still be bitch'n it almost half a century later?
 
RSX11M+ said:
When I consider how long ICs were in common use at the time this was designed, it really doesn't surprise me. I'm not defending the design, but I'm inclined to filter it with that understanding.

After all - how could he have known we'd still be bitch'n it almost half a century later?
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Yep!
The other guy's designs always have something wrong with them, while our own are perfect!:)
 
AteEm said:
Yep!
The other guy's designs always have something wrong with them, while our own are perfect!:)
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Well I am pretty certain none of those folks would have believed that forty years later their boxes would still be running. I know I would not have believed it. I still think it is somewhat remarkable these systems still run as designed.

I really do not expect to see any working iPhone5's 40 years from now.

My only 'complaint' in this instance is someone used a peculiar feature of a 74175 chip that was not in the documented functionality, when they easily could have done it other ways that were much simpler.

It was fun reverse engineering the hack, however.

Don
 
AK6DN said:
Well I am pretty certain none of those folks would have believed that forty years later their boxes would still be running. I know I would not have believed it. I still think it is somewhat remarkable these systems still run as designed.
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Interesting perspective. I wonder, on the other hand, if engineers back then purposely tried to design systems that would have a long reliable life. And, that they are proud that some of their product is still running as designed!

I really do not expect to see any working iPhone5's 40 years from now.
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I agree 100%. I don't think long life is much of a design goal with most electronics these days. In all fairness, many technologies are obsoleted so quickly that there is little point in engineering in long life. So, I guess today's products are still 'well engineered' :)

My only 'complaint' in this instance is someone used a peculiar feature of a 74175 chip that was not in the documented functionality, when they easily could have done it other ways that were much simpler.
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Well, maybe in historical perspective (which is what I think a previous poster pointed out) the engineers at DEC had 'grown up' with discrete transistor flip-flops; I doubt if I had seen such a discrete RS circuit being set with an output pull down I would have even thought twice about it. I suspect that engineers in that time period were quite a bit more familiar with the internal workings of ICs than designers of today. Doing what they did may not have been seen as all that unusual.
As far as it only working with some manufacturer's parts, that's not uncommon for discrete analog circuits even today. Many components are configuration controlled with strict requirements on vendors not to change their mask sets/processes; sometimes only certain vendors components yield the desired circuit performance.
Back then, I'm not sure the designers made such a clear distinction between 'digital' and 'analog.'

It was fun reverse engineering the hack, however.

Don
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Well, I wouldn't call the circuit 'elegant' but I'm not so quick to judge and label it a 'hack'.
IMHO, naturally.
 
Thanks Lou,
that looks fine and can see all the diodes and links. Im in the middle of a "core a thon" right now, trying to see if I can get 32k working to put in one of my 8m's but as soon as thats done, or I give up and go solid state, i will dig out my spare M847 and give it a go.
Dave H
 
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