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Homebrew TVT I picked up

390±5%.

I always found it easier to memorise the values of the colours than to use the foulmouth mnemonic. To me the mnemonic is akin to trying to decode Morse by counting symbols.
 
Yeah, but recognizing values on sight is a skill developed over time. Sort of like knowing the names of the cranial nerves and being able to identify them. Forget Old Olympus.

If you're familiar with the colors of the spectrum, resistor color code is just a subset and follows the same order. Black, Brown, (Red Orange Yellow Green Blue Violet) Gray White.
 
That is the wrong EPROM. I think that is the one to do the reset.
The EPROMs next to the UART are the ones I thought you'd want to bypass. Those may also have pulldowns though.
390 ohms is a little heavier pull down than I've normally put on a 1702. 1K resistors would be fine.
Dwight
 
Chuck(G) said:
Yeah, but recognizing values on sight is a skill developed over time. Sort of like knowing the names of the cranial nerves and being able to identify them. Forget Old Olympus.

If you're familiar with the colors of the spectrum, resistor color code is just a subset and follows the same order. Black, Brown, (Red Orange Yellow Green Blue Violet) Gray White.
Click to expand...

Having a large chart on the wall above the bench makes it easy, I think. I still have one, even though I don't need it. Although the main reason is the capacitor information that it also contains.
 
Dwight Elvey said:
That is the wrong EPROM. I think that is the one to do the reset.
The EPROMs next to the UART are the ones I thought you'd want to bypass. Those may also have pulldowns though.
390 ohms is a little heavier pull down than I've normally put on a 1702. 1K resistors would be fine.
Dwight
Click to expand...

Ok. I sort of assumed the 'reset' one was the one ww wanted to look at, since it seemed to react to the logic probe most directly with a change in onscreen output. I should try it on the UART EPROM and see what happens, but yes, it too has resistors.
 
I did some checking and those resistors are at least 5 times too low a value. They should be at least 2K. The EPROM outputs only output a few milli-amperes.
Dwight
 
falter said:
So you're saying in order for the bypass of the 1702 to work, you'd need stronger resistors?
Click to expand...

No, in order to use the EPROMs as they were intended, you need to increase the resistance of the resistors. 390 ohms is way too low a resistance for the EROMs to drive. Remember when you were probing the EPROMs and they'd start to work. It may have been changing the loading just enough so that the weak drive of the EPROM could pull the level high enough to be seen as a 1 on the TTL inputs that the EPROMs were connected to.
For wire bypass, you need to completely remove the resistors. They were there to give pull down for the weak pulldown of the EPROMs. For a TTL to TTL, wire connection, you don't want them there at all.
Dwight
 
Thanks Dwight. Interesting. I'm guessing this thing must have worked at some point though?

I'm still trying to wrap my head around this resistance stuff. :) I know it makes sense but in my head it sounds backwards.. resistance to me is like pressing a brake, so if the EPROM is too weak with the existing resistors in place it must need less brake, not more. I am making an effort to really read this stuff and try to understand it but it's like Grade 12 math all over again. :)
 
The point of the resistor is to pull the line "low" when the chip isn't pulling it "high". Think of it as a spring. If you stop pulling, the spring pulls the other way. Else the logic level goes wherever the wind blows it.

A lower value resistor is like a stronger spring. Right now the spring is too strong for you (the EPROM) to overcome.
 
Not to bad but I'd say it was more like a weight. The EPROMs are trying to pull the weight off ground but they are old and tired. Their just not what they used to be. To be a one, they have to pull against the heavy weight to make a '1'. Still, the TTL input needs something close to ground to be a '0'. The TTL input pulls up lightly on each input and expects the driving device to pull down strongly. The EPROM is a PMOS device and not all that good at pulling down. How can we make both sided happy. We want a solid '1' when the EPROM pulls up but also a good '0' when driving a '0'. The 1702(A) was the only EPROM that used PMOS. Most all of the later ones, that I know of, use NMOS process. These were strong pull down for '0', making happy connections to TTL.
When using 1702s, to drive TTL, it was necessary to improve the pull down to get a clean '0'. Normally the pull down was done with a resistor to the -9V rail of a few K. Using the ground to pull down requires a lower value resistor to pull down harder. This has to balance with the EPROMs ability to pull up high enough.
You might consider it a hack but it generally works. The output of the EPROM is considered good if it pulls up with 4 ma to 12 ma or so.
Now, for the fact that your meter is showing 470 ohms. These are carbon composite resistors. They are known to age. Most digital now use carbon film that have almost no tendency to age. When carbon comp resistors were first invented, they were mostly used for tube circuits. Many of these applications could withstand 100% increase in resistance ( except many cathode resistors ). The newer carbon film have only one issue. They have higher inductance that can be a problem for high frequency RF.
Anyway, I'd tend to trust your DVM more than carbon comp resistors.
For straight across, I'd expect you'd need to remove the resistors while for the EPROM, I might consider using a higher value. Again, this is a case that a working scope would be handy.
Dwight
 
I'd be rather surprised that all of the 390 ohm resistors have gained +20% across the board, given that they're originally 5% devices. A sampling of all of them should show somewhere between 380 and 400+ ohms, but not all the same out-of-tolerance value.
 
I wouldn't use the "weight" analogy because that adds an extra force, gravity. Meaning that the intended level would have to have the extra properties to float, or then they'd always rise (above supply voltage) or fall to ground (and never go negative, which a floating logic point sure can).
 
KC9UDX said:
I wouldn't use the "weight" analogy because that adds an extra force, gravity. Meaning that the intended level would have to have the extra properties to float, or then they'd always rise (above supply voltage) or fall to ground (and never go negative, which a floating logic point sure can).
Click to expand...

Soft ground!
Dwight
 
Ok so it's not about the EPROM trying to drive things up to "1" per se, it's about trying to hold back the TTL logic and prevent them from dragging the EPROM up to that level?
 
falter said:
Ok so it's not about the EPROM trying to drive things up to "1" per se, it's about trying to hold back the TTL logic and prevent them from dragging the EPROM up to that level?
Click to expand...

No. It's that an output (in most cases) has one low impedance state and one high impedance state. In the high impedance state, the impedance may be higher than that of the rest of the circuit. This can cause inputs, nearby circuits, RF noise, etc. to affect the voltage, potentially causing an unknown or unpredictable logic level (high or low). The resistor "pulls" the voltage toward the logic level of the high impedance state. When the output goes low impedance, it overcomes this. So the level is now much, much more likely to be an intentional high or low, solely influenced by the output.

Here's a visual example of what can happen when logic circuits are left high impedance: they're pretty random, in this case possibly controlled by radiated power line frequency in the air beating against the power supply ripple (my guess).

 
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It'll take me a while to wrap that around my brain.

In the meantime, I looked at the resistors on the other EPROMs. In fact, only the EPROM on the UART board also has resistors attached. The one that is directly handling keyboard I/O has nothing on it.

20190704_200132.jpg

My trouble with reading resistors is I have hard time with the colors. To me the bottom band looks gold, but according to the charts that can't be. It says you read the three closest bands first, the first 'digit' is the band furthest away from the last band (which is separated from the other three by a greater distance). So if I assume that top band is brown rather than top ban, I get 12ohm? Or the other way, brown black red and gold = 102ohm? I dunno. I find the colours a bit ambiguous and hard to distinguish sometimes on these things. My ohmmeter gets 500ohm.
 
From the top: Orange = 3
White = 9
Brown = 1
Gold = 5%

Think of it as scientific notation, if that makes you more comfortable: 39 x 10[sup]1[/sup] = 390 ohms. +/- 5%

In the ones you have pictured:

Brown = 1
Black = 0
Red = 2
Gold = 5%

or 10 x 10[sup]2[/sup] = 10 00 ohms = 1Kohm

The ones that are brown, black, red, gold, yellow are the same 1K, but with a temperature coefficient of yellow=40 ppm/K (not seen much on modern resistors).

There are other exceptions; for example, 6 color bands for high-precision resistors, where the first three bands are the three significant digits, the fourth, the multiplier, the 5th, the tolerance (percent as a single digit) and the 6th, the temperature coefficient.
 
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