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Hardware diagnostic tool sol-20 keyboard

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Hugo Holden

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Recently, working on a SOL-20 keyboard, I figured it would be handy to have a module that would plug onto the keyboard's connector and power the keyboard independently of the computer and be able to fully check all the key functions.

So I made one out of parts (albeit retro parts) that I had on hand. I wrote a brief article on it with the schematic:

http://worldphaco.com/uploads/HARDWARE_DIAGNOSTIC_TOOL_FOR_THE_SOL.pdf
 

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Those 311's would be hard to find now days but that is really clever for minimal parts. For me, I'd most likely use an Arduino or BluePill and LCD. You are clearly a true tinkerer. It is great to have your knowledge on this MB.
Stay safe
Dwight
 
Dwight Elvey said:
Those 311's would be hard to find now days but that is really clever for minimal parts. For me, I'd most likely use an Arduino or BluePill and LCD. You are clearly a true tinkerer. It is great to have your knowledge on this MB.
Stay safe
Dwight
Click to expand...

Thanks for the kind remarks.

It is amazing how easy the TIL311 is to get, I think there are over 30 listings for them on the bay, many from China but also inside the USA.They are not too expensive:

https://www.ebay.com/itm/TI-TIL311-...945669&hash=item46abf68534:g:GB8AAOSw8~5eimrS

There are two other options,

Avago make a beautiful hexadecimal display that comes in vivid green, or red, of mil spec like quality I have two of these, but for some reason they are crazy expensive ,like more than $50 from the usual suppliers, but they are on ebay sometimes but still more than 3 or more times the price of a TIL311:

https://www.ebay.com/itm/Avago-HDSP...042532&hash=item259df8d332:g:8C4AAOSwxYxUur4n

Then I also noticed that the TIL311 had been replicated in this project:

https://www.hackster.io/news/build-a-replica-til311-with-22-leds-and-an-atmega328p-e2fac423b974

So its interesting the options for these hex displays.
 
I just went through every key combination and the output it should produce according to the SOL-20 manual. The are around 170 possible output bytes.

I found a misprint in the manual where it says m is 60h, and M is 40h, but that is a typo in the manual it should read 6D and 4D.

Also the sol-20 manual says the output of Control-E should be byte 15, both my sol keyboards produce 05 for this, which is correct for the character symbol generated by the 6574, so 15 in the manual should read 05.

So my keyboards are ok, but three typos found in the Sol manual.
 
Most people are not that thorough in checking every byte of the keyboard directly. Most would be happy to see the right character on the screen. One wonders how long the typos were there in the manual.
40h for M is clearly wrong. I had to look at an ASCII table to check ^E. 5 is right.
Dwight
 
Dwight,

I'm also working on an article for the keyboard to help with general repairs for others, with diagrams & scope measurements, circuit theory etc, especially in the region of the key scan system and the discrete transistor capacitance detector circuit they created. It will take a while longer to finish it.

Hugo.
 
bobstek said:
Nicely done, Hugo.
Click to expand...

Thanks Bob.

While I have been working on a keyboard I found an interesting "not quite" fault.

Firstly one of the CD4051 analog switch IC's had failed, replaced that keyboard works.

But as I have been going through it with the scope, to document things, I found something quite interesting. The other CD4051 IC (the keyboard uses two) is actually defective too. It is outputting a large voltage transient on the switching edges of the control (channel select) data. Once processed by the transistor amplifier, it is about 3V peak. The other transients due to small amounts of the capacitive coupling with the keys open are about 300mV.

It turns out though that this amplitude of this abnormal pulse is just a little below the trigger level for generating an output pulse from the transistor threshold detector. So the keyboard works normally. But if this problem/pulse was 700mV to 1.4V higher the keyboard would have malfunctioned. The new CD4051 does not have the issue.

It is being caused I think by failure of one of the Fets inside the CD4051, at a gate junction, that is allowing the control data bit switching transitions to leak into the the switched analog information. I have heard of these analog switches doing this sort of thing before.

It is interesting too to find a fault that is just about to, or hasn't quite happened yet. It is not unlike the movie Minority Report, with the department of Pre-Crime, where you find something about to go wrong and fix it before it happens.
 
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Most parts are relatively immune to small static discharges but the 4051/4052/4053 are especially easily damaged when being handled and assembled. These can be solid failures or partial failures. Just using a cheap ungrounded soldering iron can damage them. Sometimes, circuits using them are poorly designed to provide a leakage path. Some end of the gates need a path someplace to +5 or ground. I rarely use a grounded work surface or a wrist strap but when dealing with these, I treat them with respect.
It doesn't surprise me much that you find one with the control voltage leaking to one of the gate pins. Unlike most logic gates, the analog paths are completely unprotected. There are no protective resistances or reversed junctions to either rail. There is a body diode to the bulk but its breakdown voltage is more than the gates can handle.
Dwight
 
Dwight Elvey said:
Most parts are relatively immune to small static discharges but the 4051/4052/4053 are especially easily damaged when being handled and assembled. These can be solid failures or partial failures. Just using a cheap ungrounded soldering iron can damage them. Sometimes, circuits using them are poorly designed to provide a leakage path. Some end of the gates need a path someplace to +5 or ground. I rarely use a grounded work surface or a wrist strap but when dealing with these, I treat them with respect.
It doesn't surprise me much that you find one with the control voltage leaking to one of the gate pins. Unlike most logic gates, the analog paths are completely unprotected. There are no protective resistances or reversed junctions to either rail. There is a body diode to the bulk but its breakdown voltage is more than the gates can handle.
Dwight
Click to expand...

Yes , this makes sense. It might also be that the earlier generation 4000 cmos IC's were not as robust as later ones. I'm still waiting for some more CD4051's to arrive, I only had one in my stocks. Who knows what sort of electrostatic abuse they could be subject to before I get them. Most IC's I get on ebay are not packaged well for electrostatic protection, but it doesn't matter much for TTL as they are so hardy (which is why I like them). But for these CD4051's its another story, a very vulnerable part indeed.
 
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