Monday, June 18, 2018

Fixed ASR33 keyboard, built USB to teletype interface device but still debugging it


I partially disassembled and adjusted my keyboard for my teletype model 33 ASR, getting it to work smoothly and reliably. I verified the contacts using a VOM while pressing many of the keyboard keys. It is now ready to be reinstalled in the machine, while I go on to work on the call unit, printer unit, tape punch and tape reader mechanisms.

Restored keyboard module for my ASR33 teletype

The board for the open source TTYLoopDriver project uses many small surface mount components, in addition to a few through hole parts. I carefully put down solder paste and inserted all the components in place, ensuring the polarity of diodes and similar parts.

I ran this through my reflow oven, another open source project by the way. The board came out with solder bridges on three of the tiniest integrated circuits, something I had to clean up with solder braid and manual soldering under the microscope. There are also a couple of skewed parts I can adjust.

Board coming out of the reflow oven, needing tweaks
The large metallic coil in the center was enough of a heat sink that it kept the solder paste from fully flowing. I touched that up with the soldering iron. The small ICs U2, U11 and U9 had solder bridges on the leads, which I repaired by judicious use of solder braid after flooding the surface with flux. Finally, I used the hot air rework tool to remove the capacitor that was pivoted and shorting to the pivoted resistor, then cleaned up the resistor end and manually soldered the capacitor back in place.

The remaining components were through hole parts which I mounted and soldered in place. The final part was the USB board that fits in the upper left. It comes with a right angle header that must be removed. My Hakko desoldering gun vacuumed away the solder and allowed me to remove the only header. I then installed small straight headers as a mount for the USB daughterboard onto the main PCB and cleaned up all the solder.

Finished board ready for testing
After an inspection to rule out any solder bridges or other errors, it was time to do the checkout steps. First is to plug in the USB board and verify that it is supplying proper power to the rest of the board. The board is not properly reporting its ID to Windows (or my Mac either). The RX led is stuck on.

The USB board must be reprogrammed to implement the 45 baud speed and to request 400ma from the USB host, which requires the daughterboard maker's development studio software. I downloaded it but until the USB board is talking properly I can't reload the firmware or do anything else.

It seems to me that the CP2102 daughter card should be talking to the PC even if the connection is not working to the main board. Since I had to desolder the original right angle header and install a straight header, I may have a connectivity failure on one of those pins. I can see that the top side is soldered well, but if the actual signal lead attaches to the underside of the daughter board, there might be problem if I broke the via when extracting the old pins. 

My action plan is to:

  1. find the CP2102 schematic and test connectivity from the chips on the daughter board to the pins
  2. failing to find the schematic, I will unsolder all the headers, separate the daughter card and solder the pins on the underside. If I reattach it after this change, and the header modification was the problem, then it should work.
  3. in parallel I will do more connectivity and short testing of the main board, just in case it is somehow holding the USB card in reset or keeping it busy. 

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