Serial Port issue

[jcneumann]

I built my IMSAI 5 years ago and recently I have been re-acquainting myself with it. It's loads of fun! But it suddenly developed a problem with serial ports. I am no longer seeing the Z80 startup screen over the DB9 serial port and I am no longer able to access CP/M using Minicom on Linux or Tera Term on Windows.

Parameters are 115200,n,8,1 (as they always have been), and the jumpers are correct. Serial Connection was working fine while I was using the IMSAI, and then the lights suddenly went out (went dark) in the middle of that session. After that I noticed my serial port no longer worked, so I think it may be a bad MAX3232 chip. Everything else seems to work fine. I can change disks, boot CP/M etc.
I even keyed in the old kill-the-bit program on the front panel and ran it.

What are your thoughts; a bad serial chip or something else?

[ronaldlw]

Hey Jcneumann, glad you've brought your IMSAI out and are messing around with it again!

You might mention what kinds of tools / test equipment you have access to and what level of electronics expertise you have. I'm sure there are some ways to test to see if the chip is working, I used an oscilloscope with my issues, but you can probably get some info with just a voltage meter.

I'd check voltages to make sure they're good / within spec at a minimum.

I had some problems with my connection between my IMSAI and VT132 board. I ended up replacing the MAX232 on the IMSAI and it's been working great since.

I had a hard time finding an exact replacement, so ended up getting a newer version I found on amazon:
https://www.amazon.com/dp/B087Y7TCH2

With this one It said to use 1.0 uF caps instead of the .1 uF ones on the original chip, so I replaced those as well:
https://www.amazon.com/dp/B08BF91KY3

Someone else might know of a good source for the original chip, hopefully they'll speak up, but if not these parts are working well for me.

I hope this helps!
Ron

[jcneumann]

Hi Ronald,

Thanks for the info. I found the correct chip on eBay: MAX3232CPE and will order it. I believe that model is a 0.1uf version. I do have a multi-meter and a cheap Rigol scope. I haven't used the scope in years, though. I might break it out and see what I can. In the mean time I'll poke around and verify some voltages. Do you know if TheHighNibble provide an IMSAI schematic that I can download?

Thanks again,
Jaosn

[ronaldlw]

I don't believe there is any schematic available

[ronaldlw]

Oh, btw, if you want to do the replacement, I have extra chips and caps that I'd be happy to mail to you for the cost of postage to wherever you are.