HP 41C Video interface test. Howto?

[Dave Frederickson]

(07-12-2020 01:33 PM)Sylvain Cote Wrote:  

(07-12-2020 10:56 AM)tribidouille Wrote:  Side question: The antenna adapter is useless until you don't want to plug/unplug cables all the time isn't it?

It is necessary if you want to plug the 82163 TV output to your TV.
To my limited knowledge, the 82163A TV output has the big part of the RF modulator in it but it need the VHF adapter to finish the job.
I would assume that some old TV did had a VHF adapter in it with a RCA plug but I do not remember having seen that.
I have never opened an VHF adapter, but since it is passive, I would assume that it only contains basic components in it. (resistors, caps and coils)

The antenna switch is NOT needed with a TV with a coax antenna input. All it does is switch the input to the TV between the antenna input and the input from the 82163.

The output of both the analog monitor and the RF TV output is 75 ohms. If you want to connect the 82163 to the coax input on your TV all you need is one of these and a coax cable.
https://www.amazon.com/VCE-2-Pack-Coaxia...ef=sr_1_14

The U.S. 82163A version includes a switch adapter which apparently also contains a 75 ohm to 300 ohm matching transformer (balun) to connect to the twin-lead antenna input on vintage TV's.

The export 82163B version includes an adapter which I presume is a matching transformer similar to this one, but with an RCA jack.
https://www.amazon.com/RCA-VH54R-Matchin...ref=sr_1_4

Dave

[tribidouille]

Well, to sum it up,
-The video converter sends an image to the TV, clear enough, full of triangles.
-The problem of this 82163 seems to be in the HP-IL communication part of it.
-The tests Sylvain kindly posted show the HP-IL is working (empty closed loop is ok) but printer is not detected by the 41: flag 55 and 21 are "NO".

I should try to find schematics to understand where to investigate further.

[vassilisprevelakis]

hi,

I have looked at the usual places for the schematics of the HP-82163A and did not find it.

If anyone has them, please post them here.

Thanks

Vassilis

http://www.series80.org

[Dave Frederickson]

I don't think the schematics are going to be that much help.

The major components are the video controller, the microcontroller, the video RAM, and the HP-IL controller.

The indication is that the display is all triangles, which sounds like a RAM issue, but I don't see that any of the tests have caused the display to change. If it were simply a bad RAM the display should change to some other garbage.

BTW, what's the difference between the ASCII code for a space and a triangle?

Examining the photo, it can be seen that the RAM is connected to 1LF4 chip, the video controller. I'd change the 1LF4 but there's no harm in changing the RAM chips first because it's easier and cheaper.

As mentioned, there's a 1LF4 available at Sphere and I have one I bought a few years ago while trying to fix a jitter problem that I'm willing to sell.
https://www.sphere.bc.ca/test/hp-parts/hpparts3.html


Dave

[tribidouille]

(07-17-2020 08:53 PM)Dave Frederickson Wrote:  ...
BTW, what's the difference between the ASCII code for a space and a triangle?
...

Hi,
Here is a closer view:


Space's ASCII code is 32 IIRC but I don't see triangles into the ASCII table actually.

Ok for the RAM and the 1LF4 (and thank you for proposing to sell it) but would they also be responsible of the fact that the Video Interface is not detected by the HP-41?

Regards,

[Dave Frederickson]

(07-17-2020 09:14 PM)tribidouille Wrote:  Ok for the RAM and the 1LF4 (and thank you for proposing to sell it) but would they also be responsible of the fact that the Video Interface is not detected by the HP-41?

Sorry, I thought that part worked. No it wouldn't. Power supply, maybe.

[vassilisprevelakis]

(07-17-2020 08:53 PM)Dave Frederickson Wrote:  ...
BTW, what's the difference between the ASCII code for a space and a triangle?
...

Hi,

the triangle is non-standard and could be anything (e.g. on the 82143 printer its 8, while on the HP-86B is 7). Its not mentioned in the manual of the 82163 as available.

So we must assume that if the video controller is working, then the code for the triangle must be one that the 82163 considers unprintable (i.e. anything from 0 to 31 and 127).

My guess is that the video RAM is all FFs and the video controller prints a triangle when it sees FF.

Now the video RAM is big enough to store just the ASCII map (31 lines with 32 chars per line) of the screen, leaving only 8 bytes free. So I do not see the CPU requiring access to these 8 bytes in order to work (it has its own internal RAM).

Nevertheless, my chief suspect is still the RAM, but to see what is actually going on, I'd use a logic analyzer to look at the signals being exchanged between the CPU and video RAM, to see if a RAM error is causing the CPU to stop.

Then I'd look to see if the HP-IL IC is working. There are some simple tests you can do, described in my HP-3421A repair video (https://www.youtube.com/watch?v=G4UJPeiKOQE).

Complex stuff and perhaps not really necessary as your only hope of getting this to work is that the CPU is OK. So, frankly, if I were in your shoes, I'd have already replaced the ram.

Best

**vp
http://www.series80.org

[tribidouille]

Yes, and as I am in my shoes too, i'll do that when I'll be back. Order some ram and change it.
Thanks a lot to everybody for all these answers.
I'll be away until end of august and will continue the repairs then.
Best regards