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Garbled video Signal from my 256TC - thags - 05-05-2022

Hi All

On the weekend I replaced a few components on my CRT and plugged in the 256TC. It powered on the lights but didn't seem to be booting, or perhaps wasn't sending a video signal. Thought I would post some images and see if anyone recognized the issue and had any suggestions of where to start looking before I just started blind testing everything. 

[Image: IMG-2232.jpg]When I first powered on I got this signal which didn't change for quite a while so I pushed the reset button the bee and got

[Image: IMG-2236.jpg]this one, stayed like this for a while, reset button again and then

[Image: IMG-2244.jpg]this one

Thags

RE: Garbled video Signal from my 256TC - MbeeTech - 05-05-2022

Hi Thags.
The fact that you get some video out [garbled or otherwise] means that the processor is executing the boot rom to a certain extent.
I say this because to get any video out, the video controller (6545 CRT controller) has to be programmed which happens while executing
the boot rom code.
It looks to me to be some problem with accessing the video ram (screen character ram, colour ram, attribute ram).

Have you checked your 256TC to see that the NiCad battery has been removed & that there is no PCB damage from leakage?

RE: Garbled video Signal from my 256TC - someone - 05-05-2022

For starters, it appears that the wrong half of the FONT ROM is being selected.
The most likely culprit is the MA13 line of the CRTC line not being asserted.
PCG update looks fine.

Someone finds the best way to fix bees is to use their set of simple diagnostic ROMs for non-destructive testing.
Do you have a ROM burner & some spare ROMs (or a ROM Emulator) by any chance?

The ROM just needs to boot the bee to a wanted steady state.
e.g. Just enough to init the CRTC with the desired FONT & then remain static.

Secondly it appears that the screen is a bit too far to the left causing the wraparound effect.

RE: Garbled video Signal from my 256TC - thags - 05-05-2022

Thanks for the replies! I have had a look at the board and it looks like there has been some very minor leakage from the battery Sad hopefully surmountable as it's only directly around where the battery was mounted. I've removed the battery and assume I need to source a replacement now? Thank you for your advice and patience with a newbie like me who is learning the ropes. I will upload photos when I have a chance over the weekend.

RE: Garbled video Signal from my 256TC - Ernest - 05-05-2022

(05-05-2022, 09:14 PM)thags Wrote: Thanks for the replies! I have had a look at the board and it looks like there has been some very minor leakage from the battery Sad hopefully surmountable as it's only directly around where the battery was mounted. I've removed the battery and assume I need to source a replacement now? Thank you for your advice and patience with a newbie like me who is learning the ropes. I will upload photos when I have a chance over the weekend.
Hi thags,
This maybe a super basic question, but what monitor are you using?
Is this a CGA monitor or are you using a CGA to VGA converter?

Ernest

RE: Garbled video Signal from my 256TC - thags - 05-05-2022

(05-05-2022, 10:02 PM)Ernest Wrote: Hi thags,
This maybe a super basic question, but what monitor are you using?
Is this a CGA monitor or are you using a CGA to VGA converter?

Ernest

Hi Ernest

I've got a CGA to VGA converter and just fixed up an old Thomson CGA Monitor over the weekend (the one in the pictures). Getting the same results on both

RE: Garbled video Signal from my 256TC - Ernest - 06-05-2022

(05-05-2022, 10:27 PM)thags Wrote:
(05-05-2022, 10:02 PM)Ernest Wrote: Hi thags,
This maybe a super basic question, but what monitor are you using?
Is this a CGA monitor or are you using a CGA to VGA converter?

Ernest

Hi Ernest

I've got a CGA to VGA converter and just fixed up an old Thomson CGA Monitor over the weekend (the one in the pictures). Getting the same results on both
Great thags,
I though it best to ask as it would not be the first time I came across someone trying to use a VGA monitor on these old systems and to do so can give some strange results depending on their vintage and capabilities. I even saw an old NEC multi sync that kind of gave a semi viewable display.

It is quite some time since I have repaired a battery damaged 256TC but from memory there is a crucial signal that traverses the battery area maybe even a feed through which are notorious week points for acid damage.

Ernest