Model 1 Video Problem

[cbmeeks]

Hello all.  I’m restoring a Model 1 TRS-80 and the first issue I came across was the picture is nothing but a thin horizontal line across the screen.  
 

I have checked the voltages and I believe they are OK.  However, some are a little low (checked Z1 and Z2 and some 5V show 4.6V).  
 

I have the manual that talks about the video divider so I thought I would put a scope there and check the chips.  
 

Any idea where to start?  
 

Thanks!

[+sm3]

How about reaching out to Ian Mavric from AU? Chances are, he will have some info for you:

 

http://ianmav.customer.netspace.net.au/trs80/

 

The issue (if you have the same one) seems to be common in older TVs.  The TRS-80 monitor is just an old TV with the tuner removed if I'm not mistaken. I forget the brand it was though.

 

Look at this YouTube video. Just an example, the TRS-80 Monitor doesn't have vacuum tubes: 

 

 

NOTE: I found out the brand of monitor that Radio Shack used was RCA. Look at this picture I found on the internet:

 

 

 

 

 

[cbmeeks]

Thanks, I will check that out.  So I found out that it only shows that line when connected to my Apple II monitor (which works great).

When I connect it to the original Tandy TRS-80 Monitor, I get the pics below.

 

 

I have tested the frequencies with my scope and came up with the following...I think the video chain is close but not perfect.  The end result is *ALMOST* 60Hz.

 

[+sm3]

So it's not the vertical collapse issue. I'd still shoot Ian an email. He refurbishes a lot of TRS-80s. Maybe he's seen this particular issue before.

[MrDave]

You should really check all the capacitors as that effects the voltages.

[BillO]

Looks to me like a horizontal sync issue.  Is the HS pulse voltage within spec?

Edited May 9, 2020 by BillO