No video on Model I.

[pebecker]

I got started with TRS-80s back in about 1979 at high school. First one I owned was a Model III 4K L1. Still have it, though it got upgraded over the years and became a Model 4.

Today I got a Model I with an expansion interface and monitor. Checked +12 and +5 and they were good, so was -5. No video on monitor (can't believe the monitor even had a raster). Started troubleshooting with the SAMS material and I see a strong waveform on Z41 pin 6 but nothing but noise on pin 5.

The pulses on pin 6 grow in length as I type a few characters, so I think the computer is alive inside. I ordered a new Z41 and it should arrive in a few days.

Question: could a defective Z41 be caused by plugging in the power supply to the video output DIN connector? I bought this computer at an estate sale and it is in remarkable condition, but the original sale photo showed connectors plugged in. Not certain what was plugged in where, but I suspect it could have been wrong.

Thanks for your help (and sure wish I could make it to Tandy Assembly this year),
Paul

 

[Bruce Tomlin]

From the recent Adrian videos, make sure that the +5 looks good on a scope, a voltmeter is not enough. Is the big transistor cold? It could be bad contact with that screw.

What is Z41? The Model I tech ref manual says "relay driver", are you sure you have the right chip number?
EDIT: oh there's the other half of it on the final video output
Anyhow, look at the schematic, only two of the pins are even in common with the power supply, but half of the AC goes to pins 1-4, and there's a couple of transistors before it gets to Z41.

 

[pebecker]

Thank you!

 

[Eudimorphodon]

You have to give credit to the sheer sadism of the guy that chose to use the same DIN socket for all three of those functions on the back of the Model I.

 

[Trash-Eighty]

Don French in an interview on TRS-80.com: "We made many decisions in the design related to keeping the cost down. We left out having upper and lower case to save $1.97 per computer. [Charles] Tandy would never have accepted a product we lost money on."
I think cost reduction measures were the reason for chosing the same kind of DIN sockets. It's just cheaper to order thousands of connectors from the same design type.