Hi,

I am restoring a TRS-80 Model 4 computer and adding a number of additional internal drives to it.

When testing the computer with just one Teac 55B 5.25" floppy drive with the stock power supply I see some 'shrinkage' on the CRT display even though I've rebuilt the stock power supply and replaced the caps.

I've already added the following +5v and +12v power supplies to power the additional 3.5" floppies and a cooling fan. I chose the separate power supplies because of space restrictions in fitting all of this in the original case.

https://www.amazon.com/gp/product/B00F2B8XXI/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00XYYNB78/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

I've figured that I can just bypass the original p.s. altogether now to save strain on it and power everything from these 2 additional power supplies but how would I go about powering the 5.25" floppy which requires both +12v and +5v from separate supplies supplying these voltages?

Thanks!
Tim

 

Welcome to AAC!

Do you have a schematic for the power supply showing how high voltages for the CRT are generated?

My guess is that you're overloading the power circuitry by adding loads the original design wasn't intended to support. Have you calculated total current draw from each of the supplies?

 

Hi and thanks for the reply!

Well, the original ps was to power 2 internal 5.25" floppy drives using both +5v, +12v @ about 1A max, the computer itself and +12v for the crt.
The Teac 1/2 height drive I used to replace one of the full-size 5.25" originals has nearly the same power specs. I am powering all of the additional drives with the additional power supplies that I have added.

Just powering the 1 Teac 5.25" from the original supply and I get some slight crt 'shrinkage' from disk activity hence my reasoning for eliminating the original power supply from the equation entirely. Now to figure how to power the TEAC from the separate +5v and +12v supply is the tricky part for me.

 

I initially missed the part where you were powering the additional drives from external power, but didn't think it was important enough to edit my post.

Have you checked voltages to see what's drooping? Do you happen to have a schematic for the power circuitry?