TI Solid State Printer - Power Supply Issues?

[Cory]

I've been poking around looking for some articles, blogs or videos on how to diagnose / repair a TI Solid State Printer.  Symptom - no power.  Are there schematics available?  Has someone gone through the diagnostic steps previously?  I'm pretty handy diagnosing digital circuits, but have been less successful when diagnosing power supply issues.

 

Any thoughts or helpful tips?

[GDMike]

Verify voltage test at regulator output pins to start.

[Cory]

51 minutes ago, GDMike said:

Verify voltage test at regulator output pins to start.

I'll need to do a little disassembly to get to the voltage regulators, but I did get the reading from the following connectors.  In the picture, the yellow box on the left:

 

Black and White = + 15V

White and Red + 5V

 

For the connector on the right from left to right:

 

Left Black and Left Red = +13.75V

Right Black and Right Red = -13.75V

 

 

The connector on the right goes to the motor that feeds the paper.  However, the paper doesn't feed if you press the paper advance button.  This is why I thought I had a power supply problem, which now it looks like it may be something else.

 

Edited May 22, 2022 by Cory

[+Ksarul]

Here's the schematic for it, courtesy of @acadiel.

thermal_printer_1979.pdf

[GDMike]

Sounds like logic issues..

[Cory]

If I read the schematics right, it looks like P3 should be +15V.  It’s reading 13.75.  Is that enough variance to effect the motor?  Would that be a failing regulator? 

[GDMike]

2 hours ago, Cory said:

If I read the schematics right, it looks like P3 should be +15V.  It’s reading 13.75.  Is that enough variance to effect the motor?  Would that be a failing regulator? 

Looking at your schematic just now..I'm guessing there's a controller somewhere..ahh here. Possibly..

Can you check the logic in U3 and that transistor and diode combo, G2 and 12

Or disconnect the motor and check your voltage and logic, does the motor ever come on at all during power up or init?

Edited May 22, 2022 by GDMike

[GDMike]

Is that motor the only issue?

 

[GDMike]

Could be in the advance circuit too?

Edited May 22, 2022 by GDMike

[GDMike]

Ahh. I think I'm looking at the print head motor instead of the form motor. Sorry. I'm not seeing that. I guess look at the advanced circuit.. my bad

[Cory]

23 hours ago, GDMike said:

Ahh. I think I'm looking at the print head motor instead of the form motor. Sorry. I'm not seeing that. I guess look at the advanced circuit.. my bad

Will report back what I find.  Thanks for the pointers. 

[Willsy]

What's the purpose of the diode? It looks superfluous.

[GDMike]

I suppose it's for pulse width modulation,(PWM).

These diodes protect the transistors when switching occurs. < 0. < 0. This results in large voltage spikes across the transistors.

 

Edited May 24, 2022 by GDMike

[Stuart]

On 5/24/2022 at 9:28 AM, Willsy said:

What's the purpose of the diode? It looks superfluous.

Each coil can be energised in either direction. Current flows through the transistor in one of the blocks, and the diode provides the return path past the transistor in the other corresponding block.

[Cory]

Thanks for the tips, I've identified and fixed the issue.  The issue appeared to be a bad solder joint for one of the regulators.  When I was testing voltage I noticed that most of the readings were accurate, but occasionally I got a bad reading which I assumed was me being clumsy.  

 

I spent some time inspecting the circuit board under magnification and found what looked like a cracked solder joint.  So I desoldered and resoldered the joint and it's back in business.

 

Thanks again for the tips and input from all!

Cory