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LA120-DA (DECWriter III) - Line Feed Fuse

treehouse3911

New Member
Joined
Dec 6, 2015
Messages
9
Hi all;

I recently acquired an LA120-DA terminal printer and need a little help with some diagnosis on the Line Feed stepper motor.

As soon as the terminal is powered up, the LINE FEED fuse blows. I have checked the resistances of the stepper motor coils according to page 5-26 of the EK-LA120-TM-001 Technical Manual, and they all confirm as about 7.5 ohms. Using an LED, I have also confirmed that the stepper motor winding pairs light the LED (as shown in this video:
https://www.youtube.com/watch?v=1U9pf7S_ov4).

The printer powers up and does a self-test that reports no other problems. It will print during a self-test print, but simply does not advance the paper. The Local Line Feed and Form Feed buttons do nothing in Local Mode.

Before I assume that the logic board for this terminal is at fault, is there any further diagnosis I can perform to help isolate the problem?

Can anyone guide me in checking the transistors that would be controlling the stepper motor? (see photo)

LineFeedComponents.jpg

Any help would be appreciated. I know it's so close to working perfectly!

Thanks,
Tim
 

Christoffer

Experienced Member
Joined
Oct 15, 2014
Messages
299
Location
Denmark
Is that the line feed motor control board? Doe it have a separate power supply/winding/circuitry? is that known to be good?
Do you have the schematics? - Checking transistors might require you to desolder them, unfortunately, what model of transistors is it?
What are the two huge TO-3 metal can packages, and the lone transistor or reg. to the left?
 

treehouse3911

New Member
Joined
Dec 6, 2015
Messages
9
Hey Chris;

Alas, I can't answer your questions back to me... I just don't know enough about the setup for the stepper motor yet, nor which of those components on that part of the logic board are directly related or involved. I'm still trying to track down a schematic for the terminal. If anyone can point me to one, I'd be much obliged.

Tim
 

gslick

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Joined
Dec 30, 2010
Messages
2,112
Location
Seattle, WA
LA120 Field Maintenance Print Set
MP-00663 Rev. C
http://www.bitsavers.org/pdf/dec/terminal/la120/MP00663_LA120_Oct79.pdf

Printer Logic And Power Board, Component Layout, Page 38 of the PDF
Printer Logic And Power Board, Schematic Sheet 4 of 6, Page 46 of the PDF

Those five TO-220 package Q11, Q12, Q13, Q14, Q15 are all TIP121 NPN Darlington Transistors

The Line Feed fuse F1 connects +24V to Q11. Q11 also connects to the stepper motor common. Q12, Q13, Q14, and Q15 each connect to one phase of the stepper motor. There is also a 1N4004 diode D1 connected to the stepper motor common. If D1 is shorted that would probably blow the fuse. If you disconnect the stepper motor then you could check diode D1 from either of the common pins 2 or 7 of the stepper motor connector across to ground.

On the component layout it looks like D1 is below Q12 and next to J2.

(The TO-3 Q9 MJ3000 NPN and Q10 MJ2500 PNP Darlington Power Transistor pair drive the carriage DC motor)
 
Last edited:

treehouse3911

New Member
Joined
Dec 6, 2015
Messages
9
Hi gslick!

Superb! Thanks so much for your response and identification of the components involved. That's a tremendous help to me given my limited electronics experience and inner workings of this terminal. You've shown me a few things to focus on in my diagnostics, and I appreciate the link to the schematics.

I will report back so others might benefit from my experimentation.

Best regards,
Tim
 

treehouse3911

New Member
Joined
Dec 6, 2015
Messages
9
Hi all;

I just wanted to report back that we seem to isolated the problem with the linefeed on the DECWriter III (LA-120) terminal printer to the Q11 transistor, which appears to have failed. A short is being detected between is Base and Collector. The stepper motor appears to checkout okay, as does diode D1.

I have to thank gslick again for your reference to the schematics for this printer, although (as luck would have it), there is a drafting error on the schematics, in that all five transistors have their Collectors and Emitters (Pins 2 and 3) labelled backwards. Still, without the schematic, we'd not have made much progress. So, thanks again!

Now, we have to figure out what caused the Q11 TIP125 transistor to fail, so we're checking for leakage now somewhere back in the 7406 IC. There might be more to the story yet!

Regards,
Tim
 

gslick

Veteran Member
Joined
Dec 30, 2010
Messages
2,112
Location
Seattle, WA
I have to thank gslick again for your reference to the schematics for this printer, although (as luck would have it), there is a drafting error on the schematics, in that all five transistors have their Collectors and Emitters (Pins 2 and 3) labelled backwards. Still, without the schematic, we'd not have made much progress. So, thanks again!

Yeah, I was wondering about that. It didn't make any sense to me how the individual stepper mode phase transistors Q12, Q13, Q14, and Q15 were drawn on the schematic, but I thought the common transistor Q11 made sense when I was looking at it.
 
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