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asr-33 Teletype Trouble

R

ryaninspiron

Member
Joined
Jun 29, 2011
Messages
33
Location
NY
Hello,
I just picked up an asr-33 and it won't stop printing a null character.(ascii 0x00 to be exact)

I took a video of the problem to save time of trying to explain it.

http://www.youtube.com/watch?v=ODdcUSgReYM

the only time it does anything different is when I hit the "here is" key.

any advice would be greatly appreciated.

Thanks, Ryan
 
Last edited:
That's what they do unless they are "on line."

If you flip the knob in front to Local it should, hopefully, work properly. If it's on the Line setting it's expecting a signal on the current loop interface.

I don't understand these beasts enough to know why that's the case, but that's my experience.
 
modem7 said:
I just picked up a girl and she won't stop chattering. :)
Click to expand...
:lol:, I realize that I could have picked better words but I was at a loss at the time.(I updated the post now that I know what its doing)

Since yesterday I have been reading the manual and the theory of operation section in particular, it is amazing how much detail it goes into.
Thanks to that book I am have made a discovery, the stop bit of the distributor that electrically creates the marking condition which engages the selector magnet holding the start lever must be the source of the problem. either the distributor, the magnet itself, or the circuit between the two.

I stuck a screwdriver into a gap behind the selector armature and turned on the machine, sure enough the only thing moving was the motor and the main shaft.
which I believe is the only thing that should be moving in the stop state of local mode and line mode.

here's a picture of the selector trip mechanism held in the marking position
wedged armature.jpg

I have no doubt that this is not going to be the only problem with this machine but this is a step in the right direction.

The next question is that while in local mode the machine should be internally set to a loopback, right?
 
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Hi
When on line it requires the current to hold the
coil.
Since you have a problem with local, it may be that the keyboard
is not resetting. Look the see that the "H" bock is properly connected
between the keyboard and the printer. If it was shipped without locking
down the printer, the "H" block usually pops out.
If it is there, you can try manually reset the keyboard by rotating
the "H' block ( forget which direction ).
Check that there is current flowing in the local mode to the coil.
Dwight
 
Just checked it and the h bridge visually appears to reset after each key press.

No current in local mode.

I just noticed that the selector magnet driver card is clearly having serious issues, resistor r8 is heating up to 250 degrees Fahrenheit in just about a minute of the machine running and all the resistors appeared to have smoked up at one point. the resistances are mostly within spec upon measuring them.
R8 is "transistor q1 load" but measuring the temp of that reveals it is only a slightly warm 90 degrees.

Here is the section of the schematics I'm reading.
selector card map.jpg

selector card parts list.jpg

selector magnet driver card schematic.jpg

and the card itself
Blackend selector magnet driver card.jpg

now for the hard part,
What could be causing this?
my guess a short somewhere between this card and the magnet itself.
 
Update: it the resistors are staying hot as long as the machine is plugged in
temp checks on the resistors with a infrared temp probe reveal

(moving top to bottom)
r7 is at 140 degrees
r6 is 270
r8 is 250
r2 is 220
r5 is 130
everything else on the card is around 100
 
Well I disconnected the selector magnet and then I disconnected all the connectors on the back and both times the card still heated up the second it was plugged in so that must not be the problem.
maybe it's transistor Q2....
these temps can't be normal right?
 
Hi
Use a volt meter and make voltage measurements
using the + lead on pin 2 of the board and fill in the
voltages around the board. Form there we can make
better guesses.
( make a copy of the schematic and write the measured
levels )
Dwight
 
Here is the schematic with my measured voltages
Measured voltages.jpg

why does this card stay fully powered when the machine is "off"?
 
Should I desolder and remove Q1 and test it using my transistor check mode on the meter?
While its out I can see if things stay cool.

If Q1 is the problem then what transistor should i get to replace it?
I cant find a spec sheet for it because the number on top is the teletype part no.
 
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Didn't test it yet but the only change after taking it out is that now r2 and r5 heat up way faster than all the other resistors on the board.
they hit 150 within 30 seconds of the machine being plugged in.
 
The board layout diagram and the board don't match exactly. The DIAC is not in the place shown in your layout. Does the schematic match the board with regards to Q1 and the DIAC? What were the results of your Q1 out of circuit tests? That will be a more valid indicator than resupplying power to the board.

Kipp
 
ryaninspiron said:
Didn't test it yet but the only change after taking it out is that now r2 and r5 heat up way faster than all the other resistors on the board.
they hit 150 within 30 seconds of the machine being plugged in.
Click to expand...

You should not power the board with the transistor out, it could blow the coil!!!!!!
Dwight
 
Just checking, the voltages your measuring, are you using a DC scale
or an AC scale.
You should never power a circuit with pieces missing unless you are sure you
know what each part does and can do.
Dwight
 
I was looking at the circuit and trying to figure how it works.
I believe I may have erred on the transistor being bad.
In fact, I don't think there is any problem on the board at all.
The coil should energize when circuit board pin 2 is shorted by the external switch
to circuit board pin 7. From the looks, there is no external
switch closed.
Remove the power from the machine ( unplug ) and use an ohm meter to
measure between these two pins.
Typically, in the local mode, this come from the distributor that samples
the switch code from the keyboard.
It could be a bad brush contact on the distributor or something close.
The way it works in local is that entering a key trips the distributor
that then samples the parallel keyboard output and then serializes
it. Check to ensure that the distributor is starting and stopping in the
right location.
Dwight
 
kyeakel said:
The board layout diagram and the board don't match exactly. The DIAC is not in the place shown in your layout. Does the schematic match the board with regards to Q1 and the DIAC? What were the results of your Q1 out of circuit tests? That will be a more valid indicator than resupplying power to the board.
Click to expand...
Q1 seems fine from what I can tell also here are a some better pics of the board

this is the transistor area backlit
backlit transistor section.jpg

and the trace side
trace side of card.jpg

and here is the card at a better angle for reading the lower labels
better angle for reading lables.jpg


Dwight Elvey said:
Just checking, the voltages your measuring, are you using a DC scale
or an AC scale.
Click to expand...
on the transformer side of the rectifiers I used AC and on the rest of the card I used DC.

Dwight Elvey said:
You should never power a circuit with pieces missing unless you are sure you
know what each part does and can do.
Click to expand...
I really am smart enough to know that but I must have been experiencing a mental lapse this morning.(Won't be doing that again)


I see the reason this card is heating up must be because it is only designed to sit idle in with the selector magnet energized (marking) because because it is being left in an open (spacing) state the resistors are over heating.

the distributor is stopping in the right spot (second stop bit position) but sure enough in the rather than providing a marking signal I just get all spacing. a meter to the pins of the distributor reveal that none of the pins are conducting. The inside ring brush has no contact issues but the outer rings are not getting any continuity to the center

Here are some pics of the distributor,
1. in the stop state
distributor at idle.jpg

2. rotated to show under stop 2 position
distributor moved to reveal rest of contacts.jpg

looks like I should do some cleaning and may be should use some fine sand paper to clean the bottom of the outer brush. what should I use to clean the distributor contacts? alcohol and Qtips?
 
Shorting pins 2 and 7 does engage the selector magnet but I still have no signal reaching pin 7. after my first attempt at cleaning the distributor I got it to improve from not making contact at all to intermittently making contact.
Also I need to track where the signal goes from the distributor to the selector magnet data pin(7).
I assume in local mode it should have a relatively straight path between the two.
Right now even with the distributor is slightly working I dont get the slightest output from the selector magnet.
 
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