6 Jun
2009
6 Jun
'09
5:39 p.m.
On Saturday (06/06/2009 at 06:28PM +0100), Tony Duell wrote:
yes... I had to clean it out of the raceway of the solonoid as well as
off the plunger. It was really like glue. WD-40 seemed to do a nice
job cutting it actually.
That was absolutely it. It was a fun afternoon tracing out the circuit
and finding a bad TIP31B (NPN, 3A, 40W) on the driver board. Of course
I didn't have a TIP31B in stock so I stuck a TIP29B in there temporarily
and we're back online. A pile of TIP31B have been ordered for spares.
I remember having to change one of them once before in the distant past
and the evidence of that (solder rosin) was still on the board so I've
been here before... can't understand why I would have forgotten after
30 yrs of not using this thing :-)
Great stuff. Cool to see all those TTL parts and not a one of them is
LS or HCT or anything close. 74XX was it in 1973.
oh it's fun! Great to see the thing printing crisp and clear and feeding
the paper as it should.
Note that I have really good luck with modern thermal FAX paper in these
units. Around here it is readily available at office supply stores on
98 ft rolls. They have a 1/2" dia core which needs a piece of dowel
to support it in the 725 (and probably in the 733 too) and it is a much
smaller roll than the 725 can take... but it works. I also use it in
my 745 and there the roll is a perfect fit.
http://www.officemax.com/catalog/sku.jsp?productId=ARS27286&history=vu0…
6 rolls for $17.
Chris
--
Chris Elmquist
However, the
paper does not feed correctly. On inspection, I've already
found that the grease in the little solonoid that lifts the head from the
paper when it advances has turned to something more sticky than honey.
I'm in the process of addressing that.
This is a well-known problem on all sorts of devices. Someitmes the
grease will soften with common solvents (propan-2-ol, etc). There's a
green grease used in some camera lans mounts that goes rock hard with age
and nothing wil shift it short of scraping it out.... I also see
that the stepper motor that actually turns the platen (or equiv
of) is not making a full increment each time. Sometimes it advances but
other times it de-advances. So, something it messed up in the stepper
circuit or the grease in that stepper motor has turned to honey too.
Maybe you've lost the drive to one of hte stepper phases. That can make
them step in thwe wrong direction sometimes. There's no
microprocessor in the thing-- not even a UART. It's all done
with 7400 TTL and electronically, it's extremely fixable.
The 733 is like that too, although there's probably at least one ROM (the
character generator). From what I rmember there's a cardcage of PCBs
between the 2 tape drives in the ASR add-on (behind the swtich/LED panel
in the middle -- in fact I think that's mounted on the frontmost PCB in
the cage). Anyway, about the most complicated chip on those boards is a
little bipolar RAM. No microprocessor. Everything
else is on the back burner now-- gotta get this baby
printing right! :-)
I'm almost convinced to dig mine out and have a go with it... Too many
projects, not enough bench space...