7 Apr
2016
7 Apr
'16
10:50 p.m.
Hey folks,
I've been on a tear trying to resurrect old projects here, and next on
the list is an ADM 3a.
This particular 3a has no horizontal scan. None at all. I just get a
vertical line down the center of the screen. First thing I checked was
the horizontal deflector on the yoke, which seems secure.
Before I go diving into the schematics, I figured I'd ask here: Is
this a common failure mode? Has anyone else experienced no horizontal
scan on a 3a? Any tips on where to look first?
-Seth
--
Seth Morabito
seth at loomcom.com
8 Apr
8 Apr
12:34 a.m.
On Thu, Apr 07, 2016, Seth Morabito wrote:
Hey folks,
I've been on a tear trying to resurrect old projects here, and next on
the list is an ADM 3a.
This particular 3a has no horizontal scan. None at all. I just get a
vertical line down the center of the screen. First thing I checked was
the horizontal deflector on the yoke, which seems secure.
Before I go diving into the schematics, I figured I'd ask here: Is
this a common failure mode? Has anyone else experienced no horizontal
scan on a 3a? Any tips on where to look first?
Is this the same one that was showing all exclamation points on every
other line?
--
Eric Christopherson
4:49 a.m.
I've been on a tear trying to resurrect old projects here, and next on
the list is an ADM 3a.
This particular 3a has no horizontal scan. None at all. I just get a
vertical line down the center of the screen. First thing I checked was
the horizontal deflector on the yoke, which seems secure.
Before I go diving into the schematics, I figured I'd ask here: Is
this a common failure mode? Has anyone else experienced no horizontal
scan on a 3a? Any tips on where to look first?
I have an ADM 5 (which might have the same monitor circuit) which did something
like this but the vertical line faded out after a while. There was a bad joint
on the little driver transformer just ahead of the horizontal output transistor.
I guess the joint was managing to make intermittent contact allowing EHT to be
generated and the CRT capacitance to be charged up for a while.
If your vertical line is not fading out, EHT is being generated which rules out
pretty much everthing (unless it's an unusual monitor circuit) except the scan
coils (unlikely), connections to the scan coils and any correction components
in series with the scan coils.
Regards,
Peter Coghlan.
10:41 a.m.
This particular 3a has no horizontal scan. None at all. I just get a
vertical line down the center of the screen. First thing I checked was
the horizontal deflector on the yoke, which seems secure.
I am going to assume this is a conventional design, I would be very
surprised if not. In which case, to get anything on the screen the
horizontal output stage must be working, as the CRT electrode
voltages come from the flyback transformer.
Normally the horizontal deflection yoke is capacitively coupled to the
horizontal output stage (often straight off the collector of the transistor).
There may be a width control inductor and a horizontal linearity inductor
in series too. So it's about 4 components (yoke winding, the 2 inductors, the
coupling capacitor) and the interconnections to check.
If the coils (yoke and inductors) show DC continuity then most likely they
are OK and in any case you would get some deflection. Most likely, actually,
is the coupling capacitor. It will be non-polarised (maybe plastic film, maybe a
reversable electrolytic) and a fair voltage rating. They do fail...
-tony
1:24 p.m.
OK, I made a very interesting discovery today.
When I'm working on a project on my workbench, I normally have it
plugged into a Variac. This ADM 3a is no exception. I bring up voltage
slowly on any unknown old electronics, and then let it sit at our
normal wall voltage - which currently reads 121V.
Today, for whatever reason, my Variac was turned down to 110V, and I
didn't notice. I turned on the ADM 3a, and... it's fine. It beeped, it
has a cursor, and scan looks perfect.
I turned the Variac slowly up to 121V, and at around 118V the 3a
goes nuts and loses horizontal scan, I just get the vertical line.
So, some part is really marginal, and I need to figure out which one
it is. What fun!
-Seth
2:33 p.m.
Today, for whatever reason, my Variac was turned down
to 110V, and I
didn't notice. I turned on the ADM 3a, and... it's fine. [...]
Fascinating!
I have trouble imagining what sort of fault could cause those symptoms.
I would have agreed with whoever (tony, I think) said that if you have
HV then the horizontal scan is almost certainly operating in some
sense, but this makes me doubt that. I have trouble coming up with a
failure mode in that design that could kill the horizontal scan but not
the HV based on the power supply input voltage, so there is probably
something very unusual in there.
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2:59 p.m.
* On Fri, Apr 08, 2016 at 02:33:25PM -0400, Mouse <mouse at Rodents-Montreal.ORG>
wrote:
It IS odd.
One more observation: Two of the three +5V regulators cut out
when their input voltage exceeds about 11.5V. The input voltage
comes from a 9.4VAC winding on the primary transformer. Input ripple
is VERY low, maybe 200mV. So, I suspect those regulators need to be
replaced.
I'm sure it needs to be recapped, too, so I'll do that before I
investigate too much furtther.
-Seth
--
Seth Morabito
seth at loomcom.com
Today, for
whatever reason, my Variac was turned down to 110V, and I
didn't notice. I turned on the ADM 3a, and... it's fine. [...]
Fascinating!
I have trouble imagining what sort of fault could cause those symptoms.
I would have agreed with whoever (tony, I think) said that if you have
HV then the horizontal scan is almost certainly operating in some
sense, but this makes me doubt that. I have trouble coming up with a
failure mode in that design that could kill the horizontal scan but not
the HV based on the power supply input voltage, so there is probably
something very unusual in there. 
11:47 p.m.
On Apr 8, 2016, at 11:59, Seth Morabito <lists at
loomcom.com> wrote:
One more observation: Two of the three +5V regulators cut out
when their input voltage exceeds about 11.5V. The input voltage
comes from a 9.4VAC winding on the primary transformer. Input ripple
is VERY low, maybe 200mV. So, I suspect those regulators need to be
replaced.
Could they be going into thermal shutdown? I don't recall whether linear regulators
from that era commonly had thermal shutdown features yet.
--
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/
9 Apr
9 Apr
11:28 a.m.
Try renewing the thermal compound.
Also, look for any really hot components on the 5V
lines. Tantalums don't always smoke.
A lot of older circuits also used a shunt resistor across
the regulator to share part of the load. These were usually
the square wire wound resisters. These often fail because
of internal corrosion. When open the regulators carry the
full load.
Dwight
4:21 p.m.
Well, since it was such an easy job, I took a chance and replaced the
two suspicious 7805 voltage regulators. The terminal works just fine
now!
http://imgur.com/a/wharu
I'm fairly surprised that the 7805 regulators died, and that they
present this symptom (no horizontal scan) upon their death. I'm sure
there are other marginal components that will need to be replaced, so
the restoration is not yet complete until I'm confident I'm not
stressing the new 7805s too much.
-Seth
--
Seth Morabito
seth at loomcom.com
5:51 p.m.
On 04/09/2016 01:21 PM, Seth Morabito wrote:
I'm fairly surprised that the 7805 regulators
died, and that they
present this symptom (no horizontal scan) upon their death. I'm sure
there are other marginal components that will need to be replaced,
so the restoration is not yet complete until I'm confident I'm not
stressing the new 7805s too much.
Good job!
You might check to see how much current and the input voltage of the
7805 is being fed. Too high a current or input voltage can cause the
regulator to run hot. There are some remedies for that, such as
replacing the regulator with a "simple switcher" type regulator module.
--Chuck
3032
days inactive
3033
days old
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