12 Aug
2014
12 Aug
'14
8:42 p.m.
My best Commodore monitor, the 1084, went belly-up on me today. It's been
showing sings of trouble for quite a while, but this time it finally failed
over.
Nothing worse than an intermittent..
Symptom was a little non-intuitive. The video would go out, randomly, and
all at once - like flipping a light switch. But when it would return, it
returned more slowly - it faded in, as if the CRT heater were warming-up.
Now, if the heater were losing power, why wouldn't it fade +out+ as well?
But I checked, and the heater was definitely off - no glow, no voltage at
the heater pins 9 & 10.
Anyway, it turns out that this is one of those sets that draws its heater
supply from a second winding on the HV flyback transformer. After some
inspection of the board, I found that one of the flyback's soldered
mounting pins had cracked at the joint, causing an intermittent connection.
This was the cause of both symptoms - a quick cut-out of the video when the
HV disappeared, and the slower fade-in as the connection re-made and the
heater warmup lagged the HV rise.
12 Aug
12 Aug
8:53 p.m.
On Tue, 12 Aug 2014, drlegendre . wrote:
My best Commodore monitor, the 1084, went belly-up on
me today. It's been
showing sings of trouble for quite a while, but this time it finally failed
over.
[snip]
Anyway, it turns out that this is one of those sets
that draws its heater
supply from a second winding on the HV flyback transformer. After some
inspection of the board, I found that one of the flyback's soldered
mounting pins had cracked at the joint, causing an intermittent connection.
This was the cause of both symptoms - a quick cut-out of the video when the
HV disappeared, and the slower fade-in as the connection re-made and the
heater warmup lagged the HV rise.
This seems like bad-news / good-news. It sounds like the problem is
fixable.
--
David Griffith
dgriffi at cs.csubak.edu
13 Aug
13 Aug
1:07 a.m.
Oh, I thought that part was self-evident.. =)
Yes, it's repaired and back in service - time will tell if I +really+ got
it, but what I found most certainly could have completely accounted for the
symptoms.
Flyback primary opens up = video goes off like a switch; no HV = no CRT
current, no image
Flyback primary re-closes = video comes back slowly; HV is there
immediately, but image still waiting on the CRT heater to supply some
electrons
On Tue, Aug 12, 2014 at 7:53 PM, David Griffith <dgriffi at cs.csubak.edu>
wrote:
On Tue, 12 Aug 2014, drlegendre . wrote:
My best Commodore monitor, the 1084, went belly-up on me today. It's been
showing sings of trouble for quite a while, but
this time it finally
failed
over.
[snip]
Anyway, it turns out that this is one of those sets that draws its heater
supply from a second winding on the HV flyback
transformer. After some
inspection of the board, I found that one of the flyback's soldered
mounting pins had cracked at the joint, causing an intermittent
connection.
This was the cause of both symptoms - a quick cut-out of the video when
the
HV disappeared, and the slower fade-in as the connection re-made and the
heater warmup lagged the HV rise.
This seems like bad-news / good-news. It sounds like the problem is
fixable.
--
David Griffith
dgriffi at cs.csubak.edu
5:13 p.m.
Anyway, it turns out that this is one of those sets that draws its heater
supply from a second winding on the HV flyback transformer. After some
That was my first thought when I read the intial fault description : 'Does
the heater get powered from the flyback, if so, a problem with the
horizotnal output stage (or driver stage) would cause just this effect.'
inspection of the board, I found that one of the
flyback's soldered
mounting pins had cracked at the joint, causing an intermittent connection.
You were very lucky. Often dry joints around this area will end up
blowign the horizotnal output transsitor. If you have not already done
so, I would resolder _everything_ in this area.
-tony
7:13 p.m.
Yup, I went ahead and re-flowed all of the pins that mount the flyback,
plus any large / heavy parts, power resistors, all of the connectors -
basically anything doing double-duty providing both mechanical and
electrical security.
But how could that bad joint blow the horizontal output? I don't know squat
about video circuitry, just never got into it.
On Wed, Aug 13, 2014 at 4:13 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Anyway, it turns out that this is one of those sets that draws its heater
supply from a second winding on the HV flyback transformer. After some
That was my first thought when I read the intial fault description : 'Does
the heater get powered from the flyback, if so, a problem with the
horizotnal output stage (or driver stage) would cause just this effect.'
inspection of the board, I found that one of the
flyback's soldered
mounting pins had cracked at the joint, causing an intermittent
connection.
You were very lucky. Often dry joints around this area will end up
blowign the horizotnal output transsitor. If you have not already done
so, I would resolder _everything_ in this area.
-tony
7:49 p.m.
On Aug 13, 2014, at 7:13 PM, drlegendre . <drlegendre at gmail.com> wrote:
Yup, I went ahead and re-flowed all of the pins that
mount the flyback,
plus any large / heavy parts, power resistors, all of the connectors -
basically anything doing double-duty providing both mechanical and
electrical security.
But how could that bad joint blow the horizontal output? I don't know squat
about video circuitry, just never got into it.
The inductive "kick" from the monetary disconnection can be really
severe in a flyback, especially for the HOT (which is driving an
awful lot of current). Often times it only takes a few arcs to
destroy the thing. Flybacks and HOTs are usually replaced in pairs
because one usually kills the other.
- Dave
8:13 p.m.
Hmm.. but.. if both the HV (25K anode) & LV (2.5V heater) secondaries share
the same primary, then where is the HOT in the circuit? There aren't many
xistors that will take 25K, so it's got to be on the low side, right?
I've deposited a copy of the service manual w/schematics at
http://nerp.net/~legendre/1084_schematics_NTSC.pdf if you'd be willing to
point out a thing or two. Dog knows, I need to learn! I always thought the
HV just provided a (static) acceleration bias for the CRT.
I believe the relevant sections are on PDF page 28.
On Wed, Aug 13, 2014 at 6:49 PM, David Riley <fraveydank at gmail.com> wrote:
On Aug 13, 2014, at 7:13 PM, drlegendre .
<drlegendre at gmail.com> wrote:
Yup, I went ahead and re-flowed all of the pins
that mount the flyback,
plus any large / heavy parts, power resistors, all of the connectors -
basically anything doing double-duty providing both mechanical and
electrical security.
But how could that bad joint blow the horizontal output? I don't know
squat
about video circuitry, just never got into it.
The inductive "kick" from the monetary disconnection can be really
severe in a flyback, especially for the HOT (which is driving an
awful lot of current). Often times it only takes a few arcs to
destroy the thing. Flybacks and HOTs are usually replaced in pairs
because one usually kills the other.
- Dave
9:20 p.m.
On Aug 13, 2014, at 7:13 PM, drlegendre . wrote:
Hmm.. but.. if both the HV (25K anode) & LV (2.5V
heater) secondaries share
the same primary, then where is the HOT in the circuit? There aren't many
xistors that will take 25K, so it's got to be on the low side, right?
The primary doesn't operate at 25kV....the anode secondary winding steps it up to that
voltage. The HOT operates at B+ which in this case is around 125V. It is TS467 in the
schematic.
14 Aug
14 Aug
11:31 a.m.
Ok I think I see what's up. The FB primary isn't driven from line-derived
AC - it's driven by 125V DC, modulated by TS467, and I'm guessing at a
frequency equal to or some multiple of the horizontal scan frequency? Did I
get that right?
See and I always thought that the HV just provided a simple DC bias,
because that's the case in oscilloscopes - which are about the only 'video'
hardware I have any experience with.
On Wed, Aug 13, 2014 at 8:20 PM, Geoff Oltmans <oltmansg at gmail.com> wrote:
On Aug 13, 2014, at 7:13 PM, drlegendre . wrote:
Hmm.. but.. if both the HV (25K anode) & LV
(2.5V heater) secondaries
share
the same primary, then where is the HOT in the
circuit? There aren't many
xistors that will take 25K, so it's got to be on the low side, right?
The primary doesn't operate at 25kV....the anode secondary winding steps
it up to that voltage. The HOT operates at B+ which in this case is around
125V. It is TS467 in the schematic.
7:14 p.m.
Ok I think I see what's up. The FB primary isn't driven from line-derived
AC - it's driven by 125V DC, modulated by TS467, and I'm guessing at a
frequency equal to or some multiple of the horizontal scan frequency? Did I
get that right?
It's the horizontal scan rate. I mentioend in anotehr psot the reaso nfor
all this...
See and I always thought that the HV just provided a simple DC bias,
because that's the case in oscilloscopes - which are about the only 'video'
hardware I have any experience with.
It does. There HV spike from the flyback trnasofrmer is rectified (often
therectifier is uncapsulated inside the flyback transformer [1]) and
smoothed (the cpacitor may well be internal and external conducive
aquadag coatingson the CRT flare). the CRT runs at DC.
[1] wWIth colour CRTs a voltage multiplier circuit is not uncommon.
There is noreaso nat all why the CRT anode votlage could not be provided
my a mains-frequency [2] trnasofmrer, rectifier and smoothing capacitor.
The problem was that hthe mains trnaformerwas large and heavy, the supply
was downright lethal, and thatenergy was wasted in the horizotnal output
stage during flyback that might waswell be put to use.
[2] The term 'line frequency' is one that has differnet meanings on the 2
sides fo the Pond. To you it means the frequency of the power line
(mains). To me it means the horizontal scan requency (the horizontal
cirucit is the 'lien circuit'becuase it produces the scan lines.
OK, what about using an HV windign on an SMPSU trnaformer to produce the
HV? It's been done.One manufacturer, I forget who (Sabs?) had soemthing
they called hte 'IPSALO' circuit. It stood fro 'Integrated Power Supply
And Line Output'. The chopper trnansformer was the flyback trannsformer.
It was powered from rectifiend mailsm the chopper ran at hteh orizotnal
can frequency and was synchronised to it. The PSU choopy transofer had
the HV widnigns, drove the dlefieciton coils, etc.
-tony
6:40 p.m.
Hmm.. but.. if both the HV (25K anode) & LV (2.5V heater) secondaries share
the same primary, then where is the HOT in the circuit? There aren't many
xistors that will take 25K, so it's got to be on the low side, right?
The HOT is the transsitor that deives the primary winding. Rememebr a
transformer cna't work on steady DC, it needs a chaning primary currnet
ot work. hThe cirucit is powered form a DC supply rai, the hOT is the
chopper trnasisotor (yes, this is essentially a specialisd SMPSU) that
interrupts the transofrmer primray current.
The idea goes back to vlave (tube) TVs. The original horizontal
defleciton stage used the fact that applying a constant voltage aross an
inductor will cause the current (and thuse the magnetic field) to rise
linearly with time. So far so good, this will sweep the beam across the
CRT. But at the end of the line, you wanto suddenly reduce that currnet
ot zero. Now, you have sotred significant energy in the magnetic field of
the yoke (and in the case of valves, matchign transformer), what do you
do with it?
Wellm, the drist ieea was just ot dame it with a reissotr, and waste said
energy. Then Alan Blumlein (I beleive) realised you could step up the
votlage produced when the circuit was suddenly opened by the output
valve (now a trnasisotr, of course) for the flyback period, by putting extra
widings on the transofermer. And thus get the EHT for the CRT. Note that
when he thoguht of this, SMPSUs were not really even thought of.
-tony
6:27 p.m.
But how could that bad joint blow the horizontal
output? I don't know squat
about video circuitry, just never got into it.
Well, there are 2 main possibilites. One is that the sudden break i nthe
circuit form said dry joint causes a back emf 'spike' form someinductive
device (The flyback transformer, etc) which zaps the transitor. The
second (and more common)_ problem is tha tthe flybak is resonated to
(normally) 3 thimes the horzontal scan rate by an exernal capacitor. IF
that oges open-circuit, the transsitor noramlyl passes too mcuh current
and fails.
-tony
3664
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11 comments
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participants (5)
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ard＠p850ug1.demon.co.uk -
dgriffi＠cs.csubak.edu -
drlegendre＠gmail.com -
fraveydank＠gmail.com -
oltmansg＠gmail.com