Repairs are not always easy
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Being at somewhat of a
loose end, and having finally stopped my regular lift repairs
back in April this year (2024) I decided to try and fix a cinema
surround sound amplifier I'd bought a few years ago. I'd actually
got the amplifier and its remote working satisfactorily but the
HDMI output was bad. I'd searched fruitlessly for a repair manual
so had decided to just put it on one side for the moment (3 years
ago). |
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As I knew what the problem was
I needed a way of checking HDMI pictures so went off in search
of a suitable monitor. The best option was a TV I'd bought many
years ago but where was its power supply? The TV was helpfully
marked with its requirement of 12 Volts 5 Amps.. but where was
the original PSU? A 30 minute search revealed it in a box with
others, mainly 19 volt. |
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I plugged it in and found
it was dead. It actually has a green LED that should be lit when
working properly but this was not illuminated so I opened the
case to figure out why. Only two ordinary philips screws held
it together but the circuit board was encased in a wrap-round
copper clad cover. I unsoldered a pair of grounding wires (mains
safety earth and negative output), detached the cover and studied
the components. Nothing seemed amiss so I powered it up and measured
across the thick wires emerging from the mains smoothing capacitor.
Nothing registered on my voltmeter so I unplugged the IEC mains
lead. Oops... I got a shock (the case does have a warning! I
peered at the thing and realised the thick wires from the capacitor
were insulated with transparent sleeving and the capacitor voltage
was actually present (circa 320 volts hence the shock as I'd
grasped simultaneously the metal shields on either side of the
circuit board). |
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Not to be put off I carried
on fault-finding, but all the components tested OK... at least
the diodes, FET, resistors and capacitors all measured fine.
I looked at the controller chip
and spotted it was marked "UC3843". I looked at the
UC3843 device spec then checked the pins... all the voltages
seemed OK with the Vcc pin sitting at around 9 volts. As far
as I could tell the FET wasn't receiving any pulses hence the
reason for zero volts output. |
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At this point things went from
fair... to middling... then pretty ropey. The chip was an SOIC8
but my spares box had yielded only a DIP8 version of the UC3843.
Shall I order a new chip? This would mean a long wait so I decided
to remove the old chip and solder in its place the larger dual-in-line
device on the grounds that, if it worked, I'd order an SOIC8
version, knowing my time and money wouldn't be wasted.
Leaving the thicker device in-situ
wasn't an option because the board wouldn't fit in the case.
Anyway.. I fitted the DIP8 (below) and I saw 12.48 volts output
so the old chip had been faulty (not a common fault).. so I need
a new surface-mount UC3843. |
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However, logistics are
against me as a I found new SOIC chip would cost nearly as much
as a brand new 12 volt 5 Amp power supply. Although these chips
are around 50 pence each they're either supplied in tens with
high postage or at ridiculous prices unless one opts for a Chinese
supplier and a fortnights delay. So.. an option would be to try
a near equivalent or hunt through my scrap board collection.
I found and fitted a new UC3844BD1G and it tried to work but
failed because it was turning on and off, probably because it
had a different Vcc requirement. I actually tried to measure
the pin voltages but as I approached pin 4 with my fat meter
lead there was a quiet "pop" and the randomly changing
12 volt output sank to zero.
I put it on one side. |
The next day I checked
the continuity across the 320 volt HT circuit and found zero
ohms. After thinking about this for a millisecond unsoldered
the FET and found it had a dead short across all three pins.
I also found the mains rectifier bridge had a shorted diode and
the mains fuse was blown. So, instead of a new chip, I now also
needed a new FET and a new bridge rectifier.. certainly double
the price of a new PSU because of postage costs. What I needed
to do was to make a more thorough search of my scrap boards.
There must be more than 500 of these of which about half use
surface-mount components. Finding a PSU chip was made easier
because all I needed to do was to check anything using a switching
transformer. Not too easy though because chip markings are hard
to read and often truncated. My search did turn up a new FET
and a new bridge rectifier before noticing a chip marked "13844".
The FET was marked "K2645" and will be a good option
for the original 2SK2996. I hunted around and discovered the
13844 was probably an FA13844. The data sheet carried a picture
showing that its pin connections had the same names as the UC3843
which seemed promising, at least worth a try. |
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Disconnecting the replacement
parts was straightforward and I was able to fit the new FET and
the new SOIC8 without difficulty. Unfortunately the new rectifier
was a millimeter too long so I had to mount it vertically and
wire its pins to the board. I then unsoldered the old fuse and
soldered the thinnest wire I could find across it before replacing
it. Checks on the HT circuit proved the short had gone and it
was now worth plugging it into the mains.
Before going further I'll mention
a strange feature I'd discovered. Ordinarily these control chips
are initially triggered from a voltage derived from the HT supply
via a high value resistor and smoothed by a small capacitor in
the range 10 to 47uF, but this design used a strange alternative.
The Vcc pin was connected to one of the raw mains connections
at the bridge rectifier via a pair of high value resistors in
series. This theoretically supplies a rather roughish voltage
of about half the fully rectified 320 volts.
Once the control chip has started
driving the FET an isolated voltage is fed back from the chopper
transformer to maintain operation. One drawback in the odd scheme
was that my accidental short of the chip pins had destroyed the
FET, the bridge rectifier and the chip rather than much lesser
damage.
A rather funny lapse as a tail
end to the story... I connected my voltmeter to the output lead,
picked up the end of the IEC lead and plugged it into the circuit
board only to instantly see 12.37 volts output. I'd missed the
correct mains lead (unplugged from the mains supply) for one
that was already live and was rather lucky not to have repeated
the shock I'd had earlier. Anyway... all's well and I can now
(hopefully) fire up the TV to commence tests on the audio system. |
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But first I should put it back
in its case..
The TV is a Digimate LTV-1929WHTC
and a quick check revealed it supported up to 1440 lines. I started
by checking to see if i was serviceable as it hasn't been used
for over a decade. I connected a short length of wire and it
picked up digital Tv with perfect sound and vision. But could
it respond to the HDMI input driven from the Teac? The result
was a screenful of noise and a legend 722 x 481 (= Standard Definition).
I expected information much as provided my Teac AG-15D except
perhaps in higher definition, but no ... just noise at SD. Was
the HDMI feature defunct? My plan was to now first connect my
Asus Mini PC to the TV VGA input and prove that was working...
it was fine. Then to connect the Asus to one of the HDMI input
sockets and see what was sent to the TV. The answer was fairly
surprising as a display popped up with a 1920 line notification
(=1080). In fact, each HDMI socket provided the same results
but wasn't being decoded by the TV as the screen was just a mass
of lines.
This is interesting because
it indicates that the HDMI circuitry is working OK at 2K or with
HD definition. No "on-screen-display" of information
may merely be due to my lack of understanding of the Teac or
a fault. I need to study the Operators
Manual in more detail. Click to read this.
Thankfully I've now learned
enough to abandon my plans to buy a new AVD7623BSTZ
HDMI chip which are available for a few pounds but would
mean a pretty tricky replacement as it has no less than 144 pins
! What is interesting though is its TV spec extends only to 1080
lines with up to 1600 line capability and is therefore not compatible
with the 1920 image I fed into the HDMI socket. It's use is therefore
restricted to nothing better than High Definition TV and, unless
downscaled, my PC HDMI 2K output will not work with it.
Reading the TEAC manual appears
to say that the signal at the HDMI output socket will be commensurate
with whatever HDMI input source is selected. In other words the
output may well be 720 (or SD) with noise if the selected input
has no signal. I would hazard a guess that the noise shouldn't
be present but, instead a message indicating something.. mabe
"no signal" or similar? Reading the ADV7623 spec seems
to indicate that a message can be overlaid on the raster. |
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pending |
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