The owner of the Goodmans, having
a technical bent had followed my instructions but had been unable
to sort out the problem. His set would go off from time to time:
it had done this for ages, but had got a lot worse lately, culminating
in it going off but not coming back on.
Under a strong light and a good magnifier dozens of cracking
solder joints were revealed. All, bar one, were, in my opinion,
just about to make their presence known. The rogue joint seemed
to be in the base-band area.. in other words it was possible
the picture had gone away intermittently leaving a blank raster.
Yes this was true said the owner, and for some time, but this
wasn't the current problem.
After a good resoldering exercise I switched on the TV. There
was a familiar rustling noise from the EHT, bringing with it
that feeling that all was well. Not for long however as within
a few seconds the set went off with a whine.
I turned it off and thought for a moment.
While I was thinking, the owner then volunteered that he'd changed
a resistor in the power supply that had looked frazzled. Maybe
this hadn't been done properly and the HT was high, I wondered.
I looked at the new resistor. It was actually two resistors soldered
in parallel and they read 500 ohms. That seemed to fit in with
what he'd said about the original.
I looked at the model number and went to look up the chassis.
It was a Decca chassis, otherwise found under the Tatung brand
name and it seemed to be a model 170 or 180. I fished out the
U-View 1987-88 circuit compendium and soon found that it was
a 180. The resistor should have been… 500ohms so that was
a red herring.
The design was pretty standard. A chopper power supply followed
by a line output stage, which produced several other supply rails,
including the one that powered the RF circuitry and the sound.
In the few seconds the set had been on there had been sound so
the line output stage had been running. As a matter of course,
almost without thinking, I'd checked the temperature of the metal
plate behind the S2000AF transistor and found this to be very
hot. In fact this had been so hot that the base-emitter junction
was now reading a lot lower than it ought. I fitted a new one
just to be safe. I then noticed that the clamping diode was dead
short so in went a new BY228 as well.
My golden rule, which I don't always remember, is to look for
distressed components before getting underway with random checks.
Better late than never, I looked around the chassis and I noticed
a distressed looking capacitor adjacent to the line output heat-sink.
It was a 35 volt 1000uF component whose sleeve had shrunk leaving
a clear eighth of an inch of can showing at the top. It's capacitance
measured OK but to be safe, as its impedance was probably way
too high, I fitted a nice new capacitor. Absolutely no improvement.
The set came on for a moment and went off with a whine, taking
out the new BY228.
Usually there are a number of common faults that can produce
this effect. One is a faulty line output transformer. If this
develops a leak between turns in the primary winding the stage
will usually try to work but the output transistor driving the
winding will get very hot because it will draw too much current.
Because this is so common the major manufacturer of these transformers
sells a tester that can carry out a check of the various windings.
The device emulates a TV set and a display shows the likely EHT
value, or if there's a fault, a code indicating what's gone wrong.
Typically, this can be a faulty primary winding or a short between
windings.
I unsoldered the old transformer and fired up the workshop computer.
Once I'd checked the pinning for the transformer from the CD
that came with the tester I connected it and found that…
all was well.
Maybe there was a breakdown under the stress of the higher voltages
in the TV compared with the tester so perhaps I shouldn't give
the LOPT an absolute clean bill of health?
I decided to continue fault finding as there are other reasons
for output transistor heating.
Maybe the scan coils have developed a short? Some of these use
a glue that decomposes, eating away insulation until the coils
short out and draw excessive current from the output transistor.
I replaced the transformer, disconnected the coils and tried
the set. The same series of events happened again and the BY228
clamping diode bit the dust and went dead short. I fitted a replacement
and plugged in the coils.
What about the HT line? When this goes high you can get this
pattern of events. Sometimes there's a special zener diode arranged
to go dead short if it sees say 10 volts more than the design
value of HT. Not in this chassis though so I disconnected the
line output stage and connected a 60 watt lamp. This lit up and
the voltage across it measured exactly 150, corresponding to
the value in the circuit diagram.
I removed the lamp and replaced all the connections and decided
to check all the major components with my test meter. All the
diodes were OK, resistors were fine and when I removed the four
or five innocent-looking capacitors around the line output stage
they also measured OK. That was unfortunate as the high voltage
tuning capacitors can go low in value and blow up the line output
transistor but these are simple to diagnose and replace.
Next I fished out my trusty book of TV stock faults. Nothing
was listed other than things I'd already checked so I went to
the workshop computer where there's a listing of all the faults
in Practical TV from the year dot. A couple sounded interesting
and I retrieved the relevant volumes from the filing cabinet
in which resides copies from the last umpteen years.
One report sounded just like our fault. "Repeated failure
of the transistor and clamping diode". This talked about
the tuning capacitor though and these had measured OK. Another
report talked about the E-W correction circuit and the final
output transistor. I'd already checked this but just to be safe
I removed it and tested it properly. My transistor tester proclaimed
it was in fine shape so back it went.
What now? Well, when I'm at a loss as to what to do next, I usually
wheel out the Variac and slowly crank up the mains input whilst
making checks on the HT line etc. I connected everything up,
and just to be ultra safe I wound down the HT pot in the chopper
circuit. Was this a UK designed set? I mention this because the
pot needed to be wound clockwise to reduce the voltage. I slowly
cranked up the mains voltage. The HT settled at 135 volts. This
set needs it's on/off switch to be pressed to kick it out of
standby. I cranked down the mains to about 180 and pressed the
on/off switch gently. The set rustled into life and the sound
appeared. For more than a few seconds I thought it was OK then
suddenly the sound went off and it whined at me. I disconnected
the Variac and checked the output transistor and the diode. Both
dead short. By now my stock of transistors and diodes was getting
low.
I reckon it must be the transformer I muttered and looked it
up. The price was around £20. That's a bit expensive I
thought, especially as I imagined fitting it only to be rewarded
with a few seconds of operation followed by a whine. In the past
I'd discovered that this premonition invariably comes true so
I shelved the idea of a new transformer until I was really sure.
The next day I decided that logic and proper fault finding should
be the order of the day. I swept litter from the bench, which
was now getting reasonably clear since an adjacent 33 inch monster
had just been collected.
I removed the LOPT, fitted a new output transistor and a new
diode, and prepared a number of lengths of wire. I then connected
the transformer coils to the circuit board one by one. First
the primary winding. I switched on the set. Nothing happened.
At least there wasn't a whine and the HT measured 135. I reset
it to 150 and carried on… so far so good and without secondary
connections I hadn't expected to get sound. I connected the CRT
heater winding and switched on. Still nothing untoward. Next
I connected the over-winding of the primary… still OK. Things
were looking up. In this set there were only a couple of secondary
windings so I connected the last and the set came on briefly
then whined.
There was the answer. The fault lay in the winding for the RF
and sound circuitry.
What was the link to the line output stage though? It can't be
an overload from the secondary circuit because I'd heard the
sound build up… unless it was some sort of thermal runaway,
say in the audio output stage?
I examined the circuit diagram. There was a 1mH choke connecting
the secondary winding to the line output and scan coils circuit.
I removed it. It looked pristine but I've had these things develop
an internal short in the past and a few times I remembered that
the line output transistor had got very hot because of this.
I rigged up a test circuit and made some measurements. After
a few calculations the value of the thing worked out to be precisely
0.95mH. Too close to be faulty I thought, however I left it out
and tried the set.
On came the sound and on came the picture. True it was too bright
but that was just the brightness setting. Of course there was
no E-W correction and if I'd had the handset this could have
been seen from the text picture which would have had an egg-timer
shape.
I connected a pair of wires to the circuit and briefly connected
the choke. The sound and picture disappeared and there was whine
but quickly disconnecting it restored both.
I turned off the set and studied the circuit diagram. The E-W
circuit surely couldn't be the culprit? I checked the few transistors,
resistors and diodes and all claimed to be entirely innocent.
What else? The transformer secondary winding fed the RF and sound
via a diode rectifier but it also fed raw AC to the scan circuit
via the 1mH choke. This to provide pulses at the line frequency
as part of E-W correction and width control but this part of
the scan circuit is isolated from ground by capacitors and surely
would not permit what seemed to be a short circuit. These capacitors
had all measured correctly so the fault must be at the other
side of the choke. Here there was only the rectifier. This was
a BA157 and I found it sitting next to the line output heat-sink,
next to that distressed capacitor. It measured just like any
other diode but on close examination, admittedly looked a little
brown.
I removed it and fitted a new BA159, a similar device. Why was
the rating way above 1kV when it served to produce only 30 volts?
Well I suppose the line output stage may develop that sort of
pulse and the scan circuit is connected to the secondary winding.
Maybe the high voltage pulses from the scan area appear at the
anode of the diode?
I turned on the set. Up came the sound and up came the picture,
this time with E-W correction!
The diode had been breaking down but in a "recoverable"
way. This sort of failure is an "avalanche" type. In
other words the diode conducts like a short piece of wire in
the wrong direction but fully recovers once the trigger signal
has been removed, something like a zener diode. Apart from the
slight discolouration that should have indicated something may
have been wrong, it's just like an ordinary diode when asked
to do its primary job of rectifying 30 volts. Hit it with high
voltage pulses and it becomes a virtual short circuit. At its
cathode there was the 1000uF electrolytic, whose plastic sleeve,
shrunk with past heating, clearly signalling that something was
amiss. The fault had been making itself a nuisance, it turned
out, for a long time. Each time it had happened the diode's breakdown
voltage had been getting lower and lower until eventually it
had given up each time the scan pulses had stabilised and had
plonked AC on the capacitor making it very hot.
After removing all the temporary connecting wires from the LOPT
I refitted it and tested the set. After adjusting the E-W and
the width the picture was perfect.
I looked at the bench and collected the debris. There were three
duff line output transistors and five duff BY228s, an innocent
looking BA157 and a sorry looking 1000uF capacitor. At least
I hadn't bought a new line output transformer!
The moral of the story is to pay more attention to the evidence.
For instance.. why had the capacitor been getting hot, for surely
it must have been getting very hot indeed to shrink its plastic
sleeve? |
