The observant amongst you will have noticed that my e-mail address
changed last month. This annual change helps to reduce the amount of
junk mail that I receive. It also forces readers to buy at least
*one* copy of TELEVISION each year if they want my free advice. You
don't need a computer to use e-mail, by the way. An Internet Set-top
box is available for just 399 from Satellite UK in Derby (01332
812588, fax 01332 850300). This box looks like a satellite receiver
with a cordless keyboard. It simply plugs into your TV and 'phone
line and is ready to use.
A nicely dressed lady brought me this Nokia receiver for repair.
It had "gone off" suddenly. She had taken it to Wossname up Church
Street but he hadn't managed to find the cause of the fault. However,
he HAD managed to lose two screws! On test, the channel number
appeared in the display, LNB voltage was present and on-screen
graphics worked OK. However, there was no picture or sound on any
channel, apart from an audible hiss. I deduced that the 5 volt supply
must be OK as the microcontroller was working. It was likely that the
12 volt supply was at fault. Amazing what you can deduce when your
head is kept cool by lack of hair!
I consulted the circuit diagram. According to my multimeter, 12 volts
was going into the BC327 PNP transistor, TP08, but not coming out. I
replaced this with an FXT749 that's rated at 1 Amp. It wasn't easy
because the lead configuration is different! Now the PSU was simply
pulsing. The capacitors all checked out with my "Genie" ESR meter.
After a few minutes, I traced the problem to TP13 - also a BC327 -
which switches the 28 volt supply to the decoder board. Removing TP13
brought back picture and sound and the decoder worked perfectly so I
didn't replace it. The high voltage used to be used by Sky to "zap"
out of date cards but they don't do that nowadays. How did I trace
the fault to TP13? Easy, I used my eyes. The board around it was
darker than the rest. Sometimes you don't need expensive test
equipment!
The receiver arrived courtesy of "Parcel Farce". Despite this fact,
it was intact and had been in transit for less than a week. I was
impressed. The sender had packed it in a box inside another box and
had included a two-page letter which detailed the problem. In brief,
he'd reconnected the receiver on return from his holiday and now it
simply made a ticking noise. He had enclosed a RELKIT 9 which he had
purchased but had not dared to fit.
I dared - it is simply a matter of being methodical and replacing
every part, one by one. The receiver now lit up and pictures were
excellent. Unfortunately there was no left or right audio from any
output. I was undaunted by this fact as the MSP3400 audio processor
is a common failure item. Unfortunately, replacement of this and its
associated crystal failed to produce even a whisper of sound. In
desperation I phoned Pace and a helpful young man suggested that the
3R3 resistor labelled "LK140" might be at fault. I searched in vain
for this, although I recall seeing it in an MSS500.
However, a glance at the circuit diagram indicated that R619 was a
3R3 resistor feeding 5 volts to pin 18 of the MSP3400 I.C. This
resistor turned out to be a surface mount device beneath C148. It was
open-circuit so I replaced it and was rewarded with good audio. I
measured the 5 volt rail as overvoltage here can cause the 3R3
protection resistor to fail but the measurement was 5.18 volts which
was within specification. Presumably the failure of the PSU
"start-up" capacitors had caused a surge which destroyed the resistor
but all was now well.
A stooped, bearded gentleman shuffled through the door and handed me
his Sky smart card.
"It's an invalid" he muttered. "Fix it!"
Taken aback by such a fearsome attitude I retreated quickly to the
workshop. The card worked perfectly in another customer's receiver so
I handed it back.
"That was quick. Is it fixed? How much?"
"There's nothing wrong with it, sir."
"But the telly told me 'Your Card Is an Invalid'!"
"I think you'd better bring your satellite receiver in."
"Huh? What's one of those?"
"The thing with the card slot?"
"Ah, right, yes, bathroom scales, right."
He shuffled out and returned an hour later with a Pace MSS100
receiver.
On test, the receiver gave the "card invalid" message even without
the card inserted. Sometimes this fault can be caused by the PTV113
card verifier I.C. and sometimes by the PTV115. Before "jumping in at
the deep end", however, I tried a simple test. Beneath the board I
dropped a solder blob between the pins of the card-detection switch
which is a simple leaf spring arrangement inside the card slot. The
on-screen message now gave the name of the programme and requested
the smart card. This indicated that the switch was permanently
open-circuit. Replacement of the contact assembly provided a simple
cure and, since I took it from a scrap PRD800 it cost me nothing.
"How much?" demanded the fierce little man on his return.
"Just thirty five pounds, please," I replied.
"Darn!" he muttered. "Hope it reads pounds instead of kilogrammes
now."
"Pardon?"
"Bathroom scales. Never been right sin' I got 'un."
Despite the fact that digital is "free" and analogue repairs have
decreased, there are still people willing to pay for repairs - even
if they are not necessary! I've had three SRD510s this week. The
first was reported to "ignore the remote" but I found nothing wrong
with it. The zero volt wire had been fitted; the 5 volt supply was
correct at a fraction under 5 volts; the capacitors from "Relkit 3"
had been fitted to the power supply and main board. As a precaution,
I made sure that the ground wire was still connected beneath the
card-slot board (cowboys disconnect this for some reason) and I
cleaned the power supply connector contacts.
The second one also showed no fault so I fitted "Relkit 3" and the
ground wire then adjusted the 5 volt supply with RV600. RV601 needed
a fraction of a turn to bring the 13/17 volt LNB supply back into
specification. Unfortunately, the receiver now produced perfect
pictures but no audio - apart from a loud hiss! Fearing the worst I
resoldered the TDA6160 FM demodulator IC which is beneath the board.
No better, drat!
I decided to replace it with one from a scrap receiver. To remove the
suspect IC, I removed as much solder as possible then used a
needle-tip iron to lift each of the IC's legs, one by one. A twist to
break the glue bond released the nasty little beast from the PCB. The
"good" IC had to be removed from the scrap board in a different way.
I put a knife blade beneath the IC and twisted. It broke free with a
bang, ripping tracks from the scrap board. The tracks clinging to the
IC were removed easily with the iron. I was then able to solder the
IC to the first board but - still no sound!
It took quite a while to realise that in scraping away the glue to
desolder C86, I had inadvertently broken the track that connects the
microcontroller pin 15 to the tuner via link J18. As this data line
also controls the TDA6160, it was not surprising that there was no
audio.
The third receiver had been bought in a while ago from a "digital
upgrade" customer as "perfect". I fitted the parts as above and left
it on "soak" over night. In the morning there was no picture or sound
and the LNB voltage measured 3 volts. The board around TP303, inside
the little metal screening box, was blackened. I replaced TP303, a
2SB1143, which brought the receiver back to life, but I could find no
reason for the failure.