Satellite Workshop 27
According to the nice blonde girl who brought it to me, this Nokia
receiver exhibited a blank screen when warm and had been "looked at"
by Wossname up Church Street. Fearing the worst I began to remove the
screws but the young lady interrupted me:
"He said there's nothing wrong with it," she volunteered, "so he
didn't even look inside it. But it's still faulty."
I put my screwdriver down and breathed a sigh of relief.
"In that case, Madam, it will be ready this time tomorrow," I
smiled.
Must be getting old. A few years ago I wouldn't have let her go as
quickly as that. Still, I had work to do. I left the SAT800 on the
soak test bench while I started on a Toshiba TS-540.
The TS-540 receiver is actually a Pace MSS500. This particular one
had a very dim vacuum fluorescent display. Replacing C2 (a 22 F
electrolytic) on the display panel with a 1 F/50v multilayer ceramic
capacitor, as recommended by Pace, soon had it working brightly. In
fact, nowadays, I cheat. Removing the display panel without cracking
it in half is a delicate task so, instead, I solder the new capacitor
across the pads on the back of the board. Since the electrolytic
fails open-circuit, I see no problem with this "bodge." It saves time
and avoids calamities. The trick lies in knowing the exact position
of C2.
With the LNB connected, the receiver was showing bad black and white
sparklies on several channels. This effect is quite common with some
Pace tuners when the receiver has been in a cold place for a time.
However, warming the tuner module with my (sorry, her) hairdryer made
no improvement this time. Even setting the AFC to manual failed to
remove all the sparklies. I decided that the tuner was faulty.
With a new tuner, 221-2178012, fitted, the AFC worked nicely and
pictures were free from sparklies. However, the picture looked
decidedly streaky and, after half an hour, developed very strong
broken white lines across the picture. I suspected that this might
happen when I saw the colour of the power supply electrolytics. I
dismantled the receiver again and replaced ALL the electrolytics in
the power supply area. Experience has shown me that this is quicker
than trying to find the two or three actual culprits. Conveniently,
the parts are available in the SatCure "Relkit 6" reliability kit
that contains the correct 105 C low ESR and Ultra High Reliability
capacitors which Pace recommend.
The final invoice was rather high compared with, say, a PRD800 repair
and the owner declined my offer to fit a miniature cooling fan. I
didn't mind. "See you again in twelve months" I said cheerfully to
the owner who is one of those people that likes to keep his equipment
nice and warm.
Meanwhile, the Nokia receiver had been warming up nicely and the
picture had vanished. The sound was still there (my TV doesn't mute
with no video) but the screen was blank. Out came the hair dryer and
freezer spray and quickly located the culprit as a surface mount
transistor RE101.
Since I don't yet have a service manual for this receiver, I measured
the transistor and determined that it was a PNP device. A BC856B did
the trick and, I hope, will last at least as long as the original
one. Next day I told the young lady to keep it cool and she told
me to "stay cool." Strange because I wasn't even wearing
my pullover!
(Note: use picture on floppy disc showing location of
Q58)
Why do people put their receivers inside closed
cabinets? The man who brought this Pace receiver to me said he was a
"computer engineer". I think he had something seriously wrong with
his firmware because his receiver was, apparently, installed in a
narrow gap inside a "Hi-Fi" cabinet, on top of a video recorder with
a D2Mac decoder on top of it!
Now, surprise surprise, the decoder was intermittent "except with the
contrast set at 8" he told me "and even then it comes and goes".
Well, the receiver won't usually work with the contrast set at
maximum so I guessed that the video level must be low.
Since the fault was now permanent, it took very little time with my
oscilloscope to trace the problem to surface mount transistor Q58. A
BC856B cured the fault. Must remember to order some more!
Interestingly, I get lots of calls from repair "engineers" who
complain of decoder faults, low video levels, interference from power
supplies and so on; but, when I ask them how far they have got in
tracing the problem with an oscilloscope I hear ...
"oscilloscope?"
I'm beginning to think that I am the only person in the world who
even owns one, let alone knows how to use
one! Maybe I should set up a basic training course?
I rang the "computer engineer" owner and explained the overheating
problem to him. These receivers will run happily for years if
installed on an open shelf but, inside a cabinet, they really need a
fan. Unfortunately, he was not won over by my salesmanship and
informed me that he'd already found the supplier of the fan kit "at
the web site" and would order it himself. Damn these experts!
(Note: picture on floppy disc showing location of
C41)
"I've written my diagnosis on the label" said my next customer.
"That's helpful, I said,but I can't read it and i"d prefer to know
the symptoms.
"Symptoms? Ah! Symptoms. I didn't get where I am today without
knowing how to describe a symptom. It's the tuner. It's faulty."
Now Doctor Smith is getting on a bit and I didn't like to press him
for more information so I filled in the ticket and handed him the
receipt.
The SR950+ is actually a rebadged SRD700 sold by Amstrad. Tuner
faults are usually BAD NEWS because the tuner module includes most of
the video and audio demodulation circuitry and is, therefore, very
expensive.
I feared the worst when I connected the LNB cable and saw very
sparkly pictures. I tried the tuning mode but that did not improve
matters at all.
Rather than condemn the patient outright, I decided to have a play to
see if the problem was caused by a faulty electrolytic. Inspection of
another dead tuner showed that C41 was close to the IC and the most
likely to suffer from excessive heat. Since removal of the tuner unit
takes about half an hour I conected a new 10 F electrolytic across
the appropriate pads (see picture) and switched on. Amazingly, my
guess had been correct and the picture was perfect! I phoned the
owner immediately:
"Hello, Dr Smith. The patient is cured. Yes, I know, modern
technology is wonderful. No, the prognosis is good, provided that you
keep her cool. No, you won't need an ambulance. Just bring your car
and, erm, don't forget your cheque book."