Satellite Workshop 07
The customer described his symptom as "no satellite pictures" after
decorators had been. This sounded straightforward but, when I
inspected his installation, all seemed to be well. The receiver, an
old Amstrad SRD400, had popped its 630mA fuse, indicating a possible
short circuit in the LNB or cable. A replacement fuse melted
instantly, confirming the diagnosis.
However, his terrestrial TV pictures were also very poor and this
brought up a suspicion in my mind. I traced the aerial and LNB cables
through the wall and went outside. They'd been swopped! The
decorators had removed the plugs in order to pull the cables into the
wall to allow the new wallpaper to lie flat. Unfortunately, they'd
made a superb job of re-fitting the plugs - on the wrong
cables.
An MSS1000 arrived "dead" and the customer wanted an estimate. At
first I thought that it was simply the front panel display not
lighting up but a quick voltage check confirmed that it was indeed
dead.
Replacing the main switching transistor with the recommended STP5N90
MOSFET appeared to be all that was needed. However, at switch-on the
power supply simply ticked, with pulses appearing on the
secondary.
This symptom is often caused by a shorted amplifier I.C. on the audio
board but disconnecting the supply to this had no effect. However, my
diagnosis was quite close since I traced the problem to D54 - the
power diode which feeds the audio board. Removal of the diode allowed
the receiver to work nicely, but without loudspeaker function. I
couldn't tell if the audio output I.C.s were damaged so I quoted the
customer accordingly and ordered the diode, fully expecting the
estimate to be accepted.
The customer complained that my price was far too high for a receiver
which cost only three hundred in the first place and insisted on
taking it away! I charged him my standard amount for an estimate and
he marched off with the receiver under his arm. I'd taken the
precaution of putting back the faulty diode but removing the mains
fuse as well.
I do a lot of repair work for local TV shops and a week later an
MSS1000 was brought in. Sure enough, it was the same one. I gave my
estimate and this time it was accepted. Replacing D54 was all that
was required - the audio board having survived intact. I still don't
know what the owner paid in the end but I bet he wished he'd accepted
the original estimate!
The Amstrad SRD510 has a download facility which allows you to
connect it to another, via a Scart lead, to transfer the memory
contents. The transfer is initiated by holding the Standby button
whilst applying mains power, when the red and green LEDs will begin
to flash alternately.
The unit which arrived at my workshop did exactly that when plugged
into the mains but I didn't need to press the Standby button. A quick
inspection revealed that someone had already "had a go" but they'd
been looking for a microprocessor fault.
Thinking that one of the two Standby buttons might be damaged, I
removed the front panel and looked. The buttons operated freely but
measured less than 1k when off! There was a lot of glue spread across
the little printed circuit board and it had gone completely black
with heat. Scraping off this carbonised glue cured the problem.
The receiver had another fault (don't they always!) The picture was
fine but the screen remained blank for four seconds each time the
channel was changed. In addition, the menu graphics were not
superimposed on the picture but on a grey background instead.
I checked the microprocessor pin 5 for horizontal sync pulses but
there weren't any. In this model, the micro "knows" a good picture is
present when it receives good, clean sync pulses but in this unit
they were not reaching it.
The synch-separator circuit is fairly simple, comprising individual
components taking the signal from a TEA2029C. The fault was traced to
TR16 but I've known the same symptom being caused by TR13 and
TR14.
This receiver came all the way from Scotland. According to the owner,
he had taken it to a local repair shop who had removed all the power
supply components, which they had found were faulty, then returned it
to him minus components and all screws and brackets! That's how it
arrived on my doorstep but the owner had done an excellent job of
packing it (not many do) so it was in perfect condition.
I acquired most of the Power supply components from David Poole* of
Davenham Satellites but one capacitor eluded us. It's a 22nF rated at
630 volts. Amstrad don't stock it but helpfully suggested trying the
"usual distributors". Farnell don't stock it; RS list it but no
longer stock it. Finally, in desperation, I "emailed" all my friends
and Martin Pickering suggested "Tardis Electronics" at 30 Station
Road, Sandbach (01270 763029). This is ostensibly an ordinary TV and
video repair shop but it is aptly named for when I visited, Terry
Boyd, the owner, showed me around a vast labyrinth of rooms
stretching far back into the building! A radio Amateur's paradise
with every component under the sun. (How about a 15,000 F capacitor
rated at 150 volts or two unused Quad II amplifiers?). Terry says he
does "mail order on request" which apparently means "if you know what
you want, we'll look for it". I noticed computer boards stuffed with
Eproms; boxes of resistors, capacitors, valves, video heads, TV
spares....even a butter churn!
But I digress. We found a box of 20nF capacitors rated at 1500 volts.
I bought the lot.The size was "extra large" but I persuaded one to
fit by glueing it neatly (well, if Amstrad can use glue...) at the
rear of the power supply and using short, insulated wires to connect
it to the board. After the parts were fitted the unit fired up
immediately and everything worked - except that the D2Mac board gave
audio, and messages on a blank screen (didn't I say that
faults come in pairs?) - I spent a long time with my oscilloscope
trying to find why no video was coming out before a friend at Eurosat
reminded me that the SRD600 model suffers from this problem. Can you
guess the (simple) cause? *
By the time you read this, "Telepart" at Wolverhampton (01902 773122)
should be supplying a PSU kit for the SRD600.
* Sadly, David Poole died of cancer in June 1997
This model has been part of a Sky promotion recently. I can see
satellite receivers going the way of mobile telephones; subscribe and
get one free!
People think that, because a receiver is low-priced, it will cost
pennies to repair. Unfortunately, nobody is subsidising the poor
repairman.
However, this one was brought to me for repair because the customer
wasn't prepared to wait even three days for the Pace warranty repair!
Well, his loss was to be my gain.
The symptom was described as horizontal streaks on the picture of all
satellite channels. Just to make it more interesting (!) the fault
was intermittent. By tapping the unit gently I was able to reproduce
the symptom. It affected only the picture from the RF modulator.
Scart outputs were fine.
By poking and prodding with a plastic knitting needle I discovered
that the track to C51 in the modulator had broken. I made a neat
repair and the customer got it back in slightly less than three days
and at a very reasonable price.
* SRD600: The colour, brightness and contrast were set to zero in the
menu!