Satellite Workshop 10
House calls are not something I do as a rule, other than
occasional collecting and delivering. However, an installation
company, based 50 miles away, had fitted a motorised system which was
giving problems. They wanted me to check it out before they sent an
installer on what might be an expensive wild goose chase.
The receiver was an Echostar LT8700 - the very low-threshold version.
The dish was a 1.5 metre job with (according to the customer) a 0.6dB
LNB. The problem was very sparkly pictures, especially on "Sky"
channels broadcast from the Astra satellites. These are obviously
very strong signals and would not normally give problems like
this.
The dish was only twenty metres away so no losses there. However,
when I thought about the problem, it was fairly obvious what was
causing the sparklies and I proved it by hooking in a 250 metre reel
of cable between the receiver and the existing cable. Magic! The
sparklies on Astra disappeared.
The whole problem was caused, not by insufficient signal, but by
excessive signal. The LT8700 is an enthusiast's machine with a very
low input threshold. It was designed for seeking out very weak
transmissions and never intended to be belted by the power of four
Astra satellites. To make matters worse, the large dish, low
noise/high gain LNB and short cable run added up to a signal strength
that would have given even an old SRX100 problems.
I asked the customer which weak stations he was interested in.
"None", was the reply. "I just want the best pictures from Astra and
Eutelsat."
It seemed to me that somebody had made a poor choice of system. This
kit would have been fine for TV Mauritania which is a very weak
signal on C-Band in the U.K. but for Astra it was a definite
overkill. However, I said nothing about it to the customer and he was
happy for me to leave the reel of cable in place until I could get an
attenuator. The installer was none too happy with my call-out charge,
however, and even less so when I told him what I thought about a
company which would sell a customer a totally inappropriate
system.
A customer brought in an MSS500 which, he said, would record clear
channels but would only record scrambled Sky channels. I
asked the usual questions was he connecting it via Scart and was he
putting the Sky card fully into the slot?
His answer left me rather puzzled because he reckoned that his cat
kept pulling the card out of the slot each time he went out of the
house with the video recorder left on timer operation. I suggested
that he lock his cat out of the room but he said it made no
difference somehow the cat always got in.
He left the unit with me for testing. Everything worked perfectly but
I noticed that the slightest tap would open the card flap. I phoned
him and suggested that I would drop his receiver off on my way home
since it decoded perfectly. That would give me a chance to check his
VCR and connections.
When I arrived he insisted that the receiver should be installed on
top of the VCR inside his "Hi-Fi" cabinet. Regular readers will know
my feelings about this but I told him it was at his own risk and
plonked it inside the cabinet.
I tried recording a Sky channel, pushing the card firmly home as the
VCR began to record. On playback all was well. Then I set the MSS500
and the VCR to time-on after five minutes. With two minutes to go, we
sat back and stared. The cat was nowhere to be seen but I noticed my
customer kept glancing at the door, as if expecting the beast to
pounce in at any second.
Finally, there was an almighty "clunk" as the old VCR cranked into
life. The MSS500 responded by releasing its flap and ejecting the
card!
The proper answer was to replace the MSS500 catch mechanism. I would
do that later but I used the fault as an excuse to persuade the
customer to install the unit outside the "Hi-Fi" cabinet where it
would stay cooler. It had certainly run extremely hot inside the
cabinet and I decided to order replacement capacitors C11 and C12 as
well as the catch. Pace recommend this upgrade since extremely low
ESR capacitors are required.
I wonder how many people actually repair Cambridge receivers? The old
RD480 is fairly difficult. My first action, after removing the cover,
is always to run hot-melt glue along the ribbons on the main board.
If I don't do that, the wires snap off as I put the receiver back
together. The tuner is awkward to remove and worse to replace. I've
now managed to find a supplier of the bits inside the tuner which
fail so I can repair these but have you tried to get hold of
Cambridge spares? I've had several attempts to set up a spares
account with "Cambridge Spares and Service" (disguised as "T.A.S." at
Knaresborough), including writing letters and collaring manager, Alan
Dawson, after a seminar, but all to no avail. I get promises but no
action.
I've now given up and tell most of my customers that Cambridge
receivers use "throw-away calculator technology." Not too far from
the truth in my opinion. The ARD200 in its various O.E.M. guises is,
if anything, even worse in this respect. I could remove the tuner,
albeit with considerable difficulty, by using my Weller desoldering
station. However, after the iron broke (the second one in a year
whose end snapped off), I've reverted back to my trusty old station
made by an American manufacturer called Pace (coincidence). Armed
with this beast, I can pop the tuner out without any track damage at
all. A pity I can't buy new tuners!
A shop some miles away passes me its "impossibles". This week it was
an SS9200 which, they said, simply "ticked". On first inspection the
receiver looked untouched but in actual fact nearly every component
in the power supply primary side had been changed but what a nice
clean job! It was a pleasure to work on. The problem was caused by an
open-circuit 100 SM resistor beneath the board. Once it was up and
running, however, I noticed that this receiver produced "embossed"
characters in the on-screen displays. Instead of the usual stark
white, the channel names were outlined in black and not easy to
read.
The 5 volt supply was a little low and the 12 volts also gave a low
reading on my DC meter range. I replaced the usual culprits which
cause ripple, C21 and C25, but this made the problem worse! The 12
volt rail had now risen to 13 volts, although the 5 volt supply was
correct. This was clearly impossible so I replaced the brand new
transformer, T2, with one from my own stock. Problem solved.
It's often tempting to tell customers to take away their
water-damaged receivers, especially on a busy day! However, I
accepted this customer's excuses since it was, apparently, all his
wife's fault. She had been watering the plants on the shelf above the
TV and... some things are inevitable, really.
The receiver had been left in a nice warm room for a week without any
attempt to dry it out so I feared the worst. When I removed the cover
the board looked stained but no obvious corrosion showed and there
was no green fur of the type which thrives on animal urine (am I
allowed to write piss?)
Having ensured that there was no obvious water sloshing about, I
connected up the receiver and looked at the picture perfect! However,
the only sound available was a loud hiss. Oddly, when I probed around
the surface mount FM demodulator I.C. with my oscilloscope probe, the
stereo audio nearly blasted me off my stool. With one hand gripping
the scope probe, I tried to change audio modes; I couldn't so the FM
demod chip was obviously working only partially and my probe was
simply adding the correct capacitance to the varactor to tune in the
frequency for one mode.
I replaced the beastly FM demod chip but to no avail. The fault
remained. The waveform on the SDA line looked like - well - data, I
guess, and the clock pulses on the SCL seemed OK. However, the 4MHz
oscillator on pin 1 was decidedly low so I traced it back to the
"Nicky" chip where it was 4 volts peak to peak. Aha! Careful probing
revealed a tiny break where the track had corroded away. Once this
damage was repaired the audio was perfect.
However, when I reconnected the TV via the RF output from this
receiver, the output was clearly drifting slowly upwards. I pressed
"F" "5" on the handset and the display indicated UHF channel 66,
although the output was currently drifting gently past channel 34! I
could have doused the board with alcohol and given it the hair dryer
treatment but decided to leave it plugged in overnight to dry out
instead. Next morning all was well. The RF output was steady on
channel 66. I decided to leave well alone and instructed the customer
to excuse it on bath nights.