You may recall that I repaired an SRD6 for Tom in the next town. The
symptom was low video level and no decoder messages. The cause turned
out to be transistor TV04 which is connected to what I thought was
the video level adjuster, PV01. In fact this adjuster controls the
deemphasis, as I discovered once I'd replaced TV04.
Anyway, the same receiver "bounced back" to me just two weeks later.
The fault report, unhelpfully, said "same fault again" but it wasn't.
This time the pictures were "washed out" and the decoder messages
were present but heavily distorted. The decoder was working
perfectly. My oscilloscope showed that the video level was correct
until it came out of the graphics generator (pin 8 of IV02) at which
point there should have been 2v p-p composite video imposed upon 1.5v
dc. In fact the dc bias stood at 3 volts and the video waveform was
being clipped by TV22 on its way to the TV Scart socket and the RF
modulator. Now the graphics generator is a TCE10117180 and,
curiously, I didn't happen to have one in stock. My final solution
was an empirically derived "bodge". I soldered a 330k resistor from
IV02 pin 8 to chassis. That fixed it!
According to Tom, the customer indicated his grattitude, as customers
do, by whining about being charged twice and muttering about "Trading
Standards." I really must try this myself; next time I get an engine
fault I'll take my car to the garage which replaced my exhaust pipe.
"It's the same fault" I'll say. "It makes a noise again." (Wonder if
they'll fix it free of charge?)
I've just bought ten "scrap" SVS250 receivers for spares. Of course,
I won't end up with many spares because I've just mended the first
two "dead" ones (fuse melted) and jumped up and down on the third in
frustration! Well, they are frustrating machines!
I had a letter from Peter Thorneycroft, a dealer in Telford. He
described the problems that he had with his customer's SVS250:
"The original fault was the EEprom faulty" he says. "Symptoms were
"OUT OF RANGE" bars on display, the LNB offset read 25.38 and if
corrected it couldn't be stored. I replaced the 24C04 with a used
one, fitted the SatCure upgrade kit (Relkit 17) and the customer left
happy. An hour later he phoned to tell me the receiver was the same
as before. It turns out he had used it for about 2 years via RF and
wanted to Scart it. As soon as he connected the TV Scart, the
receiver went feet up. I had previously checked it on our TV Scart
socket and it was OK. I checked his TV and there's a 12 volt output
on pin 10 of the Scart socket! The SVS250 offset reads 25.38,
indicating that the EEprom is dead once again, so I shall cut the
necessary wires before re-installation. The TV is a BEKO, by the
way."
So beware Scart connectors! I have come across similar problems with
decoder Scart connections on Amstrad receivers but never with the TV
Scart. It seems that the SVS250 has a data line conected from the
24C02 to pin 10 of the TV Scart socket.
Left to themselves, these Pace receivers will give trouble-free
operation for years but the Pace designers failed to consider the
ingenuity of the average customer who believes that equipment should
be kept hot! And, in my experience, it seems that the more money they
have the less common sense they use.
This particular receiver belonged to a gentleman whom I shall refer
to as "Lord Potts" to save him from embarrassment. He designs
clothing--or, at least, he employs others who design it. Consequently
he has, so he tells me, "a feel for quality."
The MSS1000 had been squeezed into a custom-built brick fireplace
arrangement, together with a 28 inch TV, two video recorders and a
Hi-Fi audio system. Just to ensure that there was absolutely no
possibility of nasty draughts (or "ventilation" as we call it in the
trade) the chimney stack had been blocked and the whole affair was
hidden behind velvet drapes.
The fault symptom was obvious. The audio from both left and right
channels was very faint and extremely distorted. In addition, the
vacuum fluorescent display flickered in time with the audio. Very
pretty.
Back in the workshop, I discovered that the fault vanished when the
Dolby board was disconnected. Before I plunged in head-first,
however, I tried another working Dolby board. The distortion
returned. Hmm.
I decided to replace all the power supply electrolytics since these
looked decidedly black. I see little point in trying to trace a
dificult fault when such an obvious clue is staring me in the face.
Relkit 10 contains all the high reliability capacitors required,
apart from the 100 F/400v which seldom fails. I replaced each
capacitor in turn, reassembling and testing after each change. Since
this symptom was new to me, I was interested in determining which
part or parts were contributing to the fault. In fact it was C10, a
100 f/35v electrolytic but I replaced the complete kit as a
precaution. In addition, I fitted a miniature fan kit since Lord
Potts was determined to install the receiver in its original
'oven'.
The end result was an excellent picture and sound for Lord Potts and
a nice big cheque for me!
This receiver had been "pizzafied". If you've ever left a pizza in
the oven for slightly too long, you'll understand exactly what I
mean! Anyway, it didn't light up.
I spent some time scraping away what looked like burned, melted
cheese from the board then fitted the PSU repair kit. The receiver
remained stubbornly dead until I discovered two cracked tracks near
the fuse. The power supply board is wedged quite tightly in place and
has to be wiggled and twisted for removal. I don't think I had been
heavy handed, however, (yuk! aliteration) and assume that someone
else had "had a go" before me.
With the receiver now working and giving pictures, I discovered that
there were no decoder messages. Another half hour passed while I
replaced all the decoder capacitors and other bits supplied in the
kit. Usually these kits save me a lot of time but this one didn't
seem to work until I heated the decoder board with my hair dryer. My
impression was that the board was damp so I scrubbed it with
isopropanol then dried it thoroughly with hot air. Finally, since the
PTV111 area seemed to be the most senistive to moisture, I dripped
hot beeswax around it, the bench and my trousers, leaving nice white
stains. (My wife gave me a funny look later on. I can't imagine why
they were very old trousers).
Next morning was cold and damp but the receiver worked perfectly.
The fault report from a local dealer was "dead", although the
receiver lit up when plugged in. However, my first impression was
that the tuner might be dead since I could get nothing but "snow" on
most channels. Tuning was tricky because the menus did not appear on
screen. Clearly there was more than one fault.
I fitted a second-hand tuner with no effect and scratched my head in
confusion. Flicking through the channels produced a few pictures but
only on horizontally polarised stations. Aha! The power supply noise
was switching my Universal LNB to high band. I'd seen this before and
the cure was to replace all the power supply electrolytics. I did,
but the fault remained!
Looking at the circuit, I traced the LNB supply to the tuner and
noticed that C128, a 100 F, was used to decouple the LNB supply as it
reached the tuner. Replacement of this capacitor solved that problem
and vertical channels now appeared. However, there were no decoder
messages, channel idents or menus. I guessed that there were no sync
pulses (yes, I should have used my 'scope but sometimes guessing is
quicker if you are right!)
I was right. Replacing the TEA2029C sync separator IC restored normal
operation.
Finally, for good measure, I replaced C125 with a 1000 f electrolytic
in order to get rid of some horizontal streaking on decoded pictures.
Sometimes a lot more capacitors need to be replaced to effect a cure
but I was lucky this time.