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Satellite Workshop 36


Amstrad SRD400

Why do public houses have the oldest systems? Maybe because the SRD400 has been a reliable workhorse and publicans know how to be careful with money! Bert Wetherill who runs the "Horse and Hounds" was no stranger to thrift.
"It won't cost more than a tenner?" he told me, forcefully. "'Cause I can get another at the car boot sale on Sunday. Only it'd be real nice to have it back fer the match tonight. Plus, you don't want the price of your pint to increase, do you?"
This last statement was made without a smile so I decided to get on with the repair immediately!
The apparent symptom was low LNB voltage, switching between 10.6 and 13.4 volts on my meter. Of course I wasted half an hour messing with the regulator circuit before realising there was a dry (corroded) joint on the big 4700uF electrolytic. If I'd plugged the TV in I'd have seen the hum bars
immediately - but I'm far too clever for that! I had to measure the blooming voltages first and confuse myself by assuming I was measuring smooth d.c.
I've known other competent repair men to be caught out by supposedly "low secondary voltages" on power supplies. I always recommend using an oscilloscope to check the power supply outputs because this will instantly show if the voltage is regulated or has sawtooth spikes! On day I'll take my own advice.
An additional problem was evident; although the channel number changed as I pressed the "up" button, the actual picture didn't. It alternated between just two programmes - one on vertical and one on horizontal polarisation. Measurement indicated that the regulated 27 volt supply was only giving 3 volts. Replacement of the 2SK301 and the AN431 cured this problem.

Flashing lights GRD150

"The power went off and now the red light is flashing. I'm afraid to turn it on in case it goes bang!"
Now, Jimmy is a timid person at the best of times. He's been a gardener all his life and still prefers gas mantles to electric lights, so his fear was understandable. However, all modern Grundig-based receivers and "BT" receivers flash an LED when first connected. It's there simply to remind you to set the internal clock in case you decide to use the timer function for recording a programme. Even the old SRD510 flashes its lights after a power cut if the timer was set. It never ceases to amaze me how many dealers phone me about this type of "fault". Am I the *only* person in the world who reads the User Manual???
I stuck a piece of black tape over the offending LED and handed the receiver back to Jimmy. He doesn't own a video recorder so he wouldn't be setting the timer.
"Thanks, Mr Armstrong. I'll cut your grass, shall I?"

Pace PRD800

This unit arrived by carrier. I do contract work for a chain of public houses and receivers often arrive unannounced! The decoder message bars on screen were brilliant white without any text, although the decoder worked perfectly. Replacing the graphics generator, U27 (TCE-VCT01) had no effect. According to a helpful young man at Pace, the message graphics are inserted onto the video signal in U19 (4053). In this particular case, U19 pin 1 was permanently high at 12 volts. The IC was faulty and replacement effected a cure. Even the lads at Pace were surprised as this is not a fault that had been reported previously. By the way; if you use Pace's friendly help line and subsequently find the cause of your fault, please take the trouble to fax them with details of the original symptoms and your cure. This helps everyone.

Amstrad SRD2000

This receiver appeared on the market as a direct rival to the MSS1000 from Pace. It has similar features in a slightly smaller box and the price was somewhat lower. Recently, the price fell even further and you could buy one for the price of a PRD800. As with anything, you get what you pay for. The reliability, in my experience, is not too bad but, if the receiver *does* fail, some spare parts are difficult to obtain. For example, IC602 (labelled "AMS42577") controls the LNB voltage and is "unavailable" from CPC as a spare part. Strange because Pete Gurney tells me it's just a standard LM2574N switching regulator!
But I digress. When Betty from the cake shop demonstrated her receiver to me, it was clearly not well but nor was it a power supply fault. The picture was dull and would occasionally go even more dull or flash bright horizontal lines then give a blank screen. It looked very much like the "Q105" symptom in later Pace PRD receivers so I rubbed my hands together and thought "goody".
Two hours later I was less confident. The oscilloscope showed that the baseband coming out of the tuner and entering TR5 was "jittering". Both the dc level and the peak amplitude were varying. Scraping my plastic toothbrush handle inside the tuner would correct the fault for a few minutes but then it would return. I got out my magnifying glass and resoldered a few joints as a token gesture but, it seemed to me, that a semiconductor was failing.
I replaced the tuner but it made no difference. Then I noticed a tiny connection on the circuit diagram which took the tuner baseband output straight to... where?
Straight into the decoder, in fact, but the decoder circuit is not shown in the service manual. So, the problem was definitely in the decoder, which meant that the receiver was a write-off as I was not prepared to spend any more time on it. Sometimes you just have to cut your losses. Knowing when to stop is all part of running a business. I'll find Betty a good second hand Pace receiver instead.

Grundig GRD150

During the warranty period, the Grundig receivers apparently had an incredibly low failure rate - but they are making up for that now!
When Derek, a local installer, brought me a GRD150 and described the fault as "horizontal line on picture" I had my suspicions that he was describing a "hum bar" and plugged the mains cord in at arms length.
A horrendous buzzing noise errupted from the power supply so I pulled the plug, quickly!
I didn't need a magnifying glass to see that C201, the 47uF/400v capacitor, was bulging.
"Wow" Derek breathed. "It was giving quite good pictures before it cooled down. Just that line across and occasional scrambling. Is it repairable?"
I assured him that it was and quickly replaced the offending capacitor with one that I'd robbed from a "B.E.R." Minerva, earlier. The result was perfect pictures and no more buzzing. Derek paid up with good grace and drove off to fleece his customer.

Grundig GSR1 mk 2

This is a cheaper version of Grundig's GRD150 but uses a different board from the "Omni" chassis used in the Minerva and Matsui variants. Last week I saw my first one and I am still puzzled by the fault. Both red and green LEDs lit permanently and the only response to button presses was a slight dimming of the LEDs. On removing the cover I could see that ALL the large electrolytics were bulging. This smacked of power supply output voltages having gone too high so I replaced all the faulty capacitors then applied power while monitoring the 5 volt output. It was perfect but the symptoms remained the same. I put the lid back on, quickly, in case I was tempted to spend more time on it. The customer had set a 35 limit on the cost so now was the time to give up! It seemed to me that some of the ICs had been damaged by the overvoltage. If anyone has come across this problem in a Grundig receiver, I'd be interested to know the cause. There was no obvious fault on the primary side so why had the output voltages risen so high that the electrolytics had been damaged! (I suspect foul play).