Items Covered This Month Loewe Studio 70 (110C91 chassis) Sharp VC-A200X VCR Pye radio/cassette/CD player Teac CT-M144 34cm TV set

Did you know that Loewe is a very popular brand of TV set in Canberra? This is because our Lords and Masters have them installed throughout Parliament House which means that they must be pretty good performers.

In fact, Loewe colour TVs have been available in Australia since 1974 and are highly-respected, German-made sets with many advanced features. However, this information really has nothing to do with the Loewe Studio 70 (110C91 chassis) which Mr Canaris (not his real name) reluctantly allowed me to take back to the workshop. He initially complained of the sound dropping in and out, as well as half vertical deflection, but when I switched it on, it was dead.

"Oh yes, that too", he replied – offhandedly dismissing this all too obvious fault when I phoned him back to ask why it hadn’t been mentioned.

Fortunately, I was able to obtain a service manual for the set (they are available from a firm called Interdyne in Melbourne and cost around $40). That at least was a good starting point and when it arrived, I wasted no time in opening up the set for a preliminary investigation.

The reason the set was dead was that the power supply had blown, due mostly to the electrolytic capacitors (this set was now about 10 years old). Getting it going again involved replacing IC611 (TDA4601), the switchmode transistor, four electrolytic capacitors, R613 and the start-up PTC resistor (R622) – see Fig.1.

This done, the set came on perfectly. I adjusted variable resistor P633 (4.7kΩ) and set it for 155V at point UB, as shown on the circuit. (Note: some European manufacturers use the designation "UB" to indicate the main HT rail, while some German manufacturers may sometimes just use "U").

Unfortunately, there was some initial confusion regarding this HT rail value. The circuit involved is simple enough, though. Diode D651 rectifies the output from the switchmode transformer secondary at pin 18 and the resulting DC is then filtered by R651 and C651 (47μF) to chassis. The HT is then further filtered by coil L651 and a second 47μF capacitor to chassis to give the 155V HT rail (UB).

Although this value is clearly marked on the circuit diagram, the parts layout diagram indicated 145V at what appeared to be the same point. However, closer analysis of the layout diagram clarified this; the 145V reading was the voltage at C651, whereas the reading on the circuit indicated the voltage at C652.

But this only created further confusion. Why was the reading across the first filter capacitor less than that across the second capacitor?

The logical explanation is that the waveform across C651 still contains sufficient ripple to upset the reading. It needs L651 and C652 to produce pure DC. Anyway, the set was working for the moment and I put it aside for a soak test, hoping that the sound and vertical height faults would eventually show up.

Impatient customer

Unfortunately, I hadn’t counted on the impatience of Mr Canaris who was on the phone the very next day. I explained that I had fixed the total failure but hadn’t been able to observe the other faults he had mentioned and that obviously they were intermittent. I would need time for them to show up so that I could find and fix them.

He was very disappointed and although he didn’t actually say so, I got the impression that he thought I was incompetent. Apparently, I was supposed to wave some sort of magical wand and all the unseen faults in his set would disappear. Anyway, I told him that he would have to wait a few more days and that I would phone him when I had fixed all the faults.

Unfortunately, the message didn’t sink in because his wife phoned the next day. Once again, I explained the situation and said that I would ring when the job had been completed.

The next day, I switched it on and it came on with only half a scanned picture. I heated it and I froze it and suddenly it worked OK, before I had time to work out which component was responsible. Then Mr Canaris was back on the phone wanting to know when he could pick up the set; his wife had said it would be ready today.

Feeling somewhat frustrated with him by now, I told him that she had misunderstood me and that the set was still not ready. The next day the fault was back, so I changed the vertical output IC (I561, TDA8175). The fault didn’t show again until three days later but Mr Canaris was still phoning every day. This was a man who had trouble understanding plain English.

Finally, I decided to replace all the electrolytics in the vertical timebase. After that, the set worked well and there were no sound problems but I was not convinced that I had solved all the intermittent problems. The manufacturers and agents are generally more familiar with difficult problems, particularly intermittents, than individual servicemen, so I phoned Loewe in Melbourne for advice.

As it turned out, they were very helpful. In particular, they suggested I replace IC I441 (APU2471) and fit a special kit to the horizontal output transformer to connect the ferrite core to chassis, which might be flashing over. I had already reworked the entire chassis for dry joints.

I acquired and fitted IC I441 plus the special kit as suggested, but then just when things were looking good, the next disaster hit – the set went dead again. In the meantime, the Canaris were still phoning every day and getting very shirty, which didn’t help matters.

This time, both the horizontal output transistor T534 (S200AF) and diode D536 BY228 had gone short circuit. Replacing these restored the picture but the horizontal system was overscanning and there was no east/west (E/W) pincushion correction.

By going into the service mode with the remote control, I found that I could adjust the picture but not enough to correct the problem. I also noticed that the E/W output transistor’s heatsink was getting very hot. By now, Mr Canaris was no longer phoning me but complaining long and hard to Loewe in Melbourne, who then did their best to help me.

The voltages seemed correct everywhere and so did the waveform on pin 24 of IC I511, except for some horizontal pulses superimposed on the lower part of the parabola. I removed and checked all the transistors and replaced a number of electrolytic capacitors (including C558, C546, C583, C581, C512 and C542) but it was all to no avail. I then replaced C594, C536, C537, C538, C541 and C531 but still no joy. It wasn’t until I noticed some of the old brown goo on L538 (incorrectly marked as R538 on the circuit) that I realised I had a clue.

This large coil (1.6mH but marked 14323) measured shorted turns when checked on my shorted turns tester, so I acquired a replacement from Melbourne and fitted it. Success at last – the picture was good and the only thing left was transistor T594 (BD537B) in the E/W correction circuit, which (I felt) was running far too hot. I replaced it and its other half of the Darlington pair, transistor T593 (BC546B) and rechecked the voltages. There was 15V on the collector and 0.65V on the base. In addition, the waveform on T594’s collector was 100% correct.

I rechecked all the transistors in the E/W correction as well as all the resistors and everything was correct. In the end, I felt I had taken all possible steps to solve this problem and the only lame idea I had left was to add more aluminium to the heatsink to get rid of the heat (it was literally too hot to touch). This was done and after soak testing for another 24 hours, I finally agreed to let the set go home.

Mr Canaris complained long and hard about the service, the cost, the delay and how I didn’t know what I was doing. For my part, I was thoroughly fed up with him and did little to hide my annoyance. In fact, I doubt very much that I will hear any more about this set. There’s so much bad blood between us that even if it does fail, I’m sure he will take it somewhere else.

That’s a pity really, as I would like to know if any more faults subsequently showed up. Some customers really are their own worst enemies.

Perverse inanimate objects

I’m in a whinging mood at the moment. I don’t usually whinge, at least not in print. But fair dinkum, I’m getting fed up with components that keep giving different measurements.

During the last few months, I have been beset by several such frustrating experiences. Perhaps they are due to what an acquaintance calls "the perversity of inanimate objects". His philosophy was that some objects have mind of their own and that if you want them to behave in a certain way, they will do all they can to frustrate you.

A questionable philosophy? Well, maybe it is. But have you ever tried to fit a nut to a screw, in an awkward corner of a chassis? Or have you tried to fit a pigtail through a hole in a PC board from the hidden side?

Of course, these are relatively simple mechanical situations. It’s when these inanimate objects are part of an electronic circuit that the fun really begins.

This month, I had a Sharp VC-A200X VCR with no display. I didn’t have a circuit but I felt it shouldn’t matter as the circuit is so simple. Apart from disassembling it, it wasn’t difficult to establish that there was -28V on the segment legs of the fluorescent display panel but no filament volts on the ends.

Moving along to the switchmode power supply, I measured the voltages on each diode. There were appropriate positive or negative voltages on all diodes with respect to chassis, except for diode D921. As I quickly discovered, this diode rectifies an output from the switchmode transformer and feeds the fluorescent display.

The output from D921 is filtered by a 100μF 6.3V capacitor, C921. There was no voltage across this capacitor, the capacitor wasn’t short circuit and it made no difference when I connected another capacitor across it.

Diode D921 (FR103) couldn’t really be measured in circuit because of the low impedances everywhere, so I unsoldered one end and found that its forward resistance was too high for my liking. This looked like the culprit but when I removed it completely and measured it out of circuit it measured perfectly.

Still, I really didn’t have any other clues so I fitted another diode, a BYV96E, in its place. This immediately restored the 3.5V rail needed to drive the 3V filament and the display with the word "SHARP" came up at full brilliance.

So that solved that problem. But it really cheeses me off, having to keep remeasuring components because of the uncertainty that the first reading was correct.

So that’s my whinge for the month. I know it’s nobody’s fault and there is nothing I can do about it. And having had my whinge, I feel better already.

And now here is a story from a colleague, P. K. I’ll let him tell the story in his own words.

The Ghettoblaster

This story concerns what is often referred to as a ghettoblaster; in this case, a Pye radio/cassette/CD player with two tape cassettes. The unit had originally come in for service about two weeks previously and I diagnosed the problem as being in the CD player, which required cleaning and testing.

This time it was a cassette problem. I replaced a fuse which had blown, after which there were some signs of life. The radio worked, as did the CD player and the "A" cassette. But when I pressed the play button for the "B" cassette, everything went dead.

Closer inspection revealed that a switch associated with the play button had failed. It was a leaf switch and one of the two leaves had broken off at the base and was hanging loose. And in order to understand the implications of this, a brief description of the switch’s associated mechanisms should help.

The play button for each cassette – the "B" button in this case – activates the mechanical loading functions, moving the tape against the head, closing the pinch roller against the capstan, etc. At the same time a lever – at chassis potential – activates the leaf switch, which is suitably insulated, closing its two contacts. The intact leaf carries the 12V supply, while the broken one connects to the load; the motors, the audio, oscillator and other circuits.

But now, when the button was pressed, the lever contacted the remaining, live 12V leaf, taking it to chassis. The result was inevitable; a blown fuse.

It was a simple enough diagnosis but what could be done about it? I contacted Philips but I was advised that a replacement was not available. But even if one had been available, it would have been a major job to pull the unit apart to fit it.

My next idea was to simply bridge the two leads. This would mean that the "B" cassette drive would function continuously, while ever the set was switched on. This was not as a wild an idea as it sounds; a number of other model cassette players use this arrangement.

Well, it was worth a try. And at first I thought that it had worked. In fact it had, to the extent that the "B" tape worked perfectly. But now the "A" tape would not play – the wheels worked but there was no sound. When i disconnected the two switch leads which I had bridged, the "A" tape came good but, of course, the "B" tape was dead again.

So I could make one or other cassette work, but not both at the same time. Why? – I don’t know; it would have been too time-consuming to figure it all out. All I knew was that while the 12V rail to the "B" cassette was activated, the "A" cassette would not work.

Doubtless, given the time and enough technical backup, one could analyse the device in sufficient detail to work out how it functioned and perhaps find a solution. But, at a practical level, this approach was out of the question. Considering the age of the unit, I was beginning to fear that the customer might be forced to cut his losses and settle for only one cassette player. After all, he could only use one at a time!

Then I had another thought. Was it possible to substitute an electronic switch for the faulty mechanical one? In fact, this looked to be relatively simple.

I selected a BC327 PNP transistor as the switch, connecting the emitter to the 12V supply rail and the collector to the load. The base was connected to the emitter via a 1kΩ resistor, which would ensure that the transistor was switched off unless otherwise instructed.

Obviously, an "instruction" would be needed to turn the transistor on when the "B" play button was pressed. And this could have been tricky. Fortunately, the remaining switch leaf came into its own. It was no longer connected to anything but still made electrical contact with the lever when the play button was pressed. So the leaf was connected to the base via a 3.3kΩ resistor, applying forward bias to the base and turning the transistor on.

Did it work? Yes it did – just like a bought one! And I had another happy customer.

Spring crisis

When it is a glorious day in spring, one arrives at work feeling euphoric, convinced that nothing could possibly spoil your day.

So it was last Tuesday – the birds were singing, the temperature and humidity were just right and I was full of bonhomie when I booked in Mrs Townsend’s Teac CT-M144 34cm TV set. All that was wrong was a broken RF socket on her tuner and she still had the broken parts. It promised to be a simple fix for a simple lad on a sunny day – if only life could always be this sweet.

On removing the covers, I felt I might be able to resolder the coax socket onto the tuner in situ. Unfortunately, I soon discovered that I would have to remove the tuner and its covers to resolder the centre pin to the PC board.

I pulled the chassis out and placed it upside down, happily whistling
a little ditty while I prepared the solderwick to desolder. Mrs Serviceman wasn’t quite so happy – I don’t know whether it was because she didn’t like my ditty, because it was out of tune and rather repetitive. Or perhaps it was because I was happy and she wasn’t.

Anyway, all this was about to change because, unbeknown to me, the set had been switched on in the last 12 hours or so and when I placed the solderwick braid across the PC board pattern, there was a bright flash, a spark and a crack. Whoops! Well, there was nothing I could do until I had replaced the tuner.

It didn’t take long to do this but a degree of anxiety was creeping into me and my whistling ditty had stopped. Where had the spark come from and more importantly, what had it struck? I was praying it was just a direct short across an electrolytic capacitor but unfortunately this wasn’t the case. When I switched on there was no sound or picture.

There was EHT and voltage on the CRT filament heaters but not much other activity. Fortunately, I had a schematic diagram and I soon established that all the obvious voltage rails were intact (115V, 24V, 15V, 12V, 5V, etc). And the source of the spark was eventually traced to a residual voltage across C260, a 2.2μF capacitor associated with the video output transistors (Q601, Q602 & Q603). But it was one thing to know where the discharge originated and quite another to know what it had struck.

By turning up the screen control, I established that the raster was scanning correctly and touching the pins of the audio IC (IC205) produced noise in the speaker. At this point, I wished I had insisted on having the remote control. Because the fault had seemed so simple, I hadn’t seen the need for it when the set was brought in. Now I hesitated to ask the lady for it, in case she suspected the worst.

The front controls were having no effect and there was no effect when external signals were applied to the SCART socket on the rear; neither was there any on-screen display. By now, I was beginning to suspect that the microprocessor IC201 (TMP47C434N-R214) and/or the EEPROM had been damaged.

I checked that Vcc of IC204 (vertical output) and Vdd of the microprocessor were both getting 5V, and that crystal XT201 was oscillating correctly at 4.19MHz. I changed IC202 (TC89101P) first as it was cheaper and simpler but even replacing IC201 as well made no difference. The CRO confirmed that the video was getting to the TA8717 jungle IC (IC206) from the SCART socket but no further.

By switching the IC to the TV mode, I could also put the set into the preset tuning mode and tune stations, using the CRO to monitor the video input to the jungle IC on pin 16. But, as before, the signals were going no further. I checked the voltages to IC206 and then used the CRO to check crystals XT202 and XT204. Unfortunately, this provided no clues and it was now obvious that I had overlooked something, but what and where?

I went back to microprocessor IC201 and decided to check each pin. I discovered that even though nothing could be seen or heard, most functions were working and responding to the front controls, and these could be measured on the appropriate pins.

I finally checked pin 26, marked HD, and found nothing on it. I did not know what HD - or indeed VD next to it - stood for but they suggested horizontal and vertical pulses. I followed the horizontal circuit back to the collector of Q218 and then checked the base circuit. This was fed from horizontal output transformer T201 (pin 10) and so I expected to see horizontal pulses - but there weren’t any!

Following the circuit further, I found a branch feeding diode D233 (MTZ208) and resistor R330 (10kΩ) to pin 17 of the jungle IC (IC206) which wasn’t getting any pulses either. It took some time to follow the PC track to find where D233 was situated on the mother board but I finally found it nestled right next to connector CN203. And guess what was on pin 1 of this connector?

Yes, the 200V rail to the video output transistors (Q601-Q603). This rail is derived from pin 3 of T201 via diode D229 and my old friend capacitor C260. Obviously, the desoldering braid connection had shorted this rail directly to D233, the residual voltage in C260 instantly destroying it and turning it into a short circuit.

The next step was to identify D233 and I worked out that it was a 20V zener diode. Fitting a new one restored all the set’s functions.

Mrs Townsend was spared my anguish and so remains blissfully unaware of the trials and tribulations involved in fixing her wretched antenna socket.