Items Covered This Month Ingersoll Rand SCD25EA compressor. Weconic VX-4270 4-channel car amplifier. Akura CAN1 3-1 stereo system. Clarion RT-4042B car stereo receiver.

The Christmas holiday season can bring on some weird repairs, especially when combined with hot summer weather and high humidity. In my case, there was the added complication of purchasing and moving to a new location. The exquisite bad timing of this move during the busy period was bad enough. However, you would have thought that not many people would be watching TV during the non-ratings period and hence there would be less repairs.

Not so! – there is something very fishy about the logic involved with TV programming over Christmas!

As I said last month, I’ve just returned from a fabulous holiday around the world. This coupled with the onset of the festive season and my move has disrupted my normal routines and I’ve found it difficult to get back into the swing of things. To make matters worse, several unusual repairs came in but at least they break the monotony.

The first unusual repair I had came about when the painters wanted to use my compressor to blow away the dust inside the new premises. Now my compressor is getting on a bit – it’s about 13 years old and all it’s ever used for is blowing dust out of VCRs and TV sets.

In greater detail, the compressor is an Ingersoll Rand SCD25E8, which is a portable 50-litre electric motor belt-driven model. Unfortunately, the extra work load imposed on it by the two painters turned out to be too much for it. They had used it non-stop all that first day to blow out the sawdust from what used to be a cabinet maker’s workshop and, according to one of the perpetrators, the motor "just stopped and smoke poured out of it".

Of course, they had switched it off immediately – or even sooner!

The job, of course, was uncompleted and this was a major blow with so much sawdust to get rid off. By the time I got there to check it out, the compressor had cooled and so I plugged it in once more to confirm the symptoms.

Now let me tell you that I am the first to admit that I know nothing useful about motors and so I reluctantly carted it off to the nearest authorised service centre for a repair quote. Two days later, I phoned them to find that it would cost in excess of $350 to fix as the motor was burnt out.

I was aghast at this dreadful news and – just like some of my own clients – blurted out stuff along the lines of "I can get a new one for that!" I was politely informed that oils ain’t oils and I wouldn’t get one this good for that sort of money.

The problem was I was already short of cash due to the cost of opening and decorating the new premises – not to mention actually paying for it! However, I needed a compressor right now – today – and so I went down to Repco and bought a small 25-litre unit for $200.

Well, of course, the repair centre was quite right; this poor little beast had to pump almost continuously to keep up the same airflow as my old one and it got very hot. This was only going to be a stopgap answer and I really needed to get the big unit back into service.

I called around to pick up the old Ingersoll Rand and arrived to find that the AC motor was in pieces. I was shown the armature coils which had become so hot they had melted the string that had kept them in neat bundles. The reason it had failed was that a safety cutout switch had been activated and the switch wafer had hit the metal casing of the motor and spot welded to it – probably because the thick sawdust had jammed it open.

The AC motor is an Australian-built 300W single-phase Betts Motor EP1987 15 HO13-21 (James N. Kirby) and looks well made. I don’t quite know how the safety switch works but it is either bimetallic or, I suspect, centrifugally operated.

I subsequently took the disassembled motor around to all the local electric motor repair shops to see if I could get it fixed more cheaply. However, in each case I found that it was quite difficult to get past the "expert" receptionist who to a man/woman said that the motor was burnt out, shorted and beyond repair and I would have to get a new one. The cost would be around $500 plus fitting and the shaft or pulley might have to be modified to suit the compressor.

This was all very depressing but it forced me to examine my old motor more carefully. Perhaps I could repair it myself and so I blew out the remaining dust and cleaned it. Despite what everyone had been telling me, the coil windings didn’t look that bad and the insulating shellac still looked intact.

OK, so the coil windings had become hot and melted the coil formers – but wouldn’t you if you were shorted to earth where you shouldn’t be? I decided to reassemble it to see what happened – after all, what else did I have to lose? The thing is so well made and has so many safety features that even if it did catch fire it would be confined to its metal case.

Before starting the assembly, I checked the two capacitors and the coils for shorts and continuity. Although some shops had told me that the coils had shorted, I couldn’t find any resistance between the windings. Uunfortunately, I don’t have a shorted turns tester that works at 50Hz and so I could only assume that the coils were OK.

My next problem was to reassemble the motor, which was entirely in pieces with 13 unidentified colour-coded wires to reconnect! I went back to the original repair centre and asked if they had drawn out the wiring for the motor. Although not 100% delighted to see me, they politely said they would ask their electrician – who was subcontracted – if he had drawn one and kept it! The problem was, he only came in when there was enough work and I was told to try again at the end of the week.

In the meantime, I had to work out how the safety switch fitted back in around the bearings where it had arced and welded onto the back plate. I filed away the metal to give it better clearance and removed all the welded excess metal caused by the accident.

The main problem here was how to get the safety mechanism into the correct position. It has two springs and a multiple axle system that causes it to move axially along the shaft (please excuse the layman’s terms here – if I was an expert in electric motors I would probably be a lot wealthier than I am now). Eventually, after a lot of coarse language, I managed to get the whole thing back into its case and turning freely.

All I had to do now was connect the 13 wires. There were three for the 240V input and earth, two for the reset switch, two for the starter capacitor, two for the running capacitor and two for each of the two windings. These were all connected to five studs – A1, A2, 71, 5 & 4 – on a mounting assembly connected to the cutout switch. This switch was between points A1 & 71 and A1 was permanently wired to point 5 which was all extremely confusing.

Fortunately, when I returned to the service centre, I was given the hand-drawn wiring diagram by the electrician, which I duly followed. I was still somewhat nonplussed by one of the capacitors (Plessey P419 900V DC 15μF) as its orange wires go to A1 and 5 which are permanently wired together. I can only assume that this is superfluous to requirements or that the diagram is wrong.

Not feeling very confident, I connected all the colours as shown on the drawing and with the motor on the bench and my hand on the main switch, I finally plucked up enough courage to switch it on.

The motor leapt into life and settled down to a quiet 2835 rpm purr. It was working perfectly with no distress and no heat. Then I refitted it into the compressor, connected and tensioned the belt and tried again. The whole compressor was working like before. As I write now, some six weeks later, it is still going well and had blown out all the rest of that sawdust.

I can’t help feeling that sooner or later the problem may re-occur and this time the motor will be really "cactus" but until then, every day is a bonus.

As for the repair centre, they pointed out that they would have been unable to guarantee such a repair and I fully agree with them – you can’t. In fact, they had gone to a lot of trouble for me and had correctly diagnosed what had happened and quoted a lot lower than anyone else. As a result, I will certainly recommend their services to others.

Ron’s amplifier

The next "weird" repair that came in was via a friend of my kids called Ron.

Ron loves his car, or rather, his car stereo. The car is somewhat less than average but it is one of those vehicles that "throbs" from the subwoofer at the rear and you can hear – or rather feel it – from two blocks away.

However, to Ron’s consternation one day, there was peace in his entire area as his amplifier wasn’t working any more. Apparently, I was the obvious choice to "have a go". I did explain that my dubious expertise lay more in the field of TV and video but no, as it had wires and electricity, I was definitely the guy to fix it.

The amplifier is an enormous Weconic VX-4270 which advertises its audio power output on its large heatsink as 800W. It didn’t explain what sort of watts these are and I would have dismissed them as being peak music power with a following wind but for the fact that this unit is made in Germany and is really quite heavy. I assume therefore that these are 800 genuine RMS watts (ie, 800W RMS).

It is also a 4-channel amplifier, so I would say that it probably is 200W RMS per channel into a 4-ohm load.

The symptoms were fairly straightforward, with the amplifier drawing virtually no current when connected to +13.6V. The only thing that happened was that the red "protect" LED came on. This probably meant that the power supply was being disconnected from the amplifier stages by an internal protection circuit.

Unfortunately, I knew very little about this amplifier and so I thought that the best course of action would be to find out who the agents were and try to obtain a circuit diagram. After two weeks of searching, it became apparent that there was no local agent and even searching the Internet failed to reveal any trace of Weconic. I was on my own.

Ron’s a nice guy and my kids like him, so I persevered. After removing something like 100 screws, I removed the PC board and transistors from the heatsink. The amplifier looked reasonably well made and I could distinguish four separate 12-transistor audio amplifiers and a large 8-FET power supply with a couple of ICs for protection and regulation control.

With the amplifier laid out on the workshop bench, I found that I could momentarily measure +24V and -24V rails before they decayed as the protection circuit cut in. My guess was that the power supply was probably OK but one or more of the amplifier stages wasn’t.

There were no signs of any distress on the board due to overheating, nor could I see anything else that was obviously wrong. I therefore decided that the best course of action was to disconnect each amplifier in turn from the power supply until the protection was released – assuming that the protection circuit itself was OK.

I was fairly lucky and soon discovered that it was the left rear amplifier that was causing the problem and that it was probably due to the offset sensor circuit which is activated via R363. A quick check with a multimeter in circuit failed to reveal any faulty active devices but this wasn’t a good way of checking them. Rather than remove each transistor and measure it out of circuit, I decided that it would be far quicker to replace them all one at a time.

I started at the beginning of the amplifier and after replacing seven of the small signal transistors, I finally found that Q308 (2SD600) was the culprit. This transistor is a flatpack device and its base-emitter junction was open circuit. Replacing it cured the problem and the amplifier burst into life.

Ron is now back to terrorising his neighbourhood and there is no longer any peace.

The Coca Cola can

The next unusual repair was the Coca Cola can! I kid you not.

It is 900mm high, 510mm in diameter and weighs 26kg. It is bright red with Coca Cola all over it, has two concealed doors and contains a stereo system!

The reason I reluctantly became involved was through another friend of a friend routine because no sound was coming out of the can any more. I helpfully suggested that Coke and electricity didn’t mix, but this wasn’t appreciated and the missus insisted that it wouldn’t be any trouble at all for me to look at it. Wonderful, I thought, there goes the better part of my summer holidays.

When I got it onto the bench, I discovered that it was a 1994 Akura (model CAN 1) 3-1 stereo consisting of an AM/FM stereo tuner, dual cassette deck and CD player, with a bass reflex speaker system built into each of the doors. Surprisingly, considering its age, they still make them and you
can see them on the web at http// www.akura.com.

Removing it from the can wasn’t too difficult except for the mains lead – the plug has to be removed in order to get it out. I then tested the unit on the bench and found that a low-level noise could be heard through the loudspeakers which indicated that the main amplifiers were OK. However, none of the three sources could be heard.

I ran my fingers over the main amplifier IC connections and found that this produced loud noises in each channel. Similarly, loud noises were produced when I touched the PC board tracks around the volume control but the unit was quiet as soon as I moved closer to the source switching.

The main volume rotary control (VR305) is of an unusual construction, with six in-line solder connections. Noise could be induced by touching its third and sixth connections but not on any of the others. I felt sure this was where the problem was and so I removed it and checked the connections out of circuit. I could find no continuity between these two pins and any of the rest of the control in any position.

By prising off the metal clamps that hold the shell onto the wiper board, the construction of the control could easily be seen. It is a double-gang 100kΩ potentiometer with the outer and inner tracks being the wipers and the two inner tracks being the carbon resistors. The plastic wiper former had cut through the inner and carbon tracks on the righthand side due to constant wear.

The diagnosis had been easy but where was I to get the part for a 7-year old stereo that was so unusual?

Eventually, I purchased a spherical 16mm 100kΩ conventional PC-mount ganged pot (R7612) from Dick Smith Electronics. I drilled a hole through the PC board and mounted it. The next part was to extend the splined shaft to a "D" shaped one. Fortunately, I found an old plastic extension shaft from an ancient black and white TV set and cut it to size. The whole thing fitted together excellently and worked perfectly.

The boss was exceedingly pleased with my efforts so I’m in the good books.

A tricky car stereo

I’ve mentioned before that I really don’t like servicing car stereo systems. Like video cameras they are now becoming so specialised, small and hi-tech, that it is best to stick to what one knows.

Jim’s stereo system was a Clarion model and was housed in an old Mazda bus which he used to carry children to and from school. Jim needed this fixed as it helped keep the rowdy kids quiet at the back.

This outfit was more of my vintage; like the bus – old, crotchety, and large. I mean, I could actually see the components with the naked eye – and not a microprocessor in sight.

The complete system consisted of two Clarion units – an RT-4042B receiver and PT-8039F cassette deck – and separate front and rear amplifiers. He complained that it was dead. Happily, he had taken the vital section out of the bus for me.

What landed on my bench was simply the two Clarion units. As such, it was almost a self-contained system. All it needed was a 12V power supply and a pair of speakers to become functional. The front and rear amplifiers, and their associated speakers – still in the bus – were extras.

My first problem was to work out which of the many sockets and plugs was used for what. Normally, there is an in-line fuse which indicates 12V in but this fuse was internal. The 12V supply is applied via a 3-pin plug/socket connection and the presence of the third pin – and its wiring – added to the confusion. Tracing out the wiring and working out which lead was which was rather tricky.

As I eventually traced it, a volume control/double-pole switch (S3) combination controlled both the positive (active) and negative (chassis) lines, which seemed reasonable enough. One of the three pins was the positive 12V ACC line, going to a 3A fuse, then to a choke (CH1) and finally to the on/off switch, on the main PC board. The second pin connected, via the other pole, to chassis.

But what was the third pin’s function? Apparently, this is an output connection to operate a separate device; eg, an external power antenna.

Having sorted out this much, I encountered the first real problem. The on/off switch was obviously faulty and needed replacing. It therefore seemed logical to bypass this switch and check the rest of the system first, before changing it.

With the switch bypassed, only one globe lit – the AM indicator – and I still had to guess which socket was the speaker output. There are in fact no less that six unmarked sockets – including three DIN sockets – on the back of the radio and two on the cassette deck, involving 32 possible connections in all! In the end, I conceded defeat and ordered a set of service manuals, a volume-on/off control assembly and seven replacement lamps.

Not being familiar with this system, it wasn’t easy replacing these items but the service manual at least made it possible. As it turned out, the three DIN sockets feed the front and rear external power amplifiers and the cassette deck. Another two 3-pin sockets are used to feed a stereo speaker pair from an internal audio amplifier IC (IC7) and to provide illumination and power to the power antenna.

Replacing the volume control-on/off assembly meant removing or at least loosening the top PC board and flexing it to let the old unit out and the new one in. Everything was put back and reconnected. It glowed like a Christmas tree and I connected the workshop speaker to one channel via a pin in the 3-pin socket and there was sound. Then I checked the other channel in the same way, only to find it was low and distorted.

The circuit itself consists of two tuners – one AM and one FM – either one switched in as required. Their outputs are fed to a stereo preamplifier stage consisting of three ICs (IC4, IC5 & IC6), several transistors, the treble and bass controls, and the gain and balance controls – see Fig.1.

From here the signals go to an audio output IC (IC7, TA7264), the output of which feeds one of the 3-pin sockets. And, at that point, the system is essentially self-contained. But this is not the end of the story. As well as going to IC7, the signals are also fed – via transistors Q8 & Q10 in one channel and Q9 & Q11 in the other – to the front and rear output DIN sockets and their associated amplifiers and speakers. The whole system could, in theory, supply no less than six speakers if everything was connected.

But it was IC7 that was the prime suspect now. Signals into it from both channels were normal and I could not measure anything abnormal around it. This was a blow because I hadn’t really budgeted for the cost of an output IC – let alone the time and effort involved.

It would also involve some effort, since IC7 was on another PC board underneath the controls and the top board. Once again I loosened the top board, removed the screws, stressed the metal supports and pulled out the wiring harness. Then I inserted a screwdriver and loosened the amplifier board and released the IC heatsink. Finally, after lots of wiggling, I managed to remove the board for a close examination.

What a relief – there were faulty joints on almost every other pin of the IC. I resoldered and reassembled everything and switched it on. Both channels came up in stereo.

Checking further along the line, to the rear and front amplifier DIN sockets, cleared that section and the system worked like a bought one.

Jim was pleased, the kids were quiet and I was worn out!