Recently, I had a 1977 Sony KV-9000UB 22cm (9-inch) portable TV in for repair. I really couldn't believe anyone would want to fix such an old set but it had sentimental value for its owners. They were complaining of a seized UHF tuner and so were unable to get anything on the UHF bands.

Fortunately, access to the rotary tuner was easy and after removing its cover spring, I was able to get right inside it. The bearings of the variable capacitor had seized and the resulting strain (from trying to turn it) had broken the solder joints to the mounting plates. A hot iron soon fixed this and the bearings were freed with CRC.

Now the tuner could pick up channels easily. I then had to modify the UK specification sound system to the Australian system. This meant replacing ceramic filter CF201 to convert the 6.0MHz IF to 5.5MHz. I then had to tune T213 and T211 until the sound was loud and clear and the colour was free from herringbone patterning.

When I'd finished, I was amazed at how a vintage TV a quarter of a century old could give such a good sound and picture with such a basic circuit. One wonders how long all the new technology will last?

Items Covered This Month Sony KV9000UB TV set. Sony SLV-EZ7AS VCR. Samsung SV-641 VCR. Philips VR 250, 350, 450, 550 series and JVC HR-J240 series TV sets. 1993 Mitsubishi Diva CT-29ATS(A)TX TV set. NEC FS-59T90 TV set. Philips 29PT2255/79R TV set (L01.1A chassis). Philips 29PT4873/79R TV set (L9.1A chassis). Akai CT2007A TV set. Yamaha VR-5000 100W guitar amplifier.

Philips VCRs

I've had a number of Philips and JVC VCRs in for repair. These include the Philips VR 250, 350, 450 & 550 series and the JVC HR-J240 series, etc. Philips uses the same JVC mecha-deck (PMC 0011A) in their models and they no longer service them. Instead, they offer exchange units and long warranties.

One common problem that develops in the 1994-1997 range of models is that the VCR is unable to eject the tape. Discovering the cause and fixing this the first time around was long and arduous but now I am quite adept at this repair.

What happens is that the tension spring to the change arm assembly comes off because the support bracket clip breaks on the latter. The part numbers are 4822 403 71304 (Philips) and PQ46353A-2 (JVC). You have to remove the whole deck and the cassette ejector cage assembly first before you can get access to it. You then have to remove the slide plate assembly.

That done, you can remove the change arm and swap the change gear onto the new one. Care needs to be taken refitting the spring as the new change arm doesn't look any different from the old one and will probably break again in exactly the same spot in a few years time (you would have thought they would have improved this assembly by now). Refitting the slide plate can be a bit fiddly but with gentle perseverance, it will soon all slip in with spring-loaded brakes (place in OFF position). Reassembling the rest is a breeze.

What can cause a lot of confusion is the tension spring from the take-up lever. The reason for this is that it isn't fitted on all models and yet the spring support clips are still there. This has been known to cause panic as you carefully comb the entrails of the VCR looking for the missing spring. My policy is that I haven't ever known this to break off, so if it's not there, it's not supposed to be there.

On the later JVC decks, (eg, HR-J457MS), I have also found that the drive gear axle (LP30243-001B) can sometimes break (this connects onto the cassette holder assembly).

I also had a Sony SLV-EZ7AS video (S-MECHA) with a right old mess inside, which I think can only be put down to forcing the mechanism. Anyway, the main cam and stopper, the slider and reverse brake arm were all smashed. The whole thing had to be stripped right down and reassembled and it was a good job I had the complete set of service manuals to do this.

Going crackers

A Samsung SV-641 hifi stereo VCR came in with the complaint that it was chewing tapes. This wasn't too surprising considering that I eventually retrieved a cheese cracker from within the mechanism - one to add to my collection of diamond engagement rings, false fingernails, cake, money, an ants' nest and, of course, the proverbial cockroach infestation.

When it came to testing the VCR, however, I discovered I couldn't tune it in on a TV set. In fact, it wasn't until I removed the antenna that I found the answer - it was slap bang on an SBS channel. The owner was unaware of this, as that SBS station was not available in his area but he did have a lot of patterning and interference on his VCR. Relocating it to Ch68 and using AV leads fixed this problem.

Faulty NEC

Mrs Jocelyn Gale, a charming old lady of 84, asked me to look at her 2-year old NEC FS-59T90 TV set (actually a Daewoo CP-785A) which had intermittent sound.

In deference to her age and her polite manner, I agreed to call round but I said there was a good chance it might have to go to the workshop, as it was intermittent. I told her to try giving the set a good belt when it played up and tell me about it before I called. In retrospect, this was bad advice, as osteoporosis may have done her more damage than the set.

Anyway, she told me this procedure hadn't broken her arm and did have an effect. I was delighted with this news because it was highly probable there was a crack or dry joint and I probably had a good chance of fixing it in her unit.

When I arrived, I noticed she was using headphones on the set, so as not to disturb her neighbours. Experience told me straight away that it was highly likely that the headphone jack socket had been damaged due to tension on the cord.

I removed the back and withdrew the chassis. It was soon apparent that my suspicions were spot on. I resoldered the PC-mounted socket (HP01) and also checked the audio output IC (IC602) and anything else that looked suspicious.

That fixed that problem but Jocelyn then wanted to try an infrared cordless headphone set to take the place of her wired headphones. I set them up for her and then spent some time patiently explaining that the receiver unit's batteries needed recharging when they weren't being used.

An old Panasonic

I had another 1991 Panasonic TC68A61 (M16M chassis) to attend to the other day. The owner complained of poor colour, which could mean anything.

When I got there, it was immediately obvious from the pink picture that there was no green. This fault can be caused by anything from the jungle IC to the picture tube. I started with the output transistors and measured the voltages around them and on the picture tube cathodes (M68KPH167X).

I was mostly comparing the voltages between the three colours and looking for differences, particularly on the green gun. Unfortunately, there was very little to pick, which really meant there was a problem with the tube itself. Just in case, I swapped the red and green transistors over and also tried using links to swap entire colour amplifiers over but to no avail.

The CRT analyser quickly showed the problem. Unusually, it reported that the green gun had G2 open circuit and G1 to cathode short circuit.

Apart from changing the tube (which isn't cost effective), the only other option is to try to "boost" the tube by "blasting" the cathode with an AC voltage.

I did this and was extremely happy when the green came up immediately. I put the set on soak test but then, 10 minutes later, the picture (all colours) went very dull.

Over the next couple of days, I tried again and again to boost the tube and on each occasion I was successful for up to 10 minutes before it went dull again. Obviously, when the tube got sufficiently hot, the electrodes were touching again under the influence of gravity. In the end, there was nothing for it but to return the set to the owner and advise that it be scrapped.

Dead & ticking

A 1993 Mitsubishi Diva CT-29ATS(A)TX (ATMV691 chassis) was delivered to the workshop with the complaint that it was "dead and ticking". Its owner, Mr Plumley, thought it was the on/off power switch. Well, of course he would. I mean, there are only four components in a TV set, aren't there? - the fuse, the switch, the picture tube and its valve!

After all, what else would you expect for the money?

After spending half the morning undoing the 50,000 screws that held the back on, I finally managed to remove it. Next, just to satisfy my curiosity and Mr Plumley's conviction, I measured the on/off switch and the fuse. After all, they do occasionally fail. But not in this case - they were perfect.

The power supply is a complex twin IC (master and slave) switchmode unit, designed to give good regulation of several rails in both manual and standby conditions. It also features full over-current, short-circuit and over and under-voltage protection.

So, where do you start? I measured full voltage going in and nothing coming out. There were no short circuits on the secondaries, only the usual load conditions.

I resoldered any likely dry joints but to no avail. Everything looked pretty good, so I diverted my attention to the electrolytic capacitors - and there were an awful lot of them.

There were two options here - I could either get all scientific and make lots of measurements to try to track down the faulty capacitor, or I could simply adopt a blanket approach and change them in batches. I opted for the latter.

Judging by the smell of fish as I unsoldered the first batch, I was immediately on the right track. After replacing just seven of them (C9B7, C9C1, C9C2, C9E1, C9E5, C9E9 & C9F1), the set was working perfectly. I soak tested it for a week before returning it to Mr Plumley who didn't know what a capacitor was. However, he did understand a 90-day warranty - I do hope the switch lasts that long!

Philips L01.1A chassis

I am beginning to see a few late-model 29PT2255/79R and 29PT2252/79R Philips TVs. I am talking about the L01.1A chassis, which came onto the market about 2001 onwards. The usual problem is that the set is dead due to the fact that the switchmode power supply is cactus.

The cause is not yet fully understood but this fault normally occurs during power surges. To fix it, I order and install what I call "Fred's Mod Kit", named after a friend of mine who repairs these sets under warranty. He found that eight parts usually need replacing - the chopper transistor (7521), its driver (7522), IC7520, R3523, R3530, D6523, D6524 and possibly D6525, which is not always fitted.

Of course, strictly speaking, this isn't a modification kit but only a replacement parts list. Perhaps a modification will come out in due course?

Another problem that is occurring is hum in some Philips models. One in particular comes to mind, the model 29PT4873/79R (L9.1A chassis), which can get particularly bad if placed on a resonating wooden table.

The cause is a deflection yoke with vibrating windings and the solution is to remove it and dunk it in Estapol varnish. Beforehand, it is a good idea to mask any areas that might be critical such as where it contacts the picture tube and the three rubber spacers.

The procedure should be repeated three times, allowing it to dry after each dunking before reassembling it onto the tube neck. That should stop the windings from vibrating and fix the problem once and for all.

Kong Wah TV sets

I see a lot of the Kong Wah or similar Chinese-manufactured TV sets made for Teac, Akai and others. Most problems stem from electrolytic capacitors, particularly those located near heatsinks and other heat sources.

The other day, I had a slight variation on this theme that caused me to lose even more hair. It was an Akai CT2007A and it was dead. To begin with, I replaced the main culprits - ie, C911 and C909 (47μ). That done, I got stuck into the collateral damage, namely R917 and R421 (0.68μ), plus 12V zener diode ZD401.

I now had sound and picture but unfortunately the picture was really crook! I connected a colour bar generator to the TV to try and make sense of it all. This resulted in large vertical black bars, a floating picture (intermittent horizontal sync) and colour reversal on every alternate bar.

All in all, it looked like a titanic failure of the jungle IC (IC301, AN5601K) - ie, a general flooding in all major compartments! However, before ordering, paying for and replacing this 42-pin high-density IC, I thought I would first check out some of the voltages and waveforms being fed into it.

This turned out to be a good move, because I measured just 3.2V on pin 10 (the 5.2V Vcc pin). This voltage didn't improve even after I had desoldered the IC pin. Following it back, I discovered ZD301 (a 5.1V zener) and R357 (270Ω 1W) coming from the +12V line. The latter was getting hot and it was fairly obvious that the zener was more than half dead.

Replacing it fixed all the symptoms and restored a healthy picture.

And now, here is a contribution from one of our readers. It comes from A. P. of Kuranda, Qld. Here's how he tells it . . .

Yamaha guitar amplifier

I recently started to do repairs for the local music store. So far I have had a steady stream of faulty cables, crackling volume control pots, faulty phone sockets and the like. Occasionally a more juicy morsel turns up.

The Yamaha VR-5000 100W guitar amplifier came with a brief note saying that the "power board" had "blown up". This amplifier has two fuses accessible from the rear panel. There is a 1.6A fuse which is in-circuit when the unit is set to run on 240VAC, plus a second fuse, rated at 3.15A, which is always in circuit and protects the unit when it is run on 110VAC.

In this case both fuses were intact but the 1.6A fuseholder held a 3.15A fuse. When I removed the amplifier from its case, it was clear that this overrated fuse had permitted extensive damage to occur. There were two charred areas on the PC board where 1/8W resistors had formerly resided and it was clear that a couple of 0.22Ω 2W resistors had also overheated.

I tested all the components in the power amplifier section and made an inventory. The damaged parts were the two 2SC3181 NPN output transistors (Q711 & Q713); their 0.22Ω 2W emitter resistors (R717 & R719); a 2SC1980 transistor (Q652) in the speaker protection circuitry; two 150Ω 1/8W resistors (R655 & R656), connected between the base of Q652 and the emitters of Q711 and Q713 respectively; and two 100Ω 1/8W resistors (R741 & R742), spanning the emitters of the driver transistors.

So what had caused the failure? Everything pointed to PNP driver transistor Q709 (2SC3421) but this tested OK. In the end, I shrugged my shoulders, replaced the damaged parts and repaired any PC-board tracks that had been damaged by the heat. But I hedged my bets a little and installed only one set of output transistors.

When I picked up the mains plug to insert it into the wall socket, I noticed that it wasn't the original OEM plug and that the neutral and active wires had been transposed. I made a mental note to correct this before returning the amplifier to the customer but left it as it was for the time being. Why? The active line visits both fuses and the connections are not in any way protected against accidental contact. It would actually be safer for me while I worked on the amplifier if all these connections were at neutral potential.

The switch-on was an anticlimax - there was absolutely nothing. The supply rails were showing 0V and I traced the problem back to the transformer. The 150°C thermal fuse in the transformer primary was open circuit. I couldn't easily get at it but there was electrical access to both sides, so in the interests of getting things going, I simply bridged it for the time being.

This time, when I switched on, there was again no apparent activity. However, when I checked the 1.6A fuse afterwards it had blown and so had the replacements for Q711 and its emitter resistor. So there was still a fault somewhere but at least I had demonstrated that the correct fuse protects most of the circuitry.

I decided to try the amplifier without the output transistors, to see whether the feedback was effective at keeping the DC levels on the driver transistors in check. To maintain the feedback, which is critical to the DC balance, I disconnected R721 from the output line and connected it instead to the junction of R741 and R742.

At the next switch on, I was immediately greeted by a stream of smoke coming from R741 and R742. I quickly switched off. Q709, the NPN driver transistor, was very hot but both it and the resistors seemed to have survived. Just to be sure that there wasn't a fault in Q709, I lifted one end of R713 (its base resistor) and switched on again. This time, Q709 remained cool.

Clearly, Q709 was being turned hard on. With R713 still out of circuit I measured the voltage at the junction of the two 100Ω emitter resistors. It was -44.9V. Perplexed, I checked the base voltage of Q710. It was -41.1V. -Q710 should be off.

And then the penny dropped: Q710 must have emitter-collector punch-through. I hadn't spotted it in my initial testing because I had only tested for diode action between base and emitter and between base and collector. Now wiser, I checked the collector-emitter resistance - it was a dead short in both directions.

So Q710 was dragging the output very negative. Q709 was getting hot because it was driven hard on into a low resistance load. And Q709 was driven hard on by the fully functional first stages of the amplifier, which were desperately trying to correct that
-44.9V output to match the 0V input.

Replacing Q710 brought everything back into line. I replaced Q709 too, just to be safe, since it had been seriously overheated.

I now considered what I should do about the power transformer, which no longer had its thermal fuse. I could easily fit a new thermal fuse but it would have to sit on the outside of the transformer where it might not respond quickly enough to prevent the wooden cabinet from catching fire.

This put me into an ethical dilemma. I didn't expect that the owner would be happy forking out $150 for a new transformer when the old one was working perfectly. But I also didn't want to return the amplifier in a state where it might cause a fire - even if that situation was precipitated by someone again replacing the 1.6A fuse by one with a higher rating.

Fortunately, when I contacted the owner he gave the go-ahead to replace the transformer. The replacement arrived in a few days. I fitted it, swapped the active and neutral connections in the mains plug and we were back in business.