Items Covered This Month Akai CT2590A-T "Jet Reflex" TV set. Deitron CTV74ST TV set. Akai TV2570 TV set. Philips 25PT448A/75A PV4 TV sets (2).

Mrs Dawson is very proud of her TV set and its features. This is not just any Akai set; this is the "Jet Reflex" model (CT2590A-T) because of its "Super Bass Woofer System" and its comprehensive dubbing facilities. She also had an Akai hifi video but unbeknownst to her, she wasn’t getting the full effect of this as it wasn’t connected to the sound system via the AV leads. As a result, when she replayed it via the RF lead, she was only getting duo-mono!

Anyway, it was a rainy day when her beloved machine died shortly after its fourth birthday and I was asked to give it first aid in her home.

I hadn’t actually ever seen this model before but by good fortune, I already had an original service manual. Judging by the quality of its picture it looked as though the set was designed and built by Akai in Japan but I was surprised to discover that the set was actually made in China. I was also disappointed to find that the set was crammed inside a black home entertainment unit in the darkest corner of the room, with tight leads connected to the rear of the set.

Anyway, I persevered, straining my kryptonite vision night glasses and really struggled to get the back off. The black screws are in deep dark recessed holes in the black cabinet but I finally succeeded, to realise I had just opened a Pandora’s box.

The set was almost completely dead; it would only switch the red LED power on light to green, which would then fade off to nothing. Access inside the set wasn’t brilliant and with the way it was installed, it was difficult to get to the underside of the PC board.

But despite this, I managed to determine that the power supply was working OK and that there was 150V on R508 (interestingly this 59 cm set has a higher HT than the 68cm model, which has an HT of +135V). I also managed, again with great difficulty, to measure the 8V, 5V, 27V, 33V and 38V rails and found nearly 150V on the collector of Q52, the horizontal driver transistor. This meant that this transistor was well and truly switched off (normally it is 63V) and so there was no horizontal drive from the jungle IC (IC201).

There was only one other thing left to check before I called a halt and that was whether there was any power getting to IC201. I measured 8V on pin 35 of this IC, so that wasn’t the problem.

It was time to take this baby back to the workshop but Mrs Dawson was not happy with this news. However, I had already spent at least an hour on the set and so I insisted. When she finally agreed, I partially replaced the back on the set (I have a life – I think?) and got out as fast as I decently could.

Back at the workshop, I was able to relish what space and light were all about and could at last examine the set more carefully. Though the service manual is good, both it and the set can only be described as "bitty" – with lots of flying leads, plugs and sockets and small modules.

The main board is screwed onto one of those black plastic support brackets which always make large parts of the circuit inaccessible – usually, the very parts you are trying to investigate. In this case, there was also an annoying control board screwed onto the front of the bracket.

This was all removed until, finally, I could analyse the naked board for hairline cracks and dry joints. This also allowed me to use the CRO to check that the crucial crystal clocks - X201, X202 and X301 on 3.58MHz, 4.43MHz and 8MHz – were all working. They were which led to a quick analysis of where I was going with this.

We had all the voltages correct (well, nearly all), we had an oscillator but no drive and we had limited control (the set would switch on and off with the remote). To me, it left three major areas to investigate – the jungle IC, the micro and EEPROM and the signal paths between them, especially the I2C (Two-line Inter IC bus), SDA (Serial Data) and SCL (Serial Clock) pulses.

The latter turned out to be extremely interesting, as this is the first set I have come across that has two sets of data lines (which is surprising because I thought the whole idea was to keep it simple by having only one set). As far as getting the set going though, we only needed SCL1 and SDA1 from the jungle IC (IC201; TDA8366), the EEPROM (IC302; XL24C04P) and microprocessor IC301 (TMP87CM39N). The SCL2 and SDA2 lines control the small signal circuits only.

There is not much you can do to check data lines other than to check that they are at 5V and that there is data on them. The CRO confirmed that everything was OK here, so I then checked for continuity between the ICs and the line driver transistor (Q052). Again, everything was OK, so there was nothing for it but to replace the ICs themselves.

This also meant inflicting pain on the old overdraft, because they are invariably expensive. Basically, because I didn’t have a clue, I decided to replace the ICs in order of their expense, starting with the cheapest – the EEPROM.

This 8-pin device is an enigma – you can buy them from a couple of dollars up to over fifty dollars each for what is essentially the same device. Like so many other aspects of the devices used in TVs, I wish there was more information available on them and what their numbers mean. But basically, it is whether the EEPROM is sold preprogrammed or not that decides its cost.

I socketed the new IC and slipped it in when it eventually arrived. To my relief, this fixed the problem instantly and the set burst into life.

However, despite paying the price for what surely must be a preprogrammed IC, I still had to reprogram it. The service manual describes this as presetting the MCU option code for the E2 P-PROM.

First, you engage the TEST MODE by pressing the volume + and - button on the front panel while switching the set on. Nothing happens except that the set comes on in the Standby red LED mode. Next you switch on the power button on the remote control. Next we get a "TEST MODE" written on the screen with a list of five digits starting at 000 00.

The first three are addresses and the last two data. From a table in the service manual, the Akai CT-2590AT model requires address 3A0 to have data 3A and address 3A1 to have data CE. When you have finally learnt how to navigate this list and edit it, you find that the replacement KS24CO4P, which has a white paint spot on it, is set for the CT-2990AT model as a default, which meant changing only the 3A0 from 2A to 3A. The secret of doing all this, I find many lifetimes later, is to save the data immediately after you have entered each digit by pressing the centre button on the remote.

Not quite the end

I thought that this would be the end but wait, there is more – quite a lot more. Despite setting the MPU option for this model, I found that the geometry was incorrect, especially the east-west pincushion.

This meant finding the address for the EWPW (E-W parabola/width to you in Akai talk) which is 369 and changing it from 33 (29-inch) to 19 for the 25-inch set for pincushion, and finding address 368 and changing it from 25 to 33 for the width.

When the job is done, you have to turn the set off and on to get back to an operating mode. After that, it is just a matter of using the on-screen menus to do the channel presetting.

The set was now working perfectly – all I had to do was reassemble the boards, replace the back and return it to Mrs Dawson.

Pretty simple stuff, eh? Well, unfortunately it wasn’t meant to be. Putting the boards back together again was fine but fitting the chassis back into the front shell of the cabinet wasn’t quite so easy. The problem was the on-off power switch and its dog-leg extension and trying to line this up with the pushbutton knob in the front. After several attempts, ducking the degaussing coils and other leads, I finally got it in and checked its function before putting the back on.

To digress just a moment, the back has two red round turbine adjusting bass reflex holes which look like jet engines – hence (I assume) the name "Jet Reflex"!

Then it was on to getting the back on. It is long overdue, but everyone should give the manufacturers a treatise on how to make backs that can be removed and replaced easily. Half my grey hairs are directly attributable to the problems of poorly fitting backs. Nowadays, they are even worse because they are heavy with the subwoofers and even more difficult with their flying leads to disconnect and rejoin.

Well, I did fit the back only to then find the on/off switch wouldn’t work. It was only on my third attempt after I had lubricated the latch mechanism that I finally got the on/off switch to work. I then left the set on soak test for two days, switching it on and off regularly before arranging to return it.

Mrs Dawson was very excited that her set was back – she had been phoning every two minutes for the last three weeks to find out when it would be ready. And now it was. After climbing the stairs, getting it down the narrow passage and past all the chairs, I steered it gingerly into its tight home. I finally got everything plugged in and ship-shape before switching it on.

And would you believe it – it was giving exactly the same fault symptoms as when I picked it up! Actually, I lie a bit – at least the set was trying to fire up momentarily before going back into its standby state. And the reason was that that wretched on/off switch wouldn’t latch into place again. The only way I could get the set to work was by using a piece of paper to jam the switch into the on position.

Considering the strife this set had caused us both, I think we were both pretty restrained in our language and demeanour. However, there was nothing for it but to take it back, order and fit a new switch.

All I can say is come the revolution, I am going to be out looking for the designer of this set and won’t be satisfied until Madame Guillotine is!

Deitron CTV74ST

I had never heard of Deitron before (except for an old English brand many years before), until suddenly there was a 68cm stereo TV sitting right plumb in the middle of my workshop table. I had no idea how it got there but there it was with a scrap of paper stuck to the screen which said "dead".

At the time, I was the only one in the shop and I had finished my coffee and so I figured I had to be the one to fix it. The set was a Chinese-built Deitron CTV74ST employing an RCA picture tube.

I soon found out the cause of the problem. Q302 (2SD1556) was short circuit and the flyback transformer had been arcing. The former was easy to fix but getting hold of the latter was harder. However, a Samsung FCR-2SA015 was an exact replacement and I soon had a pink raster and sound. Finding and replacing Q57 (JC558) with a BC558 and changing the pin orientation got me nearly there.

All I had left to do now was fix the uncontrollable bright white raster and retrace lines.

First, I tried adjusting the screen control on the new flyback transformer but it could only reduce the intensity slightly. Next, I measured the 180V video B+ to the CRT board and for good measure replaced C57, a 10μF 250V smoothing electro. I also measured the 12V rail and replaced C51 (470μF 16V) as well. Neither of these steps made any difference at all but I did notice that unplugging the plug to the colour decoder board removed the raster completely.

Finally, I did the sensible thing and measured the voltage to the three cathode guns to find they were all at nearly 0V.

The three colour amplifiers are from transistor push-pull amplifiers driven directly from IC824 (TDA3505) on the colour decoder board. I fitted a socket for the 28-pin IC and before putting a new one in, switched the set on. This time the screen was dark but after fitting a new IC I was back to square one.

By now, I was desperate for a circuit diagram and just by chance I found that an AWA 6990S was extremely similar and so I used that.

I then decided to investigate the beam-limiting contrast and brightness circuits and noticed a negative voltage rail was developed from the flyback transformer (EM301, pin 8) via R345, D309, C332 and R346 - except of course there was no voltage getting to the destination of R90 and Q65 on the CRT board. I found a 220kΩ resistor (R346) that was covered with brown glue to be open circuit and felt sure that replacing it would be the panacea for all my problems. Unfortunately, it made no difference at all except apart from putting a negative voltage on the SK line to the CRT board.

It was only then that I paid attention to a mysterious 3-transistor circuit on the CRT board. It consisted of Q65 (BF423), Q51 (BC547) and Q52 (BC558) - plus D51, a 10V zener diode. 12V is applied to the emitter of Q65 and a negative voltage to its collector, which is also connected to G1 of the picture tube. This apparently forms part of a brightness stabilisation control circuit but I can’t quite see how it is all meant to work.

I decided to check each transistor out of circuit and eventually found Q52’s collector-emitter junction to be leaky. Replacing this and readjusting the screen control finally restored the picture.

One of the main problems I had with this repair was being unable to measure the screen G2 voltage accurately, getting extremely low voltages (100V) with all sorts of meters, making me suspect the new flyback transformer again. However, I now think that this was so low because the intense white raster on the tube was loading the voltage.

Akai TV2570

The Akai TV2570 is an ITT/Nokia European-designed and built digital set and I have seen quite a few over the years. By and large, it has been pretty reliable and is a good performer but the remote and panel controls are difficult to comprehend and not at all user friendly.

The most common fault has been flyback transformer (TV53 4515 10344) problems and for a while, there was a shortage of replacements. And more often than not, they take T511 (S2000AF) with them.

Recently I had a case where D514 BY218-400 was also short circuit, giving no east-west correction.

The problem is getting general-purpose equivalent replacements for these diodes, because you just cannot stock every diode known to man and genuine replacements are often no longer available anyway. I’ve written before about the huge variety and types of diodes available.

I chose to fit a BY228 diode as a replacement but it didn’t fix the problem. I also replaced T63 (BD237) which measured fine with an ohmmeter but had totally lost its gain (ie, hfe = 0).

Next, I replaced C781, C548 and C546 but this made no difference. I could see the waveforms arrive at T562’s base but disappear on the collector of T561. I replaced these transistors and checked all the components around them but got nowhere.

I tried freezing and heating but only managed to blow D513 (BY448), which made the set go dead. I even tried replacing the EW tank coil. However, once or twice, the width did come good with the freezing treatment but only for a few seconds.

Finally, I managed to crack it with another set in for repair, swapping the components over one after the other. The culprit was D514 – which measured perfectly. BY218s are hard to come by and I solved the problem by using a BYX55-600 General Purpose Switching Diode (600V 3A) instead of my first choice of a BY228 (a TV damper diode rated at 1500V 1.75A).

The silly thing is I have seen similar TVs with a BY228 fitted in this position. I even tried replacing the BY228 with another but it made no difference. The set has been working fine with the BYX55-600 ever since.

Mr Sandman’s Philips

Mr Sandman is a wealthy man, consequently he can afford to live right on the ocean waterfront.

He had two 4-year old Philips sets (25PT448A/75A PV4 chassis) which weren’t working and though he could easily afford new ones, he asked me to fix them. I guess he wouldn’t be where he is if he did not appreciate the value of money.

Both would try to start and then go into standby mode with the red LED pulsating. Both sets were also somewhat corroded inside but not excessively.

I already had a service manual for the PV 4.0AA chassis (4822 727 20962) and the first thing I did was to measure the main HT rail. The service manual – as is Philips’ wont – is fairly confusing, giving the VBATT voltage as 95V for 14-inch sets, 100V for 21-inch sets and (120) in brackets at point P1 across C2551 or D6550 K.

Well, I was getting 134V, so I felt it was fairly obviously in a protection mode because the VBATT line was so high. I also figured that once I’d fixed one set, the other would be very easy since the fault would probably be the same.

In order not to inflict damage, I shorted the base and emitter leads of the line output transistor (7448) and hung a 100W globe across the HT line. The power supply looked pretty conventional, though Philips love to put some SMD parts underneath the chassis, making circuit tracing just that bit more difficult. I tried adjusting VR3540 and noticed that although the voltage changed, it was still too high and remained so – even in the standby mode.

I spent a lot of time investigating the power supply and could find nothing seriously wrong. I replaced all the small electros and IC7520 (MC44603P) and checked all the resistors. Obviously all the voltages were out – but not by much. I tried this with both the sets but just wasn’t getting anywhere.

I was fooled by the circuit involving Q7510, especially when I found resistor R3510 (39kΩ) to be high in value. Actually, a 56kΩ resistor was fitted in both sets but one was still too high at nearly 80kΩ. Fitting new ones and various variations made no difference and in any event, this circuit is part of the degaussing circuit. It’s obviously provides a soft start-up to reduce surges but I am not sure how it works.

In the end, I decided to fudge it by altering the value of the pot (VR3540) but even with the right voltage output the set was still closing down. As a result, I decided to concentrate on the line output stages. Accordingly, I removed the dummy load and base-emitter jumper and found a whole stack of faulty components: (1) R3482 and R3329 were open circuit, preventing the +165V rail being applied to the CRT output stages; (2) D6444, D6447 and D6497 weren’t even shown on my circuit diagram but all were short circuit; and (3) L5479 was open circuit on the +13V VDEF line.

When I replaced the latter, the new one started to get hot which was consistent with the HT being too high. The set was still almost immediately going to standby, so measuring voltages on the secondary was extremely difficult.

It was about this time that I had the good fortune to meet and talk to a Philips technician who was familiar with these sets. (Because Philips has produced well over 5000 models since 1974 it is hard to be that familiar with any of them). He informed me that although I did have a service manual for this chassis, it was still the wrong one. Instead, there is a separate manual for the 25-inch model and being an extremely obliging fellow, he gave me a photocopy of the circuits I needed.

This breakthrough informed me that VBATT is actually +134V at P1 and the circuits also showed me all the differences I had already noticed, including the location and detail of D6444, D6447 and D6497, as well as their values.

It also meant I had to write off a huge amount of time spent investigating a total red herring. I reset the power supply and could see the +13V VDEF rail was under stress. Following the rail along, I could see it mainly fed the vertical output IC (IC7960, TDA9302H). Replacing this restored the sound and picture and I now had
everything working except that there was no east-west correction.

I put the set into the Service Default Mode SDM by shorting M25 (pin 1 of the EEPROM) to ground and switching the set on. I checked out the Options Code Address/Data with the list stuck on the inside of the back of the set. The East West data was different to that in the list but varying it made no difference to the picture geometry.

I then spent another huge chunk of time investigating hardware failure in the east west circuit, which is half on the motherboard and half on a separate module J. I soon discovered – oh joy! – that leaving this module unplugged destroys the line output transistor!

Eventually, I conceded that it had to be a faulty jungle chip (IC7200, TDA8375) and ordered one in. This IC fixed the first set completely. I just needed to set up the east-west geometry, so I went back into the Options table (which incidentally told me this was an Anubis-PV4 PM4.3) and reset address 111 (width), 112 (para), 113 (corner), and 114 (trap) for the best square picture using a test card. Address 246 (height) needed adjustment as well. I also had to tune in the set using the installation menu afterwards.

Having finished the first set, I got back onto the other one, hopefully wiser than I was before. Thankfully, it had very similar problems, with the vertical output IC (IC7960 TDA9302H) being the main culprit. Replacing it and the other coils and resistors restored the picture and sound but there were no east west problems this time.

These sets certainly caused me grief but I’ll know better next time.