Email Address:
Password:

Lost your password?

This is the legacy website; please use the new website.

Serviceman's Log

Who said servicing was dying?

By The TV Serviceman

I have a full house this month, starting with a 1999 Panasonic TC-14S15A (MX5 chassis). It was dead and the horizontal output transistor Q551 (2SD2499) was short circuit. A new one was fitted but it became extremely hot. The horizontal output transformer T501 was also replaced and all the components around the horizontal output stage were checked thoroughly. Nothing amiss was found but it was still blowing the transistor.

The only clue was a some ringing around the positive horizontal pulse on the collector of the horizontal output transistor. This problem was solved only when a sister set was brought in and the two compared. A smart pair of eyes noticed that there were four ferrite beads fitted on the good set - L552 in the emitter, L558 in the base and L551 & L557 in the collector. In the crook set, someone in the factory had left out L551 and L552 and fitted only links.

The question is, how did it last for so long before it reached this stage, because the transistor was very hot? Anyway, the ferrite beads fixed it quick smart and the transistor now runs quite cool.

Items Covered This Month
  • Panasonic TC-14S1SA TV set (MX5 chassis)
  • Panasonic TX-79P100Z TV set (MD2 chassis)
  • Hitachi 5-inch VT-LC50EM (AU)
  • Sony KV-XF29M35 TV set
  • Panasonic NV-FS90A TV set
  • Sony KV-XF29M35 TV set
  • Philips 28CE1965 TV set
  • Philips 28GR6775/75R TV set
  • Sony KV-1415AS TV set
  • NEC N-3452 VCR

Another Panasonic

At about the same time, a similarly aged (1999) Panasonic TV set also came in with a similar fault; ie, it was dead with the horizontal output transistor short circuit. This model was a more upmarket TAU set, model TX-79P100Z with an MD2 chassis, and advanced features such as computer and DVD inputs (B-Y & R-Y), etc, which one might expect at $4700.

The cause of the failure was unusual, as the frequency of the horizontal drive was far too high. In fact, it was double the correct frequency. Surprisingly, it wasn't the jungle IC that was the culprit. Rather, it was the Digibox that had somehow become stuck in the 100Hz mode. This module is non-serviceable and was replaced under warranty which fixed the problem.

A mysterious customer

Mr Armstrong was a rather mysterious customer. He was a single man in his late thirties and spent a lot of time travelling overseas. And he was on his way to another overseas trip so there was no forwarding address - just a mobile telephone number.

He brought in an Hitachi 5-inch LCD/Video Cassette Recorder VT-LC50EM (AU) which was completely dead. This is a rather nice little toy, consisting of a truly portable battery operated miniature multi-system TV receiver and VHS video system, all in a neat 370 x 90 x 220mm case. It was a set I had never seen before.

Mr Armstrong was convinced that it was just a fuse or switch and left
it with me after I had checked the external AC adaptor/charger (VM-AC600EM) was delivering a healthy 9.6V DC from a rather frayed cord.

I knew immediately that this wasn't going to be simple; it was far too compact and it would be like a notebook computer - all surface mounted components and tricky access. I shot around to my mate who is an Hitachi agent and borrowed his service manual(s) - and that's when I started having second thoughts.

Maybe I had been too courageous in taking on this repair? Mr Armstrong had given me the impression that the unit was only a few years old and so I was rather disillusioned when I found out that it was in fact nearly 12 years old.

The first thing I did was to confirm that the 2Ah 9.6V nicad battery, VM-BP63, was completely shot and that a new one was rather expensive and obtainable only from Hitachi.

A tape was also stuck inside but the video cassette was unable to eject it or even show any signs of life.

I removed the bottom cover by undoing seven screws to reveal just what I had expected - a fair whack of miniature electronics in a small box. After a little careful reconnaissance and surfing the service manual, I discovered that there are two main boards on the left looking from underneath - ie, PC boards JAS and TTS - plus a further board (SWS) on the right under the video deck. The TTS board could be unclipped and folded upwards to give access to the JAS board below.

Click for larger image
Of course, the boards were double-sided with surface-mount components - but the thing I noticed most, which filled me with fear, was the vast number of subminiature electrolytic capacitors, many of which were leaking electrolyte on all the boards.

At this stage, all I was intending to do was to diagnose the fault(s) so that I could give Mr Armstrong a quote for the repair cost. I already had a good idea what had happened but I was determined to cross a few "t"s and dot a few "i"s. And I needed to know where the power came in and where it went, which I thought was going to be fairly simple.

It wasn't. The circuit was very complex and it took a long time to work out that the AC adaptor came in via JK1501. The battery came in via PG502 and the line then went via fuses FU1501 and FU501. These are 2A picofuses and both had blown.

The fuses, which look like resistors, were located some distance away from the DC jack and battery input. Unfortunately, it was too difficult to trace the path, not only because the board was double-sided but also because the parts were tightly packed.

Even with the fuses blown, there were voltages that could be measured at random on all the boards at places that were accessible. Unfortunately, replacing the fuses had no positive effect - the unit was still as dead as a doornail.

Next, I followed one rail from the fuses to the TTS board and then to another switchmode power supply. In the process, I checked a lot of other fuses but I was getting nowhere.

By now, I was feeling rather frustrated. All I had achieved so far was to find two open circuit fuses and determine that there was no 5.1V where it should have been. Nor was the 9.6V reaching pin 11 of IC581, the switching regulator. What's more, there were a lot of electrolytic capacitors to replace.

I then checked some of the IC regulators that fed the microcontrollers, to find they were OK (eg, IC1902 that fed IC1901 on the TTS board; and IC906 to IC901 on the SWS board which controls the power-on function).

By now, I could see that a lot of work was needed to replace the electros and possibly fix the corroded tracks underneath them. Then there was the NiCad battery, plus the memory back-up lithium battery. After adding 10% GST, I thought it hardly worth continuing with what was essentially a toy.

Click for larger image
Anyway, I had to wait a few months before I could finally contact Mr Armstrong, when he resurfaced back in Australia. I was agreeably surprised when he accepted my expensive estimate. I guess he is one of those blokes who doesn't have many other interests and this was one of the luxuries he felt he had to have.

While waiting for his verdict, I had been planning my campaign of attack and now I was ready to go. First, I removed his jammed video tape by unplugging the loading motor (CN904) and connecting a 9V battery to it to release the tape.

That done, I concentrated on changing the worst of the electrolytic capacitors. There were 10 brown electrolytics, eight of which were 47μ 16V (C584, C587, C588, C589, C597, C1855, C1857 & C1939) and there were also two at 100μ (C585 and C586). C1855 and C1857 had corroded the tracks badly underneath and it took a lot of effort to work out which "micro-thin" tracks were which. The main one was VDET from PG1902-6 to pin 22 of IC1901 via R1958 and pin 1 of D1901.

Unfortunately, I wasn't having much luck and still couldn't get the power switch to work - or even get the power-on LED to light.

I have to confess that much of my work was done with the unit upside down and I was operating the power switch by toggling it with my fingers under the half-opened LCD screen lid. After changing a few more electros, it was getting late so I thought I would clear the bench and partially reassemble the unit, ready for the next day.

That done, I turned the unit over, opened the lid fully and was staring at it hatefully while I again hopelessly pushed the power switch. To my total amazement, the whole lot came on - even the video system was working. I tuned in a channel (when I worked out how to do it!) and the picture and sound were perfect.

When I came in the next day, I couldn't help feeling that it had all been a mirage - but it was still working!

What I hadn't realised before was that the set would only work when the screen was fully opened. There is another panel switch (S2801) - not mentioned in the manual - on the LCD board that controls pin 17 of IC1901. So I'm no too sure just when the set had actually been fixed as I worked on it.

There was no question that the screen was fully open at the start and the set was definitely "no go" then!

The problem was that the power switch doesn't just switch the power straight on. The power switch (S017, FSW board) controls IC1901-18 on the TTS board which, if everything is OK, will somehow talk to IC901. IC1901-61 then "wakes up" Q904 and activates IC901-49 on the SWS board. And that in turn switches on Q581, Q585, Q582 and IC581 on the JAS board which then switches on regulator transistors Q588-Q591

Of course, that's all assuming that nothing is wrong and that the protection circuits don't switch on!

In the future, jobs like this are definitely for the birds.

Seaside problems

Disasters can happen to everyone and to every type and make of set. Mr Byron had one such experience with a newish (1999) Sony KV-XF29M35, which lived with him in a stylish mansion by the sea.

I guess one can have too much of a good thing, because the humid salty sea breeze plays havoc with anything containing metal and electricity.

Inevitably, his set died prematurely and ended up on my bench. The power supply was dead and blowing IC601 (STR-F6656) repeatedly. I checked for shorts on the secondary of the chopper transformer and found the horizontal output transistor, Q511 (2SC4927-01), was short circuit also but this didn't stop the switchmode control IC from destroying itself. It was only after I had committed the third IC to the bin that I woke up to the fact that the sensor amplifier, SE-135N (IC602), was giving incorrect feedback information.

However, I wasn't completely out of the woods. The picture was extremely dark, with no contrast. And the horizontal output transformer, T503, was looking particularly dodgy, as it was hissing and spluttering.

While a new one was on order, I decided to chase the ABL circuit and see if there was anything untoward there. It turned out to be a fairly involved circuit but my hunch was correct in the end - several surface mount components on the A board, including Q312 (2SA1162-G), D315 (ISS3565) and D316 (a 6.8V zener diode), were faulty. This was rather surprising because I would have expected Q512 to have been destroyed as well but it was OK.

The new horizontal output transformer finally restored the set until the next onshore sea breeze. And the customer was happy.

The SAMPO chassis

The Philips Group has produced over 5000 different models of colour TV sets in the last 30 years. Almost all have been designed and made by the company but there are a few that have been made outside.

Two that come to mind are made in Taiwan - the SAMPO-1 and SAMPO-2 chassis (Models 26CE1991 and 28CE1965). Anyway, Mr Tennant phoned for a home service call on his Philips set, a 28CE1965, complaining that when it was cold it was hard to start. And he was convinced that it was the on/off switch. Well, of course - if it isn't the fuse, the switch or the tube - it can't be anything else!

When I arrived, he had the set on and so it was switching on and off perfectly every time. I just couldn't accept that the switch was faulty only when it was cold, so I told him that I suspected the power supply and probably the electrolytic capacitors in it and that it would have to go to the workshop.

Back at the workshop with the back off, I could see the set was very well made and that it used a Toshiba IC chip set. There is no master power switch - the set is controlled by a subminiature push switch going to a microprocessor and then a relay.

I found and replaced capacitors C713, C714, C717, C735 in the power supply which were dried out. I then put it back into operation and left it to soak test on the bench but with back off just in case I had to do any further work. After a few days, it was still working correctly and so I put the back on.

However, with the back on, I found it difficult to turn the set off with the remote control. I didn't discover this until it had run all day and it was time to turn it off, so I wondered whether this new problem might be caused by additional heat affecting something. I tried it for another day, before opening it again. And with the back off, I couldn't fault it, so I was even more suspicious of the heat factor and replaced even more capacitors - C740, C741, C744 & C745 - in the auxiliary power supply for the relay and microprocessor.

Well, I still couldn't fault it with the back off but once back in its case, the set intermittently wouldn't switch off with the remote control, even when it was cold. However, it did respond to the switch on the set.

Consistent with my old age, it took a while for the cause to sink in. The chassis slides in from the back, on plastic rails, until the escutcheon mounted pushbutton controls just touch the push switches. However, the combination of the chassis being pushed too far forward and a slightly distorted power knob meant that the microswitch was permanently switched on. So, when the remote control was used, it could only mute the sound but not turn the relay off. However, when the switch itself was pressed, it released itself properly.

So Mr Tennant was right; or at least partially - there was a problem with the switch assembly.

Another Philips

I had a Philips 28GR6775/75R G110-S chassis in for repair at about the same time. This model was very popular in Australia and there are a lot of them about.

Mr Brady brought this one in and, originally, it had intermittent vertical deflection and linearity but the fault was now permanent.

I replaced capacitors C2813 and C8214 (both 1500μF 40V electrolytics), which were leaking badly and thought that that would fix it. However, before replacing them, I had to clean off the excess electrolyte on the board. It had even leaked underneath the chassis but no visual damage to anything was apparent. Unfortunately, having done all this, the fault was still there.

I measured everything in sight with the ohmmeter but could find nothing wrong. I then spotted a surface mount component link (4815) under C2813 and C2814. This had 000 printed on it, denoting zero ohms, and connects C2813 and C2814 together.

Anyway, when I measured this, it did indeed measure zero ohms but when I hit it with freezer, while the set was on, the vertical timebase began to scan. I felt that it was telling lies and linked it out with a fair dinkum wire link. This fixed the fault completely and when I measured the surface-mount link again, I found it was high resistance.

I can only assume that the corrosion from the electrolyte had damaged it in a manner such that it failed under load.

Dark NEC

A Thai-built NEC portable 34cm remote control TV set was brought with a dull dark picture. It was an N-3452 model with PWT-101A chassis.

The voltage on TP91 was only 85V when on but shot up to 143V on stand-by. It should have been 116.5V so something was very wrong. The power supply uses an STK 730-80 (IC601) and I noticed that it raised the voltage when hit with freezer.

The voltage input across capacitor C604 was a very healthy 320V.

I then started looking for any electros in the primary or control section of the power supply and at first didn't see any. But then I spied C610 (10μF, 50V) on the circuit although I couldn't see it on the board. I finally found it tucked up tight behind IC601 and replaced it.

This was indeed lucky as it was the culprit and not the expensive IC I was about to order and replace.

A crook guess

I don't appear to be very good at guessing which jobs look easy and which don't. Mrs Lyon's Sony KV-1415AS (SCC-F35A, G3E chassis) had a very small dark picture and for all the world it looked like the main HT was low, which would be relatively easy to repair.

However, after I had taken the back off, I measured the HT and it was spot on at 115V, on the cathode of D608. So my main theory was quickly dissolved.

I moved to my fall-back position - when in doubt, measure all the power rails. This I did, and realised fairly quickly that all the secondaries of the horizontal output transformer were low and the EHT was down to about 15kV. There are three main voltages from the transformer: 200V for the CRT video outputs, 26V for the vertical output and 15V for all the ancillary circuits. I started with the latter by disconnecting the output of IC851, a 9V IC regulator, to see if the rail would come up when the load was removed.

I was back on track again! Disconnecting the 9V rail brought all the secondaries up to normal - but what was loading it? There were no shorts on the 9V rail but when it was reconnected, the 9V dropped to 6V.

Unfortunately for me, the 9V rail goes everywhere in the set. So in the absence of any other brainwave, there was nothing for it but to progressively disconnect the devices being fed by the 9V rail and keep track of its value.

Much later, I found that desoldering the links to Q851 (2SA1162), a surface-mount PNP regulator, made a significant difference, even though the device itself was perfectly OK. I then found that disconnecting diode D251 (ISS119) in series with the collector of this transistor restored everything.

That surprised me, as both these devices are minute and yet were holding this rail down by one third! I measured D251 to find it too was perfect - so where to now? D251 fed the bases of the two transistors, Q251 and Q252, which apply audio muting to IC251 (the audio output IC).

Once again, I had to disconnect components to find out where the current was going. I desoldered Q251 and Q252 in turn but found that it was D250, another ISS119, that was causing the problem. It was leaky and replacing it fixed the set completely.

So what was the significance of D250? As already stated, its cathode (along with D251) fed the two muting transistors. The anode goes to the emitter of Q2004, which is in the power on/standby circuit and feeds horizontal drive transistor Q801 via R057 (1kΩ).

Not being a circuit designer, my hypothesis is that a leaky diode reduced the drive to the horizontal output stage. And it was this that was causing the low output rather than a current overload problem.

It was an interesting case but unfinancial as far as I was concerned. Mrs Lyons' set had been fixed at a significant discount but she still thought it was too expensive!

Panasonic VCR

Click for larger image
With the low cost of VCRs, I am getting less and less to repair, the exception being the more expensive hifi and SVHS machines. Recently, I had a Panasonic NV-FS90A with no TV reception - the tape would play OK and all the other functions were fine.

Unlike most similarly dated SVHS machines from Panasonic, this model selected AV via the program selector in sequential order, or it could be selected on the remote control. Other models have a switch on the front panel offering Tuner, Simulcast or Auxiliary inputs.

This set was stuck in the AV mode and wouldn't switch to TV at all. I took a long time studying the service manual and in the course of tracing the circuit, discovered that C1003, the memory back-up capacitor, had leaked electrolyte onto the main board, corroding at least three tracks.

I thought that linking the broken tracks would fix the problem but it wasn't to be. The critical point was pin 4 of the microprocessor (IC6001, MN188166VDU) which should have 5V on it for the TV function.

It took a long time to realise that the corroded tracks were not the only problem. IC6001 itself was also faulty but only on pin 4. But why had such a complex microprocessor failed only on one pin. The reason, I suspect, was because the electrolyte from the leaking capacitor had shorted the nearby
-20V to pin 4.

I worked out a fix by shorting Q607's base and collector, to hold this rail at 5V. But this fix was incomplete - it fixed the TV problem but only at the expense of the AV function.

Basically, of course, the answer was to replace IC6001. But I hesitated. It was a large and fairly expensive unit and, with labour costs, would make a costly exercise. And I sensed that the customer was worried about further costs. As an alternative, I suggested that I could fit a toggle switch so that he could switch between the AV and TV inputs. However, he ultimately decided that he really had no further use of the AV inputs and that a switch would look out of place.

So we left it at that. The customer was happy and I was happy. What more could one want?

Share this Article: 

Privacy Policy  |  Advertise  |  Contact Us

Copyright © 1996-2019 Silicon Chip Publications Pty Ltd All Rights Reserved