Sony KV27DXT1U Loss of Picture when warm/cold (AE-1 chassis)

[Vectrolosys]

Hello,

My main set is a lovely old Sony 27" unit from around 1986/7. One of the first sets in the UK to be fitted with NICAM I believe. I am quite attached to the set and have recently begun to experience problems.

When warm and sometimes when cold the picture will fade out, not to return. However, some viewing sessions can be completely un-interupted with viewing being available without hitch.

I have found some links on the internet that advise R807 (1k ohm I think) is a weakpoint on this chassis, when picture loss is present, but sound is normal.

I have just dismantled the set and given everything a general dusting, including removal of the scan circuit and a quick clean of the pins on the tube neck. I checked the resistance across R807 and found it to be 1.02 K Ohms when mildly cold (the set had been on around 10 minutes before).

I put the set back together and fired her up. Picture came up within 5 seconds as normal, it then intermittantly lost red, but a sharp tap to the rear of the set restored picture, although with a flood of red on the screen which stabilised soon after.

My questions are as follows.

1 - Has anyone seen symptoms like this with the AE-1 chassis before.
2 - Is it possible I have a component failing on the scan circuit, or a dirty connector on the main multiplug?
3 - Could this be the end of the tube itself?

I am thinking of retiring the set to lighter duties (i.e. not the main set) as a B&O LX5000 has come up at a reasonable price, but I am keen to get to the bottom of this fault and repair it if it is a simple, non tube related fault.

Many thanks in advance,

James

[tellyman]

Hi James.
It would be worth resoldering the RGB output transistors on the tube base as this sounds like it may be the cause of your problem. This was a very common cause of failures on this set. Otherwise check very carefully elsewhere on the main chassis for dry joints. Good luck.

[Vectrolosys]

Originally Posted by tellyman

Hi James.
It would be worth resoldering the RGB output transistors on the tube base as this sounds like it may be the cause of your problem. This was a very common cause of failures on this set. Otherwise check very carefully elsewhere on the main chassis for dry joints. Good luck.

Thanks for that , sounds a relatively simple thing to start with. I may retire the set anyway so I have time to get everything out, dismantle and re-solder. I did clean away some rather interesting looking dust around the line output stage, but I assumed this would attract a lot of dust due to the amount of electricity in this area. So far, the set has been on for over an hour since re-assembly.

[Michael Maurice]

As the picture is fading in and out, you should check and resolder the heater plug connections on the CRT board, it may also pay to resolder the relevent joints on the main board.

Whilst you have the set in pieces I suggest you check/replace the following:

Replace C615 1000µF/25V C531 680µF/50V, C532 100µF/50V

Resolder the chopper transformer, the audio output IC, frame O/P IC, coils in the IF section.

The tube is unlikely to be the cause of these faults.

[ballistic]

This is a similar species to mine I believe. With the disclaimer that I the token complete novice around here, I think it was resoldering the main ring of connectors on the CRT board on the back of the tube that fixed this for me, although I have some other issues I am still working on, on the same TV.

[Vectrolosys]

Originally Posted by Michael Maurice

As the picture is fading in and out, you should check and resolder the heater plug connections on the CRT board, it may also pay to resolder the relevent joints on the main board.

Good call, I see where you're coming from as regards it being a possbile dry joint around the heater connections. I'll get it open again at some point and check all these problem areas and replace as required.

[ben]

It's a sony - so RESOLDER RESOLDER RESOLDER! ;-)

[Vectrolosys]

Originally Posted by ben

It's a sony - so RESOLDER RESOLDER RESOLDER! ;-)

It would seem that way from the replies I have had . Excellent picture quality and well built (well, the 1980's examples I have) but prone to dry joints and niggly faults, but, with a proven repair technique and predictable faults.

The only Sony I have been less than happy with was the KVX2162U my parents rented from Radio Rentals in 1993. We eventually bought it from Boxclever, last year for £30!!!. Had one callout in that time for a field output chip that had gone dry jointed. My reason for un-happiness was the tube, the picture was blurred at the corners and had been like that from new. I heard that mid nineties Sony tubes were either spot on, or out so bad that they'd be difficult to set back up again.

[ben]

Originally Posted by Vectrolosys

It would seem that way from the replies I have had . Excellent picture quality and well built (well, the 1980's examples I have) but prone to dry joints and niggly faults, but, with a proven repair technique and predictable faults.

I'd say that's pretty much spot on. I've worked on dozens of sonys and on 99% of them, had to do fairly extensive resoldering while I was in there. In 90% of the sets, the reason why i had them open in the first place were problems caused by dry joints, usually on the frame or tuner/IF areas. the other 10% were that N15 ic protector opening up in the b+ line....

[dsergeant]

In this case it will be dry joints on the rgb transistors and heater connections (crt base/pcb or lopt). But note that failure of R807 (in the G2 supply circuit, not your problem) is ALWAYS due to failure of the associated diode D803 RGP01-017. They upgraded it to a RGP02-17 (available from Chas Hyde and elsewhere). Never just change the resistor.

Dave

[mhennessy]

Originally Posted by dsergeant

But note that failure of R807 (in the G2 supply circuit, not your problem) is ALWAYS due to failure of the associated diode D803 RGP01-017. They upgraded it to a RGP02-17 (available from Chas Hyde and elsewhere). Never just change the resistor.

Thanks for this info Dave - I did ask on another thread for causes of this failing. In my admitedly limited experience of this (maybe only 3 cases, IIRC), it's been for no reason that I could identify, and a replacement fixed it. But does this mean that the diode can give (very) intermittent problems? All these sets are still own by or known to me, and I've not heard or seen any further problems...

[dsergeant]

You usually can see some sign of arcing/tracking on the body of the diode if you remove it. I have done loads of these repairs in the past and always replace both resistor and diode.
Dave

[mhennessy]

Thanks again Dave - I'll watch out for that in future

[Vectrolosys]

Hi All,

Just wanted to say thankyou for all the replies on this. It's certainly useful to know that a lot of the problems I am having have all been seen before.

I shall post an update once I have completed the soldering, just to let you know how the repairs went, but I doubt there will be any issue as it's all (it would seem) run of the mill failures for this chassis .

Thanks again,

James

[Vectrolosys]

Hello,

Some of you may remember my first post about this set, as I was having problems with the picture fading out (at random intervals).

Last night, I decided to pull the set apart and try to rectify the issue using some of the very useful advice I had received .

On examing the trouble spots on the scan circuit, I was greeted with numerous dry joints. I repaired these, put the set back together but received the same error.

Today, as I have a day off, I decided to delve deeper and resolder the joints around the Lopt and connectors on the main board. I was greeted with this, which seems to be centred around the mains (chopper?) transformer stage. I have attached photos of the problem area.

I think, and please correct me if I am wrong, but this is pretty much terminal for this board!!.

Pictures below.

http://i60.photobucket.com/albums/h3...r/IMAG0006.jpg

http://i60.photobucket.com/albums/h3...r/IMAG0007.jpg

http://i60.photobucket.com/albums/h3...r/IMAG0008.jpg

I apologise in advance if these pictures have been uploaded incorrectly. I have examined the rules but could not find a reference to correct picture upload.

Anyway, as you can see, the area appears to have been burnt, and I am not 100% certain, but I am sure there should be some circuit board and track there!!

Thanks in advance,

James

[Nickthedentist]

Hi James,

Nice clear pics

Yes, this seems to be the primary side of the swtich-mode power supply.

This is a classic example of what happens if you carry on using a telly that's got intermittent problems. The arcing (sparking) causes the board to burn away, as you have found. This is also a potential fire risk, of course.

The "official" advice is that the set should be written off, especially if it's a repair for a member of the public.

But in practice, a safe and reliable repair can be carried out by removing the affected components, using a small file to get rid of the carbonised circuit board, then re-fitting the components and using wire to replace the tracks that have been damaged.

I'd use tinned copper wire, and scrape off all the green "varnish" so that the wire can be soldered along the whole length of damaged track, from a pad either side of the break (i.e. NOT just across the break, and NOT just soldered from pad to pad).

A picture of an undamaged board is invaluable. Often the service manual includes such a picture, showing how the tracks run. Do you have a manual?

Nick.

[dsergeant]

Those 10W resistors caused all sorts of problems on Sony sets and yours is typical of what happens. But it has obviously been repaired already and is probably OK. Not pretty and you are not supposed to do those sorts of mods on the primary side of the chopper but we all have done some in the past. I think you will find it is unrelated to your current problems. By the way you were supposed to use high temperature solder on the centre pins when replacing the resistors.

Dave

[Vectrolosys]

Hello,

I bought the set around 3 years ago, so I guess the repair had been carried out before then.

If it is un-related to my current fault, before writing it off, I'll resolder the LOPT and terminal points for the Scan circuit on the mainboard.

I was a bit shocked to find this on dismantling!! still, one of the joys of owning a 19 year old set with unknown previous life .

Fortunately I've managed to get a service manual for the AE-1 chassis. I'll leave this 'previous repair' for now and concentrate on trying to get a picture back on the screen.

On the flip side, the baby Sony of the family, the KV21XRTU has had a new lease of life. More on this in another thread .

James

[Michael Maurice]

Many years ago Sony brought out a mod kit which consists of the two resistors mounted on a seperate board with thick wires to the main board.

If the main board was so badly burnt as to be considered unrepairable, Sony supplied - free of charge - a new main PCB.

Yours is OK

[Vectrolosys]

Originally Posted by Michael Maurice

Many years ago Sony brought out a mod kit which consists of the two resistors mounted on a seperate board with thick wires to the main board.

If the main board was so badly burnt as to be considered unrepairable, Sony supplied - free of charge - a new main PCB.

Yours is OK

Thank goodness .

Time to continue the solderfon to repair the picture, or lack of.

Just to re-confirm. The picture fades out now as soon as the set is turned on, whereas previously it would come back on after 5 minutes and stay on.

To add, the heaters appear to be glowing on the guns.

I have resoldered the following.

3 x large transistors on the scan circuit
1 x Connector (200V plug which goes to the mainboard) only at scan circuit end so far
1 x small multi pin connector that goes to a daughterboard, think it's the tuner circuit (only soldered on the scan circuit end)
1 x Tube socket on the scan circuit

And that's it so far, the results have not improved. With this in mind, I shall now resolder....

Chopper transformer (as recommended, may not solve the issue but it is a problem spot)

All connectors from the scan circuit to the main board

The LOPT

Is there anything else anyone can think of? It was recommend above, replacing a pair of capacitors. Are these related to the problem I am having in any way?

Thanks again,

James