Horizontal Output Transistor Question.

[IKC2E51R8]

Hi All

The ICC5 in my 51K5 has developed its first natural fault IE not caused by me or shipping damage as its using the perfectly intact 59P7 Board...

It had been working fine for a few hours every day until two days ago, I had it on for about 4 hours watching some old TV shows and I left the room for a moment and when i came back the set was dead, tried turning it on again and 3 trips condition.

I took the back off and after about a minute of testing suspect parts i found the HOT was shorted its a BU508AF. I replaced it and set came straight back to life.

Had the set running again yesterday, this time it ran for about 40 mins and the picture suddenly stretched a bit at the bottom, then brief flyback lines then the set died. I let the set cool down and tried powering on again, to my surprize it came on, lasted about 20 mins then died again, now its back to three trips condition.

I havn't had the back off yet but im guessing the HOT has popped again

I know there is a stock fault that points to CL48 as being the culprit for repeated HOT blow ups so im going to change it but im gonna need a replacement HOT.

I dont have any BU508's but i have an S2055AF from a scrap IKC2 board, this one has the internal damper diode, could this be used as a substitution? the ratings are the same from what i can see and the only difference is the damper diode, i dont know enough about how these work to judge whether id be making a mistake by fitting it in place of the old BU.

[G6ONEDave]

Can't remember if the BU508AF has an internal protection diode, if not is it the correct transistor and if so is the protection diode on the pcb. Should the protection diode be on the pcb I would replace that as well, probably something like a BY228.

I'll have to dig out the service manual to study the circuit and get the ref numbers.

Dave

[IKC2E51R8]

BU508 has no internal diode, there's a BY228 on the PCB Position DL46 which forms part of the EW Modulator circuit. on the circuit below an S2000A3 is shown as the HOT but a BU508AF could also be used.

[G6ONEDave]

Not had chance to get the manual out yet but something that comes to mind, is to check that the line driver transformer is not dry jointed, also likewise the line output transformer. Anything that causes overvoltage on the BU508AF will kill it. Things like base emitter and collector emitter overvolts and or reverse voltage spikes, the latter is why the BY228 is there. It might also be worth checking the ratings of the 2 transistor types incase there was an upgrade.

I'll try and dig out the manual tomorrow morn.

Dave

[Red to black]

IIRC the S2055A/AF not only has the internal integral diode across C-E but also an internal integral resistor across the B-E junction as well, and so will not work in the ICC5 chassis for long, if at all.

Yep it does, I remembered correctly see attached.

[IKC2E51R8]

success! I found a working S2000AF in the Power supply of the Scrap IKC2 board which is compatible. popped it in and I replaced CL48 with a Brand new 10n5 cap and the set came back to life. Now to test it for a couple hours and see if it manages to stay on without cooking another HOT.

[MickMcmichael]

Luke,

PM me if you need anymore S2000AF or BU508A. Got plenty so you're very welcome. Good luck with that Thomson thing!

Regards,

Mick.

[IKC2E51R8]

Thanks Mick, I may take you up on that, depends if the HOT survives.

[IKC2E51R8]

Ok so there's still something else wrong, it lasted 7 minutes before dying again and now its back to three trips. anyone any ideas what could be causing this? when the set was running everything seems to be perfect until it suddenly dies.

[MickMcmichael]

Do you know if the line output transistor is getting hot before it fails or is it suddenly dying at normal running temperature?

If it suddenly dies then the line drive could be disappearing for whatever reason. Poor connections on the line driver transformer is more than suspect. In the "old days" we would resolder the connections by rubbing the pins of the transformer with the soldering iron tip until it literally squeeked. You then know that you've got a good soldered joint. ICC5's were famous for problems like these. All the best. Give me a shout if you need transistors.

Mick.

[IKC2E51R8]

Im not sure if its getting Hot but given that the way its failed at different times i suspect its running at normal temperature but suddenly 'something' happens and the set goes off

[Nuvistor]

I don’t know the set but the Bush T20 series had similar problems when the base resistor went high in value, from memory it was a 1 ohm and drifted to 10ohm. This resulted in the transistor overheating and going short.
From the circuit in post#3 that would be RL30 but worth checking other base components.

[murphyv310]

Unsolder the line driver transformer and give the pins a good scraping with a Stanley knife. Also replace the line driver transistor as a matter of routine. An internal flashover in the LOPTX will also give the same issue. Sit and watch it and don't even attempt to blink! Sudden drop in width could indicate a duff tuning cap.

[Red to black]

Originally Posted by IKC2E51R8

Ok so there's still something else wrong, it lasted 7 minutes before dying again and now its back to three trips. anyone any ideas what could be causing this? when the set was running everything seems to be perfect until it suddenly dies.

you will need to monitor the temperature and the HT, see what the flyback pulse time is measuring on a scope as well as checking the line drive wave form.

Also check the soldering on both the power and line output heatsinks as these are part of the chassis returns on this chassis.

[IKC2E51R8]

Originally Posted by Red to black

you will need to monitor the temperature and the HT, see what the flyback pulse time is measuring on a scope as well as checking the line drive wave form.

Also check the soldering on both the power and line output heatsinks as these are part of the chassis returns on this chassis.

Unfortunately I dont have a scope so cant monitor the wave form.im going to try clean the legs on the Line drive transformer and I'll change the HOT (Found a BU508 from a scrap Philips chassis) and drive transistor and RL30 while im at it and see how it gets on, if it goes again then i'll have to keep looking which i realise without a scope is gonna be difficult to pin down. I'll check out the soldering on the heatsinks too.

[Red to black]

without a scope you are seriously handicapped as you will end up chewing through transistors, check all of the scan path components for dodgy joints, including any on the scan coils themselves and the connector on the main chassis, line lin coil etc. replace the decoupling capacitor for the V5 23V line too as this supplies the line driver stage.

[IKC2E51R8]

Ok so I took the back off and surprisingly the HOT was spared this time (probably because i didn't try and turn the set on and off a few times like i did with it last time).

I took out the line drive transformer, cleaned the legs as suggested and popped it back in, then i replaced the line drive transistor TL29 and I replaced the HOT Base resistor RL30 (supposed to be 0.47ohm and it read at 3.2ohm when hot after taking it out and when it cooled slightly it read 1.5ohms so it had to go)

I also replaced the decoupling cap for the 23v line as suggested (1000uf) and when i took the old one out it measured 784uf with an ESR of 11 ohms.

turned the set on and let it run for 15 mins, everything seems to be ok, I'll give it a proper soak test tomorrow, too late to be having things go boom if it decides to go again, I'll report back tomorrow on how it gets on, hopefully ive narrowed down the cause to that resistor and capacitor.

[murphyv310]

Hi
Good work. Yes I've had the resistor Go high and you are correct it will cause the issue, the drive waveform with not be the correct shape changing the operation of the HOT away from a switch, I'd put my money on it's a cure.

[G6ONEDave]

Sounds hopeful. Having checked the service manual there are several differences in the east west circuit between the 51K5 and the 59P7. There are 3 versions of the 59P7 that I know of being 59P7A, 59P7P and 59P7V, they have different parts in some areas of the east west circuit.

Do you have copy of the ICC5 east west correction circuit repair guide, ref T0011/1, /2, /3? It lists all the differences between several Ferguson sets.

Dave

[IKC2E51R8]

Originally Posted by murphyv310

Hi
Good work. Yes I've had the resistor Go high and you are correct it will cause the issue, the drive waveform with not be the correct shape changing the operation of the HOT away from a switch, I'd put my money on it's a cure.

its back to the drawing board as it wasn't the cure, set ran perfectly for 20 mins and shut down again. No visual indication on the screen to give any clues what might be going on, didn't loose any width etc. just went straight off.

I'm wondering now what else could be going on and as the back was on it when it happened im not sure how hot the transistor was getting but it may be temperature related and if so what could be causing the transistor to be getting so hot in the first place and why is it happening at random times rather than always after the same amount of time.