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Old 5th Nov 2020, 12:36 pm   #121
Welsh Anorak
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Default Re: Philips G8 G22K522

T4524 is the line blanking amplifier. Not sure if that would go with a crack! Very low voltages around this.
More likely you've had EHT flashover that damaged it, especially as you noticed a fizzing noise. Now have those green capacitors mentioned earlier been changed? If not they'll cause the EHT to rise alarmingly, flash over and cause general havoc to the transistors and ICs. Don't switch on again unless they have been changed.
If they have and the HT and EHT are normal then you might have damp around the anode cap. Get your hairdryer out!
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Old 5th Nov 2020, 2:31 pm   #122
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Default Re: Philips G8 G22K522

I’m not sure where the crack itself came from, I assumed the transistor as it’s backs been blown off.

Those green tuning caps were the first thing I did to the set, even before turning it on with everything connected. Might be a daft question, but what exactly is EHT Flashover? I’m assuming a very fast build up of EHT? Maybe a sudden drop? I’ve heard the term with the CRTs themselves, too.

I couldn’t tell you if the EHT is normal or not as I don’t have the appropriate gear to test it. I think you can measure HT with an everyday DMM?

I’ll make sure to give the set a blast with the hairdryer before next switch on, i’ll order some BC148s in the meantime.

Thanks for the help
‘77
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Old 5th Nov 2020, 6:05 pm   #123
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Default Re: Philips G8 G22K522

EHT flashover is due to a discharge from the high voltage area, usually the anode cap, to the CRT's aquadag coating. You generally see a flash of blue lighning before you duck behind your chair. This can be caused by excessive EHT or else correct EHT jumping across through dampness or damage to the lead or cap. The ionisation of the air gives rise to ozone (O2 to O3 in the presence of an electrical discharge) which has a characteristic smell.
It gave lots of problems back in the day, especially when damp houses were much more common due to unvented gas fires, paraffin heaters or just good old damp!
As you've changed the green meanies then it's either dampness or else I'm completely wrong and it was just the transistor exploding.
Bear in mind the HT has a direct effect on the EHT voltage, so a small increase of HT can increase the EHT by a couple of kV - not a good idea, so do check your HT. Remove the 800mA fuse on the output panel and measure the HT and reset if necessary.
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Old 5th Nov 2020, 8:01 pm   #124
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Default Re: Philips G8 G22K522

Can't say I saw any blue light, but I wasn't behind the set at the time, which might be lucky. It didn't seem to have a characteristic smell, just that of burning. I had put the HT on the PSU at about 195-200 volts, which I think is slightly underpowered, should it not be at 205V?

I'll definitely give it another check, after all, some caps or something might've reformed and raised its value? Just an idea

Thanks for the help
'77
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Old 6th Nov 2020, 10:50 am   #125
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Default Re: Philips G8 G22K522

Yes, it's possible. No harm will come from running the set at a low HT - say 180-200v while you're getting it up and running. As it is you'll just need to get the transistor and see what happens.
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Old 8th Jan 2021, 1:23 am   #126
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Default Re: Philips G8 G22K522

Hi all, another update:

So I ordered another BC148 but it just wouldn't fit in place as it was "back to front" and clumsy here managed to snap one of it's legs off in the process. I mentioned it elsewhere talking about G8's in general and someone recommended a BC108, which I bought and have now fitted into place.

Gave the LOPT and anode cap a blast with the hairdryer, just in case, and the set's working just the same as it was before the BC148 threw its back off. I never turned it on after that and before replacing it. This new transistor hasn't sorted any problem out, but I didn't really expect it to anyway.

I'm still really stumped :/

Thanks
'77
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Old 8th Jan 2021, 1:48 pm   #127
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Default Re: Philips G8 G22K522

I think you need to remind us of the faults. Just a summary as I for one am a bit lost!
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Old 8th Jan 2021, 4:54 pm   #128
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Default Re: Philips G8 G22K522

Sorry, My bad, I meant to have attached a picture again on my previous post as a reminder.

The main fault for now is there's not horizontal sync? (I think it's the sync, or is it lock?) It can be a stable image, (as seen in the picture) then turn it off and on again and it's unstable. The picture on screen is the best I could get it. There are a few smaller faults which can be sorted out in due time, such as an unresponsive volume control. Luckily it's stuck to a comfortable volume.

Luckily I don't think anything else has come to fault after the potential LOPT flashover.

(The picture on this looks slightly better than the picture in post #108 it seems, but that might just be the camera)

Thanks
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Old 8th Jan 2021, 6:58 pm   #129
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Default Re: Philips G8 G22K522

Hi 77,
I'm sure I may have said this before, but your line oscillator frequency looks way, way too high! Hence the multiple images of the test card on the screen.

It's way too long ago since I had the pleasure of working on a G8 so I can't remember what the line oscillator/ sync separator stages look like. I don't know if it actually has a user line hold (horizontal hold) control?

That's the part of the circuit I would be directing my attention!

Cheers
Nick
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Old 8th Jan 2021, 8:00 pm   #130
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Default Re: Philips G8 G22K522

I’d not know what the oscillator and sync separator stages look like either. There’s not a user Horizontal hold, only vertical. There doesn’t seem to be a lot for horizontal other than the coil which doesn’t do much.

Thanks
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Old 8th Jan 2021, 8:13 pm   #131
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Default Re: Philips G8 G22K522

Ah! I've just found the link to the circuit diagram, so I will have a good look at it later on this evening and see what secrets it can divulge!

Cheers
Nick
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Old 8th Jan 2021, 11:23 pm   #132
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Default Re: Philips G8 G22K522

Right then! I've just spent a very pleasurable hour re-reading this entire thread and studying the circuit.

It would seem you are now back to the point you were at in post#108 with the line frequency way too fast.

This had happened because you had changed C4498 (4mfd) decoupling cap for two 2.2mfd ones in series. I can't quite read the circuit reference but I think it's C4498. I suspect the original 4mfd cap was actually OK!

Before you changed that cap, the line speed was correct, but there was a sync issue. Look at the picture in post #90.

So the first thing to check is what value capacitor is actually fitted now to C4498? It would seem it's value has a radical effect on the line frequency, so it needs to be at least roughly near to 4mfd. 4.7 should work.

You need to be able to get the set back to the point you were at in post#90.

As an aside, you mentioned your new BC148 wouldn't fit. Some transistors come in different packages with the leads being in a different layout. The original BC148 would have been a 'lockfit' whereas the new ones would have been a TO 92 package which is totally different. You would need to work out which was the base, emitter & collector and then bend the leads to get them in the correct holes in the board.

Hope that helps,
Cheers
Nick
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Old 8th Jan 2021, 11:28 pm   #133
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Default Re: Philips G8 G22K522

Quote:
Originally Posted by 1100 man View Post

This had happened because you had changed C4498 (4mfd) decoupling cap for two 2.2mfd ones in series. I can't quite read the circuit reference but I think it's C4498. I suspect the original 4mfd cap was actually OK!

Before you changed that cap, the line speed was correct, but there was a sync issue. Look at the picture in post #90.

So the first thing to check is what value capacitor is actually fitted now to C4498? It would seem it's value has a radical effect on the line frequency, so it needs to be at least roughly near to 4mfd. 4.7 should work.


Hope that helps,
Cheers
Nick
If you did fit two 2.2uF capacitors in series then the overall value would be 1.1uF they would need to be in parallel.

If so this is a good spot by Nick.

Cheers

Mike T
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Old 9th Jan 2021, 12:06 am   #134
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Default Re: Philips G8 G22K522

It’s definitely been entertaining rereading this thread, even if it has been several times.

The line frequency was fast even with the original decoupler. The picture stability in #90 didn’t last very long, and I’ve never been able to get it back to that stability, and even then it wasn’t the most stable. The cap in place is now at 4.7 as this is the closest I could get. If needed I can get two more 2.2s and put them in parallel - and properly this time.

I’d figured out the BECs on the BC148 and it was back to front, E and C had swapped over. As I say was recommended a BC108 which has the right BCE configuration. This is in place and appears to be working just fine.

Definitely does help as I’d be completely llost otherwise.

Thanks
‘77
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Old 9th Jan 2021, 12:10 am   #135
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Default Re: Philips G8 G22K522

Quote:
Originally Posted by Cobaltblue View Post
If you did fit two 2.2uF capacitors in series then the overall value would be 1.1uF they would need to be in parallel.

If so this is a good spot by Nick.

Cheers

Mike T
I did originally do this, but the mistake was noticed and rectified with a singular 4.7uF cap.

Would it be recommended to get two more 2.2uF’s and actually put them in parallel and not series?

Thanks
‘77
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Old 9th Jan 2021, 12:00 pm   #136
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Default Re: Philips G8 G22K522

Quote:
Originally Posted by 19Seventy7 View Post
I’d not know what the oscillator and sync separator stages look like either. There’s not a user Horizontal hold, only vertical. There doesn’t seem to be a lot for horizontal other than the coil which doesn’t do much.

Thanks
‘77
The sync separator is contained within IC2001 which is a TAA700, so far as I can make out pin 2 is line out and pin 15 is field out, T4500 is the reactance transistor and T4511 is the line oscillator transistor.

The value of C4498 shouldn't be super critical, two 2.2uF's connected in parallel will be ok.

I don't have the full manual, just a few circuit details, but there should be a procedure in the manual for setting the line oscillator frequency etc once you've reached that stage.

Lawrence.
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Old 10th Jan 2021, 12:04 am   #137
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Default Re: Philips G8 G22K522

That's interesting, I'd always thought it'd have been a more 'obscure' component, rather than a transistor, to be an oscillator, maybe more along the lines of a crystal type component.

I've a copy of Practical Television with the G8 Servicing article. It mentions the line oscillator and says that "T4500 is phase-shifted through C4506. C4507 and R4508." and that "C4496, R4495 and C4498 form the time-constant filter."

Personally, I'd be pointing the blame here? But of course I don't know anywhere near as much as others on here, so wonder about your thoughts?

Thanks all for the help
'77
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Old 10th Jan 2021, 12:10 pm   #138
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Default Re: Philips G8 G22K522

Hard to say without being there, chicken/egg etc, basically it's flywheel sync, line oscillator>line output>sawtooth to phase detector>phase detector output>reactance stage>line oscillator.

Sync separation etc is within the IC. The phase detector is gated to operate during line flyback, the reference sawtooth from the line stage is at pin 2 of the IC as is the control voltage for the reactance stage, the gating pulse from the line stage is at pin 3 of the IC. The sawtooth is obtained by integration of a pulse from pin 2 of the the line output transformer, the gating pulse is obtained from pin 6.

Might be worth checking any relevant plug/socket/connections and work you've done etc, if ok I would do some voltage checks.

As said before there should be a procedure in the manual for setting the line oscillator frequency, I would check that out as well.

If that's any help.

Lawrence.
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Old 10th Jan 2021, 3:55 pm   #139
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Default Re: Philips G8 G22K522

Quote:
Originally Posted by 19Seventy7 View Post
That's interesting, I'd always thought it'd have been a more 'obscure' component, rather than a transistor, to be an oscillator, maybe more along the lines of a crystal type component.
Transistors can be made to do many things- oscillators being one of them. If a crystal is involved in an oscillator circuit, it will run at a very precise frequency determined by the crystal.

Whilst a line oscillator in a TV needs to run at a precise frequency, it also needs to be controllable in both frequency and phase. The line (horizontal) sync pulses ensure that the line oscillator runs at the correct frequency but also that each line starts at the correct time. In the absence of sync pulses, the oscillator should run at nearly the correct frequency.

So the two transistors (T500 & T511) form an oscillator which can be controlled by the sync pulses.

As everything changed when you changed C498 (4mfd), I still think this is the cause of the oscillator running way too fast. After all, it was running at the correct frequency before C498 was changed, but there was a sync fault.

Look very carefully at the value on the new capacitor- is it really 4.7mfd? Not 0.47 or 47mfd? Is it actually connected? Check for lifted print or hairline cracks around its legs. Do you still have the original capacitor you could try?

All the best
Nick
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Old 11th Jan 2021, 11:00 am   #140
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Default Re: Philips G8 G22K522

I'd pay close attention to both Lawrence and Nick above.
It doesn't help that the circuitry is on two boards. As the timebase panel's been out it's worth checking the edge connectors - the pins can come away. The circuit description from the maunal might help:-
"The line oscillator under normal operating conditions is fed from the 25v rail through R4517 and D4526, T4511 with its associated components forms a sine wave oscillator whose frequency is set by means of the variable inductor L4501. Fine control (sync) is achieved by means of a capacitive reactance stage using transistor T4500."
The voltage on T4500's collector should be 17.8v, base 7.35v and emitter 6.9v and T4511 C 13.0, B -2.45 and E .014v - are they close?
Although basically a straightforward circuit there are quite a few components here. Do check the line coil for continuity. An important voltage is at TP47 - 7.35v - this is the control voltage to T500's base.
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