Philips G8 and G11 Psu circuit

[zak]

Does anyone have the circuit for the power supply boards for these sets?

[gdTV]

G11 PSU Schematic.....

[gdTV]

G8 PSU Schematic.....

[Mike Phelan]

Apologies if you already know, but the worst offender in the G11 PSU is the reservoir capacitor - the rivets corrode.

Regarding the G8, if it is an older one without this component, stick a small silicon diode - 1N4001, say, across C1376, anode to chassis.
This prevents HT flutter is the BR100 diac is a bit out of tolerance, which most are.
The problem is that the charging capacitor C1376 only discharges partially on negative half-cycles and therefore the diac breakover is indeterminate.
HTH

[Sideband]

Originally Posted by Mike Phelan

Apologies if you already know, but the worst offender in the G11 PSU is the reservoir capacitor - the rivets corrode.

Quite right and if you don't change it, there is a very real danger of cracking the CRT!

The rivets corrode and cause a very high pulse to be generated in the line stage. This will cause an arc of sufficient strength from the CRT electrodes to scan-coils resulting in a pinhole rupture in the CRT neck!




Rich.

[Danny]

the Gll cap used to eat the frame chip as well oh the memories of these sets you just dont get things like these in for repair any more . these were the bets dayes for me as i used to make money at the bench.

[Steve_P]

How about 'Never replace the chip (TDA2600) without changing the socket too'. And there was a cap or two you'ld be advised to change too!

[AndiiT]

Originally Posted by vince

In my 2nd year at college i asked why does the G11 have (2) 3.15mA? fuses inline with the neg/pos mains i/p he said he did not know and it was left at that do you know why.

I am not too sure about this, but I think the reason for the two mains fuses was due to a bridge rectifier being used in the PSU circuitry, and/or possibly also to dual thyristors in the circuit.

Regarding the "system switch" that has been mentioned elswhere in this thread, I don't ever recall the G8 or G11 having these; the G6 however had a "safety" switch on the Line output stage screening can due to the use of a valve (tube to those of you over the water lol) stabilizer in the EHT generator stage, which gave off X-ray radiation, hence the switch to render the line output stage inoperative once the screening cover was removed.
The G8 and G11 both having transistorised line output stages did not need protection such as this.

The main thing you need to worry about with the G11 is the "half live" chassis due to the use of a bridge rectifier as mentioned above.

Andrew

[Steve_P]

Well I've been beaten to it! The G11 power supply was a strange beast and I'm glad AndiT got in first!! It saves me getting the Manual out.....

There was a cap on the Frame Board that I found could cause the same fault as the chip and in fact out of 4 chips I replaced, along with their sockets, only 1 was actually faulty. There is a 15uF electrolytic on the board which also went faulty, especially as the sets got older. Changing this would bring about a cure as well!!!

Also the socket was often more of a problem as well as it went conductive...!

System switches have been mentioned in this thread but only as a signature. This is from the Dual Standard era and the shock of changing from 10 to 15kHz in a split second would be a death knell for a LOPT. Unfortunately most customers didn't know this. Nor did some Engineers.

The protection system on the G6 (Now there was a good set!) was to reduce the EHT to a non-operational level when the Frame Stages went Caputzel, to protect the CRT. There was a spare triode in a PCC84 and this was used as a switch to knock the line stage out when the frame collapsed.

And that lecturer must have been bad. If a student asks a question like that and he does not know the answer, then it is up to him to FIND OUT!

Cheers,

Steve.

[Steve_P]

Well I'd never put a fuse in neutral or earth. The set may be dead but still live.

I don't know is the simple answer, but as an educated guess, I'd say that they put fuses in both sides of the mains incase some idiot wired the mains wrong on either side of the mains plug, or in those days safety was not what it is now.

As to why the neutral fuse would go, well maybe that's because of the sort of mains used, where the earth was, or just luck!! Who knows!!!!

Perhaps someone could verify this?

Cheers,

Steve.

[AndiiT]

Upon scrutinising the G11 circuit a little further, I notice that the neutral side fuse does not come in to circuit until after the degaussing circuitry, which leads me to suspect that it may well be to "protect" the thyristor in the neutral side of the circuit as opposed to the bridge rectifier (I have seen many sets using a bridge rectifier power supply with only "live side fusing")
Also if I remember correctly the resevoir capacitor in a thyristor power supply takes quite an inrush of current upon switch on; with two thyristors is it perhaps the case that this current is increased and the fuses being anti-surge devices are there to protect both thyristors against this initial current surge.

I seem to recall reading the reason why there was a live and neutral fuse in the G11 in an old copy of "television" magazine some years ago, maybe I should have a dig through my old copies and see if I can confirm this.

Andrew