Improve the Philips G8. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 10th Mar 2008, 11:50 am

One of the best TV sets of the 1970s was the Philips G8 Even I liked them.
But it was let down by the thyristor power supply.
Here's a project for someone. Design a better power supply for it. What comes to mind is the Siemens power supply employed in the Bush T20.
This PSU supplied the receiver with a regulated 200 volts. It was reasonably reliable, the regulation was good and did not cause any problems to the mains supply.

A good idea?

DFWB.

Studio263 · 10th Mar 2008, 2:01 pm

Was it really that bad? Apart from being a half wave design (which causes problems as it distorts the mains waveform) simple thyristor supplies like this have many advantages, not least of which is that you don't need a power transistor. Thyristors are very robust devices, though the ones used for TV sets seem to have been a bit marginally rated.
I think Philips got it right with the G11, the full-wave thyrsitor power supply they used was a good design and very efficient (no mains droppers or big heatsinks). The B&O 20AX chassis uses a silghtly more refined version of this arrangement (it includes a current trip rather than a fuse and has an active filter on the HT rail) which has proved to be very reliable. Both have just one drawback, the horrible 100hz racket that comes from the choke!
The TX9 shows the ultimate development of the single thyristor regulated supply (the second one in there is the crowbar). In these sets the thyristor is preceeded by a bridge rectifier which gets round a lot of the problems.

Mike Phelan · 10th Mar 2008, 6:22 pm

Hmmm ... I thought the G8 power supply was quite decent, once you got around the 'shivering' problem by putting a diode across the charging capacitor as was done in later ones.

Heatercathodeshort · 10th Mar 2008, 8:17 pm

The G8 power supply was typical of the British approach to set manufacture. It had to be cheap as British customers have very strange monetary values. They were quite willing to spend £50 a week on cigarettes but would shop around to save a fiver on something they would keep for ten years!
The G8 worked very well and once the stock faults were sorted out and ran for long periods without trouble. How they made it for the money is one of the 1970's wonders.
Regards, John.

Hybrid tellies · 10th Mar 2008, 9:51 pm

I think the G8 used the least watered down version of the original Mullard Thyrister power supply. As a result it was one of the best examples of a typical thyrister power supply from this era. It was reliable and very effective.
I would leave it well alone, after all it would not be a real G8 with a different type of power supply.
Just a further thought if you were to fit a switch mode power supply in place of the original you may have problems isolating the chassis etc as the G8 uses the AC/DC principle (as did most sets of this era) ie the chassis was connected to mains neutral.

murphyv310 · 10th Mar 2008, 10:58 pm

Hi.
Thyristor PSU's as we know were were universally outlawed by the various electricity boards, sub stations were known to be destroyed by these poorly designed power supplys!
I have to agree with Fernseh, a well designed PSU for the G8 and others using Chopped half wave thyristor PSU can only be good.
I shall be working on a design for the G8 and will post it as soon as I can.
Trevor

FERNSEH · 10th Mar 2008, 11:33 pm

Quote:

Originally Posted by murphyv310

I shall be working on a design for the G8 and will post it as soon as I can.
Trevor

Hi Trevor,
It might be possible to introduce a bridge rectifier into the power circuit. At least the negative and positive waveform of the mains supply would be used. It would be similar to the G11 arrangement and the early TX9.

For those who wish to see the awfulness of the simple tryristor power supply. Fit a 1 ohm resistor in series with neutral side of the mains lead, connect up a 'scope across the resistor and observe the waveform. You'll see a narrow spike of a few milli-seconds on the positive wave. I did this experiment in the early 80s.

DFWB.

gec2110 · 11th Mar 2008, 10:18 am

There was an artical in the television mag to provide the G8 with a soft start circuit
to over come the ht line shooting over the 200v some times at switch on
from memory it is a smiple circuit added to the print side of the power stage

Welsh Anorak · 11th Mar 2008, 3:19 pm

Hi
I think it would be a good experiment to see how picture regulation is improved (or not) by the SMPS. I was never a great fan of the thyristor PSU - it always seemed a barbaric way of producing a voltage, and as for the noise....
As said above, the G11 and B&O were better as was the Rediffusion Mk3, but thyristor PSUs were doomed as the switching regulator was about to become universal.
The G8 was a basic design - cheap, effective and, as said, typical of the period. That doesn't make it good, though!
Glyn

10th Mar 2008, 11:50 am	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Improve the Philips G8. One of the best TV sets of the 1970s was the Philips G8 Even I liked them. But it was let down by the thyristor power supply. Here's a project for someone. Design a better power supply for it. What comes to mind is the Siemens power supply employed in the Bush T20. This PSU supplied the receiver with a regulated 200 volts. It was reasonably reliable, the regulation was good and did not cause any problems to the mains supply. A good idea? DFWB.

10th Mar 2008, 2:01 pm	#2
Studio263 Octode Join Date: Apr 2007 Location: Hampshire, UK. Posts: 1,576	Re: Improve the Philips G8. Was it really that bad? Apart from being a half wave design (which causes problems as it distorts the mains waveform) simple thyristor supplies like this have many advantages, not least of which is that you don't need a power transistor. Thyristors are very robust devices, though the ones used for TV sets seem to have been a bit marginally rated. I think Philips got it right with the G11, the full-wave thyrsitor power supply they used was a good design and very efficient (no mains droppers or big heatsinks). The B&O 20AX chassis uses a silghtly more refined version of this arrangement (it includes a current trip rather than a fuse and has an active filter on the HT rail) which has proved to be very reliable. Both have just one drawback, the horrible 100hz racket that comes from the choke! The TX9 shows the ultimate development of the single thyristor regulated supply (the second one in there is the crowbar). In these sets the thyristor is preceeded by a bridge rectifier which gets round a lot of the problems.

10th Mar 2008, 6:22 pm	#3
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Improve the Philips G8. Hmmm ... I thought the G8 power supply was quite decent, once you got around the 'shivering' problem by putting a diode across the charging capacitor as was done in later ones. __________________ Mike.

10th Mar 2008, 8:17 pm	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Improve the Philips G8. The G8 power supply was typical of the British approach to set manufacture. It had to be cheap as British customers have very strange monetary values. They were quite willing to spend £50 a week on cigarettes but would shop around to save a fiver on something they would keep for ten years! The G8 worked very well and once the stock faults were sorted out and ran for long periods without trouble. How they made it for the money is one of the 1970's wonders. Regards, John.

10th Mar 2008, 9:51 pm	#5
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Improve the Philips G8. I think the G8 used the least watered down version of the original Mullard Thyrister power supply. As a result it was one of the best examples of a typical thyrister power supply from this era. It was reliable and very effective. I would leave it well alone, after all it would not be a real G8 with a different type of power supply. Just a further thought if you were to fit a switch mode power supply in place of the original you may have problems isolating the chassis etc as the G8 uses the AC/DC principle (as did most sets of this era) ie the chassis was connected to mains neutral. __________________ Simon BVWS member

10th Mar 2008, 10:58 pm	#6
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	Re: Improve the Philips G8. Hi. Thyristor PSU's as we know were were universally outlawed by the various electricity boards, sub stations were known to be destroyed by these poorly designed power supplys! I have to agree with Fernseh, a well designed PSU for the G8 and others using Chopped half wave thyristor PSU can only be good. I shall be working on a design for the G8 and will post it as soon as I can. Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

11th Mar 2008, 10:18 am	#8
gec2110 Retired Dormant Member Join Date: Feb 2008 Location: Croydon, London, UK. Posts: 408	Re: Improve the Philips G8. There was an artical in the television mag to provide the G8 with a soft start circuit to over come the ht line shooting over the 200v some times at switch on from memory it is a smiple circuit added to the print side of the power stage

11th Mar 2008, 3:19 pm	#9
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,916	Re: Improve the Philips G8. Hi I think it would be a good experiment to see how picture regulation is improved (or not) by the SMPS. I was never a great fan of the thyristor PSU - it always seemed a barbaric way of producing a voltage, and as for the noise.... As said above, the G11 and B&O were better as was the Rediffusion Mk3, but thyristor PSUs were doomed as the switching regulator was about to become universal. The G8 was a basic design - cheap, effective and, as said, typical of the period. That doesn't make it good, though! Glyn