Philips k35 - Dead on start-up - UK Vintage Radio Repair and Restoration Discussion Forum

cornboats · 10th Dec 2006, 10:58 am

Having resolved bright blue picture, I started to take scope results from test points.

I started with test point 13 -supply board and got a good likeness. Couldn't get a result from test point 10 -terminal 14 - so moved the probe hook on the terminal I'd soldered on to aid testing. The set was heard to power off then on again as I moved the hook, then the set went off completely.

I checked on-board fuses and found rectification board -U2, 2 amp blown. Changed, but set will not power up. Red light comes on when mains is switched on but when channel is selected, light goes off.

Some voltages are present on rectification board.

12v to supply board U11 (this included overprotection circuit)
There are 2 x 3.5K resistors in series before this output that are getting really hot, though.
300v is present which goes to chopper.
There should be a 40v coming in from LOPT, but as this is driven from supply board I thiought this could be why it's only 26v.

Any ideas would be gratefully seized upon.

flyingtech55 · 11th Dec 2006, 2:25 pm

Hi Cornboats

I think (note think, my forgettery becomes more and more capacious daily) the two resistors are only used to start the set up. When the line stage starts up, it takes over and supplies the running supply. However as the line stage isn't running now the start supply is sweating a bit.

I am however happy to stand corrected if I am getting confused (again).

TimR

Welsh Anorak · 11th Dec 2006, 5:32 pm

Hi
Check the chopper transistor isn't short - it doesn't blow the fuse if it is, unusually. Then suspect the TDA chopper control IC on the power board (second one in). As the set isn't tripping, I'd expect the fault to be very near to that IC.
Good luck!
Glyn

cornboats · 11th Dec 2006, 6:26 pm

Glyn and Tim R

Many thanks for your comments I'll check out and advise via forum.

cornboats · 12th Dec 2006, 10:37 am

Glyn. Checked chopper out. It was shorting on all three pins. The fuse did blow but only the once. I only hope it didn't take the IC out with it, but I am not going to know until I find a replacement transistor.
"BU426V" .

Dave

Welsh Anorak · 12th Dec 2006, 6:48 pm

Hi
You may find a BU426V a bit hard to find - a BU508A will work fine in that position if you've one to hand.
Hopefully the IC will be fine.
Glyn

10th Dec 2006, 10:58 am	#1
cornboats Triode Join Date: Nov 2006 Location: Bromsgrove Posts: 31	Philips k35 - Dead on start-up Having resolved bright blue picture, I started to take scope results from test points. I started with test point 13 -supply board and got a good likeness. Couldn't get a result from test point 10 -terminal 14 - so moved the probe hook on the terminal I'd soldered on to aid testing. The set was heard to power off then on again as I moved the hook, then the set went off completely. I checked on-board fuses and found rectification board -U2, 2 amp blown. Changed, but set will not power up. Red light comes on when mains is switched on but when channel is selected, light goes off. Some voltages are present on rectification board. 12v to supply board U11 (this included overprotection circuit) There are 2 x 3.5K resistors in series before this output that are getting really hot, though. 300v is present which goes to chopper. There should be a 40v coming in from LOPT, but as this is driven from supply board I thiought this could be why it's only 26v. Any ideas would be gratefully seized upon.

11th Dec 2006, 2:25 pm	#2
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	Re: Phillips k35-Dead on start up Hi Cornboats I think (note think, my forgettery becomes more and more capacious daily) the two resistors are only used to start the set up. When the line stage starts up, it takes over and supplies the running supply. However as the line stage isn't running now the start supply is sweating a bit. I am however happy to stand corrected if I am getting confused (again). TimR

11th Dec 2006, 5:32 pm	#3
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Phillips k35-Dead on start up Hi Check the chopper transistor isn't short - it doesn't blow the fuse if it is, unusually. Then suspect the TDA chopper control IC on the power board (second one in). As the set isn't tripping, I'd expect the fault to be very near to that IC. Good luck! Glyn

11th Dec 2006, 6:26 pm	#4
cornboats Triode Join Date: Nov 2006 Location: Bromsgrove Posts: 31	Re: Phillips k35-Dead on start up Glyn and Tim R Many thanks for your comments I'll check out and advise via forum.

12th Dec 2006, 10:37 am	#5
cornboats Triode Join Date: Nov 2006 Location: Bromsgrove Posts: 31	Re: Phillips k35-Dead on start up Glyn. Checked chopper out. It was shorting on all three pins. The fuse did blow but only the once. I only hope it didn't take the IC out with it, but I am not going to know until I find a replacement transistor. "BU426V" . Dave

12th Dec 2006, 6:48 pm	#6
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Phillips k35-Dead on start up Hi You may find a BU426V a bit hard to find - a BU508A will work fine in that position if you've one to hand. Hopefully the IC will be fine. Glyn