Bush TV12AM - Frame lock fault - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 11th May 2007, 12:58 pm

I have a bit of an odd fault with my Bush TV12AM which I'd like to ask for advice with please.

If memory serves me correctly the service data for the chassis is the same as the TV12 whereas the service data for the RF unit is the same as the TV22.

The problem is with the frame hold. It has to be at the far end of its travel to lock, but, oddly enough, it locks at either end, so if turned all the way clockwise or anti-clockwise, it will lock.

The problem is that, as the set warms up, it is not possible to keep it locked after a while and it just rolls.

I do not think the frame hold control is the original, the pot used here has a purpose-made "click" at the centre of travel.

Any suggestions as to what is best looked at here would be appreciated. Should I replace the frame hold control? If so, do I need a wire-wound pot, or will a carbon one be alright?

Any advice appreciated.

Peter.

Steve_P · 11th May 2007, 1:42 pm

The frame hold control on these is 50k. Fit either kind, it does not matter.

Also check R33. R35, R30, C25, C23. Just so you know, I'm working from Trader Sheet 971.

Cheers,

Steve P.

beery · 11th May 2007, 2:01 pm

Hi Peter,
for the TV12AM it is the ther way round.
The set has a TV22 top deck and a TV12 RF unit, well the ones I've heard of are like that. Basson Bloke has a TV12AM.
Trader sheet 1003 shows the TV22 top deck. The frame hold is shown as being 1Meg in which case it will definitely be a carbon pot.

To tell which top deck you have...
The TV12 top deck has some EF50 valves on it and a two stage line output transformer. The line scan transformer sits sealed up in a can and the EHT transformer sits on top of this.

The TV22 top deck has ECL80 valves for the timebase oscillators. All the line output valves and the line output transformer share one very large aluminium housing.

I can't explain why you would get lock at both ends of the hold control (unless at one end you are locking to 25Hz). It is possible that the centre click is due to a damaged track inside the pot. After having replaced all the 0.1u caps on the frame tag board (left-hand side of chassis when viewed from back) then do as Steve suggests and check resistor values around that tag board. Remember a good old type resistor is usually only within +/- 20% of the value marked on it.

Cheers
Andy

DoctorWho · 11th May 2007, 3:46 pm

Steve - Thanks for your help. I will check the areas you have highlighted.

Andy - Thank you, yes, when I took the cabinet off just now I realised that I had indeed got it the wrong way round. The RF unit is the TV12 one, it has a number of EF50's, the main chassis is indeed the TV22 Mark I chassis. The centre click on the control definitely feels like one that is suposed to be there, a bit like a balance control which clicks when in the centre. Examining the control it looks pretty old. so if it is a replacement then it was replaced in the 50's I imagine. It is indeed odd to get lock at both extreme ends of the control, it doesn't look like 25c/s at one end, both ends seem to be proper lock, there's no flicker. The condensers have all been replaced some time ago, and the set has worked very well until recently, the problem of not being able to frame lock after a short while is a recent development. I will get on to the resistors next and let you know what I find.

Thanks again for the advice.

Peter.

DoctorWho · 11th May 2007, 4:14 pm

This is now resolved - I think. I reduced the value of one of the resistors connecting to the Frame Hold control and it now locks with room to go the other way a little. I'll now run it for a while and see how it performs.

Thanks for the help.

11th May 2007, 12:58 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Bush TV12AM - Frame lock fault I have a bit of an odd fault with my Bush TV12AM which I'd like to ask for advice with please. If memory serves me correctly the service data for the chassis is the same as the TV12 whereas the service data for the RF unit is the same as the TV22. The problem is with the frame hold. It has to be at the far end of its travel to lock, but, oddly enough, it locks at either end, so if turned all the way clockwise or anti-clockwise, it will lock. The problem is that, as the set warms up, it is not possible to keep it locked after a while and it just rolls. I do not think the frame hold control is the original, the pot used here has a purpose-made "click" at the centre of travel. Any suggestions as to what is best looked at here would be appreciated. Should I replace the frame hold control? If so, do I need a wire-wound pot, or will a carbon one be alright? Any advice appreciated. Peter.

11th May 2007, 1:42 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bush TV12AM - Odd fault The frame hold control on these is 50k. Fit either kind, it does not matter. Also check R33. R35, R30, C25, C23. Just so you know, I'm working from Trader Sheet 971. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

11th May 2007, 2:01 pm	#3
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: Bush TV12AM - Odd fault Hi Peter, for the TV12AM it is the ther way round. The set has a TV22 top deck and a TV12 RF unit, well the ones I've heard of are like that. Basson Bloke has a TV12AM. Trader sheet 1003 shows the TV22 top deck. The frame hold is shown as being 1Meg in which case it will definitely be a carbon pot. To tell which top deck you have... The TV12 top deck has some EF50 valves on it and a two stage line output transformer. The line scan transformer sits sealed up in a can and the EHT transformer sits on top of this. The TV22 top deck has ECL80 valves for the timebase oscillators. All the line output valves and the line output transformer share one very large aluminium housing. I can't explain why you would get lock at both ends of the hold control (unless at one end you are locking to 25Hz). It is possible that the centre click is due to a damaged track inside the pot. After having replaced all the 0.1u caps on the frame tag board (left-hand side of chassis when viewed from back) then do as Steve suggests and check resistor values around that tag board. Remember a good old type resistor is usually only within +/- 20% of the value marked on it. Cheers Andy

11th May 2007, 3:46 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Bush TV12AM - Odd fault Steve - Thanks for your help. I will check the areas you have highlighted. Andy - Thank you, yes, when I took the cabinet off just now I realised that I had indeed got it the wrong way round. The RF unit is the TV12 one, it has a number of EF50's, the main chassis is indeed the TV22 Mark I chassis. The centre click on the control definitely feels like one that is suposed to be there, a bit like a balance control which clicks when in the centre. Examining the control it looks pretty old. so if it is a replacement then it was replaced in the 50's I imagine. It is indeed odd to get lock at both extreme ends of the control, it doesn't look like 25c/s at one end, both ends seem to be proper lock, there's no flicker. The condensers have all been replaced some time ago, and the set has worked very well until recently, the problem of not being able to frame lock after a short while is a recent development. I will get on to the resistors next and let you know what I find. Thanks again for the advice. Peter.

11th May 2007, 4:14 pm	#5
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: Bush TV12AM - Odd fault This is now resolved - I think. I reduced the value of one of the resistors connecting to the Frame Hold control and it now locks with room to go the other way a little. I'll now run it for a while and see how it performs. Thanks for the help.