Noisy Decca TP85 - ideas? - UK Vintage Radio Repair and Restoration Discussion Forum

amillar · 1st Aug 2019, 5:08 pm

Hi,

I've been restoring my wife's TP85 from her childhood, which was in a pretty awful state. All sorts of things living inside it. With the aid of a donor set for speaker, aerial, buttons and battery connectors, and after fixing a crack in the PCB, it remarkably came back to life. But very noisy (RF noisy). I'm only trying to get the FM working.

Got hold of a TF2015 FM generator and aligned it last night - my first time ever aligning an FM set in 50 years of mucking around with analogue electronics! - and it made no improvement at all. In fact the alignment seemed pretty spot on - I just took it out of alignment and back again.

So now I need to take the PCB right out, remove the blessed screening can again, and have a proper look now I have the sig gen. But before I do I thought I'd ask here if anyone had any good guesses of where to look which could save unnecessary pain and grief - from other posts here it looks like others will have been there, done that, and got the tee shirt.

Here's some clues: The adjustments on the first RF stage, L1 and C9, did absolutely nothing (L3 and C15 on the second stage worked perfectly). So does this suggest that the first stage AF114 is suspect? Or something else? I know the dc voltages around the RF transistors all seemed ok when I first stripped it down and rebuilt it - the cracked track was in the common rail of that first RF stage so I had a good check around when I was finding and fixing it.

And also RV1, the balancing adjuster in the demodulator, did absolutely nothing on the AM rejection test, even swinging it from end to end.

I've got the "Trader" sheet for it which got me this far.

Any thoughts welcomed and appreciated!

I'm very pleased with the TF2015 (fortunately I already had a decent Philips frequency counter) - after all these years I still get FAR too much fun out of playing with a "new" piece of test kit

Cheers,

Andy

Nuvistor · 1st Aug 2019, 6:07 pm

Any AF11x transistor can be suspect, a bit more testing though before removing it. They grow tin whiskers inside the can and s/c the connections.
https://www.vintage-radio.info/whiskers

Andrew2 · 1st Aug 2019, 6:13 pm

Hi Andy. An AF114 of that vintage has got to be suspect, either through the dreaded 'tin whisker' blight or just general degradation. Tin Whisker would greatly upset the DC voltages though.
As the original PCB crack was in the RF area it's worth having another look - there may be a second crack nearby.
As for the non-operative balancing pot in the FM demod, it could be the pot itself or that old favourite electrolytic across the diodes gone dud.

amillar · 1st Aug 2019, 9:31 pm

Many thanks, as soon as I've cleared a bit of space on my workbench I'll have a go at it!

Cheers,

Andy

1st Aug 2019, 5:08 pm	#1
amillar Triode Join Date: Jan 2014 Location: Callington, Cornwall, UK. Posts: 16	Noisy Decca TP85 - ideas? Hi, I've been restoring my wife's TP85 from her childhood, which was in a pretty awful state. All sorts of things living inside it. With the aid of a donor set for speaker, aerial, buttons and battery connectors, and after fixing a crack in the PCB, it remarkably came back to life. But very noisy (RF noisy). I'm only trying to get the FM working. Got hold of a TF2015 FM generator and aligned it last night - my first time ever aligning an FM set in 50 years of mucking around with analogue electronics! - and it made no improvement at all. In fact the alignment seemed pretty spot on - I just took it out of alignment and back again. So now I need to take the PCB right out, remove the blessed screening can again, and have a proper look now I have the sig gen. But before I do I thought I'd ask here if anyone had any good guesses of where to look which could save unnecessary pain and grief - from other posts here it looks like others will have been there, done that, and got the tee shirt. Here's some clues: The adjustments on the first RF stage, L1 and C9, did absolutely nothing (L3 and C15 on the second stage worked perfectly). So does this suggest that the first stage AF114 is suspect? Or something else? I know the dc voltages around the RF transistors all seemed ok when I first stripped it down and rebuilt it - the cracked track was in the common rail of that first RF stage so I had a good check around when I was finding and fixing it. And also RV1, the balancing adjuster in the demodulator, did absolutely nothing on the AM rejection test, even swinging it from end to end. I've got the "Trader" sheet for it which got me this far. Any thoughts welcomed and appreciated! I'm very pleased with the TF2015 (fortunately I already had a decent Philips frequency counter) - after all these years I still get FAR too much fun out of playing with a "new" piece of test kit Cheers, Andy

1st Aug 2019, 6:07 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Noisy Decca TP85 - ideas? Any AF11x transistor can be suspect, a bit more testing though before removing it. They grow tin whiskers inside the can and s/c the connections. https://www.vintage-radio.info/whiskers __________________ Frank

1st Aug 2019, 6:13 pm	#3
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,037	Re: Noisy Decca TP85 - ideas? Hi Andy. An AF114 of that vintage has got to be suspect, either through the dreaded 'tin whisker' blight or just general degradation. Tin Whisker would greatly upset the DC voltages though. As the original PCB crack was in the RF area it's worth having another look - there may be a second crack nearby. As for the non-operative balancing pot in the FM demod, it could be the pot itself or that old favourite electrolytic across the diodes gone dud. __________________ Andy G1HBE.

1st Aug 2019, 9:31 pm	#4
amillar Triode Join Date: Jan 2014 Location: Callington, Cornwall, UK. Posts: 16	Re: Noisy Decca TP85 - ideas? Many thanks, as soon as I've cleared a bit of space on my workbench I'll have a go at it! Cheers, Andy