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Old 16th Apr 2015, 7:23 pm   #1
ukcol
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Default Decca TPW70 I.F. fault.

Some time ago I purchased a Decca TPW70 and I finally got around to doing an electronic restoration towards the end of last year. This set is an attractive 1961 transistor model designed to hang on the wall of a bathroom or kitchen and it responded to the usual capacitor replacements and alignment.

It hangs on the wall in our kitchen and gets occasional use. About a month ago I switched it on and it appeared to be dead. Closer examination showed that the audio amplifier was working OK and that it was receiving stations albeit very very faintly.

Voltage checks indicated that DC conditions were about right when suddenly the set burst into life to work normally again. I put the set to one side and the fault eventually returned only to clear when I took voltage measurements in the I.F. amplifier.

This cycle of working and not working continue for some time. I did a blanket resolder of the joints in the I.F. amplifier but the problem persisted. I found that the fault often cleared when I put the meter probe on the collector of the last I.F. amplifier transistor.

The last I.F transformer is tuned by a polystyrene capacitor connected across the primary. This component is C20 in Trader sheet 1541 but its value is not listed. The capacitor itself turned out to be marked 250 (250pF). I remembered from bitter experience that some polystyrene capacitors go open circuit due to failure of the end connections, so I thought perhaps this was happening and that the pulse from the meter probe was punching through the poor connection. With the can off the transformer, and the set working well, I cut the lead to C20 and got an identical symptom. I didn't have a 250pF polystyrene capacitor so I fitted a 220pF inside the can and a 33pF on the back of the print. After a retune of the transformer the set was back up to par.

It has been running a couple of weeks without any problems but I haven't put the chassis back in its case yet.

The trouble with intermittent faults is that if they re-appear you can be certain that you haven't fixed the problem but if the fault doesn't re-appear you can't be sure that you have fixed the problem.

I am optimistic though and will reassemble it if it goes another week OK.

Last edited by ukcol; 16th Apr 2015 at 7:26 pm. Reason: repeated use of word "but"
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Old 22nd Apr 2015, 12:06 am   #2
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Default Re: Decca TPW70 I.F. fault.

Nice bit of logical fault tracing. Only time will tell but I think you have cracked it.
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Old 22nd Apr 2015, 8:10 am   #3
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Default Re: Decca TPW70 I.F. fault.

Hi Simon

I've reassembled the set and its back on the kitchen wall, so now this is the time for it to go wrong again.
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Old 22nd Apr 2015, 11:57 pm   #4
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Default Re: Decca TPW70 I.F. fault.

Just one point, what type of IF transistors does it use?
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Old 23rd Apr 2015, 8:46 am   #5
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Default Re: Decca TPW70 I.F. fault.

They are STC types Simon.

https://www.vintage-radio.net/forum/...ad.php?t=81913
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Old 24th Apr 2015, 9:20 pm   #6
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Default Re: Decca TPW70 I.F. fault.

So no AF***'s to worry about.
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Old 25th Apr 2015, 2:35 pm   #7
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Default Re: Decca TPW70 I.F. fault.

One thing about AF1xx series transistors, they're very easy to fault find.
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Old 25th Apr 2015, 3:51 pm   #8
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Default Re: Decca TPW70 I.F. fault.

You find one, you've found a fault?

Yeah, but finding such a highly probable source of trouble doesn't eliminate all the other things which can go wrong. AF1xx failures are additional to all the other things!

My jar of 2N5179s is slowly emptying.

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Old 23rd May 2015, 10:06 am   #9
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Default Re: Decca TPW70 I.F. fault.

The old Decca has been going OK for a month now, so it looks like its cured. Famous last words?
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Old 29th May 2015, 3:31 pm   #10
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Default Re: Decca TPW70 I.F. fault.

Shhh! Don't let it hear you say that!

I wrote up a very similar fault caused by a polystyrene capacitor quite recently. This type of component has been responsible for several horrid intermittent faults in my experience; if I see one, I now immediately treat it as suspect. Discussion elsewhere on the Forum concluded that heat from the original soldering is the most likely cause, being conducted along the wires and softening the plastic in the vicinity of the connection to the foil.
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Old 31st May 2015, 11:57 pm   #11
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Default Re: Decca TPW70 I.F. fault.

To add to the above, I've since read that this type of capacitor should not be subjected to temperatures above 70 degrees C. A wise precaution would be to use forceps or pliers as a heat sink whilst soldering.
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Old 1st Jun 2015, 8:54 am   #12
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Default Re: Decca TPW70 I.F. fault.

I didn't do that, it sounds like a sensible precaution.
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Old 12th Jan 2016, 2:08 pm   #13
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Default Re: Decca TPW70 I.F. fault.

The Decca is still going OK.
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