Decca TP85 transistor problems.

[Darren-UK]

I've today revived an ex carboot Decca TP85 which needed a replacement for TR5 which is an AF116 ( as are TR3 and 4 ).

Problem was, this transistor needs long legs to pass through a narrow gap between two capacitors, otherwise the transistor can fouls the caps and the legs won't reach to the pcb. The only transistor I could find with long enough legs was an AF115. I fitted this and the radio now works fine.

I was wondering, will it cause any problems running the radio with AF116-AF116-AF115 instead of the correct three AF116s ? or should I replace all three with the same coded transistors ? ( At present I have no better ones such as AF125/7 etc to use ). Having fitted the AF115 I did find it necessary to tweak the alignment a little but that may have been needed anyway, transistor swap or not.

As an aside, the radio showed a non related additional fault. There was an uneven rumbling at the 'speaker and a horrible feedback-type howl with volume set roughly above midway. I eventually traced this to a duff OC71. I've not come across that fault before now

[ukcol]

The AF115 and AF116 have very similar characteristics, the main difference being in frequency response. The AF115 is able to handle signals up to around 100 MHz but the AF116 although fine at 10.7 MHz would be poor at much higher frequencies.

I would guess that in the manufacturing process they started life as the same transistor and were then labelled after their parameters had been measured.

As you have found the AF115 works fine in place of an AF116 but if you had wanted to replace an AF115 with an AF116 you probably would have had a problem as the AF116 would not have been able to handle the higher frequencies.

[Darren-UK]

Thanks for that Colin, I'll leave the radio as it is then

[Hybrid tellies]

Hi Darren
As Colin said the AF115 was designed to work on the higher frequiencies notably those used in television IF stages. We came across these in the early hybrid and all transistorised tv sets dating back to the mid 1960's.
Strangely enough despite the higher temeperatures and voltages they were reliable so perhaps you may have fitted a more reliable replacement.

[Biggles]

Sounds like a microphonic OC71 Darren. I have come across light sensitive ones before if the paint gets damaged. Can cause a buzz if working under fluorescent light with the transistor exposed. Guess who found this out after wasting a lot of time trying to find a strange fault.

Biggles.

[Darren-UK]

Originally Posted by Biggles

Sounds like a microphonic OC71 Darren. I have come across light sensitive ones before if the paint gets damaged. Can cause a buzz if working under fluorescent light with the transistor exposed. Guess who found this out after wasting a lot of time trying to find a strange fault.

Biggles.

No it wasn't damaged black paint as I checked it for that. All I know is changing the OC71 cured the problem

[Tim]

OC71 Darren. I have come across light sensitive ones before

Indeed, they can be used as photo transistors.
Just scrape the paint from an old one, pop some sleeving over it leaving the end of the sleeving proud of the transistor. Connect an ohmeter across the base and emitter. Then see what happens when the open end of the sleeving is covered and uncovered.

[Panrock]

Mine on the bench today had a duff OC71(TR6) too.

Another point I noticed... on both trader sheet 1634 and the Newnes book entry, C71 (10μF) is shown connected the wrong way round. In fact the negative (not positive) side should be facing R52. Fortunately in the actual set, this was connected correctly.

Steve

[jim_jobe]

Re-microphonic Germanium transistors. Many years ago I came across the opposite of this, i.e. a transistor which produced sound. It was an unmarked Germanium one in a TO5 can and I was using it in a class A output stage. The output transformer somehow became disconnected but I found I could still hear sound coming from the transistor itself - presumably from some sort of piezoelectric/magnetostriction effect,
Jim

[Station X]

Originally Posted by Tim

Indeed, they can be used as photo transistors.
Just scrape the paint from an old one, pop some sleeving over it leaving the end of the sleeving proud of the transistor. Connect an ohmeter across the base and emitter. Then see what happens when the open end of the sleeving is covered and uncovered.

I seem to remember that Mullard produced a light sensitive transistor ORP71? which was just an OC71 without the paint. These cost more than an OC71, so hobbyists just bought an OC71 and scraped the paint off. Apparently when Mullard found out about this they started putting the paint on the inside of the glass can.

I can remember buying OC71s etc in the mid 60's. They came in a Mullard logoed box about 3" x 1" by 1/4". A far cry from the bagfuls we get today.

[Panrock]

Originally Posted by Station X

I seem to remember that Mullard produced a light sensitive transistor ORP71?

OCP71.

S

[jim_jobe]

If anyone wants a black-painted OC71 to scrape the paint off I have a few unmarked ones which make great OCP71s,
Jim