On 26/02/2016 19:16, Terry Casey wrote:
In article ,
says...

[quoted text muted]

OCP71?

IIRC if you carefully stripped the paint off an OC71 you had an OCP71.

Nope. inside was blue opaque gunk after the first few years

In other words after Mullard found that they weren't selling many
OCP71's and realised what everyone was doing.

That was what most people believed at the time but there was another reason:

If the paint on a transistor got accidentally scratched and artificial light
fell on it - though a ventilation grille, for example - the transistor would
respond to the 50Hz light.

I would imagine that the first engineer to have a battery operated transistor
radio turn up on his work bench with a mains hum fault was very, very,
surprised!

Especially when he took the back off and the fault got worse ...!

So, Mullard started adding the opaque gunk ...


When I was young, I worked for the Beeb as a technician. One day, I had
an outside broadcast mixer to check out.

While I was 'lining it up' - checking signal levels at various points -
I noticed a really bad mains hum. So I unleashed the oscilloscope. The
hum was definitely there audibly, but seemed to disappear whenever I
connected the scope probe.

To cut a very long story short, whenever I leaned over to connect a
scope probe, I put the PCB in the shadow of a fluorescent lamp and the
mains noise went away.

The mixer was all Germanium transistors. Of course, in normal use, with
the cover on, there was no problem, but when the cover was off, small
scratches in the black paint on the transistors was enough to make it
pick up the fluorescent lights, and hum.

It took a long time to find that. Nowadays, I'd just assume I'd gone
totally fishkettle aardvark alumni.

Cheers
--
Syd