I've been asked to rewire/repair an A8551. It's been in storage and every known critter which chews wires has been living in it.

The owner of it tried to reconnect the wires and tripped the 240v breaker. Now they're asking someone who knows what they're doing (me) to have a look at it.

Since it's tripped a breaker I'm thinking maybe he's damaged the unit so I don't want to tell them I'll have a look unless I can get a circuit.

I've googled and found nothing, can anyone help with one or point me in the right direction?

Michael.

It would be a good idea to pull the rectifier & examine the primary's mains wires before moving on.

Marc

Schematic for STC 8551 (no A) is here, if that's the model you have: http://www.kevinchant.com/uploads/7/1/0/8/7108231/8551.pdf

Thanks Marc and GTC.

If I do the job I'll start looking for shorts first. I have no idea what the owner has done with his wiring attempt.
That circuit is the model in question.
I'll contact the owner tomorrow with the news.

The next question will be where to get valves if any of these are crook. I haven't googled yet.
I remember in the old days we used to use a valve tester to check the valves. They're 60 years old.

You may be surprised: valves can last many decades. It's usually other components -- especially capacitors -- that die first.

I still have a 1938 Valve & circuit tester in service. Many of the shorts are obvious and caused by deteriorating wire, rubber in particular.

Electrolytic caps are chemical. The insulation is a layer of oxide; This disappears if the cap is left unused as it requires current to maintain it. When this scenario occurs, they present as a short circuit and can and have destroyed transformer HT secondaries.

That is one reason why you never power sets until you inspect them. That STC has a shielded transformer, fit a tree wire cable. I.e. one with earth & ground the chassis. Make sure that you have cleared all of the shorts before powering & do not reinstall the rectifier until you have proven the mains side & the heater windings. Note that the 6.3V heaters only ground via the "humbuck" R33, so do make sure globe holders are not shorting as well.

Do not overlook that the short is associated with the turntable. I have a 1940's turntable here where some cheapskate made the gearbox out of "Die Cast" aka "Pot Metal". Typically is is disintegrating and the wiring terminals and it are no longer intrinsically safe. It is therefore dangerous & a write off as is.

It is normally that around a valve that fails. Age is not a primary factor, more how it was built in the first place. I have put genuine pre 1930's (numbering changed 1930) UX-201-A valves back into service & they are still working.

Marc

Hi all,

I'm an ex broadcast tech having working in ABC radio and TV transmitter sites for 11 years but it's been 22 years since I was in that area so I'm a bit rusty.

Thanks for the help and I'll take note of everything you guys mention.

I've just sent the owner a message telling her I'm willing to give it a go so I'll keep everyone informed of progress.

Michael.

There is a person in the local radio club here that was on the TV transmitters at Mt Major Vic. He was also replaced about the same time with new equipment.

Marc

I worked at Mt Major doing relief work and part of my training back in 1984 or 85.

I lived in Shepparton for a few years working at Radio Australia and met my wife there.

In fact I worked at every ABC TV site in Victoria except the one out in western Victoria.

You probably would not recognise RA now its gradually being squeezed out of existence.