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 Help wanted on a Breville 730T Radio
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 Return to top of page · Post #: 1 · Written at 12:46:40 PM on 1 June 2019.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 272

Hi all,

I have a Breville 730T Wireless I'm restoring. It was built in 1948.
I have the circuit for it but am unable to identify what I think is a choke or transformer that is marked as 12H on the circuit - also, the audio output transformer is missing - I have a few spare output transformers but am not sure what would be the best to use.

Appreciate some advice.

Thanks
Scott


 
 Return to top of page · Post #: 2 · Written at 3:11:04 PM on 1 June 2019.
Gandhn's Gravatar
 Location: Windella, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 319

The item marked 12H on the circuit (12 Henry inductance) is indeed a choke and will look like an output transformer, but with only one winding. It may have been replaced with a resistor at some time, if you cannot see something on the chassis.
The 6V6 used a 5000 ohm transformer, they are pretty common.
Harold


 
 Return to top of page · Post #: 3 · Written at 6:38:46 PM on 1 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

Don't know a bout the "T", however mine was featured in "Silicon Chip" May 2012.

The Choke is between the power transformer & speaker magnet. Speaker transformer is on the 6" speaker. Use 600V Electrolytic caps on "B" (C25, C26, C27: They actually specify 525V so don't go below it) and toss the ones on the cathodes as well. RB ones will replace the cathode ones, as you leave the leads long insulate the positive lead and mount them on the socket.

Mine was not powered to see if it worked as it was very obvious that the dial light wire (rubber) was shorted so it immediately failed appraisal. I.e. Work needed to be done before it was safe to power. Take photos before pulling anything to bits: Even if its wrong. I do have a photo of the content of the pan before I attacked it.

The dial light bar is the first thing to remove, if you have not removed the chassis. Be prepared to toss the power cable & all rubber wire.

I always check resistors as I toss the paper caps R14, R16, R18 are liable to be out of spec. 470K replaces 0.5Meg.

Do not remove ECH35 by the glass envelope. The Metalisation is the shield and the wire to pin 1 is easily broken if the bond between base & envelope is dodgy. Metalisation loss & wire break are normally repairable.

Marc


 
 Return to top of page · Post #: 4 · Written at 8:09:10 PM on 1 June 2019.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 272

Thanks guys,
I think the choke is open circuit as there is no voltage going to where OP transformer would have been. Is there a mod to replace the choke? If so, I’d rather do that than get the choke rewound
Cheers
Scott


 
 Return to top of page · Post #: 5 · Written at 9:03:11 PM on 1 June 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

Thinking the choke is open is not a solution: That needs to be proven. Running it with no speaker transformer is liable to destroy the screen of the 6V6, after which you will need to replace it.

I do think we need to know a lot more about what you have, or have not done to the set. Running it with the old electrolytic caps for instance, is a recipe for certain disaster, as it places the transformer rectifier & choke in peril. Many transformers have been destroyed this way and rectifiers and I have had to fix the aftermath.... expensive.

I have noted that on looking at mine, that a Mouse has been in it: That's the penalty for booting Mrs Black out: She liked them. So I have been obliged to slip the chassis, and place Mouse bait: It will kill the snake via the Mouse, which is why this work area had none.

Now, the choke on mine with nothing disconnected comes out at 544 Ohms, meaning with 500 Ohm & 30V drop we have 60mA and 55mA at 550 Ohms. Tolerance for these sets is around 20%. So a 5Watt WW resistor should be fine (dissipation is just under 2W) a pair of 270 Ohm in series is 540.

There is a discrepancy with the circuit I have; it quotes 270-0-270VAC secondary & the transformer in this one is marked 285-0-285 . 60mA.

Black choke wire is hooked to 5Y3 (pin 8? otherwise its 2) and the yellow wire to pin 4 (screen) of the 6V6. Where is it on yours since its been Monkeyed with?

I expect the grid resistors on 6V6 to be finished & the plate one on the 6G8 to be similar. I check resistors as I rid the set of old Electrolytic & paper caps. That saves rework & time and substantially increases the probability of the set firing up first go. There are at least 10 new resistors in my set: Take heed.


 
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