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 Return to top of page · Post #: 16 · Written at 3:07:31 AM on 14 September 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1553

a tap of the transformer

That doesn't sound right.

Perhaps you could draw for us a schematic of the area around the power supply as it currently is.


 
 Return to top of page · Post #: 17 · Written at 2:26:57 PM on 14 September 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 359

Thanks Marc, at the risk of sounding ignorant, what’s PSU and CT abbreviated for?


 
 Return to top of page · Post #: 18 · Written at 4:29:53 PM on 14 September 2021.
JFB's Gravatar
 JFB
 Location: Milton, NSW
 Member since 27 June 2016
 Member #: 1945
 Postcount: 50

PSU = Power Supply Unit
CT = Centre Tap

Joe


 
 Return to top of page · Post #: 19 · Written at 8:25:32 PM on 14 September 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2050

You have a B+ short to ground. Suspect the speaker transformer. Also look for a paper cap that's across B+ to ground. The back bias resistor that's cooking is saving you from total disaster.

All you need is an ohmmeter...

I think that set is a KG Harris (Titan) by the way.


 
 Return to top of page · Post #: 20 · Written at 9:44:57 PM on 14 September 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1553

Scotty never got back to us with the voltage on the 24μF capacitor.

A short is a definite possibility. Perhaps the speaker transformer went open circuit and now excessive current is being drawn by the output valve via G2. This is easily tested by unplugging the 6BV7.

There's many tests that can be done by multimeter with the power off.


 
 Return to top of page · Post #: 21 · Written at 10:30:17 PM on 14 September 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 359

Thanks Gents,
It will be a couple of week before I can access the chassis again as it’s not here in my workshop at present. The info you have all provided has been invaluable. I’ll keep you posted.
Cheers


 
 Return to top of page · Post #: 22 · Written at 10:26:31 AM on 15 September 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 359

Quick question to follow up from the comment made already. I’m thinking of disconnecting the OP transformer and seeing if the short clears as well as then checking the voltage on the + side of the 24μF cap. If the OP transformer is faulty, what’s the replacement need to be rated to work? The current OP transformer looks like it’s already been replaced once.


 
 Return to top of page · Post #: 23 · Written at 7:13:34 PM on 15 September 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2050

7000 ohms primary is correct for a 6BV7 as I recall. Secondary - to suit your speaker.

If the tranny looks like it's been replaced, possibly the wires have been trapped under it (because the access hole was in the wrong place) and shorted to the chassis? Those pitch potted transformers are generally fairly reliable.


 
 Return to top of page · Post #: 24 · Written at 8:09:09 PM on 15 September 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1553

You can disconnect the red and blue wires of the speaker transformer, and measure the resistance from one to the other, should be a few hundred ohms. Also measure the resistance of those wires to the casing, should be infinity.


 
 Return to top of page · Post #: 25 · Written at 10:23:41 PM on 15 September 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4602

On the centre tapping 6X4 will be a duo diode (2) The transformer high voltage secondary is wound such that there will be a centre that goes to ground, or perhaps via the two 120 Ohm resistors and the two other halves have their ends going to the plates of the 6x4 Pins 1 and pin 6

Positive exits pin 7 (read clockwise base or wiring side) 6BV7 has the diode plates for detection & The pentode is listed as 8K primary on the speaker transformer (that does not mean 8K DC) The Loctals are both RF pentodes 7G7 is low gain and both are pin interchange able so will not short if in the wrong hole. They will be the RF valves. 7C7 1st IF.

Measuring B+ with an ohm meter should not be done with electrolytic caps in circuit, on it.

Those IF cans are unique I have seen them & have an American set here with them. The lot over the ditch used quite a few American parts.


 
 Return to top of page · Post #: 26 · Written at 11:44:02 AM on 24 September 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 359

Looks like the main transformer is cooked. Removing the OP transformer from the circuit and reinstalling the 6X4 sees the transformer immediately start to get hot and smell, there does appear to be a short. I wonder if I can find a replacement transformer??


 
 Return to top of page · Post #: 27 · Written at 12:34:55 PM on 24 September 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1553

But did you remove the 6BV7 when you did that? It's essential that you do.

I'm not sure you're reading all our suggestions.


 
 Return to top of page · Post #: 28 · Written at 12:48:08 PM on 24 September 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4602

Many have them in wrecked sets. Its all about loading. Not happy with methodology: Not convinced.

The rectifier itself could be shorted. As could be a valve, heater or dial lights. Dial light sockets do short. I would measure the voltages with all valves & dial globes out. Do be wary of bayonet globes, they have a spring in the socket which on some can short with no globe, or insulation erosion, or shrinks back.

Unloaded; If the transformer over heats, or the voltages are substantially low; then you have a problem. Unloaded means every wire except those on the primary, disconnected even the ones to ground. Sockets do short.

I normally test the insulation of the transformer per Tag & Test and between windings, If its isolated. Bit of work with sets Valves out needed.

Powering B+ & heater wiring with a 12V battery with a globe in series can aid in finding a B+ dead short (Valves & globes out).

Marc


 
 Return to top of page · Post #: 29 · Written at 12:50:26 PM on 24 September 2021.
Scotty's Gravatar
 Location: Melbourne, VIC
 Member since 26 December 2010
 Member #: 794
 Postcount: 359

Yup, 6BV7 removed and I completely isolated the 6x4 so it had no load and it still shorted.
The transformer has the following connections:

Primary side
Blue & Green - 240v in
Yellow - ground
Red & Red - 273v pins 1 & 6- 6X4
Black - 120ohm resistor to ground

Secondary side
Black - ground
Black - 6v pin 3 6X4

The above voltages were measured with 6X4 out
The transformer is quite black around the top looking like it’s overheated previously. Though when I recorded the above voltages, the transformer wasn’t overheating. The dial lamps are also not shorted.

Incidentally, I have another Royaltone branded chassis with the same transformer and 6X4 rectifier which has burned its main transformer out too, it’s completely open on the primary input. I suspect the rectifier valve is shorted in both cases


 
 Return to top of page · Post #: 30 · Written at 2:21:10 PM on 24 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

If you isolated the 6X4 then maybe its got a short between the two plates? or more probable a heater to cathode short.


 
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