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 Help re New Zealand Radio
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 Return to top of page · Post #: 1 · Written at 11:17:49 PM on 5 September 2011.
Vk3pb's Gravatar
 Location: Boronia, VIC
 Member since 5 September 2011
 Member #: 998
 Postcount: 4

Hi all

I have just purchased a New Zealand radio - a Skyscraper which was made in 1938 by Radio 1936 Ltd.

I am seeking a ciurcuit diagram for this radio. if you have one can you plaase send a copy to p.berrett at optusnet.com.au

That aside my radio seems to work and be amplifying ok. The odd pop and crackly is quite loud but on am I can only pick up a couple of stronger stations and they are quite weak. On SW iI coudl get a coupel fo chinese stations but these were weak too. I was using an external antenna with an antenna tuner.


The valves in the radio are

- 5Y3G - a little black at the bottom
- 7S7 - somewhat blackened
- 7A7 - good nick
- A7W Silvania - seems ok
- 7C6 - a little blackened at the top
- 7C5 - good nick

My best guess is that the amplification is fine but the regeneration section may have blown a tube or be misaligned.

Any ideas? I am new to these radios but they have a wonderful feel about them

cheers
Peter VK3PB




 
 Return to top of page · Post #: 2 · Written at 11:25:20 PM on 5 September 2011.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5309

If you don't find an answer in Oz for the circuit, then this Kiwi collector/restorer may be able to help:

http://www.radio-restoration.com/bottom.html


 
 Return to top of page · Post #: 3 · Written at 6:57:57 AM on 6 September 2011.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 729

As you're new to vintage radio restoration and repair, one thing everyone needs to do is replace all the wax paper capacitors. Usually easy to spot as they are covered with beeswax. Over the years they absorb moisture from the air, and this moisture reacts with the acids in the paper, making them electronically leaky. They have a lifetime of around 30 years, but they stopped making them about 50 years ago, thus all wax paper caps are guarenteed to be bad today.

You should replace one at a time, so you don't forget where it connects to. Use within 20% of the capacitence value, and the same OR higher voltage rating. Not lower.

The power suppply electrilytic caps sometimes are bad as well. If the radio has a loud hum, or if the caps get warmer than the adjacent chassis parts (except the tubes or power transformer) they may be leaky inside and need to be replaced. They can go shorted, which is bad for the power transformer.


 
 Return to top of page · Post #: 4 · Written at 9:49:53 AM on 7 September 2011.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3814

I would re think regeneration. That has to be a Superhetrodyne 7S7 is a Pentagrid (Triode Heptode)
7C6 is the detector (& likely first AF) it may be reflexed but with a triode?.

The only Australian company that I Know of with a Year in it was AIrzone.

If any thing is out of whack It is likely that a fiddler has messed with the IF cans.

With 5Y3 I would not use electrolytics below 500V

Marc


 
 Return to top of page · Post #: 5 · Written at 12:16:37 AM on 9 September 2011.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3814

Something is not sounding right with A7W. compare it to the 7A7.

7A7 can be found in audio or RF as it is a voltage amplifier. I could predict one either side of the 7S7.

Albeit a Sylvania creation, Philco was probably the biggest user of Loctal valves.

"Snap Crackle & Pop" were likely named after leaky Mica caps Amercan ones being the worst I have found for failing. They are the noise they generally create.

SW usually fails when the valve (7S7) gets tired, or it runs out of volts.

In this case I would expect leaking paper & by the sound of it the odd Mica capacitors. It is not unusual for a few resistors to go high as well, compounding any problems. Mica's with HV DC on them are the likely ones to fail. If they have based this on a Philco & used Micamold caps (USA) replace every one of them.

All of these leaks will drag down the Voltage or impinge on the valve's bias and subsequently, performance.

7C5 (output valve) may have a Mica cap on it's plate. Plate bypass caps on Audio output valves have a very high attrition rate as do the resistors.. grid and cathode (cathode may not always have one)

A multi meter is useless for testing leaking caps unless thy go dead short.

Black in the valve is often the "Getter" & does not indicate tube failure. White, then I would worry especially if it acompanied by a pale blue glow, outside of the plates.

Marc


 
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