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 HRSA Radio Waves article on STC 831
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 Return to top of page · Post #: 1 · Written at 9:42:43 AM on 15 August 2018.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

There is an article on an STC 831 in the latest issue of HRSA Radio Waves.
The main problem encountered was degredation with time of the zinc die cast ("pot metal") tuning dial drum. This was causing binding in its operation. I don't know if the author is a member of this forum, but a solution to his problem may be in Vintage-Radio services directory:

http://silversprings.net.au/STC_Dial_drum.html

Such a service is not currently listed in the July 2018 issue of Radio Waves Yellow Pages.

Another problem was degredation of the wave change switch by application of WD-40, echoing warnings in this forum. I was guilty of this when restoring my STC 830 resulting in a loud bang when the radio was turned on, though I was lucky the radio continued to work.

Maybe WD-40 formulation has changed since the late '80s as has been indicated by someone previously in the forum. I rather think so since in the late '60s/early '70s I drove a Morris mini. With its transverse motor with the distributor right behind the radiator grill, WD-40 was a necessity in the boot to revive the car if the distributor and HT cables got wet. WD-40 worked every time in this application. It is hard to believe that it would have if it was conductive in any way. Maybe that is why I was lucky with my radio.


 
 Return to top of page · Post #: 2 · Written at 9:01:38 PM on 15 August 2018.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 2011

Silicon Chip article on STC 831

Silicon Chip, or Radio Waves?


 
 Return to top of page · Post #: 3 · Written at 10:13:55 PM on 15 August 2018.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
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 Postcount: 7290

Even though not everyone is tooled up for this, such parts can be hand-made on a lathe when they are not available from a supplier. Brass is a suitable replacement metal and more durable than pot metal.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 4 · Written at 10:17:09 PM on 15 August 2018.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

Silicon Chip, or Radio Waves?

Radio Waves, July 2018, page 23, "A Story With A Not Entirely Happy Ending" by Graham Parslow.

.STC830: you can edit the thread title to correct it.


 
 Return to top of page · Post #: 5 · Written at 9:13:35 AM on 16 August 2018.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

"you can edit the thread title to correct it."

Edit done. My apologies.

Pot metal castings are zinc alloy die castings. A web site that covers causes of long term failures is
https://www.globalcastingmagazine.com/index.php/2013/10/16/zinc-die-casting-defects-their-cause-and-elimination/

"Very small levels of some elements have a severe effect on casting quality and may affect the long term performance of the alloys. Lead, tin and cadmium are typical of these; being controlled down to a maximum of 50 ppm or less in zinc alloys. The grain structure of a zinc alloy allows these elements to sit at the grain boundaries. If one or more of these elements is present above the maximum specified in the standard then certain conditions (warm with high humidity) will promote inter granular corrosion within the casting. This is engendered by the formation of a minute galvanic cell stimulating the zinc to sacrifice itself at the grain boundaries. The effects in severe cases of this can be catastrophic evidenced by the total collapse of the casting structure – effectively converting the casting to oxide dust.

In less severe cases corrosion stress cracks are formed promoting premature part failure in service. The presence of these impurities is often indicated by hot shortness cracks during casting. These cracks can also be caused by other process based problems so it is always advisable to investigate all potential causes before taking action. In all instances, control of metal chemistry by use of a reputable zinc alloy supplier and implementing regular machine pot analysis procedures with good house keeping will limit the risk. The die caster should also ensure that foundry personnel are adequately trained and understand the risks associated with the presence of certain impurities."

The very low level of impurities that will cause long term failures ie 50ppm must have caused problems for manufactures in the thirties, both in analysis procedure and cost, and in manufacturing practice to keep the impurities out.

Also I don't think that they would have envisaged or cared about radios in service 80 years later. It would be interesting to know if cars such as Holdens manufactured in Australia in the 40's - 60's which used zinc die castings for items such as door handles, speedo surrounds, brake and tail light surrounds etc are showing signs of such failures.


 
 Return to top of page · Post #: 6 · Written at 9:28:24 AM on 16 August 2018.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
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A point worth noting is that I have in repairing a lot of sets & seen this phenomena frequently. It is not confined to radio. Some genius made the tongue of my MKII Zephyr's glove box latch out of it. I made a new one out of real metal.

I normally Machine new flywheels that fall apart & that can often be done with drums as well. 3D printing is getting better as is CNC machining, the latter not necessarily being cheap.


 
 Return to top of page · Post #: 7 · Written at 7:59:27 PM on 16 August 2018.
Brad's avatar
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Car parts, such as door handles were chromed so there'd be a lesser chance of failure due to less exposure of the castings to the air. I personally haven't seen such issues on Holden cars or any other. That isn't to say there's never been cases of it though.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 8 · Written at 8:47:49 PM on 16 August 2018.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

"Car parts, such as door handles were chromed so there'd be a lesser chance of failure due to less exposure of the castings to the air."

Reading more of the link I posted chrome plated parts may not have been a good example since the castings need to be highly polished before plating, and impurities existing as very hard particles on the surface showed up on polishing,and had to be of pure alloy. Marc's glove box latch and other unplated parts would probably be the ones to look at since insufficiently pure alloy would not be obvious because they weren't polished.


 
 Return to top of page · Post #: 9 · Written at 8:56:59 PM on 16 August 2018.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

Handles do break, seen quite a few do it. Die cast ( Pot metal) can be a curse. It was the answer to plastic before we had it. Everything about it from forming it etc. spelt: Cheap.

Nowadays a lot of stuff in the photos can be cut on CNC machines, or printed.


 
 Return to top of page · Post #: 10 · Written at 9:10:00 PM on 16 August 2018.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
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 Postcount: 7290

One thing I won't forget is when chrome plating peels, fingers get cut.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 11 · Written at 9:56:20 PM on 16 August 2018.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

One thing I won't forget is when chrome plating peels, fingers get cut.

Indeed. Razor sharp.


 
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