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 Hi and a Radiola 573MA
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 Return to top of page · Post #: 1 · Written at 1:58:31 PM on 8 December 2012.
River's Gravatar
 Location: Sydney, NSW
 Member since 8 December 2012
 Member #: 1256
 Postcount: 20

Hi,

My late father-in-law collected a lot of things and I have been gifted some of his old radios. He wasn't into any of this type of thing. He just collected stuff, and amid the items in his packed garage were some radios.

I never had anything to do with tube valves. My background is digital electronics... mainly old processors (ie fix, repair, program 8080, Z80, 6502 etc systems). So, this is my first foray into valves.

The radio I am trying to work on at the moment is an AWA Radiola 573MA, circa 1955. I checked for short circuits and the transformer, as well as re-wired the mains lead with a proper 3 pin unit, with earth attached to the metal chassis, and gave it a general clean up.

It powers up fine but its volume is very low and it can't pick up any stations below 873kHz. The caps look pretty ordinary, though the valves all seem to glow - however not being familiar with valves I would not know if they are good or not.

So, I have decided to replace the caps and get a new set of valves. The caps are those cylindrical yellow wax looking units and they have a band at one end, which, from using the multimeter, is the -ve side. So, I assume these are electrolytics. Some look okay but I do know electrolytics don't last forever and while they may look okay they could be far from good internally.

The caps are stamped with values such as ".047MFD 200V" which I assume means 0.047μF at 200V. Like, the M doesn't mean millifarads, instead of microfarads... does it?

So, I am looking around to find equivalent caps but most places don't have such small capacitance at the required voltage. Most electrolytics are in the order of 1 to 47μF and not 0.047μF. However, there are metallised polyester tube caps with the required capacitance and voltage, but these don't appear to be electrolytics. Is it ok to use these caps or do I need to find electrolytics?

I hope someone here can give me some guidance on this, so I can have this cool old radio barking out the cricket in my workshop for the summer... and for many, many summers onwards.

regards
River


 
 Return to top of page · Post #: 2 · Written at 2:36:26 PM on 8 December 2012.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1038

Hello River

The wax covered caps are paper caps and can be replaced with the polyester caps. There should only be three or four electrolytic caps in a typical valve radio, two large ones for HT filtering and a couple of smaller ones for bypassing etc. Electrolytics must be used for these.

You are right the values are micro farad. Don’t skimp on the voltage ratings as the surge voltages on turn-on are considerably higher than the operating voltages, especially if the rectifier valve is not indirectly heated.

Hopefully someone will come up with a circuit if you need one.

Don’t try to align the radio unless it becomes obvious that the alignment has been interfered with – the tuned circuits are remarkably stable even in radios from the thirties.

A good guide to the repairs and restoration is here:
http://www.ling.mq.edu.au/~robinson/Information/Repairing_Valve_Technology/

Your path into this pass-time is the same as mine – my first was my grandfather’s 1938 STC830 console, which takes a beating when the cricket is on.

Watch out – old radios can become a passion. Nevertheless, welcome to steam radio.


 
 Return to top of page · Post #: 3 · Written at 2:59:43 PM on 8 December 2012.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1136

Kevin Chant has a circuit for this radio: http://www.kevinchant.com/model-numbers-501---600.html

Evatco should be able to help you for your capacitor needs:
http://www.evatco.com.au

Also note, Valves should be the least of your worries when restoring a vintage radio. That said, I'd still pay some attention to the output valve (6AQ5) & the rectifier (6X4)

Cheers


 
 Return to top of page · Post #: 4 · Written at 5:25:11 PM on 8 December 2012.
River's Gravatar
 Location: Sydney, NSW
 Member since 8 December 2012
 Member #: 1256
 Postcount: 20

Hi,

Thanks for the quick replies, much appreciated.

Checked the links you gave me. Excellent! Also got me a list of paper caps to be replaced, including the electrolytics (there are 2 of them).

So I will replace the electrolytics (24μF 350V) with 22μF 500V units. Will replace the other caps with new metal film tubular units, all rated at 630V. This is from Evatco.

I assume the mica and ceramic caps are okay, or should these be changed also?

I will also get a full set of valves, but first upgrade will be the capacitor replacement.

That schematic and info from Kevin Chant is brilliant, though I cannot see any 24μF electrolytics in my unit - they are labelled .1μF, but are in the same place and connected as per diagram. So, maybe they were replaced a long time ago, but I would of thought such a vast change in capacitor value would not have a desirable effect on the circuit.

And yes, this is interesting.. playing with valves and this sort of stuff. After I get the Radiola going I've got a smashed up STC Emperor, but I fear it is beyond repair. And I have a large Eddystone, which is a beast of a thing and I bet it's got a gut ful of parts that need replacing.

regards
River


 
 Return to top of page · Post #: 5 · Written at 9:15:24 AM on 17 December 2012.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4539

Old radio's are wonderful for teaching you where to not put your fingers.

The fact it makes loud hum is suggesive that it is liable to work, but you have made a significant error in the wiring.

The waxy caps are "are wax paper ", or metalised paper and should be replaced, The black "Muds" are notorious for cracking their jacket and I have noticed some seventies polyesters (cream) also cracking.

All of the electrolyics (not just the filters) need to be replaced.

As per previous, even when experienced in fixing, a circuit diagram is desirable. Some who attempted to fix these things abandoned them, with serious errors and I have seen some absolutely bizzare errors.

My initial prognosis is that you have wired in a filter cap incorrecly. The set is "Back biased" which is not common in Solid State.

With back bias the most negative B voltage point is the transfomer secondary centre tap. which floats above the chassis: The chassis is positive relative to it.

In that configuration the first filter cap has it's negative end on the centre tap, not the chassis. If wrong or reversed, you get "a loud bang" or hum.

These things are not like a modern car, they are more akin to a Vintage car. You can't plug it into a computer to see what is wrong, you actually have to know, or find out.

Finding out, is what this forum is about, all you need to do is ask for direction.

Marc.


 
 Return to top of page · Post #: 6 · Written at 1:33:19 PM on 17 December 2012.
Redxm's avatar
 Location: Tamworth, NSW
 Member since 6 April 2012
 Member #: 1126
 Postcount: 448

As Marc said, there is obviously a wiring error somewhere. Did you replace the caps one at a time so you remember where they go, or did you pull them all out at once.

Bit like changing spark plug leads...

The 573MA is a nice radio when running correctly. I've owned a few over the years but alas, I only have one now (nice grey one)
Are you able to take a photo of the chassis and post it up (may have to email it to Brad)

If you were a bit closer I'd offer to have a look at it for you.

'IF' I get time tonight I'll try and take a few under chassis photos of mine as reference

ben


 
 Return to top of page · Post #: 7 · Written at 7:21:18 PM on 17 December 2012.
Redxm's avatar
 Location: Tamworth, NSW
 Member since 6 April 2012
 Member #: 1126
 Postcount: 448

 
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