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 STC A- 141A
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 Return to top of page · Post #: 1 · Written at 6:02:51 PM on 18 June 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Hi all I have just started repairing this radio for the guy who gave me the little TRF receiver .
Noticed a couple of things I would like your options on. Firstly the earthed end of the choke has a dial light globe in series with it ,cannot read the voltage at the moment I am leaving it in circuit unless you advise otherwise. The other point is the output valve should be a 6AG6G the circuit is still wired this way. However there is a 6V6 in it`s place. The pin outs are similar the specs are not,I can see the biasing is different . If I can't find a 6AG6G in my collection can I get away with the 6V6? The A-141 used both but a few circuit differences are noted as always thank you and Kind Regards JImb.


 
 Return to top of page · Post #: 2 · Written at 6:53:02 PM on 18 June 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

If the dial globe looks like it’s been added by monkeys, then I would remove it.
And providing the cathode of the 6v6 has a couple of hundred ohm resistor to deck, then it will work fine and not get stressed.
Probably need to study circuit for details of one end of a choke going to earth. May have something to do with biasing or AGC circuit.
JJ


 
 Return to top of page · Post #: 3 · Written at 7:13:35 PM on 18 June 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Hi Johnny. Yes it has a 330 ohm resistor between cathode and ground.
Thanks for your reply.
I meant to mention the speaker has a few very small holes in the cone. I am tempted to leave them. In the past I have patched up a few cones. I have difficulty in finding the right suitable material. I have used coffee filter paper but it tends to be too stiff. I sometimes use silastic should I leave or patch.
Regards Jimb.


 
 Return to top of page · Post #: 4 · Written at 7:30:48 PM on 18 June 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

Holes in speakers will reduce bass response, similar to not having a baffle plate.
Use PVA and coffee filter paper, but keep away from the suspension edges.
The cone needs to move in/out easily for good loudness and response.
JJ


 
 Return to top of page · Post #: 5 · Written at 9:07:19 PM on 18 June 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Thanks Johnny.
I still have some filter paper from previous repairs.


 
 Return to top of page · Post #: 6 · Written at 9:15:23 PM on 18 June 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

I do not use PVA glue on speakers even I have made tonnes of it. For use on a corrugated rim that moves I prefer Medical Micropore & Acrylic type adhesive as it never really dries & is more flexible. You can put the micropore over a hole, or mould it into the grove and paint it with the acrylic adhesive.

As that adhesive does not dry so it can pay to dust it with talc .


 
 Return to top of page · Post #: 7 · Written at 7:09:40 AM on 19 June 2020.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 882

In the 141 the choke is in the center tap return to negative, is 255 ohm and provides back bias for the 6V6 only.
This is not used as a pedestal voltage for the front end AGC "delayed AGC" like most sets.

They also gain control the front end by tapping off the oscillator grid bias voltage and driving the grids of the 6J8 and 6G8 and use that as the volume control! Must try that some day.
Why have they done that? Answer: because they could, or it was somebodies pet circuit or the set was more stable that way.

Jims 141A may have a different set up and the lamp put in the choke circuit to get a dull globe. Wacky.

For speaker repair I use a "glue" that is water base, goes on white, and as it dries goes clear but remains flexible.
It is called "gorilla snot". I have no idea what it is called or the chemical base.
I got it in a big bottle, 5L) from the speaker supplier when working for Jaycar.
Speaker cones and surrounds I repaired 20 years ago are still flexible so once it dries it is stable.
Is slightly tacky to the touch once cured.

Marc may have a clue as to what it is?

Fred.


 
 Return to top of page · Post #: 8 · Written at 10:14:41 AM on 19 June 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

To my knowledge it is a co-polymer originally for binding soil: But it can apparently be used as a hair binder & lots of other things. Now if that is what Jaycar sells the sting is in the tail.

Its biodegradable aka; it self destructs.... Now that will be interesting to follow up. Other than the hype: Any clues on the bottle.

Marc


 
 Return to top of page · Post #: 9 · Written at 12:14:16 PM on 19 June 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Hi all .
On this circuit one end of the choke goes directly to ground.There are many different versions of this circuit.
I will remove the globe , I thought it may be acting as a fuse. What is the problem using silastic to repair small holes and tears in the speaker cones.I have been guilty in using it many times ? There must be a reason non of you use it.
Regards Jim.


 
 Return to top of page · Post #: 10 · Written at 12:25:17 PM on 19 June 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1905

Provided you don't use it on the flexy bit around the edge of the cone, PVA glue is easy to work with and bonds best with porous materials like wood and paper. Dries clear, cleans up with water, doesn't shrink, doesn't stink, non-toxic, lasts forever and its cheap and readily available. What more could you ask for?


 
 Return to top of page · Post #: 11 · Written at 1:22:36 PM on 19 June 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

Well said, Ian.
JJ


 
 Return to top of page · Post #: 12 · Written at 1:33:25 PM on 19 June 2020.
Vintage Pete's avatar
 Location: Albury, NSW
 Member since 1 May 2016
 Member #: 1919
 Postcount: 1993

I use to always use it for Grill cloth,but I found that it can stay white in parts on cloth if not heated with a hair dryer. It also does not hold grill cloth as well as others,but I know it's good with speakers .
I now use a different Glue for cloth.
I forget the name of it ,but I will post it's name here later.

Pete


 
 Return to top of page · Post #: 13 · Written at 1:44:49 PM on 19 June 2020.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6349

The good thing about PVA glue is that if you set the cloth a bit out of whack you can wet the glue and correct the mistake. It'll take a bit longer to dry the second time but it'll get there with some patience.


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

 
 Return to top of page · Post #: 14 · Written at 1:52:14 PM on 19 June 2020.
Vintage Pete's avatar
 Location: Albury, NSW
 Member since 1 May 2016
 Member #: 1919
 Postcount: 1993

Yes that's true Brad.
A lot of the old glue used on cloth is Hide Glue,
It's the same glue I use on veneer or cabinets.
It can also be removed by heat even after it has dried. But Hide glue does not come in a tube , you heat it up on the stove and then use it. It's the best veneer glue as you can always take the panel off again

Pete


 
 Return to top of page · Post #: 15 · Written at 2:01:47 PM on 19 June 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

Fred, the D141 circuit I am looking at shows the centre tap circuitry as part of the AGC system.
The audio output has a cathode resistor to self bias. The back bias is NOT applied to the output valve.
Strange circuit, would love to know the reasoning and the previous and later models differences.
Cannot find the A141.
JJ


 
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