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 STC 536 Radio Output Transformer.
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 Return to top of page · Post #: 1 · Written at 3:17:16 PM on 22 April 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

I've finally set myself up to being able to repair vintage electronic gear with an isolation transformer, variac and a dim bulb balast.
Now that I'm semi retired I have the time to do what I'd like to do
.
The first cab off the rank is an STC 536 large mantle radio which I would guess is from the mid 40's

I gently applied power via the above rig and got he HT up to about 150V or so with no sign of distress or smoke.

Heard some crackles from the speaker and got a healthy blurt when touching the wiper of the volume pot.

I left it that way for an hour or so, checking from time to time just to let the electrolytics wake up gently.

They along with all the paper caps would be replaced later but I just wanted to get an idea what the important bits were like ie mains transformer, speaker transformer etc.

I thought I was on a winner so I upped the volts on the variac to 230V with the dim bulb still in cct.

A few more crackles and then nothing.

No more noise from the speaker when touching the volume pot wiper and no more volts on the 6V6 anode!!

A continuity check of the output transformer showed my worst fear.

It's a potted one on a metal housing so rewinding it would be extremely difficult.

Does anyone know where I might obtain a replacement output tranny.

The circuit says it has a primary of 5K and the speaker measures 2 ohms but is most likely a 3.5 ohm unit and is electrodynamic rather than a permanent magnet one.

Any help would be greatly appreciated.


 
 Return to top of page · Post #: 2 · Written at 5:17:38 PM on 22 April 2023.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 558

If its just a repair and not a restoration, then a $12 Jaycar line transformer would do the job.
It would be difficult to tell the difference in sound quality and loudness from the original.
And being small would be easily mounting adaptable.
JJ


 
 Return to top of page · Post #: 3 · Written at 6:27:33 PM on 22 April 2023.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6720

a $12 Jaycar line transformer

Now just $9.95:

https://www.jaycar.com.au/5w-line-speaker-transformer/p/MM1900?


 
 Return to top of page · Post #: 4 · Written at 7:42:52 PM on 22 April 2023.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 558

Yes, should really stock up at that price.
They won't be available at that price forever.
JJ


 
 Return to top of page · Post #: 5 · Written at 8:35:56 PM on 22 April 2023.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Johnny is spot on.
The MM1900 I have used with success on small output pentodes.
The tranny has enough taps on it to match anything to anything close enough.
Works just like a regular OPT.
Check out some of my builds for comments on real world application.
Fred.


 
 Return to top of page · Post #: 6 · Written at 11:57:37 AM on 23 April 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

Many thanks for that info.
I'll go shopping tomorrow.


 
 Return to top of page · Post #: 7 · Written at 2:11:49 PM on 23 April 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

That STC is 1946 and the circuit in AORSM's has a short wired across the speaker transformer. I would not trust it.

Time and again I warn that powering a set in exuberance to see if it works, is never step one and I have written off a few, or initiated expensive repairs, where that has happened: You don't do that. I cannot remember, when the last time I used a "Dim Bulb" was.

Audio output transformers have an attrition rate. One can expect two in eight to be good in Kriesler's. The Audio transformer works on inductance & ratio: Not resistance. Therefore, unless it is marked on the diagram it is meaningless for anything else than continuity.

One expects any old abandoned electrolytic cap to have lost form (polarity) and present as a short circuit. Usually killing the rectifier & often taking the transformer with it. It is a radio not a capacitor reformer and they usually don't reform anyway. It should also be noted that most of these sets were abandoned because they broke down. One of the tubes could be shorted internally? The paper caps are also bad news; 99.9% of waxed paper caps will be electrically leaking and that in itself is a serious problem. Replace all of the electrolytic caps 600V for "B".

Marc


 
 Return to top of page · Post #: 8 · Written at 4:54:27 PM on 25 April 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

I bought 2 of those 100V PA transformers from Jaycar as they were only $9 each.
Fitted the transformer and fired it up with the dim bulb in series and the variac set to about 180V whilst monitoring the Anode volts on the 6V6.
Didn't take long before I could hear a slight hum from the speaker.
Placing a finger on the grid produced a healthy blurt.
None of the electrolytics were getting warm so I upped the variac to 230v.
Had no RF response when adjusting the tuning gang however.
Measured no volts on the anode of the 6B8. Traced that to an open circuit 50K resistor and leaky .5 MFD cap.
Replaced the resistor with a 47K and temporarily lifted one leg of the leaky cap.
Fired up again and was greeted with RF hash so I then put a few clip leads on the antenna leadout wire and was able to tune in the ABC loud and clear with plenty of volume. Success!
I will now go about replacing all the paper and electrolytic caps as well as replacing the broken dial string, IF alignment etc etc.
Thanks to all who steered me in the right direction.


 
 Return to top of page · Post #: 9 · Written at 8:31:55 AM on 26 April 2023.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Thats a "well done" from me.
Nice and logical.
Fred.


 
 Return to top of page · Post #: 10 · Written at 4:19:57 PM on 29 April 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

Just waiting on some parts from Carls Capacitors to arrive before diving in.
I downloaded the circuit diagram from Radiomuseum but it doesn't have any voltages.
Does anyone have a circuit diagram with expected voltages?
I'd just like to know that I'm in the ball park.


 
 Return to top of page · Post #: 11 · Written at 4:52:01 PM on 20 May 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

Work is progressing nicely on this one thanks to a delivery of caps and resistors from Carls Capacitors.
This radio is a beast to work on however.
Everything is in the way of everything else.
It has a vertically mounted tagstrip which is heavily crowded.
Removing parts without disturbing others is a real headache not helped by the differences to the circuit diagram causing confusion.
The dial cord was broken but still in situ so lots of photos taken with my phone.
The RF/IF tuning gang assembly mounting grommets disintegrated and were replaced with great difficulty.
Everything is so brittle and I'm so scared of breaking something but with lots of care so far I've dodged that bullet.
I'm not going as far as stripping the chassis to repaint it as it cleaned up pretty well and the risk of breaking something just isn't worth it.
I've removed the dial facia assembly to re finish it and gingerly removed the dial glass which is starting to flake around the edges but will present quite well once back in its frame.
It receives stations quite well even without alignment but I will touch that up once I identify all the relevant tweakers and match them to the circuit so that I know what I'm tweaking.
I'll post some pictures when it's looking presentable.
I have to say I'm enjoying the process.
Last time I repaired a mantle radio I was a teenager.
I'm 65 now.

STC 536 valve radio
STC 536 valve radio


 
 Return to top of page · Post #: 12 · Written at 10:03:59 PM on 22 May 2023.
Brad's avatar
 Administrator
 Location: Naremburn, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 7342

Photos uploaded to Post 11.


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

 
 Return to top of page · Post #: 13 · Written at 10:52:36 PM on 22 May 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

The Australian & many American circuits in "Rider" are notorious for no voltages on many circuits.

With the Filters and a 5Y3 it is advisable to use 600V Types. The old ones are likely 525 Surge Volts, or Peak Volts

The transformer is shown unconventionally as a shielded type. Ground the chassis.

The AORSM circuit has glaring mistakes yours is different & preferable.

If you use RB electrolytics on the cathodes they can be mounted on the socket with the leads a bit on the long side for cooling.

Grid resistors on the 6V6 will normally be out of spec.

Normally those run at a 250V on the 6V6 screen. startup will produce an initial surge close to double that voltage that is why 600V. Very few modern Electrolytic caps are marked with a surge voltage.


 
 Return to top of page · Post #: 14 · Written at 2:41:08 PM on 24 May 2023.
Irext's avatar
 Location: Werribee South, VIC
 Member since 30 September 2016
 Member #: 1981
 Postcount: 482

All the electrolytics and poly caps have now been replaced with 600V ones (thanks Carl for supplying them).
Most of the old "body tip and dot" resistors are within spec surprisingly with only a few needing replacement.
The 5Y3 was a bit sad and replacing it gave 50V more on the HT rail.
Just waiting on some dial cord to arrive and once fitted alignment will begin.
After the chassis is finished I will tackle the cabinet which has lost all the shellac bar a few patches.
Luckily it's all complete.
Even has a full set of knobs with the felt washers underneath.
Listening to 3LO 774 loud and clear as I write this.
An aluminium window serves as an antenna quite well.


 
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