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 STC A5235 Radiogram circuit
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 Return to top of page · Post #: 1 · Written at 6:15:58 AM on 6 December 2018.
MarkTwo's Gravatar
 Location: Sydney, NSW
 Member since 9 September 2013
 Member #: 1407
 Postcount: 11

Hello everyone

I was asked to look at reparing an STC radio gram for a family friend while I was visiting my family in Wagga last week
. Pulling the chassis out was very easy, and judging by the amount of dust and cobwebs, I am the first person to peer inside in the last 50 years...
I am after a circuit diagram if anyone has one. While I have recapped a few old radios over the years, I had the entire radio to work on. It simply wasn't practical to take the entire radio gram with me, as it's about 2 feet wide, and about 3 feet high!!
With the speaker still mounted in the cabinet, I will have to whip up an suitable sutstitute for the audio transformer, and speaker, and hope that the radio tolerates a degree of mismatch while I test it out.
Do we have any members from Wagga that might be able to help if things get tricky?


 Return to top of page · Post #: 2 · Written at 7:50:06 AM on 6 December 2018.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1030

I'll send a PDF copy of the schematic to Brad to upload here.

STC A5235 Circuit Diagram

 Return to top of page · Post #: 3 · Written at 1:27:44 PM on 6 December 2018.
Brad's avatar
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5203

Document uploaded to Post 2.

A valve a day keeps the transistor away...

 Return to top of page · Post #: 4 · Written at 11:03:51 AM on 7 December 2018.
MarkTwo's Gravatar
 Location: Sydney, NSW
 Member since 9 September 2013
 Member #: 1407
 Postcount: 11

Thanks Monochrome TV and Brad.

Now to start making a shopping list of parts, and just hope that none of the valves need replacing!!


 Return to top of page · Post #: 5 · Written at 9:06:29 AM on 7 February 2019.
MarkTwo's Gravatar
 Location: Sydney, NSW
 Member since 9 September 2013
 Member #: 1407
 Postcount: 11

Hi Guys

The good news is that after "sleeping" for more than 30 years, this radio is back working again.
All the circuit voltages are pretty close to the documentation I recieved from Brad, the plate voltage on the 6AT6, is about 40 volts higher, even though the cathode voltageis only high by 0.1 volts.
After about 10 minutes though the sound became distorted at low volumes, and the volume control has to be turned up to get sound.
I was working on the radio in high 30's temperatures while being in Wagga during a hot summer, but none of the voltages seemed to change during the fault conditions.
I eventually had to admit defeat, and brought the radio back to Sydney, but I still haven't been able to repeat that distortion fault, probably because it's cooler .
I might try and see if I can get a replacement 6AT6, it would seem to be the main suspect, but I did notice, that the original wiring of the amplifier output, (6BW6) grid is wired to IC on Pin 1, and not to the grid on Pin 2.
Even though it's been working fine for all that time, should I fix that wiring error, or leave it as it is?


 Return to top of page · Post #: 6 · Written at 9:15:24 AM on 7 February 2019.
Gandhn's Gravatar
 Location: Windella, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 306

Both pins connect to the grid, so you can use either. Just leave it as is.


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