Needing input on an output transformer fault!! - UK Vintage Radio Repair and Restoration Discussion Forum

Paul Mathew · 3rd Jul 2014, 9:22 am

I'm working on a Philips 171A, from a 170A schematic, but it seems identical. The output transformer has a double primary winding c, d, e. The section d - e, has gone open circuit. However, I am getting a clear signal, using a crystal earpiece, on the secondary winding h - j, used to apply feed-back. The secondary winding f-g is producing no signal to the speaker. I'm assuming that I'm hearing the audio from the grid of V4. Is there any quick fix?

Herald1360 · 3rd Jul 2014, 1:02 pm

You might get away with an ordinary OP TX if you float the LS winding and use that for the F/B as well as the speaker. Adding some extra C to the smoothing (if the rectifier can stand it) could help with the loss of the hum cancellation function.

Otherwise you need a secondhand TX or a rewind

Leon Crampin · 3rd Jul 2014, 1:08 pm

My fix for the 170A output transformer problem is to fit a further standard transformer to the set - preferably under the chassis as there is plenty of room.

Wire this in the conventional way, but leave the existing transformer in circuit, just disconnect the lead to the anode of the EBL31 to avoid possible intermittent re-connection of the old winding. Now connect the secondary of your new transformer to the secondary of the existing transformer (and of course, the LS and socket).

If you get the phasing correct, the set will operate as designed, the existing transformer being used in reverse to generate the feedback voltages and the hum cancellation voltage.

This scheme works well and also allows proper operation of the tone control.

Leon.

Paul Mathew · 3rd Jul 2014, 1:39 pm

Thanks, I'll give it my best shot and report back!!

Paul Mathew · 9th Jul 2014, 4:56 am

Hi Leon,
Just reporting back! I followed your advice (more or less!) and connected another TX across e - c, disconnecting e from the anode of the output valve. I then connected the secondary of the new TX directly to the speaker and all is working fine. Not sure if I was just lucky here, as I never connected the secondary of the new TX to the old as you suggested?! I assume the feedback loop is still functioning from one half of the old TX's primary ( c - d )??
I'm eager to learn, and am very appreciative of your suggestions!!
Regards, Paul

3rd Jul 2014, 1:02 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Needing input on an output transformer fault!! You might get away with an ordinary OP TX if you float the LS winding and use that for the F/B as well as the speaker. Adding some extra C to the smoothing (if the rectifier can stand it) could help with the loss of the hum cancellation function. Otherwise you need a secondhand TX or a rewind __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

3rd Jul 2014, 1:08 pm	#3
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Needing input on an output transformer fault!! My fix for the 170A output transformer problem is to fit a further standard transformer to the set - preferably under the chassis as there is plenty of room. Wire this in the conventional way, but leave the existing transformer in circuit, just disconnect the lead to the anode of the EBL31 to avoid possible intermittent re-connection of the old winding. Now connect the secondary of your new transformer to the secondary of the existing transformer (and of course, the LS and socket). If you get the phasing correct, the set will operate as designed, the existing transformer being used in reverse to generate the feedback voltages and the hum cancellation voltage. This scheme works well and also allows proper operation of the tone control. Leon.

3rd Jul 2014, 1:39 pm	#4
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Re: Needing input on an output transformer fault!! Thanks, I'll give it my best shot and report back!!

9th Jul 2014, 4:56 am	#5
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Re: Needing input on an output transformer fault!! Hi Leon, Just reporting back! I followed your advice (more or less!) and connected another TX across e - c, disconnecting e from the anode of the output valve. I then connected the secondary of the new TX directly to the speaker and all is working fine. Not sure if I was just lucky here, as I never connected the secondary of the new TX to the old as you suggested?! I assume the feedback loop is still functioning from one half of the old TX's primary ( c - d )?? I'm eager to learn, and am very appreciative of your suggestions!! Regards, Paul