Burned audio inverter xfmr for '37 American Bosch...

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Burned audio inverter xfmr for '37 American Bosch...

Post by Dion on Sun Mar 18, 2012 10:32 pm

Hi all: I could use some guidance on how to fix or replace an audio phase inverter transformer for a 1937 American Bosch radio I'm restoring. After replacing all paper/wax and electrolytic caps and out-of-tolerance resistors, I determined that I have another big problem: The radio uses a push-pull audio output stage. The output pair is fed by a single 1st AF tube running through a transformer to invert the signal for half of the push-pull pair. That transformer has an open primary, completely disabling the audio section. The xfmr is supposed to have DC resistances of 3200 on the primary and 3800 on the secondary. I'm sure that this impedance is critical for the 1st AF plate loading and also for the matching input impedance of the final stage. Is anyone aware of a person out there who is able and willing to re-wind transformers? I suspect that this particular animal is going to be very rare and likely impossible to find. Thank you in advance for your help. -Dion Question
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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Motorola man on Mon Mar 19, 2012 7:04 pm

Have you checked the connections to the transformer windings? Could be broken there & may be a simple fix.
Dave
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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Dion on Mon Mar 19, 2012 9:21 pm

Motorola man wrote:Have you checked the connections to the transformer windings? Could be broken there & may be a simple fix.
Dave
Dave: Yes, I had hoped that that might be the problem, but it is not. I gingerly cut the layers of paper wrap off of until I exposed the actual transformer wire where it connected to the leads. No luck; it's still open circuit. Sad -Dion
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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Alfred Corbin on Mon Jul 02, 2012 5:50 pm

Here's an old trick you might want to try: Disconnect the primary and install a resistor, say 10K, where the primary was, between plate ane B+. Connect a capacitor, maybe .05, from the AF plate to either grid of the push-pull pair. The secondary of the grid transformer gives the necessary phase inversion for the other grid. You'll lose some gain, but it should work.

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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Dion on Tue Jul 03, 2012 2:21 am

Alfred: That is a great idea! Fortunately, though, Gary at Playthings of The Past was able to set me up with a transformer suitable to make the amp work. The impedance was not identical, but I hung a 'scope on the output and it reproduces a good, clean sine wave. I'm now trying to figure out why the RF section isn't working. I recapp'd the whole radio, and apparently made a mistake somewhere in the RF section. I just need a few hours' time to work on it. -Dion
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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Alfred Corbin on Tue Jul 03, 2012 10:11 am

That's good news. Just for future concerns, impedance match in this application means little. It's fairly important when considering amn output transformer.

Good luck..

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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Dion on Tue Jul 03, 2012 9:14 pm

That is what Gary (Playthings of the Past) told me; said I was worrying too much about the impedance. What he sold me works just fine.
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Re: Burned audio inverter xfmr for '37 American Bosch...

Post by Dion on Thu Feb 21, 2013 10:13 am

Here's an update on the '37 American Bosch: Solved the problem with the RF section (turns out that during restoration I had landed one wire on the wrong lug of a terminal strip, so no B+ voltage to the RF amp or oscillator). Radio works great now, but needs an antenna.
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