Audio Driver Transformer Zorching.....whys and hows

[ka4koe]

Can anyone describe to me how/why audio driver transformers in Johnson's get Zorched, ie the typical failure mode? My search for my issue is narrowing down. Good R between secondary of modulation transformer (less than 100 ohms).

From my research, this appears to be a common headache. I think Hammond makes a similar unit that is almost plug and play and won't require self-dismemberment of either right or leg arm/leg.

Thanks

Philip

[WQ9E]

It is a common failure item in many transmitters and this same transformer is also found in several Heathkit units.  The wire used is extremely fine and corrodes and opens with age, the somewhat acidic paper used for insulating the transformer is probably a contributing factor.  This same issue is found in the "modulation" transformers used in Central Electronics 10 and 20 series phasing exciters.

But always check the surrounding circuit before replacement, for example a shorted cathode bypass cap in the audio driver stage would significantly increase the primary current.

Most of these transformers survived for 30 years or more so they have long exceeded the expected life of the equipment.  Fortunately proper replacements are readily available.

[W3NE]

The Hammond replacement, as recommended in several on-line modifications, is their Type 124E. I have one ready to go into my Ranger restoration.

Bob - NE

[KB2WIG]

Has anyone tried to rewind said Heapkit/Ranger/Viking driver transformer??

There must be a whole bunch of 'em lying around.


klc

[WQ9E]

Has anyone tried to rewind said Heapkit/Ranger/Viking driver transformer??

There must be a whole bunch of 'em lying around.


klc

At one point you could buy just the windings from the equivalent Stancor A-53 and reassemble your core around them but I haven't seen this part advertised in years.  Given the low price it wouldn't be worth the time/labor to rewind.  Even Antique Electronic has a replacement for $17 and I imagine there are other places with even lower prices.

[AB2EZ]

There are a number of ways to work around the limitations of the audio driver transformer in these transmitters... but here is a simple "fix" that will:

1. Remove the average (DC) current from the primary of the transformer... to protect it from the excessive current that would flow through the primary if the 12AU7 starts drawing a lot of plate current (for example, if the cathode bypass capacitor shorts out).

2. Greatly improve the audio performance of the transformer by removing the average current from its primary (which saturates the core of the transformer).

3. Reduce the stress on the 12AU7 that results from having 300+ volts on its plate

Here is what to do:

1. Add a 1uF (or larger) 450V capacitor (with the + side facing toward the 12AU7 plates) between the top of the transformer's primary winding and the plates of the 12AU7. This will remove the DC from the primary of the transformer.

2. Remove the 300V B+ connection to the bottom of the primary of the transformer, and connect the bottom of the primary of the transformer directly to ground.

3. Connect the 300V B+ to one end of a 10,000 ohm, 5 watt (or larger) resistor, and connect the other end of this resistor to the plates of the 12AU7. This provides a path for the DC plate current, but provides a high enough impedance for audio plate current. This will also reduce the DC voltage on the plates of the 12AU7 to reduce the heating of the plates of the 12AU7. I.e. instead of heating up the plates of the 12AU7, you heat up the resistor.

4. Adjust the value of the cathode resistor from 820 ohms to a lower value (if necessary) to cause the plate voltage on the 12AU7 to be around 150 volts (i.e. about 150 volts lower than the 300V B+ supply voltage). This will correspond to around 15mA of total average (DC) current through both halves of the 12AU7.


Stu

[ka4koe]

What are the symptoms of a partially zorched driver transformer?

[KK4YY]

I addition to what Stu suggests, I've read where the driver transformer core can be cross laminated (after having removed the DC from the primary) for further performance improvement. I can claim no expertise on this practice and I'm just passing it along FYI.
http://amfone.net/Amforum/index.php?topic=5010.0
http://www.hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/negative_feedback_am_xmtrs.htm

Or if you don't want to use a driver transformer at all, there's this...
http://www.amwindow.org/tech/htm/valiant/valiantmods.htm

Don

[W1RKW]

FWIW, I rewound a couple of small transformers years ago using very fine wire available from a local shop. It wasn't thousands of feet of wire. Maybe 100 or so tops.  I did this with the notion of fixing a solenoid for one of my previous automobiles.  The dealer wanted something like $100 for a replacement solenoid that probably was worth $10.00. I didn't have money to burn so decided to rewind it. It was encased in an injection molded case. figuring I had nothing to lose I carefully rewound the solenoid. It worked for several years.  Did the same with the transformers. I didn't count turns during disassembly. I used the same size wire and filled the bobbin with an eyeballed amount of wire. Like the solenoid the transformer worked OK fine. I even rigged up a winder using a variable speed drill.

[ka4koe]

Has anyone tried to rewind said Heapkit/Ranger/Viking driver transformer??

There must be a whole bunch of 'em lying around.


klc

At one point you could buy just the windings from the equivalent Stancor A-53 and reassemble your core around them but I haven't seen this part advertised in years.  Given the low price it wouldn't be worth the time/labor to rewind.  Even Antique Electronic has a replacement for $17 and I imagine there are other places with even lower prices.

I think AES is up to almost 40 bux for a substitute Hammond 124E.

FEELEEP

[WQ9E]

[
I think AES is up to almost 40 bux for a substitute Hammond 124E.

FEELEEP

See:  http://www.tubesandmore.com/products/P-T156

[VE3AJM]

The secondary of the driver transformer should be OK. You are/were seeing the modulators with their idle current in the ballpark. ie. the 6146s are getting bias voltage. If the primary of the transformer were open, you would have no audio at all. You could have partially shorted windings as a possibilty. You could disconnect the primary of the transformer from the circuit, and measure its resistance. Then you would know if it were open or not. Take the time, or spend money on a transformer that you may not need. Thats your choice.

In any case, I would start with checking from the mic forward in the audio chain. If the mic works with another tx that you have there, as you have written here previously, then it would be OK, even if there is a mismatch with using a dynamic mic into the first audio stage of the Valiant.

Have you tested and checked all the tubes in the low level audio section downstream from the mic input? The setting of the clipper control has also been asked about before. Maybe you've already checked these out, and they're OK. There are so many threads on here now about the same project.

Al VE3AJM

[ka4koe]

No sir, haven't checked the clipper. No way to check tubes other than voltages on pins and don't have a tester.

[ka4koe]

So the Stancor listed on AES is a direct substitute for the Hammond at almost a 1/3 the price?

P

[VE3AJM]

The best tube tester often is tube substitution, if you have spare tubes there that you know are good. But you may not have any spare tubes. The tubes would be the very first thing that I would check out and or order/obtain spares for.

The clipper control is easy to find and adjust. Why haven't you checked it? You ask a lot of questions, but you aren't very forthcoming when you are asked a question. As has been mentioned on here before, you can inject an audio signal from a generator into the mic input and use a scope progressively down the line, or from the audio driver back, and be able to isolate the problem stage. Guesswork would be frustrating.

Al VE3AJM

[WQ9E]

So the Stancor listed on AES is a direct substitute for the Hammond at almost a 1/3 the price?

P

I wouldn't put it quite that way but the transformer listed on the AES site is the equivalent of the Stancor A-53C which was identical to the original transformer used in Johnson and Heathkit gear.  The more expensive Hammond unit is overkill for these transmitters and is suitable for tubes with higher grid drive requirements such as triode output stages used in "hifi" audio gear although it would certainly work.  I think the manufacturer may have changed since I bought mine about 10 years ago but I have several rigs that have over 10 years without failure using the generic AES replacement transformers.

[ka4koe]

OK, sounds good to me. As long is performance is on par and IF I have in fact smoked it. I know the previous owner had started some modifications to the first audio stages and bypassed the clipper altogether. I'm in process of documenting all changes on a master schematic.

If performance is on par this will save 2/3's. Thanks.

Philip

[WQ9E]

OK, sounds good to me. As long is performance is on par and IF I have in fact smoked it. I know the previous owner had started some modifications to the first audio stages and bypassed the clipper altogether. I'm in process of documenting all changes on a master schematic.

If performance is on par this will save 2/3's. Thanks.

Philip

You are welcome and the 2/3s you saved can be used to buy more vintage gear

Rodger

[ka4koe]

Just ordered the Stancor A53 from AES.

Philip

[flintstone mop]

I think for the adventurous Ham op. you can eliminate the weak link transformer and go with a phase inverter tube.
Am I having a senior moment? I can not find any info on that mod. I remember doing it on a table top boatanchor TX..

Fred

[KA2DZT]

Philip,


IIRC a Stancor A-53 is a single ended xfmr.  I don't think it is center-tapped for PP driver service.  What type of xfmr are trying to replace??

Fred

[Pete, WA2CWA]

I think for the adventurous Ham op. you can eliminate the weak link transformer and go with a phase inverter tube.
Am I having a senior moment? I can not find any info on that mod. I remember doing it on a table top boatanchor TX..

Fred

http://www.amwindow.org/tech/htm/valiant/audio.htm

[flintstone mop]

Thanks Pete,
I thought I was losing it.

[VE3AJM]

And who could blame you Fred if you were. I was finding the conclusions being drawn on this thread so counterintuitive, that it was bordering on Bizarro World stuff.

Al VE3AJM

[ka4koe]

Philip,


IIRC a Stancor A-53 is a single ended xfmr.  I don't think it is center-tapped for PP driver service.  What type of xfmr are trying to replace??

Fred

The version I ordered does have a center tap.

Philip