Hello all,
Have a Zenith 34P that I recapped and placed in storage a few yrs ago. The radio played fine then and plays fine now. The problem is that after about 15 min. of operation, a slight smell like the transformer(s) burning emerges.
Voltages are correct at tubes, and no smoke. Would I be wise to fuse it? If so, where would the fuse go, and what size?

Hi Bill

Here is a schematic:

http://www.nostalgiaair.org/PagesByModel/862/M0040862.pdf

Does the power transformer feel hot? Fusing is a good idea but power supply filter caps should also be replaced. They can become leaky over time and draw current. This can destroy the power transformer & rectifier tube.

Norm

:Hello all,
: Have a Zenith 34P that I recapped and placed in storage a few yrs ago. The radio played fine then and plays fine now. The problem is that after about 15 min. of operation, a slight smell like the transformer(s) burning emerges.
: Voltages are correct at tubes, and no smoke. Would I be wise to fuse it? If so, where would the fuse go, and what size?

:Hi Bill
:
: Here is a schematic:
:
:http://www.nostalgiaair.org/PagesByModel/862/M0040862.pdf
:
: Does the power transformer feel hot? Fusing is a good idea but power supply filter caps should also be replaced. They can become leaky over time and draw current. This can destroy the power transformer & rectifier tube.
:
:Norm
:
::Hello all,
:: Have a Zenith 34P that I recapped and placed in storage a few yrs ago. The radio played fine then and plays fine now. The problem is that after about 15 min. of operation, a slight smell like the transformer(s) burning emerges.
:: Voltages are correct at tubes, and no smoke. Would I be wise to fuse it? If so, where would the fuse go, and what size?

::Hi Bill
::
:: Here is a schematic:
::
::http://www.nostalgiaair.org/PagesByModel/862/M0040862.pdf
::
:: Does the power transformer feel hot? Fusing is a good idea but power supply filter caps should also be replaced. They can become leaky over time and draw current. This can destroy the power transformer & rectifier tube.
::
::Norm
::
:::Hello all,
::: Have a Zenith 34P that I recapped and placed in storage a few yrs ago. The radio played fine then and plays fine now. The problem is that after about 15 min. of operation, a slight smell like the transformer(s) burning emerges.
::: Voltages are correct at tubes, and no smoke. Would I be wise to fuse it? If so, where would the fuse go, and what size?

:::Hi Norm,
Thanks for the reply. Replaced all caps and resistors in this set with modern components when it was overhauled about 4 yrs ago. Any way to determine possible short in transformers by resistance checks? Also, do you recommend any fuses? If so, which size?

You can do resistance checks. If you have DC resistance values available, these may help you. Your transformer may or may not correspond exactly to those values, though, due to slight variations in winding, etc. If the power transformer is getting excessively hot and smells bad, then it may have shorted windings. All it takes is one or two turns shorted to cause the transformer to get hot.

Thomas

Hi Bill

Here is something else you can try. Pull the rectifier tubes. In your unit a pair of 81's. Operate the supply with rectifiers removed. Transformer may get warm but not hot. If it's only warm transformer is ok.

Hard to determine transformer quality by a resistance check. As Thomas mentioned even 1 turn shorted and the transformer will get hot.

In the case of high voltage winding both sides of center tap will have different resistance readings. The winding on inside will be lower resistance than outside but voltage will be equal.

Norm

:You can do resistance checks. If you have DC resistance values available, these may help you. Your transformer may or may not correspond exactly to those values, though, due to slight variations in winding, etc. If the power transformer is getting excessively hot and smells bad, then it may have shorted windings. All it takes is one or two turns shorted to cause the transformer to get hot.
:
:Thomas

Just to ruin the hope of the situation, I must add something to what Norm said. With my Crosley 1117 it had a power transformer that got really hot and smelled bad. It may have even been a replacement transformer, though solder connections looked original. Removing the rectifier caused the transformer to cool down, but I heard a sputtering sound in the transformer. As I said, I don't know if this transformer was a replacement or not. It handled the filaments of 11 tubes well, but overheated when the rectifier was inserted. It was bad, so I replaced it, and now the set works fine. I really think that it had some shorts in it though. What I must say, then, is that unfortunately removal of all tubes may or may not reveal a shorted transformer. It may cool down just fine and still have a short. If the transformer appears to be the original, though (original looking solder connections, etc.), then it is probably shorted if no other parts in the set are failing. If the set is working properly, the only thing that would overheat the transformer is an internal short.

Usually removing all of the tubes reveals a shorted transformer (stays hot anyway), so give it a try.

Another tell-tale is if you put a fairly low wattage bulb in series with the set (say 40 watts). If you remove all of the tubes and the bulb still lights even somewhat, your transformer is very likely shorted. I did this with my Crosley, and the bulb did light up a small amount.

Good luck,

Thomas

Hi Norm and Thomas,
Thanks for the advice. Will test out your ideas during the weekend. This power unit is enclosed in a metal case similiar to the early Majestics. It has separate Filimanet and high voltage transformers, and drives a 10 tube as the power tube.
For now, the set plays fine and has proper voltages, but as I said previously, a slight smell emanates from the power unit (I think) after about 15 min. of playing time.
I was just wondering if there was any way to conclusively prove a shorted transformer. I had always thought it either works or it doesnt.

just to ruin the hope of the situation, I must add something to what Norm said. With my Crosley 1117 it had a power transformer that got really hot and smelled bad. It may have even been a replacement transformer, though solder connections looked original. Removing the rectifier caused the transformer to cool down, but I heard a sputtering sound in the transformer. As I said, I don't know if this transformer was a replacement or not. It handled the filaments of 11 tubes well, but overheated when the rectifier was inserted. It was bad, so I replaced it, and now the set works fine. I really think that it had some shorts in it though. What I must say, then, is that unfortunately removal of all tubes may or may not reveal a shorted transformer. It may cool down just fine and still have a short. If the transformer appears to be the original, though (original looking solder connections, etc.), then it is probably shorted if no other parts in the set are failing. If the set is working properly, the only thing that would overheat the transformer is an internal short.
:
:Usually removing all of the tubes reveals a shorted transformer (stays hot anyway), so give it a try.
:
:Another tell-tale is if you put a fairly low wattage bulb in series with the set (say 40 watts). If you remove all of the tubes and the bulb still lights even somewhat, your transformer is very likely shorted. I did this with my Crosley, and the bulb did light up a small amount.
:
:Good luck,
:
:Thomas

Be sure that wax isn't dripping onto hot resistors. Be sure that tubes are clean and away from flammable items. Field coils can get hot, but usually don't smell like they're burning.

Thomas

Bill, this will not conclusively prove or disprove a shorted transformer, but since your set has the proper voltages, this is a good sign that the primary winding is not shorted. When a few turns of a primary short, this gives the equivalent of fewer effective turns, thus changing the turns ratio. This higher ratio will yield a higher secondary voltage. I can’t say if just one or 2 turns shorted will make a noticeable difference in the B+, but I can say that I have seen some hot running transformers that had a secondary voltage high enough to cause a higher B+. In other words, I have never removed the windings of a defective transformer to find out how many primary turns were shorted.

I am not familiar with your Zenith, but many of these early sets have power resistors in the supply that normally run too hot to touch. It is not unusual for some of these resistors to give off a slight odor, so if your set has a very warm or hot one, it may be the guilty culprit.