Good news. Unlikely the power transformer is bad. A bad power transformer will overheat but not usually cause distortion.
To test the transformer remove the 5Z4 tube and operate the radio for 30 minutes. Tubes will get warm but you won't hear sound. If the transformer doesn't get hot it will be ok.
Distortion is usually caused by a leaky coupling cap in the audio section of a radio. In your radio part #19 should be replaced. You may want to replace other caps as older caps become leaky over time.
The schematic can be found here:
http://www.nostalgiaair.org/PagesByModel/555/M0003555.pdf
Norm
:I have a Crosley Model 655. I took it to a radio shop and they said it had a bad power transformer in it. It does play with good volumn but the audit is distored. They wanted $240 to replace the transformer. I did not have them do the work. I checked the voltage and it's strong with 220 DC from the rectifier. The tube voltages seem good at 5 volts on the rectifier tube and 6 on the others. How do you test for a transformer short circut and if it is bad where can I find a replacement. I'm new to radio collecting and repair but enjoy the hobby. Any advice would be appreciated. AC
If all voltages are what they should be according to the schematic, then the way to tell if it is shorted is to see if it gets rather hot. Transformers usually run from comfortably warm to slightly uncomfortably warm. If, after an hour, the radio smells like it is burning up, then you either have a shorted power transformer or something in the radio is overloading the transformer (bad condensers or something else). Typically, unless the transformer is severely shorted, the voltages will still be pretty accurate. It does not take much of a short, however, to turn the transformer into a fire hazard.
Regarding distortion in the audio, unless the transformer is providing significantly less voltage than it should, this is not likely the problem. If your audio power is decent, then lack of voltage is not your problem, especially if your multi-meter shows that all voltages are okay. What usually causes distortion in the audio is first condensers, and then resistors and tubes. If all of your tubes check out okay (weak to good), then check your condensers in the audio section with your multi-meter. Using its most sensitive setting (the one that deflects the meter when you touch the leads with your fingers), disconnect each condenser to be tested and test each condenser. Do not touch the meter leads while testing or you'll get false readings. There should be NO leakage, not even in the millions of ohms. The meter will swing up a very small amount for condensers over .001 MFD in value, and then it should fall ABSOLUTELY BACK TO WHERE IT STARTED FROM. If it lingers higher than this even by the width of a human hair, then your condenser has unsatisfactory leakage. You see, tube circuits work with extremely high impedances and with items (tubes) that are EXTREMELY sensitive to EXTREMELY small voltages. Leakage in the millions of ohms is enough to pass unwanted voltages from one stage to another, which will throw off the sensitive grid bias voltages on the succeeding tubes. Condensers are only supposed to pass alternating current (audio signals are alternating currents). Replace all of your leaky condensers and then see how the audio is. It should be pretty good now.
Most radio repairmen replace all of the condensers in their radios, because condensers like to fail and destroy other parts. You may replace only the leaky ones for now for the learning experience, but if you wish, you may replace all of them. Mica condensers, by the way, usually do not leak. These are the condensers in the molded bakelite squares that have colored dots printed on them. It is rare that mica condensers short, but some manufacturers produce other varieties of condensers in the same container, which can be very deceiving, so do not rule out these condensers for failing.
After you have replaced the condensers, if you still have bad audio, check the resistors. Disconnect each and test each with your meter. They should all be within 10 or 15% of their original values, though certain ones in audio circuits can drift quite high before ruining the audio. Generally, those that supply more current are more likely to affect audio when they drift.
After you have replaced all bad condensers and resistors, you will find now that even tubes that test as weak or replace may function quite well, so it is not always necessary to replace them. However, weak tubes will provide less audio power when used in the output stages, even if the audio is not distorted and bad sounding, so it is always nice to have new tubes. Stages prior to the output stage may or may not be dramatically affected by weak or new tubes. The most dramatic affect is seen in the power output stage. Tubes, especially the older styles used in the 1930s and prior, are becoming harder to find, though, so sometimes you must use a weaker tube in a radio in order to maintain its appearance. If you use the radio regularily, save the weak tubes for special displays, and use fresh tubes for your ordinary listening. The key places to use strong tubes are in the audio power output section and in the rectifier-power supply section.
Thomas
Thank you both for some very useful testing help. If I understand correctly the radio could still perform even though it has a bad transformer? If it runs on the hot side I would need to look for a replacement with similar output. Do you know where a replacement could be purchased? Would AES have something similar? Thanks again. AC
A transformer with shorted turns will operate very hot and burn up. If you can hold you finger on a transformer for 30 seconds, after 30 minutes of the radio operating the transformer is ok.
Pulling the rectifier tube is a better test if you haven't replaced caps and checked out the rest of your radio. This way your transformer should only be warm.
Replace electrolytic filter caps first. If they are shorted they will destroy your transformer.
Norm
:::The price they are charging is not too bad, since a power transformer costs between $40 and $90 typically. Add quite a bit of mounting and rewiring labor and you have $240, which puts decent food on their table and keeps the lights on.
:::
:::If all voltages are what they should be according to the schematic, then the way to tell if it is shorted is to see if it gets rather hot. Transformers usually run from comfortably warm to slightly uncomfortably warm. If, after an hour, the radio smells like it is burning up, then you either have a shorted power transformer or something in the radio is overloading the transformer (bad condensers or something else). Typically, unless the transformer is severely shorted, the voltages will still be pretty accurate. It does not take much of a short, however, to turn the transformer into a fire hazard.
:::
:::Regarding distortion in the audio, unless the transformer is providing significantly less voltage than it should, this is not likely the problem. If your audio power is decent, then lack of voltage is not your problem, especially if your multi-meter shows that all voltages are okay. What usually causes distortion in the audio is first condensers, and then resistors and tubes. If all of your tubes check out okay (weak to good), then check your condensers in the audio section with your multi-meter. Using its most sensitive setting (the one that deflects the meter when you touch the leads with your fingers), disconnect each condenser to be tested and test each condenser. Do not touch the meter leads while testing or you'll get false readings. There should be NO leakage, not even in the millions of ohms. The meter will swing up a very small amount for condensers over .001 MFD in value, and then it should fall ABSOLUTELY BACK TO WHERE IT STARTED FROM. If it lingers higher than this even by the width of a human hair, then your condenser has unsatisfactory leakage. You see, tube circuits work with extremely high impedances and with items (tubes) that are EXTREMELY sensitive to EXTREMELY small voltages. Leakage in the millions of ohms is enough to pass unwanted voltages from one stage to another, which will throw off the sensitive grid bias voltages on the succeeding tubes. Condensers are only supposed to pass alternating current (audio signals are alternating currents). Replace all of your leaky condensers and then see how the audio is. It should be pretty good now.
:::
:::Most radio repairmen replace all of the condensers in their radios, because condensers like to fail and destroy other parts. You may replace only the leaky ones for now for the learning experience, but if you wish, you may replace all of them. Mica condensers, by the way, usually do not leak. These are the condensers in the molded bakelite squares that have colored dots printed on them. It is rare that mica condensers short, but some manufacturers produce other varieties of condensers in the same container, which can be very deceiving, so do not rule out these condensers for failing.
:::
:::After you have replaced the condensers, if you still have bad audio, check the resistors. Disconnect each and test each with your meter. They should all be within 10 or 15% of their original values, though certain ones in audio circuits can drift quite high before ruining the audio. Generally, those that supply more current are more likely to affect audio when they drift.
:::
:::After you have replaced all bad condensers and resistors, you will find now that even tubes that test as weak or replace may function quite well, so it is not always necessary to replace them. However, weak tubes will provide less audio power when used in the output stages, even if the audio is not distorted and bad sounding, so it is always nice to have new tubes. Stages prior to the output stage may or may not be dramatically affected by weak or new tubes. The most dramatic affect is seen in the power output stage. Tubes, especially the older styles used in the 1930s and prior, are becoming harder to find, though, so sometimes you must use a weaker tube in a radio in order to maintain its appearance. If you use the radio regularily, save the weak tubes for special displays, and use fresh tubes for your ordinary listening. The key places to use strong tubes are in the audio power output section and in the rectifier-power supply section.
:::
:::Thomas
:
:Thank you both for some very useful testing help. If I understand correctly the radio could still perform even though it has a bad transformer? If it runs on the hot side I would need to look for a replacement with similar output. Do you know where a replacement could be purchased? Would AES have something similar? Thanks again. AC
AES (www.tubesandmore.com) sells transformers for just about any radio. They even have some universal power transformers that will work on many standard line voltages from 105 to 250 volts.
Thomas