Bad Transformer? - Printable Version
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Bad Transformer? - Dark High - 10-28-2011
I've got a Philco 40-180 that I just recapped and now I think I have a bad transformer. The transformer smokes and crackles when power is applied. Is there a way to fix transformers or do I have to replace it? If I have to replace, does anyone know what I need to replace it with?
Thanks.
Re: Bad Transformer? - Texasrocker - 10-28-2011
First off, be VERY careful checking your a.c. transformer!, high voltage from the secondary to the plates of the #84 rectifier is lethal ! To check to see if your a.c. trans is still good, you will need to use a variac to "slow up" line voltage from 0 up to full 120 v very slowly. You will also need a ammeter set to low scale to read the current being pulled by the transformer. The ammeter needs to be connected in series on one side of your a.c. line cord that powers the transformer with ac switch on (primary side of trans).
Remove your # 84 rectifier tube from the socket. With variac set at 0 volts, slowly up the voltage to about 25 volts ac, watch the ammeter, if it starts climbing beyond 1 amps... going on up as you slowly increase the ac voltage up higher, you have a bad transformer. On the other hand, if the ammeter doesn't climb as you up the the a.c. voltage from the variac up to full line voltage (120 v), your transformer is most likely still good. In that case, look for mistakes or other problems by carefully reading the schematic. Most radios pull around 1 amp max +/- a little current on the primary side when everything is working properly. If your trans is bad, you can lookup each individual tube and add up the total amperes (filament current) of all tubes combined in the set. Same with the 84 rectifier tube filament and plate currents. You can then determine what kind of replacement to purchase. Be careful working around the a.c. trans!!
Re: Bad Transformer? - Dark High - 10-28-2011
I've removed the 84 rectifier tube and i have lights on all the other tubes and the transformer does not smoke, get hot or crackle.
What's supposed to happen if the number 84 tube is removed? Could it be that I just need another 84 tube and not a transformer?
Thanks.
Re: Bad Transformer? - Arran - 10-28-2011
Dark High Wrote:I've removed the 84 rectifier tube and i have lights on all the other tubes and the transformer does not smoke, get hot or crackle.
What's supposed to happen if the number 84 tube is removed? Could it be that I just need another 84 tube and not a transformer?
Thanks.
That means that there is a dead short or other heavy current load on the high voltage winding. It could be a faulty 84 tube but check between the B+ and B- with an ohm meter, low or no resistance means that something is shorted in the B+ circuits.
Regards
Arran
Re: Bad Transformer? - gvel - 10-29-2011
Without the 84 plugged in, you can check the AC voltage across each side of the secondary winding. First, measure at pin 2 of the 84 and the negative lead of part 61, the 12uF filter cap. Next, measure at pin 3 and the negative lead of part 61. The measurements should be similar, and I'm guessing around 300VAC.
If one or both sides are substantially less than 300VAC, your power transformer is toast.
Re: Bad Transformer? - Dark High - 11-14-2011
They read around 260. Is that too far off?
Re: Bad Transformer? - Radioroslyn - 11-15-2011
That's not to bad that should give you about 210vdc output. I think that's about what is called for.
Terry
Re: Bad Transformer? - morzh - 11-15-2011
If XFMR ever smoked, this is it - even if it no longer smokes when you pulled out the diode tube, it is damaged.
Re: Bad Transformer? - Chuck Schwark - 11-15-2011
The power transformer was most likely already shorted/damaged by the old, shorted, original
electrolytic filter caps you replaced when you re-capped. Shorted/leaky B+ bypass cap(s) could
have also contributed.
Chuck
Re: Bad Transformer? - codefox1 - 11-15-2011
Agreed that if transformer ever smoked it is compromised. However if the HV windings are checking out OK, and filaments are lighting without overheating, there is faint hope. Of course put those HV windings under normal load and you may have a problem, even if all the rest of the wiring is correct and the rectifier is good.
As a safety measure I would replace the transformer and install a fuse.
Re: Bad Transformer? - Dark High - 11-15-2011
I'm not too sure how to measure the B+ circuit. This is my first repair job and I'm learning how to use my multimeter as well. I've ordered a new 84 tube, it was cheap and an easy fix if it was the issue. Which transformer should I get for this 40-180?
At times I think I'm doing well and other times I think I'm in way over my head. Reading the schematics and making sense of them and applying what it says to the actual wiring can be a challenge.
Re: Bad Transformer? - Ron Ramirez - 11-15-2011
Don't give up, my friend; it takes time. You're getting some good advice here from folks who have "been there, done that." Absorb all you can, read, study, and you too can be a successful radio repairman.
As Red Green says, "Remember, I'm pullin' for ya. We're all in this together."
Re: Bad Transformer? - Dark High - 11-16-2011
A quick question, should I have the speaker plugged in when taking meter readings?
Re: Bad Transformer? - Radioroslyn - 11-16-2011
YES
Terry
Re: Bad Transformer? - james1095 - 11-17-2011
I've wound and re-wound power transformers, it takes patience but if the core is not welded it isn't too difficult, but for something as high voltage as a tube radio that's a LOT of turns. You may have got lucky and just boiled off some of the wax, but I would keep a close eye on the transformer and not run it unattended for a while. Add a fuse to the primary too!
Did you test the tube to see if it was shorted? The continuity function on your multimeter will do that for you. It's entirely possible that the rectifier is just fine, but that something else is shorted. There are several wax paper decoupling capacitors in addition to the two electrolytic capacitors between B+ and ground, any one of these shorted can burn up your transformer.