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    Building, troubleshooting and testing of these amplifiers should only be
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Help! transformer overheating!

HELP! I'm kinda new in tube stuff, but I've been researching a lot into it and know quite a bit about electronics and curcuits. I've actually found that while tube stuff has some values and codes you have to learn as opposed to the new values and codes, the circuitry is a LOT simpler and easier to understand...considering I understand ic circuits just fine. OK heres my prob:

I'm restoreing a 1936 RCA superhet. radio, I've done the cabinet, reconed the speaker ( I had to make my own cone, surround, and I made a whizzer cone and dust cap for it ) Now onto the chassis. So, when I plug it in it works great, sounds wonderful. Theres a smidge of hum when you turn it on when the volume is all the way down, but its only noticable when you put your ear practically on the speaker. It runs great for about 15-20 mins, then I start having probs. Theres no operational problems ( that I know of) but the power transformer starts gettin reaaallly hot, after half an hour you cant touch it and when you shut the radio off it sounds like the oil inside is boiling. :( that cant be good ...

It looks like my grandpa replaced the filter caps ( we inherited it from him, he was a ham radio operator. Im restoreing this radio for my dad) anyways, the caps are an atom-lytic sprague 80 uf 450 va and a ( cant remember brand) 20 uf 450va cap. they look like mid 60's-70's ish caps. Do you think they might be going bad? also, I think almost all the caps are the originals. I need to replace them I know. Ive done a dim bulb test , but I have no idea what the results were as I've never used a dim bulb tester before. ( I just built one recently for this radio) Also, I noted that after the 30 mins of running, when you shut off the radio then turn it back on the rectifier momentarily glows purple ( gasseous mercury i believe :confused: ). Is that a sign of trouble? I know that the rectifier also has a rattle in it when you shake it, like somethings loose, actually like shaking a cracked pepper jar, but it sounds like ( and I'm pretty sure it is) its near the base of the tube. I also noted that my grandfather added 2 diodes on 2 of the pins under the chassis and wired their other ends together into a huge ceramic resistor of what value i have no idea. It has like 4 values on it, like a 4in1 resistor, but all the leads are wired together at each end. Then, the one end is wired to the ouput to the field coil for the speaker ( its a fieldcoil, not a magnet type speaker) so.. any help would be greatly appreciated!


2003-01-18 7:57 am
Near London. UK
I'd guess it's a winding-to-winding short as well. I had this recently. The transformer would be fine for a while, then it would suddenly start heating very fast. Finally, it did it when I wasn't in the room (went to get a beer). I came back and found a very hot smell, bubbling/boiling noises from the transformer and a whole load of pitch ejected over the bench. Time for a new transformer...
Try this---

Disconnect all the secondaries from the mains Tx, then plug in for a while, and see if the Tx still gets hot--If so, Its stuffed and needs replacement:bawling:

While its on, you can check the voltages on the secondaries to compare with the schematic--They may be a little higher than listed, as its unloaded at this time.............

You should be able to get a fairly close match from one of the Tube Tx suppliers, shouldnt be too bad on cost either.:D

The schematic of the set should show all the different voltages of the Tx and its easy to find a fair match with a modern Tx.

If the Tx remains cool after half hour, then the fault must be elsewere--Check for leaky electrolytics and or chokes after the rec Your speaker coil (magnet) and hum-bucker coils could have developed a leak to chassis too and also worth checking would be fore heater/cathode shorts or any other leaks on the heater supplies--Is the rec a DH type or not?--If not, you may have a short H/C here...
As for your rectifier glowing:

After switching of, let the radio cool down before swtiching on again. Most tubes don't like plate voltages without
beeing heated. A tube recifier will provide this power-on delay
on it's own, as it only produces output voltage if it is heated itself.

Check / replace all old capacitors. This is the normal
restauration procedure of such old radios.
Those old (paper-wax) collect moisture over time
(which actually increases their measurable capacity)
but which allso causes insulation failure.
Those capacitor made from (German : Teer, English ??
Bitumen, Aspalt ...don't know) have the habbit of

Restoring old radios is a hobby on it's own and there are
lot of websites dealing with this.

As for the rattling: the socket is glued to the glas with
some (again German : Kit , the stuff you use on a old window
between the glass and the wood), some of this breaks
and flys around in the socket. Nothing to worry about then.

ok.. thanks lots for the help. DH means direct heat correct :confused: ? I'm not sure if they are, Ill have to look it up in the schem. By the way, I have noticed that the tranny is a little gooey on the outside , but its more like a thin varnish coat that you can run your fingernails through when it gets hot. It doesnt actually bubble out of it.... that is if it hasnt all bubbled out of it before I got the radio. Thats good news about the rectifier tube, I dont particulary wanna spend $12 for a new rca 80 rectifier. AYE! your gonna laugh, but I dont have a multimeter, I KNOW I KNOW I NEED ONE! lol, I really need to buy one ....like soon. When I do I will try the voltage check on the tranny. I also am going to check for the shorts you mentioned to the heaters and cathodes. Bear with me here, I'm a little new at this ;) by the way, the schematic gives the unloaded voltages too

Ok about being able to find a new tranny... thats great news because I didnt know if I'd be able to find one or not! I think I should just get a new one anyways. Where do you reccomend, and what should I expect the prices to be? I'm too lazy to get out the schematic and read off the output voltages right now :rolleyes:

" Is the rec a DH type or not?--If not, you may have a short H/C here..." whats that mean? I know the tube is a half wave rectifier. Does DH mean direct heat? because I think it is.. Im not entirely sure, I know the two larger pins on the bottom (Ive noticed that on all the tubes, the two larger pins are the heater pins ) make the filiments glow when you use a 9 volt battery.

No the filer caps arent getting warm I dont think, I havent had the guts exactly to tip the chassis up on its side and touch them while its plugged in ;) , but I dont think their getting warm after being used for a while.. I only touch them after its unplugged, that chassis has so many frayed wires and missing insulation that I dont wanna mess with it on..Id rather mess with a tv circuit while its on then this :bigeyes: :)
Now, ThSpeakerDude88, I don't mean to be harsh here, but when you claim to know "quite a bit about electronics", statements like this:

" Is the rec a DH type or not?--If not, you may have a short H/C here..." whats that mean? I know the tube is a half wave rectifier. Does DH mean direct heat? because I think it is..

and this:
I only touch them after its unplugged, that chassis has so many frayed wires and missing insulation that I dont wanna mess with it on..Id rather mess with a tv circuit while its on then this

lead me to believe otherwise. You say you want to restore this 1936 radio, and you don't even know that the type 80 is a full wave rectifier. :bigeyes: Then you talk about frayed wiring and cracked and missing insulation and wonder why the damn thing overheats? :bigeyes:

In my considered opinion, you have NO business messing around with that radio. Either turn the restoration job over to someone who knows what he's doing before you destroy it completely -- or you fry yourself -- or spend some more (lots more) time studying the subject before you plug it in again.

Here's a hint: before you even consider replacing capacitors and transformers, rewire that chassis!
ahem... I got the tube specs from some site and it said it was a half wave... My first intention is to rewire it by the way, then replace all the caps. Ive checked for shorts, I can't find any due to insulation missing. And yes I have been bitten by a fly back transformer, it hurts a lot. I took apart an old 27 inch tv and my dad instructed me to cut the cathode wire......guess my cheap chinese pliers werent exactly as "insulated" as they claimed to be. If I never mess with this stuff I will never learn, Ive had my share of shocks and destroying stuff beyond repair. I've also turned out quite a few nice circuits that work very well as well as repaired several peices of electronics, as I said and am also in the process of building another radio out of an old aa5. I've been working with electronics since I was 6, its my hobby. I've learned a lot from trial and error as well as books, and help from my dad and grandfather. You'll never learn if you dont try, so don't discourage. I came here because I needed some help with a few things I didn't already know about it, not to be critisized. So if someone could actually answer my questions maybe I wont fry the radio after all? As for messing with it while its on, Ill feel a lot safer once I rewire it and make it a little neater under there.:)

so if anyone knows the answers to my questions, please answer them! Thanks! TheSpeakerDude.

btw, I didn't say I knew EVERYTHING about electronics, just quite a bit. Edison tried a million times before he invented the light bulb, but he didn't turn it over to someone who knew more than he did, he found a million things not to do, then he succeded from learning from his mistakes ;)
heh i dont discourage rebuilding old radio's and amps.
i rebuild bogen's and it takes time to do it. try to stay with the same part's mixing old and new dropes the value and it looks odd.
most of the part's you can get on ebay iron caps tubes lots of nos
stuff. my amps i replace with new to fix it then the part's from ebay come in i replace em again with original. i never have had a bad part yet from ebay;)

even try to find a good spool of old wire that's never been used.
not the cheep platic covered copper:D

as for the iron running hot and poping after thay do it even once
thats it it's toast.

as for the other questions with old unit's like that its hard to figer out whats the problem is. could be the iron or tubes or caps or a short.

a good idea is to get a digital camera or a cam and show the guys what it looks like so we can see what your working on and
see how old the part's are.

ok! I'll take some pics here in a min and upload em to my server. And as a matter of fact, the wireing I will be useing is out of a tv, the wires to the tube. ( At least thats what I'll use for now untill i recap it) I think I'd rather go with new caps than nos, just from a reliability standpoint. I'm going to raise the voltage and capacitance tolerances on the filter caps just a bit ( ie, 500 volt 100 uf for the 80 uf and 500 volt 47 uf for the 20 uf ) Plus, the old caps are quite large for their capacitance and voltage, I can get orange drops and they will be smaller and more reliable and it should clean up the chassis and make it more easy to source out problems :)

ok here are the pics

by the way , if you cant tell already, I don't care too much about the value of the radio ( its only worth about 245 bucks anyways... well it was before I redid it..) I care more about the sentimental value, and I wanted it to look nice too. So I refinished it, you can see my other post on here all about it ( in the tube pictures section, http://www.diyaudio.com/forums/showthread.php?s=&threadid=36410&perpage=10&pagenumber=73 ) LOL Don't laugh at the Coca-Cola grillecloth, that was my moms idea.. I think its looks kinda neat anyways..
heres a good big pic of the chassis so you can see everything.
here, see the odd resistor thats been added along with the two diodes? unless its original, but judging by the soldering I don't think its original. Oh yeah, and its not in the schematic either. Neither are the diodes.
heh, and heres my othe radio I'm in the process of building
its FM too!
Those Diodes and Resistor, they are placed directly under
the rectifier Tube, aren't hey? Looks like someone
replaced the rectifier tube against a solid-state rectifier.
The resistor is to simulate the much higher output
impedance of a tube rectifier against a silicon diode.

In terms of restoring a radio to it's original state,
this is of course a sin. Technically speaking it might be
100% ok.

Having both in parallel might also be a reason for those glowing effects vou've seen.

All the brown paper-roll caps are suspect to fire and explosion,
so jou seriously should consider getting rid of them.
so someone replaced the tube rectifier with a diode bridge rec. instead? why? I mean, the rectifier tube is still there, and theres still stuff connected to the other two pins. and yes the diodes are directly under the rectifier, but their attached to 2 of the pins on it. Oh wait! the other two pins are just the heaters arent they? and the wires to the pins probably go to the other heaters right?