Hello and Happy New Year.

I am delighted to say I have got my 1929 RCA Radiola 33 working! But it has some issues.

I have replaced the paper capacitor in the box under the chassis as suggested by the "Radiola guy" from whom I purchased brand new speaker Grille Cloths and a Volume Pot, as the entire wiper was missing on mine, and once I have fitted the grille cloths, I shall post some nice photos.

I repaired a tear in the speaker with Aquadhere and tissue paper in successive layers, a method I used to use with guitar amp speakers once, when I used to Hendrix the cones with the guitar. It always held up well!!!

I am listening to it and enjoying it now for the second night and it sounds great, but...

When I powered it up, I ran it through my trusty fail safe 100W bulb, and holding the antenna wire in my left hand, got some cartoon noises, wheeeeeee wooooooo etc. and variable speed motor boating, then pure music, then ABC further down the dial, loud and clear!

It is ridiculously sensitive, to the point I cannot connect it to the long wire aerial, if I do all I get is Wheeee Wooooooo cartoon radio noises and motor boating, but if I just hang its insulated aerial wire over the long wire, without attaching it, it works lovely, tunes into all stations, but is very loud and the volume control has no effect whatsoever only there's full bass tone at half volume and all treble at full? The volume pot wiper on this TRF circuit feeds the detector valve and one side of the pot is ground, the other side, the antenna.

The only way I can control the volume is to move the antenna wire away from the long wire, if I spiral the antenna wire around the long wire it gets really loud, but if I connect it , it's just Wheeeeeeeeeee etc.

Now the funny thing.

If I remove the 100W globe from the mains supply and plug it in directly, Wheeeeeeeeeee Wooooooooooooo pop pop pop pop pop etc, Can't get heads or tails out of it, as if it needs that 100W globe to limit current to a parameter where everything works correctly?

I've had it on for hours now, nothing getting hot or anything untoward, just needs the 100W globe and I hang the aerial over the long wire lead and I have a loud bright radio...

I have one theory, perhaps our mains voltage is a tad too high? It is rated at 200-240V yet our supply is 245V and as high as 247V when I measure. I'm wondering if this may well be why it seems to love the 100W bulb? A 60W bulb in contrast it still works fine, but is very quiet...

I'm wondering if there is a type f resistor I may put in series which will reduce the voltage and/or current, without producing too much heat, or do I just keep it going through the 100W bulb?

Your thoughts appreciated!

Cheers.

If its unstable its not working: The noise is it screaming for help.

As it is RCA 33 is it actually a 240V transformer?

I have an American built Grigsby-Grunnow 15A here and it is not 240V (and is labelled 115VAC) there are several other sets that I have repaired and a couple of items of mine that are 110- 115 VAC. It is here. I am sure the damage to the 15A was done using 240 V & it had an Australian plug: Be warned.

Radiola 33AC AR-784 110 -125VAC Transformer shielded type. Four UX226; one UY227; one UX171A Rectifier 80 / 280

I would ground the chassis to get rid of any charge from the transformer. Those things have no AGC the volume control normally works on the bias, or the antenna.Volume may do better with a grounded chassis?

Rip the valves out of it and measure the filament heater voltages before you fry something. Do you have a Variac (Slide transformer) to get it down to 115V? Series bulb is no good for that test. And I do not like the idea of 240V into a 110V transformer even that it is off load.

Do not power it until you confirm what the transformer is. Then and only then do we move on.

Marc

It's ok Marcc, I measured voltages a fair while ago, all filament voltages correct as per the manual, 2V for the 26 valves. The sticker underneath says that this model is designed for 200-240 volts, not 110 volts as the American model is.

I have grounded the chassis via a 3 ply brand new cord. I have put 2 brand new 26 valves in the detector end, plus a new 227 and 171A power amp.

I listed the model number in a previous post, which indicated that it is a 220-250 volt model.

Sounds like missing or O/C B+ bypass cap.

Or could be O/C or missing plate bypass on 1st audio or detector.

Or missing valve shield?

I don't have the circuit, this is just general advice. RF energy escaping and being picked up by the antenna.

Some of those sets had Neutralising caps on the gangs that look like trimmers. Incorrect adjustment of them can destabilise. I just might have a circuit?

Subbing paper filter caps with electrolytic can also result result in problems as they do not decouple RF well. You may have to put a 0.1μF cap across the last one if you did that?

Marc

It doesn't have B+ bypass cap that I'm aware of, nor any electrolytics, the circuit is ancient, very simple and basic, all valves are in a straight line, no shielding except for the steel box casing.

I'm inclined, after having a real good think about it, as Marcc first commented, that the voltage may be wrong.

Yes it is a Model "B" with a transformer designed to run on 200-240V, but when I acquired it, it had a two (rounded) prong, American style plug. Hmmmn?

Also, there is another wire, taped up with ancient electrical tape, leading into the power transformer.

A while ago I read somewhere that this "other" wire was for operating the set on a lower voltage, possibly 105-125 volts.

Therefore It is plausible that I incorrectly assumed the plug was wired to the correct (220-240V) tap.

I want to rule out the possibility in any case before pursuing other possibilities.

Question:

If I were to measure the resistance through the primary windings of the power transformer, from active to neutral, which winding would have the most resistance to neutral?

The 110V input?

or the 240V input?

PS,

I do have a 240V-110V step down transformer, which I bought on ebay when I first acquired this set,

before I actually discovered it has a 240V transformer.

As yet I am only assuming that it was previously wired to the correct voltage selection...

Asumptions are dangerous, so I would rather confirm beyond doubt that this is not the problem!

This is my previous post about this radio.

https://vintage-radio.com.au/default.asp?f=1&th=1081

Hi JamieLee, my circuit of the 33-AC depicts a straight TRF with filter caps and chokes in the power supply and a crucial HT bypass, 0.1μF on the common rail to the RF and detector valves. The set stability will relay on this cap.

The other guys are right. When you get rid of the globe and run at full volts you get full gain from the valves and now the set is unstable.
Most likely culprit is that bypass. It is the only decoupling cap in the whole front end HT system and it its open circuit the RF will just feedback.

Clip a good cap across the HT at the front end and i'll bet the whistles stop. A CRO will show any RF on the rail.
The 1mfd caps in the power supply are suspect as well.
Should be easy to sort out.

Cheers, Fred.

It was not unusual to have the transformers changed so that they would run here, but don't forget that Broken Hill ran 110V around that era.

As I have said I have seen more than one 110 - 125VAC set.

Resistance will tell you little, with transformers it's all about inductance & turns ratio's. Off load the transformer will read high. My suggestion of a "Variac" was in the interest of safety; Not only for people, it extends to injury to the apparatus by cooking something, like heaters / filaments

There is a circuit on Radio Museum did not see on Nostalgia Air?

If you have a 240 -110V step down, I would, having bought it second hand, check it. I have one that is 220V and that gives over voltage as my input is 245V it is also an auto transformer, so works both ways & has no isolation. Its cabling setup was dangerous & had to be rewired.

The other one was an illegal sale, I bought it as I could fix it (and did) but the 110V GDO with it had a dodgy plug & there was bare terminal on the mains end of the transformer box that could be touched. That was pointed out to event organiser.

You could remove the valves & retest with this step-down tranny as that will be with in the limits & tolerances of the 110V transformer, if that is what the set has? Better sure than sorry and in any diagnosis one always checks voltages. Without the rectifier there is no B voltage to get into mischief.

It does look like the mains side needs a tidy up anyway. Glands make good cable clamps.

If it had paper filter caps it would not have the bypass: If it still has paper filter caps they will leak and are best eliminated. A lot of the ones in tin cans have PCB's as well: Beware if you open one. Playing with old radios & eating food at the same time, is a health hazard.

We are all mad...........I just re read what he said "the volume control has no effect".....................if the pot is open circuit then V1 is open grid, no bias, full signal, no control!
Its like driving a car with a flat tyre and complaining it wont steer and makes funny noises!
Fix the tyre and the problems go away.
Fix the pot, restore normal bias and the problems will go away.
Fred.

It's a brand new por from the radiola guy in America, I guess it could be open circuit, I'll need to check. It does basically nothing except increase bass response at about half volume and cut out to Wheeeee Wooooooo ppop pop pop at about quarter volume down to zero...

There was a question and doubts regarding mains voltage. Clearly that has serious implications and in the best interests of everything, that should be cleared up before all else.

It may well be that the volume control has failed and that is never uncommon with those sets. Often the grounding is via the shaft & that can be dodgy. But we do not want to cook something finding out by powering it again with valves in situ. The control can be checked with the set dead and should be, even if that involves removing it physically & from circuit and chassis.

A Triode with an open grid is a diode.A Triode with the grid tied to the plate (Lyric 70) is a rectifier.

Lets not rush in: Fix / tidy up the mains side first check the pot & then worry about powering it to run, after we sort that lot out.

Marc

OK, that is what I'll do, first establish I have the correct voltages, I have the service manual, so with valves removed I can test again to determine what voltage is getting to the filaments and rectifier, and with the rectifier in, I can see what is reaching the plate pins. I did this a fair while back, but I didn't write it all down, and it was in the ball park, between 100 and 200 volts to the plates, 2 volts to the 26 filaments, or 1.85 thereabouts from memory, in any case double and triple checking can do no harm at all, now I have the manual, I know the exact voltages to look for and if they are high, I can assume something is wrong and use my 110v step down to see what the other wire does relative to the one currently hooked up to the active.

Marcc is right in taking the cautious approach. Just remember that if you have the valves out there is no load on the transformer and voltage will read high. I usually aim for 10% high when I wind a transformer and the voltage will fall back once loaded. Thus a 2.0 volt may read 2.4 or more and on load and with cable drop may be 2.0 with all filaments loaded. The scientific thing to do is LOAD each winding with a resisitor to simulate load. Easy for me with a workshop full of dummy resisitor loads.

Just do what Marcc suggests and check if the voltages are in the ball park within 10 to 20% of nominal.
IF you were powering a 110/120 primary on 240 volt you would have DOUBLE the output on the primary and possibly excess exciting current and fast temperature rise! As you have not had 4 volts on the heaters and 600 volt on the HT rail I don't think you have a 110/120 tranny on 240 BUT TEST ANYWAY and write the values on the paper. A straight ohms check WILL suggest what part of a winding does what as ohms are pretty proportional to turns. That is how I sort out the ends of a winding when I wind one and stupidly don't tag the ends! The largest ohms will be full winding and a 120 tap on a 240 will be about 1/2 ohms. Your 240 winding may read 100 ohm and the 120 tap may read 45 so at least you are in the known ball park. As an example read what my no load voltages were on that Little General transformer article I just posted. I think I had about 7 volt heater and 325 HT which then sagged to 6.3 and 300 on load.

As Marcc suggest do all the housekeeping and testing and get the voltages known and under control.
Then patch in a HT 0.1 bypass on the HT at the detector end of the isolator resistor to snuff out any RF and investigate that front end pot. IF the pot is intact and the control is at a minimum you should have almost silence. The turning the pot upward should just admit signal, not detune the station or act as a tone control. Marcc points out some chassis use the spindle of the pot as the ground center wiper and not a wire connected back to the ground rail. if there is paint/rust or the pot now has an insulated centre ring then the pot wont work as designed and the front end of the set will go crazy. Oh, I just realised that the pot is a centre grid feed and may have been a spindle wiper type with INSULATING washers on the mounting threaded bit. Be really wary about that circuit and have a good look at the original to make sure how it was connected. The smart thing to do is grab a modern pot and solder it in the set on short leads and see if it reacts the same way.

Measure, test, fix.

Good hunting!
Fred

Ok... Good news!

Problems resolved!!!

Pulled the set apart and did all of the checks, voltages all perfect, flipped the chassis over and was figuring out where to patch the .1 mfd capacitor, when I noticed an anomaly.. In my service manual, the wiring diagram shows three resistors, one Grey, one Blue and one Red.

The grey and blue ones were there where they are supposed to be, but there is a Green one also, going from the Grid of the Detector, to ground... Hmmnn, he's just not supposed to be there and the Red, 3 Meg "Grid leak" resistor with the green wire going from Detector Grid, to the RF coil, starting to look conspicuous by it's complete absence!!! What the??? Well, let's try this: I couldn't find a 3 Meg resistor, but two 1.5 Meg's in series, soldered to some wire and hooked up where the Red resistor should be, from the Grid to RF coil, snipped out the existing green one going to earth, and I put the radio back together, hooked up the antenna, plugged it in and switched it on and it's working well as it should, stable, tuning in to all stations and the volume working as it should!

Not what we thought! But it seems somebody a very long time ago did some errant fiddling. I think somebody used the radio as an amplifier as it had an input jack fitted underneath, which I removed and corrected some wiring associated with this. Luckily along with a large schematic, the Service Manual comes with a brilliant "point to point" wiring diagram!

Cheers!