Electronic Creations Co. one tube wonder phonograph

[comradesvox]

Hi. I am working on restoring an old "one tube wonder" phonograph for my dad. The only brand name I see on it is "Electronic Creations Co." It uses a single 25L6GT tube with the 90v motor in series with the tube filament.

I already replaced the electrolytic filters. I am attaching photos. I still have pretty low volume and get mechanical humming when using 33 or 45 speeds. From what I can tell I need to do at least the following:

1. Replace the selenium rectifier with a modern diode and appropriate ballast resistor. If you look at the photos it seems there is already some sort of ballast resistor in series with the selenium rectifier but it reads pretty low (don't remember right now).
2. Clean/lube the motor and replace grommets.
3. Replace the cathode resistor. It is reading about 230 ohms but I was told by forum member radiotvnut that it should be 150. If you look at the photos you will see that the stripes are pink, green, pink - so I guess with age what I am seeing as pink bands are really supposed to be brown.
4. Volume pot is intermittent - needs cleaning.
5. Wax/paper tone cap needs replacing (haven't got to it yet).
6. The difficult part - he also advised that most likely the crystal cartridge is going bad. Part number is MT-14 and it is a dual "flippable" cartridge (78 and LP). Not quite sure how to proceed with that - radiotvnut points out that I need 3v to drive the power tube and these cartridges are no longer being made new. I really don't want to get into building a preamp stage but if I get a lower voltage cartridge just MAYBE I can build out a single-ended op-amp preamp with a voltage gain of maybe 2 or 3? Getting the proper DC though would involve a voltage divider so to keep things simple I would like to avoid this.

In any case, I just figured I would throw these ideas out. These one-tube record players are new to me. I have worked on old tube radios before, and old analog organs/amps from the 60s, but never something like this.

[bob91343]

You can observe the cartridge output on an oscilloscope to see how much you have. You can also measure the amount of signal you need to put into the amplifier to get full output. These measurements will tell you all you need to know, since the power output and gain are determined by the tube and supply voltage and such. You can get a trifle more gain by bypassing the cathode resistor.

I do not recommend a solid state preamplifier. You are going to have impedance and power supply issues. What tube is in that unit? You have a chassis that could accept a second tube socket so you can get all the gain you want.

It's highly likely that the dropping resistor for the heater of the tube is the motor. That does limit your selection of tubes, but not all that much.

The mechanical hum is likely due to insufficient lubrication or worn bearings or a hardened idler wheel.

[radiotvnut]

On these models with a 25L6 in series with a 90V motor, you won't have any problems with using a 6AV6 or 6AT6 tube for a preamp stage. The original tube+motor adds up to 115 volts. Adding the 6AV6 will bring you up to 121 volts; which, won't be a problem. For reference, you can use the schematic for just about any portable record player that uses a preamp stage.

I see that the cartridge is a standard 1/2" mount and you won't have any issues with mounting either an Astatic 89T/mounting bracket or a Pfanstiehl P228 in that tonearm. The P228 is a stereo cartridge and you'll need to wire the left and right channels in parallel to give you a mono signal. You may also have to adjust the spring under the tonearm to get the correct tracking pressure for the cartridge that you end up using.

There are many opinions on selenium rectifiers. Many people say that if one is currently working OK; then, it should be left. Others say that such a rectifier is a bomb waiting to go off and should be replaced. The main problem they give is low output voltage (can be measured with an ohmmeter) and they can short; which, will produce a foul odor of rotten eggs.

If you choose to replace the SR with a diode, be advised that a modern silicon diode has much less voltage drop than the original SR had. To compensate for this, you'll need to up the input resistor to something in the 68 ohm range.

[mstaton]

I recently installed a Pfanstiehl stereo cartrige into my maganvox collaro changer. it was only $8.99 at a local supplier. Especsially after I saw the prices for a "new" electrovoice cartrige, there was no contest. I'm not sure of the voltage output difference but it works just fine.

[comradesvox]

Thanks for all the help everyone. With some help from Gary Stork at www.thevoiceofmusic.com , we identified the replacement cartridge as the EV88. I will also remove the platter to get a closer look at the grommets and get an order together. Gary did say the cartridge is NOS - but the units he sells have been voltage tested. He does admit that these and ceramics will not last forever. At this point I can live with that for the price - especially since I am sure the needle itself definitely needs replacement in any case.

For the DC dropping resistor in series with a new silicon diode, I was going to use a 100-ohm 5 watt unit. Does that sound OK? Would I still need that wound brown ballast resistor?

Yes on the AC side the 90 volt motor does function as a dropping resistor for the tube filament voltage. IIRC I measured between 25 and 26 VAC on the filament voltage so that looks OK.

I also remember that the motor ran quite hot. I assume that is because it still needs the standard servicing/lube. Does that sound right or is there something else I need to look out for?

Does anyone know anything about the manufacturer? I was not able to find out anything. Is there any sort of "generic" schematic or serivce info for these one tube phonographs? If I can draw up a schematic competently I'll post it.

Jimmy

[radiotvnut]

You may have to experiment with resistors to see which one gives you close to the correct voltage (probably in the 125 to 135 volt range under normal load).

Those motors usually get very hot under normal operating conditions; but, I'd still suggest cleaning and lubricating it.

It depends on where the big brown resistor is as to rather or not it can be taken out. If it's ahead of the rectifier, you can take it out and replace it with a higher ohms unit needed for the new silicon diode. If it's between the output of the rectifier and the first filter cap, I'd leave it in place. If it's between the two filter caps, it needs to stay. It's unlikely that it's in the filament circuit; but, if it is, leave it alone.

One issue that I've found with NOS cartridges is that the output will be OK; but, the rubber suspension will be hardened, resulting in excessive skipping and record wear. Whenever possible, I try to use current production cartridges. If you use a current production cartridge, you'll either have to live with reduced volume or add a preamp stage.

[bob91343]

The turntable is a common type. I don't know if V-M or G-I but probably one of those two. That's Voice of Music (Victor Miller) or General Industries.

[comradesvox]

radiotvnut,
The existing ballast resistor is between the rectifier and the first filter. At least, I am assuming that it is a resistor. In the photo it is the thin brown wire wound component connected to the rectifier. No it has nothing to do with the filament voltage that I can tell - that AC is dropped by the 90v motor in series with the filament.
I am on the train now but will take some more readings later on. I will also try to get a schematic drawn up but that may take a while.
It is turning out to be a much more understandable job thanks to the forum members and people like Gary Stork who have a passion for keeping these old units alive and working. Thanks again.

[radiotvnut]

I'd leave it alone. That's a surge current limiting resistor and if it was not in the circuit, the turn-on surge might place extra stress on the rectifier.

[comradesvox]

Updated schematic attached - rev. B 6/26/2012

[bob91343]

The 150 Ohm resistor in the heater string seems high. With 300 mA through it, one gets 45 Volts, too much drop. The 90V motor and 25V heater already give 115V. Otherwise there isn't enough heater current. Maybe you made an error, and the heater needs to connect separately to the ground symbol.

Also be advised that the cartridge is hot to the power line so its connections should not be accessible with fingers while manipulating the tonearm.

[comradesvox]

Thanks for the heads up Bob. I updated the schematic.

[comradesvox]

Oh, and yes I did notice the hot cartridge. This thing can be dangerous. I can't even figure out a way to make it safer with a polarized plug and fuse - as the switch is switching the ac to the chassis! The diagram shows a polarized plug only because I couldn't find an unpolarized plug in visio, but in reality this unit is not polarized.

[radiotvnut]

Originally Posted by comradesvox

Oh, and yes I did notice the hot cartridge. This thing can be dangerous. I can't even figure out a way to make it safer with a polarized plug and fuse - as the switch is switching the ac to the chassis! The diagram shows a polarized plug only because I couldn't find an unpolarized plug in visio, but in reality this unit is not polarized.

In hot chassis radios, they used a non-polarized power cord and the switch would be placed in the neutral side of the power cord. Depending on the plug position in the wall socket, there's a 50/50 chance the chassis will be hot. This will also be the case even if the radio is off because of the series wired tube filaments.

The way we get around this problem is to use a modern polarized AC cord and wire the power switch in the HOT side of the AC line. However, this is still not a 100% safe method because if the radio is plugged into a miswired AC outlet, you have a hot chassis on your hands. It would probably be best to get one of those little LED outlet testers to make sure all outlets in your home are wired correctly.

I recently had a '48 Decca 78 rpm child's record player that had a hot chassis. Due to the shielded tonearm cable, even the metal tonearm was hot, along with the metal volume control shaft. If the plug was orientated so that the hot side of the line was going to the chassis and if the user was touching an exposed metal part while another part of their body was in contact with something that had a return path to ground, the results could very well be deadly. It's hard to imagine something like this that was made for kids passing UL inspection, even in 1948.

In later years, they connected a capacitor between the chassis and actual circuit ground. This made things a little safer; but, it's still possible for one to get nailed from one of these.

[bob91343]

I would like to say something that is only slightly off topic. That is, how lethal is a hot chassis? I have been tinkering with consumer electronics for around 70 years, been jolted with 120V and more many times, and my friends have gotten many such jolts as well.

However, I know of no case where anyone has died from it. Oh sure there are tales of taking a radio into the bathtub, etc., but barring those acts of stupidity, has anyone you know ever died from a shock? Yes, doctors warn that those with heart problems are particularly at risk, but again, I know of no case personally where a shock has killed anyone.