Electronic Creations Co. one tube wonder phonograph
 

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.

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.

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.

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.

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

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.

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.

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.

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.

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

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.

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

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.

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.

Persoanlly speaking .... Do I know anyone who was killed? no. Severely and permanently injured? Though not an accidental shock from an old radio, the answer is unfortunately yes ... And that is all I care to say about it as the story has a tragic ending.

My dad's turntable, as long as it is kept in the wood case and the plastic knobs remain affixed, I guess would be generally ok under normal use, unless someone touches those cartridge leads wile grounded. With no transformer it would be like sticking your finger into the socket from what I can tell. There is also another exposed metal part - the speed selector switch - though I am not certain if any of that mechanism can present a path to exposure. I do have the knob for it but I am missing the tension spring clip that keeps it on so I will rig something up.

Schematic updates ... Tone pot is 25K. Bob, you were correct ... Looking more carefully at it this time around ... Filament pin 7 goes right to ground. Cathode pin 8 is connected to the white cartridge wire, and the 150R resistor to ground. Thanks for catching that! Visio is on my work laptop so I'll adjust and repost the diagram later. My theory about how the cathode is biased then was in error - sorry!

Revised schematic is uploaded in my original schematic post. Thanks!

Another comment. I would guess the cartridge goes to ground and the volume control, not to the cathode.

Bob,

The red wire from the cartridge goes to the volume control. The white wire from the cartridge however is soldered directly to pin 8 (cathode). There are two things soldered directly to pin 8 - the white cartridge wire, and the 150R resistor. Does that sound wrong? You would know better than me.

I guess that's a way to get a bit more gain without adding a cathode bypass or coupling capacitor. It puts the cartridge at a slight DC potential but who cares, with so much voltage to ground anyway?

The more I look into units like this, the more unhappy I get. Sure they work and they are cheap, but my engineering sense rebels at some of the techniques.

I took the motor assembly apart to service it. There do not appear to be oiling holes for the bearings - like I have seen in similar small motors for fans, leslie speakers, etc., so I'll do the best I can to get the bearings lubricated. I usually use Singer sewing machine oil.

Since I need to order the mounting grommets too I removed one and took a closer look as per the units on the VOM page. The strange thing is that my motor has straight shafts. But, the present motor mount grommets are the ones designed for cone shafts. Was this a common practice? Should I order the straight shaft mounting grommets instead?

Also, For the mechanical parts like the shifting assembly, what sort of lubricant would you recommend?

Thanks.

You can decide on lubricant based on application. If it's a high speed situation (motor shaft) use thin oil. If it's a slow moving part (control lever) silicon grease may be the right stuff. If it's exposed to the elements, molybdenum disulfide is good.

I am not aware of the motor mount differences. If there is a need for side motion to correct for tolerances, then perhaps a sloppy grommet is the kind to use. Of course, if the motor moves during operation, we have a source of wow. And if you like the way it works, you will be saying wow.

These are the kind of grommets I use:

http://www.tubesandmore.com/products/P-R407

Hot or "warm" chassis kitchen radio, exposed chassis screws, touching these while touching the faucet with the other hand, and you have a path of juice right across your heart. Quite possible. And how many kids sat those record players out on the edge of the porch and tried to play records while standing barefoot on the ground?

Out of curiosity ... I was looking at the spec sheet for the 25L6GT tube and the cathode resistor is listed as 180 ohms. But the resistor in my unit is 150. My resistor is out is spec and will be replaced. I already have both a 150 and 180 to replace it with. Should I stick with a 150, or use a 180 as per the spec sheet?

I would use what was in there. Those specs apply to one point in the operation of the tube and the circuits usually aren't exactly at the same conditions. Having said that, however, you won't see any significant difference. The higher value will result in slightly less plate current but you'd have to measure it to see that.

Update - I got the motor mounts and cartridge in from Gary. I lubed/cleaned the motor and installed the mounts. BIG difference. I also replaced the tone cap and cathode resistor, and cleaned the volume and tone pots.
Now, I am still having intermittent issues with the volume cutting out. When this happens I can touch the red lead on the cartridge I get a hum. But when I touch the needle I get nothing. So it is probably a bad connection or the cartridge dying. I want to install the new cartridge but have never done this before. The replacement cartridge comes with small metal contacts that slip over the two cartridge pins so I guess I have to unsolder the small leads and solder them to these contacts. That looks difficult. Maybe I should mount the cartridge, slip the contacts on, and then solder? I also have no idea which needle is the 78 vs LP.
Since some of you have already done this, any tips would be greatly appreciated. Thanks.

Also I assume polarity does not matter with the cartridge. Just want to be sure.

Polarity does not matter but it's better to do the soldering when the heat can't reach the innards of the cartridge. In other words, before you slip the terminals on.

Sometimes there is a ground side of the cartridge, which has a lug that connects to the metal body. Other than that, there is no polarity.

You can tell which is 78 with a jeweler's loupe or good magnifier. It's triple the radius of the long play side, much fatter.

I heard back from Gary. The LP needle is marked SA100 and the 78 needle is marked SA250. I guess that is a micron measurement or something similar?
Thanks for the tip about soldering the connectors on first. I'll try using a vise to keep the small parts steady. I'd like to have this done by tomorrow. The only remaining job is replacing the selenium rectifier, and possibly rewiring the power to switch/fuse the hot side as per the suggestions here. Along those lines, my dad asked me yesterday if I am supposed to use a 3-prong earth grounded line, and I told him I didn't think so since I would have to tie the neutral and ground wires together (from what I can tell anyway since the chassis functions as the ground), and that would cause problems if an outlet was wired backwards in any case. Is that correct?

Correct. A major redesign would be required to use a three wire power cord.