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 Post subject: '48 Decca 78 rpm child's phonograph
PostPosted: Nov Wed 11, 2009 9:58 pm 
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This '48 Decca 78 rpm record player was an ebay purchase that came in today. It has a two tube (35Z5 & 50L6) amp with only a volume control. The cartridge is similar to the old metal cartridges; but, this one is mounted on a spring. The red sticker on top of the tonearm mentions "child proof". The way the cartridge is mounted is similar to the '60's Zenith "micro touch" cartridges. Of course, there is nothing "micro touch" about this cartridge. It feels like it tracks at over an ounce.

The record player has the usual bad caps and bad cartridge that most units of this age have. The only cap in the amp is the multisection electrolytic. I haven't decided if I'll have the old cartridge rebuilt or substitute something more modern. Since the amp does not use a preamp stage, I'll have to find something with a high output.

These is no model number to be found. The only way I know it's a '48 model is I found some date codes on the tubes.

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 Post subject:
PostPosted: Nov Wed 11, 2009 11:56 pm 
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Cool tone arm, never seen one quite the same before. Looks similar to the Webster-Chicago's of that era, but not same.


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 Post subject:
PostPosted: Nov Wed 11, 2009 11:59 pm 
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This was almost identical to the unit I mentioned owning in another post. Only difference was, mine had an octagonal speaker opening and didn't have the neat red tag on the arm. It, along with some Eddy Duchin and some Ted Heath records, urged me through my pre-pubescent years.


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 Post subject:
PostPosted: Nov Thu 12, 2009 1:11 am 
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The red label says "miracle tone arm." I wonder what made it a miracle?


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 Post subject:
PostPosted: Nov Thu 12, 2009 2:42 am 
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I like those . I have 2 .... one has 4 speeds . Problem is they are not worth much money wise . My 2 are all fixed up and just wont sell . Had them up for sale in my shop for over a year at 40$ . No takers .


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 Post subject:
PostPosted: Nov Thu 12, 2009 7:47 am 
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Ken G wrote:
I like those . I have 2 .... one has 4 speeds . Problem is they are not worth much money wise . My 2 are all fixed up and just wont sell . Had them up for sale in my shop for over a year at 40$ . No takers .


Yep, I know. The only way it would be worth restoring one of these would be if you planned to keep it (which I intend to do). Most of the stuff that I bring back to life ends up staying here. I get more enjoyment out of using the item myself than to have someone think I should let it go for three cents.

Sort of reminds me of the guy who asked me to fix an ultra cheap '60's kid's record player in a cardboard case. He claimed he gave $85 for it at an antique shop so he could play his 78's. Then, he blew a gasket when I quoted him an estimate to replace a defective audio output transformer, the caps, cartridge, and idler wheel. Then, I tried to sell him a good '70's "all in one" system with a 3 speed BSR changer for something like $50. He was not interested, claiming I wanted too much. I thought to myself, "you go out and blow $85 on some cheap kid's record player that does not work but yet $50 is too much for a stereo that works just fine".


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 Post subject:
PostPosted: Jun Sun 27, 2010 4:23 am 
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I dug this out tonight and decided to work on it. The only capacitor in it was the 3 section filter cap, which was bad. The amplifier is working and the platter is spinning; but, I need to replace the dead caartridge. I wonder how one of those current production green 78 RPM power point cartridges would work? I know the original cartridge is a 3 volt unit and the power point will only be 1 V; but, it's not like I'm expecting to blast the neighborhood with this kids record player. And, this thing would never pass UL inspection today as the chassis is connected to one side of the AC line. Most of these had a capacitor between circuit ground and chassis ground; but, not this one. The only thing protecting the user from the hot chassis is the control knob. If I can find an isolation transformer that's small enough, I may attempt to mount one inside the case.


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 Post subject:
PostPosted: Jun Sun 27, 2010 6:45 am 
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If that is a single audio tube amp you must have a 3 volt cartridge. Modern low voltage cartridges will not be enough to properly drive the amplifier. What is the tube line up???
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 Post subject:
PostPosted: Jun Sun 27, 2010 6:58 am 
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35Z5, 50L6. I know for optimum performance that a 3 volt cartridge will be necessary; but, I'm afraid a 78 rpm version of such a cartridge will be difficult to find, at least at a reasonable price. I could experiment with a transistor preamp stage and use the cathode of the output tube as a power source. I think I've seen some schematics of such a preamp on here somewhere.


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 Post subject:
PostPosted: Jun Sun 27, 2010 7:13 am 
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If there is enough room, you could re design the amp to use a third tube, either a 12SQ7, or, a 12AV6.
Filament resistor woluld have to be changed down to 150 ohms at 10 watts.
You'd need to add a third electrolytic, say 20mf, at 160 wv.
The first audio shouldn't have more than say 80 volts on the plate. Generally, on three tube phonos it's closer to 65 volts, but, not always.
I have rebuilt some amps like this, including a 1948 mannual 78 rpm Dynavox that used a 50C5, and, 35W4 rectifier.
I would also add a safety capacitor to the low end of the cartide where it goes to chassis, which , in your, and, my Daynavox case is B-.
I used a .02 mf 630volt mylar.
There are also other numerous improvements you can try which do help.
I sometimes change the cathode resistor on audio output tube to 150 ohms,.5 watt. I then install a 22 mf 35 volt electrolytic accross that resistor, with the positive + end going to the cathode.
This improves the volume somewhat, and, the tone alot.
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 Post subject:
PostPosted: Jun Sun 27, 2010 12:09 pm 
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radiotvnut wrote:
35Z5, 50L6. I know for optimum performance that a 3 volt cartridge will be necessary; but, I'm afraid a 78 rpm version of such a cartridge will be difficult to find, at least at a reasonable price. I could experiment with a transistor preamp stage and use the cathode of the output tube as a power source. I think I've seen some schematics of such a preamp on here somewhere.


Former forum member Sylvain Vanier had this little preamp schematic that I saved. I would have posted a link, but I can not find anywhere that it is currently posted.

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 Post subject:
PostPosted: Jun Sun 27, 2010 12:17 pm 
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Here is an updated drawing of the same circuit
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 Post subject:
PostPosted: Feb Fri 25, 2011 8:52 pm 
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This morning, I got back on this record player. While digging around in my junk, I found an 89T cartridge holder and I had an old Astatic 89T powerpoint cartridge with a shot LP stylus; but, the 78 side is still good. I had to extend the tonearm leads and snap off the flip lever on the holder in order to make everything fit. Even though it's a 1V cartridge, the player still has a fair amount of volume, especially with later 78's.

The idler wheel still works; but, it is very slick and rubber rejuvenator does nothing for it; so, I will soon be sending the wheel off to be rebuilt. At that time, I will also clean and lubricate the mechanism.

Just for fun, I may build the one transistor preamp circuit shown above.

And, I will be trying to locate an isolation transformer that's small enough to mount inside the case. I discovered that the metal tonearm is hot, along with the volume control shaft. Since the tonearm is also hot, I think it would really be safe to say that current day UL inspectors would go insane over such a design.


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 Post subject:
PostPosted: Feb Sat 26, 2011 3:39 am 
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OK, I built the above preamp circuit on a terminal strip. Either I'm missing something; or, this circuit is not correct. I used the output tube cathode as the B+ source for the preamp and there is about 7.5 volts there. With the above circuit, I only have about .5 volts on the collector.


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 Post subject:
PostPosted: Feb Sat 26, 2011 4:21 am 
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The only thing I can come up with is the transistor that I used. All I know about it is that it is a small signal silicon NPN type (as determined by the diode scale on my DMM) It came out of a bag of transistors that have no type number on the case. The only markings are the ID markings for the emitter, collector, and base.

I suppose the next step is to visit the local parts house and buy an NTE123AP and see if that makes a difference.


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 Post subject:
PostPosted: Feb Sat 26, 2011 5:57 am 
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I've heard of radio/phono combos, but an electric shock/phono is something new. Sounds like the kind of gift you'd give too a kid you don't like. :lol:


I'm anxious to hear if the preamp circuit works. Could you use this on any underpowered phono amp/cartridge to improve performance? Would you need two of them for a stereo phono? Are the values of the parts specific to one player or can they be used on any player? What determines the value of R1? Sorry for all the questions, but I'm a noob, I wish I had found this interest twenty or thirty years ago.


Last edited by knhoj59 on Feb Sat 26, 2011 6:14 am, edited 1 time in total.

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 Post subject:
PostPosted: Feb Sat 26, 2011 6:03 am 
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That circuit will usually not work. It sets the transistor base bias only by R1, rather than by a more sophisticated circuit using a voltage divider and an emitter resistor. This circuit requires that the transistor hfe (current gain) be exactly right, which means that you must use the 2N3904 transistor called for or one very similar, and you may have to hand-select one that puts the collector voltage at between 3 and 4 volts. Alternatively you could experiment with different values of R1.

Even then the bias will not be stable. As the transistor base-to-emitter voltage drifts with temperature, the base current will drift along with it.


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 Post subject:
PostPosted: Feb Sat 26, 2011 6:10 am 
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knhoj59 wrote:
[quote="radiotvnut" ]I discovered that the metal tonearm is hot, along with the volume control shaft. Since the tonearm is also hot, I think it would really be safe to say that current day UL inspectors would go insane over such a design.[/quote]

I've heard of radio/phono combos, but an electric shock/phono is something new. Sounds like the kind of gift you'd give too a kid you don't like. :lol:


I'm anxious to hear if the preamp circuit works. Could you use this on any underpowered phono amp/cartridge to improve performance? Would you need two of them for a stereo phono? Are the values of the parts specific to one player or can they be used on any player? What determines the value of R1? Sorry for all the questions, but I'm a noob, I wish I had found this interest twenty or thirty years ago.[/quote]


The reason the tonearm is hot is because of the shielded tonearm cable used. The outer shield of the cable, which connects between the negative cartridge terminal and B-, runs through the tonearm; therefore, the outer shield is making contact with the metal tonearm.

The primary purpose of a phono preamp is to enable a magnetic cartridge to be used where a ceramic cartridge was once used. Or, in this case, use a low output ceramic cartridge in place of a high output crystal cartridge. I wouldn't suggest using a phono preamp to compensate for an amplifier that is under-performing due to circuit faults. In such a case, the amplifier should be properly repaired. For stereo, you would need two preamp circuits, one for the left channel and one for the right. For the most part, the components of the preamp should work OK from one player to another. I suspect R1 is determined by how much gain is needed.


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 Post subject:
PostPosted: Feb Sat 26, 2011 6:14 am 
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dberman51 wrote:
That circuit will usually not work. It sets the transistor base bias only by R1, rather than by a more sophisticated circuit using a voltage divider and an emitter resistor. This circuit requires that the transistor hfe (current gain) be exactly right, which means that you must use the 2N3904 transistor called for or one very similar, and you may have to hand-select one that puts the collector voltage at between 3 and 4 volts. Alternatively you could experiment with different values of R1.

Even then the bias will not be stable. As the transistor base-to-emitter voltage drifts with temperature, the base current will drift along with it.


I probably need to pull out my old electronics textbooks from college. Back then, I was pretty good with the formulas for figuring out component values, etc. Over the years, I've gotten a little rusty in that area. I'll admit that I've always been better at repairing circuits, instead of designing them.


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 Post subject:
PostPosted: Feb Sat 26, 2011 6:27 am 
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I was looking for a quick way to boost volume, but I guess in my case, I would probably be increasing distortion and hum, too. So there's no easy fix for getting better soujnd out of a cheap player. I need to find a better candidate or learn more about theory, huh?


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