quick schematic symobol help on Beatles Record player

[LucifersTrip]

sorry to go off the stombox subject, but it's a cool schematic from a $3000 item.

I'm helping a friend repair a Beatles record player (replacing large silver can cap) and am just getting acquainted with the schematic.



just a couple quick questions:

Where is the tone arm on the schematic?

What's this symbol on the left side?   variable cap?



thanx much

[artifus]

i'd guess it's the pick up. magnetic/ceramic cartridge. arrow representing needle. it's on the input.

*edit* i take that back. is the volume pot is also the power switch? the vari cap symbol could be riaa eq? any three legged caps?

[Quackzed]

this is a guess. but if i were to write 'variable voltage' i would draw that. resistor in series with v+ 'sag control'...
another guess, but i dont know what that > 25eh5 is ,so that makes me think THAT is the tone arm transduser. ..

you say you want!!@#$@!want!@#$wa wa wa @!#$ want a revolution! ..!#@$ revolution well you na...nanana...know...

[artifus]

i think the 25eh5 is the valve - check the pin numbering and i'm going for the vari cap symbol being the pick up again - pick ups produce variable voltage. can't you trace it from the vol pot?

[PRR]

> Where is the tone arm on the schematic?
> What's this symbol on the left side?

You answered your question. The "arrow" is the needle. The "parallel plates" are a ceramic transducer. For basic repair purpose the ceramic is a small capacitor (no DC continuity) which happens to make voltage when you wiggle the needle.

> is the volume pot is also the power switch?

The switch is on the _back_ of the volume pot. Turn down to the click, it turns off. Electrically un-related, but much cheaper in production (one hole, one part, one knob).

> could be riaa eq?

No "RIAA" here. The main slope of RIAA as usually plotted is really correction the magnetic transducer's rising response; ceramics are flat voltage/displacement. There's small jogs at 500 and 2150 but popular-price phonos simply used a bright-tone speaker to bump the upper midrange.

[LucifersTrip]

> Where is the tone arm on the schematic?
> What's this symbol on the left side?

You answered your question. The "arrow" is the needle. The "parallel plates" are a ceramic transducer. For basic repair purpose the ceramic is a small capacitor (no DC continuity) which happens to make voltage when you wiggle the needle.

excellent...that's why I asked those questions. We weren't sure what it was, but by process of elimination, that was our only guess.

Now, we are left with one problem. The old metal can cap had 3 wires: black/ground, red/cap#1, blue/cap#2 and the tabs which were soldered to the chassis. I tested continuity and there was none between the ground wire and the tab.

The new metal can has 2 wires:  red/cap#1, blue/cap#2 and the tabs which are to be soldered to the chassis.

I'm confused by the grounding scenario. With the new metal can cap, the grounds of the 2 caps are connected to the tabs, so if I solder those tabs to the chassis, those caps'll be soldered to chassis ground, which is not what the schematic shows.

I can't isolate ground from chassis ground.


thanx again

[tubegeek]

Well, first off, you are messing around with a device that has no isolation from the AC line. And no fuse.

Under the wrong conditions, that means YOU could be the fuse. How do you feel knowing that? If you are still feeling like you are ready to mess with this, well, let's see.

PRR correctly described the ceramic cartridge input, and the "arrow" schematic symbol is indeed representing the ceramic pickup. No RIAA EQ required due to the reasons he cited.

25EH5 is a tetrode tube with a 25V heater. The heater is in series with the 90V turntable motor to use 115VAC total for the pair of components. Note the direct connection to the AC line there.

Note, too, that a miswired AC line socket will result in the full 115V being applied to the chassis!!

The filter caps are both connected to chassis ground. Feel free to bend the cap tabs down and connect to the chassis via the tabs, making sure the $3000 dollar paint job is scraped away there, and take the wires that used to connect to the old cap + terminals and connect them to the two new cap + terminals. There is usually a legend on the side of the can that shows a shape representing which tab has which capacitance; these shapes are shown on the tabs by shaped cutouts in the tabs themselves.

Don't get electrocuted. There will be large DC voltages on the caps even when this item is unplugged, too. This is very much a horror show with regards to safety.

Any reputable tube tech would most likely refuse to hand it back to you unless you let him/her install a three-prong line cord or a 110:110 isolation transformer, and a fuse, no matter what this thing is worth. He/she would be safe working on it because he/she would have an isolation transformer on his/her bench and a place to safety-ground the chassis that he/she could be reliant upon. Do you?

Did I remind you not to get electrocuted?

[PRR]

> I can't isolate ground from chassis ground.

You MUST!!

Go back to the can-cap shop and look for a fiber mounting plate. The cap-can mounts on the plate and the plate rivets to the chassis, no contact between can and chassis.

Even this is dubious (UN-SAFE!!) because the exposed can is "live".

There IS a reason the original was constructed with isolated can (which costs more than just letting common negative leak to the can).

Also I suspect that the fiber insulator plate would require metal-work.

I really think you should hollow-out the original can and put some modern caps inside, with added insulation inside the can and on the leads. Or since the inside is not the beauty-spot, just tack new caps under the chassis.

Re-re-read tubegeek's warnings. This phono is from an era when 2-ton cars had 360 horsepower engines and would go 130MPH on hi-Lead gas, asbestos drum brakes, pointy dash-knobs, solid steeering columns, and no seatbelts. When Polio and Smallpox were bright memories. Expectations of safety have gone up in the last 50 years.

I would REALLY like to see an isolation transformer permanently installed.

I bite my tongue only because this item is uncommonly "valuable". Somewhat like an original Guillotine in perfect sharp working order. Though I suppose most surviving Guillotines have been "safed" with soft blunt blade or track-stops.

[DougH]

Be careful with that selenium rectifier too. A) that stuff is nasty and B) the outside of it is not insulated- i.e. when it is turned on, the rectifier is *live*- heat sink fins and all (ask me how I know... Not fun).

[CynicalMan]

Heh, I own quite a similar phono amp. I got it at an estate sale, it's a little amp kit with connectors to hook up to a speaker and turntable. I've never powered it up, for the reasons that PRR and tubegeek have pointed out, I've just been hoping to salvage the OT for some other project.

For reference, here's the schem:


So this one does point out that the cartridge is ceramic. The filter cap, though not canned, is attached to the chassis by strip of metal with a ring on the end that goes around the cap. For extra protection, there seems to be a plastic or fiberglass ring between the metal and the cap.

[PRR]

> Note, too, that a miswired AC line socket will result in the full 115V being applied to the chassis!!

Doesn't have to be mis-wired. The 2-pin plug may be inserted either way.

Isn't quite "full 115V" on the chassis. Chassis connects only via ceramic element and 0.05uFd cap. I'm not too sure about the health of 50 year old ceramic needle or wax-cap. If they _are_ working properly, there's 50K of impedance between line and chassis. Assuming lower impedance through human, the maximum shock current is 2mA. Taken through skin, this is still not considered un-safe.

While I hate to defend this shock-box, we should note that, unlike a guitar-amp, this chassis is not very "exposed". It's all inside the wood/cardboard case with plastic knobs. Yes, there is possible-contact to pot-nut rim, also under the tone-arm. And for the longest time, transformerless radios were totally enclosed except two chassis-screws wide-open on the bottom.

> YOU could be the fuse.

Fuses protect from fire.

Fuses can't stop shock. The power in a killing shock is not enough to light a night-light.

Fuses "can" prempt shocks IF the exposed metal is connected via the 3rd-pin to house common. But not with 2-pin plugs. And putting a 3-pin on this shock-trap isn't a full answer, because 60% of wall-outlets are mis-wired (in one small study: my old kitchen).

[tubegeek]

Ah, yes, I missed that .05 "Death Cap" on the schematic - the cap can should indeed be isolated from the chassis and the tube circuit only tied to it via the .05 cap.

Thank God, I have very little experience with stuff this cheap and dangerous. So I wasn't quite sure how it would have been made "safe."

I still wouldn't use it in the tub if I were you.

[LucifersTrip]

Ah, yes, I missed that .05 "Death Cap" on the schematic - the cap can should indeed be isolated from the chassis and the tube circuit only tied to it via the .05 cap.

yes, exactly...that was the "monkey wrench" that made the new metal can un-useable. when I first looked at that schematic I simply saw ground.

[LucifersTrip]

Go back to the can-cap shop and look for a fiber mounting plate. The cap-can mounts on the plate and the plate rivets to the chassis, no contact between can and chassis.

The new can-cap barely fits into the metal enclosure. It's actually a little larger than the original, so the extra height of a plate would make it a tough fit.

> I can't isolate ground from chassis ground.

You MUST!!

yes, and that's exactly what I did in the end....the simplest and cheapest and first way I wanted to. I just used two new electrolytics hooked exactly as the schematic, covered all exposed leads and tucked em in underneath. Then placed the old can-cap back (unconnected, of course) for "authenticity". It worked right off. Now my friend can butcher rare records on a 50 year old needle.

thanx everyone for the help...and the warnings