unit hums loudly regardless of volume

One or more diodes in the power supply (assuming it uses them) is shorted. If it has a rectifier tube instead of semiconductor diodes, then either the tube is shorted or, if it has its own filament supply (e.g. 5U4), then that supply could be shorted inside or outside of the power transformer (assuming it has a transformer), although there's a pretty good chance the tube would be glowing bright red if that were the case. If it's all tube and has no power transformer, then you've got a potentially lethal hot chassis and you'd better be d*mned careful. Check for shorted line bypass capacitors if it's this last case.

Or it could be one or more capacitors, as earlier suggested, or a break in some shielding.

As you can surmise, additional information about the circuit involved could narrow the possibilities (and possibly suggest others).

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Thanks for your reply.

Sorry, here's some more info:

There is a power transformer. Only 1 diode that I could see. Measured voltages at cap in question was ~150v. Only 2 tubes - 12AX7 and can't remember the power tube offhand.

There was also some small device which I assume was a transformer that hadd

100mA marked on it. It looked like a trasformer with cooling fins.

it

a

values

the

"tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42) --- on the heady topic of "unit hums loudly regardless of volume"

tf> From: "tempus fugit" tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record player warm up, then insert the plug one way or the other, and then paint a mark for which was the quietest way. One way: lots of hum, the other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or sometimes a voltage doubler) and used a large value resistor bypassed by a 0.01uF to RF ground the tone arm shielding to neutral. The power for the filaments often came from a secondary winding in the motor coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

... Just a little force field zap.

If the cap had DC on it, the diode is probably ok. The capacitor(s) could be dried out (although I think you said you checked them, so that's not too likely). From there, you move toward shielding type problems as already described elsewhere in the thread.

If one of the tubes had an internal heater-to-cathode short (or for that matter, a short at the tube socket), that could cause a lot of hum. Such shorts are not uncommon.

At any rate, now you have plenty of ideas to consider.

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hadd

possibly a selenium rectifier. back in the old days they hadn't figured out to make high voltage silicon diodes yet.

the usual warnings about selenium: emits poison gas when it burns

replace with modern diodes is recommended.

Thanks for the suggestions guys. I'll try some of these things out.

tf> From: "tempus fugit" tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record player warm up, then insert the plug one way or the other, and then paint a mark for which was the quietest way. One way: lots of hum, the other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or sometimes a voltage doubler) and used a large value resistor bypassed by a 0.01uF to RF ground the tone arm shielding to neutral. The power for the filaments often came from a secondary winding in the motor coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

... Just a little force field zap.

If the voltage on it measures 150V, then a 250V cap might be a better choice for a replacement.

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Tried the turining around the plug trick, but it made no difference. I also disconnected the phono plug that connects the tone arm to the amp unit, which also made no difference.

Here is something curious though. I accidentally touched my scope while my hand was on the record player chassis and got a fairly good shock. The scope has a 3 prong AC plug (the record player doesn't). I should've measured the voltage difference between the sope chassis and the record player chassis, but I didn't think to. I'll have to do that. Perhaps he chassis is live. I didn't think this would be the case though, since there is a cap with the negative connected to the case, as well as a few other green wires.

Thanks

tf> From: "tempus fugit" tf> Xref: core-easynews sci.electronics.repair:346437

tf> Hey all;

tf> I'm trying to repair an old tube portable record player. When it is tf> on, it hums really loud, regardless of the volume.

Have you tried turning the ac plug around?

Which way the plug is inserted matters with these. Let the record player warm up, then insert the plug one way or the other, and then paint a mark for which was the quietest way. One way: lots of hum, the other: quiet hiss.

Those old tube units used to get B+ directly from the powerline (or sometimes a voltage doubler) and used a large value resistor bypassed by a 0.01uF to RF ground the tone arm shielding to neutral. The power for the filaments often came from a secondary winding in the motor coil. Don't use this type of record player near the bathtub. Lethal!

Another possibility is a broken wire at the cartridge.

A*s*i*m*o*v

... Just a little force field zap.

That capacitor might be leaking or (worse) shorted. It can be a lethal failure (I read just this week about a minister in Waco who was electrocuted during a baptism when he grabbed a microphone that was "hot."). Be careful.

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Here's a cite to that story:

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I tested the cap and it showed about a half meg of resistance. maybe something is shorted somehwere else. Oh, and I checked the voltage between the record player chassis and the scope chassis - 120VAC (I thought that's what it felt like). Odd though, I would think with 120v on the chassis that it would pop a fuse at the breaker box.

Thanks

the

is

We're talking about a capacitor between the chassis and one side of the line, right?

It should be infinite (assuming you tested it out of the circuit).

When in doubt, replace it (or just snip it out and don't replace it). Google "death cap" if you'd like some further opinions, typically from the world of guitar amps.

maybe

Reversing the plug in the wall socket should change that (but don't count on it).

(I thought that's

Not until your hair catches fire.

You really need to be careful with this device. I can't stress that enough. Death is not to be trifled with.

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I havent reopened the chassis, but it seems to be connected between the ground of the speaker and the chassis.

between

Reversing the plug did correct that. Is this a design shortcoming (if so, there's gotta be a safer way to set that up) or should I be looking for a fault somewhere?

I am being very careful, not to worry, but I appreciate your concern and helpful advice (including those on personal safety).

Thanks again

unit,

OK, that's something totally different.

That's normal for a "hot chassis" device. But such a device won't have a power transformer, and I thought you said this one did. Could you be confusing an output transformer with a power transformer?

If you are (and I suspect you are) then you are such a newbie that you had better stop now and enlist the help of somebody who knows what they're doing before you kill yourself. No kidding. I don't want to have anything more to do with this pursuit.

tempus may fugit, but not for the dead

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Hi...

Sorry for the top posting; but the rest is getting a bit convulted with a confusing mixture :)

Just wondering... has tempus or any of the rest of you other old timers considered the possibility that it might be a dc "choke" speaker?

Take care.

Ken

CJT wrote:

That's an interesting possibility I hadn't thought of. I haven't seen one of those in quite a few years. Did they even make portables before PM speakers became the norm?

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