unit hums loudly regardless of volume

Thanks again for your reply.

I'll have to pop the chassis again to see about the power transformer thing. It is possible that the xformer I saw (there is 1 outside the chassis) is an output and not a power xformer. I don't remember if there was one inside the chassis or not. Bear in mind that I just took a quick look at the thing, checked the caps real quick (as I suspected them to be the problem) and put it back together. There is no PC board, just flying wires everywhere, which I didnt trace any of. I'll take a closer look to see exactly what is connected where and post back here with the results.

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tempus fugit wrote: : Hey all;

: I'm trying to repair an old tube portable record player. When it is on, it : hums really loud, regardless of the volume. I assumed that this must be a : problem with filter caps, but they test OK for ESR and shorts. 1 of the : problems is that there are 3 or 4 in a can, so I don't know what the values : are supposed to be.

: Is there anything else that may cause this symptom? I suppose I could : parallel some caps across the existing ones, but I don't even know what the : values are supposed to be.

: Thanks

I fixed an old record player with the same problem and I used to be a TV repair tech.

I'd bet money on a bad filter capacitor. Just replace it with something of the same voltage rating and equal capacitance or slightly higher if the exact value isn't available.

It's not something w/the tone arm because that would vary with volume. It's not a shorted diode because if it were, it'd be putting AC in to the capacitor and that would be an AC short and a destructive overcurrent situation would result. It's not an open diode because you'd get no hum if that were the case.

Replace the filter capacitor. They're usually a tab mounted can and it'll be difficult to find one like it so instead, use an axial leaded capacitor and run insulated wires to it and glue it somewhere to keep it in place (silicone cauking/sealant would do the trick).

b.

tempus fugit wrote: : Hey all;

: I'm trying to repair an old tube portable record player. When it is on, it : hums really loud, regardless of the volume. I assumed that this must be a : problem with filter caps, but they test OK for ESR and shorts. 1 of the : problems is that there are 3 or 4 in a can, so I don't know what the values : are supposed to be.

Sorry for another followup.

The capacitor is a multi-section capacitor. Their values are usually stamped or imprinted on the side of the can. The leads will usually have a triangle, half circle and square markings/openings in the phenolic part of the capacitor (bottom). The "legend" of values will usually show that on the capacitor markings.

If you can't get the replacement, you can wire up individual electrolytic capacitora and tie all of the minus sides of the wires together and ground this to what was previously the capacitor can ground and use the + lead of each capacitor to connect the the wires that previously attached to the capacitors.

Here are a few sites relating to them:

barry

thanks for the reply Barry;

The cap is indeed multi section - I think there are 4 different caps. Unfortunately it is a cardboard container with no visible values. I'll look a little closer and see what I can see.

Thanks

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Hello tempus, are you still with us? Don't forget to pull the plug, eh! I suggest you search for a brand and model #, then search at rec.antiques.radio+phono, where there are expert advisors on how these units can be fixed. The transformer you saw was for the output to the speaker. You are dealing with a live chassis unit. the "death cap" may be shorted, and the hum you here is 120vac running thru the chassis. The filter caps may not be too bad, although they are always leaky at that age. You can use 50-80 uF for the first 2, then drop to 20-30 for the other 2. There is no diode tube to worry about. If the diode is a bunch of square plates with a central bolt, it is selenium. Replace it with a new silicon diode and a resistor. Cheers.

John Kogel

Still alive and kicking John, lol.

Thanks for the tips. The "death cap" I assume is the one that is connected from the live to the chassis? I'm not sure that there even is one in this unit, but as I mentioned earlier, I just took a quick peek inside the chassis and put it back together. I haven't had too much time to mess with it, but I'll definitely look for that when I do. If everyone would like, I could take a picture of it and post it on ABSE.

What do youy think?

tempus fugit

Yes, there is often a .05 or so cap from live to chassis on transformerless tube gear. It's main purpose is to turn the chassis into a sheild to keep radio interference.from being detected by the device. It's secondary function is to shock the @@#$ out of you when it shorts, (although they are rated at 600v and don't carry much current, so failures are usually due to moisture and disuse). If you have small kids, keep them away from the thing. After it is fixed, install an isolation transformer for $30. Yuo can even build your own by connecting 2 12V trannys back-to-back. This isolates the voltages in the chassis from those in your house = no more shocks. Cheers.

JK

Thanks JK.

I got a chance to take a closer look at this thing and the neutral wire is connected to ground, but there appears to be about 200 ohms of resistance between the ground on the filter cap and the chassis. I didn't see anywhere were the live wire was connected to the chassis through a cap. Forgetting about the hum for a second (I'd like to make this whole thing a little safer

Also, I mentioned a small blue thing that looks like a bunch of thin squares stacked on top of one another, that someone suggested was a selenium rectifier. I hear they emit a poison gas when they burn. Would it emit any dangerous gases during normal operation? Should I make replacing this part a priority? I should also mention that there is 1 tab on this piece that has some solder on it but no wires or anything connected to it. Could this be the source of my hum problems?

Thanks again

hum regardless of volume usually means bad or dried up electrolytic filter cap(s) and/or maybe a bad rectifier..... and no, during normal proper operation the selenium rectifiers do not emit poison gas. I would just simply bridge the filter cap with a replacement of the same or approximate value and the same or higher voltage to see if the hum goes away.... which it probably will given the symptoms you reported.

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Thanks for your reply Sofie.

I agree with your diagnosis - this reeks of a bad filter cap. The problem is that the filter cap is an old cardboard multisection with no marks of any kind on it. There are 4 tabs on the bottom, with the twistlocks acting as grounds (I think). Of those tabs, 1 shows a 300 or so ohm resistance to one of the twistlocks. Assuiming that that tab is a cap +, and not a ground, that cap definitely needs replacing. What I'd like to do is just replace the lot, but I have no way of knowing what the values of each of the 4 tabs are supposed to be. Would just replacing them all with, say, 22uF caps be OK?

I'd like to replace the rectifier as well, but I need to figure how to set up a bridge or diodes properly in the place of the old one. I've never seen one of these before, but it has 2 tabs on it - + and (-). The (-) tab acually has nothing connected to it. You can see that it once did, but doesn't anymore. So..... I'll have to try to figure out where it should have been connected at one time. Oddly, there is about 150V DC across the filter caps.

Thanks again

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Hello tempus. I've been away but so have you. From your last post, more clues are taking shape as to how to fix your phono. We need to know what power tube is in it. It will have either 8 pins, (25, 35 or 50L6) making it an early model, or a later model 9-pin 50C5 or 50EH5. Locate the pin for the plate, on the 50EH5 or 50C5 that is pin 7, looking at the bottom of the socket and counting clockwise from the gap. On the

It appears the selenium rectifier has been bypassed already. There is a solid state diode doing the job. It is likely black with a silver band but they do come in other colors, looks like a resistor but with only one band. Ideally, a resistor is included in series with the diode to bring the voltage down a little to the level of the original.

The shock you recieved could very likey be due to the filter cap shorting to ground, and replacing all the caps in the multi-cap can will likely solve both shock and hum problems. A polarized cord is a very good idea. A 3-prong cord is even better, with the green wire going to a solid ground point on the chassis. If you ground the chassis, locate the death cap, (line to chassis cap @ +/- 0.05 uF), and clip it out of there to prevent future shocks.

BTW, the tube heater supply is likely fed by a tap on the motor winding, eliminating the need for a separate tranformer. This ain't hi-fi. Take good care of that pickup cartridge.

JK

Thanks again for the replies guys.

Yet more information:

The power tube is a 50L6. The selenium rectifier has been bypasssed by a single top hat diode (I guess I was looking for a bridge, or at least 2 diodes).

I tried to rewire the power cord, and I'm getting frustrated. I wired it correctly, so that the hot lead went to the switch. This put 120v on the chassis, though so I switched it around. Guess what?? Still 120v on the chassis. This is with the power off. Before I put the new cable in, switching the plug around took the 120v off the chassis. The weird thing is that I can't locate any place where either wire is connected to the chassis, and there doesn't seem to be a death cap.

Sorry about the infrequency of the replies, but I don't get a lot of free time to work on it. Thanks

When in doubt, use 33uF or so, @450V (22uF is OK for a start. If the hum reduces but does not go away altogether, then go up). Otherwise,

Bridge: Voltage in X 1.4142 = voltage out. Single diode: Voltage in .7071 = voltage out.

Keep that in mind.

Peter Wieck Wyncote, PA

"tempus fugit" bravely wrote to "All" (13 Nov 05 22:07:08) --- on the heady topic of "Re: unit hums loudly regardless of volume"

tf> From: "tempus fugit" tf> Xref: core-easynews sci.electronics.repair:348490

tf> Thanks again for the replies guys.

tf> Yet more information:

tf> The power tube is a 50L6. The selenium rectifier has been bypasssed by tf> a single top hat diode (I guess I was looking for a bridge, or at least tf> 2 diodes).

High voltage and low current allows using only halfwave rectification.

tf> I tried to rewire the power cord, and I'm getting frustrated. I wired tf> it correctly, so that the hot lead went to the switch. This put 120v on tf> the chassis, though so I switched it around. Guess what?? Still 120v on tf> the chassis. This is with the power off. Before I put the new cable in, tf> switching the plug around took the 120v off the chassis. The weird tf> thing is that I can't locate any place where either wire is connected tf> to the chassis, and there doesn't seem to be a death cap.

There is usually a 1meg or 470K resistor with a disc cap across it.

BTW The dmm has 10M high-Z input and will read full 120V no matter where the line connects. It's safer connecting line to the rectifier circuit not the chassis. The 120V on the chassis should then be high impedance and not a widow maker. Load the dmm input with 10K to test, if it measures more than 50V then there is a serious problem.

A*s*i*m*o*v

... Real techs don't lick nine-volt batteries!

Thanks again for the replies.

I solved the 120V on the chassis mystery - the new cord I installed wasn't polarized after all. It has a white stripe on it (black cable) and one of the tips appeared bigger than the other, but it would fit in the socket either way. I installed an actual polarized cord and the chassis is AC free.

Hopefully I'll get to the caps soon. I'll keep you posted.

Thanks