I'm trying to repair the volume control on a vintage record player. We have narrowed the problem down to a faulty potentiometer (the knob is functioning purely as an on/off switch)

I can't match what is going on here (see photo) with any modern repair instructions, and I'm wondering if anyone has any advice! It looks so simple, but I just can't figure it out!

 

Looks like a rotary switched volume control, you need to find the value on the side, usually these are 100K to 1Meg log. The switch will be 230V ac rated 5amps.


https://m.ebay.co.uk/itm/Range-of-M...hash=item35d2baddea:m:mikrnoWXqcuZ5nOFI11_3ww

 

Looks like a rotary switched volume control, you need to find the value on the side, usually these are 100K to 1Meg log. The switch will be 230V ac rated 5amps.


https://m.ebay.co.uk/itm/Range-of-M...hash=item35d2baddea:m:mikrnoWXqcuZ5nOFI11_3ww

Thanks! That looks exactly right! Can I solder as normal, or do I need any special equipment when working with a/c?

 

No special equipment , just an ordinary soldering iron. .

 

Unplug the record player from the wall before soldering!

 

Unplug the record player from the wall before soldering!

Gotcha! That much I know...

(Usually - my vintage mixer is also needing repair, but since its problem is turning off when I tip it too far to the side I did accidentally leave it plugged in once. There is another mysterious machine, let me tell you! So many electromagnets!)

 

Looks like a rotary switched volume control, you need to find the value on the side, usually these are 100K to 1Meg log. The switch will be 230V ac rated 5amps.


https://m.ebay.co.uk/itm/Range-of-M...hash=item35d2baddea:m:mikrnoWXqcuZ5nOFI11_3ww

I got a 1a @ 125v pot from eBay which looked very promising, but when I finally got the tape residue off the old pot, it was 3a@125v.

What will happen if I use the 1A pot?

Thanks again!

 

A couple of things.

A= audio taper or LOG; this is what you want for audio
B is linear taper

1A @ 125V is 1 Amp at 125V which is the rating of the switch contacts. 3A is 3 Amps.

So, look at the name plate and find Watts and Volts; Convert to Amps P=V*I. Your pot should have voltage equal or higher that the voltage switched. Similar for current.

A synchronous motor doesn't really have a high inductive start-up current. Switches do have AC and DC ratings as well as currents that can be reliably switched.

Some switched can be rated at say 5 A at 40 kV, but they have to switch at 0 Amps. Many high current relays can't reliably switch low currents. e.g a 30A relay won;t reliably switch 1 milliamp.

 

So using a 1A pot instead of a 3A will have an overload, yes?

I have a second pot with this info:

• Potentiometer 24mm B250k 250k pot w/ 5A AC on/off Switch

Is this one worth a try? (No other info printed on the device or package). Looks like this (probably doesn't help):

 

In this case, 3 A is "better" than 1 Amp and 5 Amp is "better" than the 3A and 1A.

But that said, if your switching 240 VAC mains and you have a 1 A pot, this means it can switch about 250 Watts.
The turntable motor probably is less than 10 Watts. The speaker has like a 1 Watt rating on it, so it's basically irrelevant. If your switching 1 A and 120 v mains, then 120 Watts and again irrelevent. I haven;t seen 250 VAC rated switches pots with switches , but I;m in the US where the mains are 120 VAC.

The taper matters more than anything else, otherwise the volume won't vary proportionally more louder.

Position the shaft such that it's 1/2 between the end points. Measure the resistance between the two ends. It will not vary with position. If you measure CW to wiper and wiper to CCW, the values of the resistance for an audio taper will be very different than 1/2 the end to end value. e.g. It won't be 250K/2 or 125K.



if you happen to get a reverse audio taper, the volume goes the wrong way.

Hey, you don;t have to fully install the thing to make sure it works. Wire nit the power wires together.

Tack solder the potentiometer with wires as if it's connected and see if it works as expected.
then permanently install it.

Here's https://www.amplifiedparts.com/products/potentiometers_0?filters=Taper=Audio,Type/Size=Switched a place with some POTS. Some here are push-pull=ON/OFF which in the long run could be better.

Many times the value of the potentiometer isn't critical, but sometimes it changes the frequency response.

Don't be afraid of it. Just don't mangle the soldering job until it sounds right. So, remove the POT which you have to do anyway. take a wire nut and put the AC switch together so it's permanently on.

You can cut the pins off the pot if you plan not to use it. That will make it much easier to unsolder.

Then tack solder the pot with LONG wires to see if the volume control behaves as expected. Check that the switch closes when turned clockwise with an ohmmeter.

==

In reality, I think you could use some help in how to unsolder without doing damage.

You may have some mechanical issues, so let's take a peek at that first. This pot uses the leads to keep it in place. Hopefully, you could use the nut and star washer mounting?

Typical POTS have a tab that stick out that prevents rotation because it's inserted into a hole. Then, if you have the right tool, you can tighten the nut behind the panel. The front nut becomes a stop.

"Desoldering" can involve heating and pulling to get the power leads off.

You can cut the terminals off the potentiometer, so now your only dealing with one at a time.

There's basically two techniques:

1. Manual solder bulb or a solder sucker. (simplistic view)
This https://www.adafruit.com/product/148 is in the better category. This https://www.showmecables.com/60w-desoldering-iron-with-bulb might be in the "better category".
The "best" category uses temperature control and a vacuum source.

You automatically have issues with too much heat or too little heat and the risk of board damage and even lead and lead free solder. Lead solder is so much easier to work with and has a lower melting point than lead free.

Then there is the solder wick method. Some info is here https://www.eevblog.com/forum/reviews/goot-solder-wick/25/ It's basically braided wire and flux. So, solder wicks into the wire and you keep cutting the ends off as you consume the braid. They come in various widths and quality.

the point is, sometimes a combination works. Find a way to get most of the solder off.

The next pat might be counter-intuative, but it still seems stuck. the idea now is to re-flow the joint with new solder so it heats the pin down the hole and immediately suck it out or wick it out.

This isn't a lesson is desoldering, but what technique might work for you.

lead solder
You need flux and it can be in the solder already. This is cored solder.
You DO NOT want acid flux (stuff used for plumbing is a no no)
Rosin Flux was the flux of the day.

Unsoldering may require you to clean out the hole, so aluminum tools can be used to do that. Solder won;t stick to aluminum.

60/40 solder melts and solidifies at slightly different temperatures. In that ERA, that's what was used.
63/37 solder is very interesting. It melts and solidifies at the same temperature. it's necessary with surface mount parts.

Flux also comes in bottles. Flux keeps the parts clean and prevents oxidation while soldering.

But now, after you have soldered the parts, the flux has to be removed or possible corrosion my occur.
Flux can be water soluable. Methanol works somewaht OK with Q-tips and tiny wire brushes.

 

I got a 1a @ 125v pot from eBay which looked very promising, but when I finally got the tape residue off the old pot, it was 3a@125v.

What will happen if I use the 1A pot?

Thanks again!

The switch will work but as it's rated at 1amp, chances are it will burn out faster than a 3amp one, you can use the 5amp one but the volume might not adjust Linerarly when adjusting.

 

The switch will work but as it's rated at 1amp, chances are it will burn out faster than a 3amp one, you can use the 5amp one but the volume might not adjust Linerarly when adjusting.

Thanks so much! All that info was really helpful.

I was worried that I would burn something up instantaneously, since the a/c goes into the pot directly. Now that I know there isn't likely to be an immediate release of the magic smoke, I'm much more confident to give it a shot!

And the resoldering tips are great. I *was* wondering if the old solder would behave differently from the new. I'll let you know how it goes.