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Old 19th Jan 2020, 2:10 am   #1
Diniz Diniz
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Default Potentiometer in Andante 1462. Original?!?

Hi!
I have this set Telefunken andante 1462. I decided to repair it since Iíll be waiting for replacement material to my previous repair.
Despite the set seemed to have not be touched previously, I find the volume/tone potentiometer to be faulty. I realized that it is a 2 x 0.5Mohm potentiometer with only one control axis. According to the schematic it should be a double axis control with 0,5Mohm for volume and 1.3Mohm for tone, am I wrong?
Does anyone find the same in this model.
But my main question is: why does R19 (1.3Mohm) have four leads in the schematic? Because the potentiometers iíve seem only have 3.
Image with partial schematic with R19 is attached.

I was planning to replace it with one potentiometer from one telefunken jubilate 1161, but now I am confused in how to wire it...

Any help will be much appreciated.

Diniz from PORTUGAL
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Old 19th Jan 2020, 2:20 am   #2
JimMac53
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Default Re: Potentiometer in Andante 1462. Original?!?

The extra lead is a tap that provides a loudness contour to the volume control. As the volume is decreased, the bass and treble are increased since the human ear is less sensitive to the frequency extremes for soft sounds. Many of the German radios have pots with this sort of tap.

It looks as though someone tried to use a dual pot to perform the same function.
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Old 20th Jan 2020, 12:40 pm   #3
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Default Re: Potentiometer in Andante 1462. Original?!?

Would this pot (see attached image) with 0.5Mohm + 1.0Mohm be suitable for replacement?

Thanks in advance
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Old 21st Jan 2020, 6:59 pm   #4
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Default Re: Potentiometer in Andante 1462. Original?!?

The original, per the schematic you posted, is just a single gang pot with a tap. I don't see anyway to mimic the original pot with a dual gang pot, but maybe someone else has a nifty way of doing this.

If it was mine, I'd use a single gang 1 meg pot and leave the connection to tap unconnected. You'll lose the loudness contour which is effective at low volumes, but the tone controls can restore most of this.
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Old 22nd Jan 2020, 12:38 pm   #5
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Default Re: Potentiometer in Andante 1462. Original?!?

Hi Jim,
When I mentioned that the set have a dual gang pot with a single axis, I was sayng that one of them is for volume and the other for tone, is this, when you reduce the volume you also reduce the resistance of the tone control pot.

So, I replaced it by a dual pot with to axis, the 0.5Mohm for volume, and the 1.0Mohm for tone. They are independently controlled, since the dual pot has to axis (one inside the other).

But now I am dealing with the following symptoms:

- I can ear (from the speaker) an intense static sound that increases when I touch the metal part of the control of the potentiometer (plastic knobs removed).
-The sound disappears when I remove EABC80.

-It goes more intense also when I touch the point "A" (see attached image) with a screwdriver. And even more intense when I touch point "B".

Any tips of what may be causing the problem?

Note: I assume the FM module is ok, since I have already ear stations being tuned (despite with very low quality).

Any help will be much appretiated.
Thanks in advance,
Diniz from Portugal
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Old 22nd Jan 2020, 4:07 pm   #6
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Default Re: Potentiometer in Andante 1462. Original?!?

So R19, 1.3Mohm, is the volume control and R20, 500kohm, is the tone control. Replacing R19 with a 1Mohm pot is fine - you may lose a little volume, but good enough. Your dual pot should be fine.

Touching point "A" or "B" will induce a 50Hz hum into the audio. It's actually one of the ways people test the audio section of these radios when there is no sound at all. Touching A or B will give a hum and then you know the audio section is working. So what I hope you are getting is this 50Hz hum (which in english is different from static - static is a high frequency noise or more like white noise - the noise you hear on fm between stations) So confirm the noise you are hearing is hum. Touching the metal parts of the pot when it is not installed in the radio (and hence electrically connected to the chassis) can induce this same hum.
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Old 23rd Jan 2020, 12:54 am   #7
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Default Re: Potentiometer in Andante 1462. Original?!?

Ok, then I believe its hum.
Since 50uF+50uF electrlytics have already been replaced, what can be causing this?

When I touch the pot, even with the pot not connected to the chassis, the saund become very intense (particulary in one of the axis of the pot).
I invite you to see the following video where i recorded the problem:

https://youtu.be/mrzQAKDaddk

Thank you in advance for your help.

Diniz from PORTUGAL
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Old 23rd Jan 2020, 1:22 am   #8
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Default Re: Potentiometer in Andante 1462. Original?!?

I watched the video. That is 50Hz hum. The hum comes from the voltage induced in you from the 220vac 50Hz in your house. The 220vac wiring in your house creates an electromagnetic field - much like the primary in a transformer - and it induces a voltage in you (you can see this on any oscilloscope). You are like the secondary of the transformer. It's very low voltage, but more than enough that once amplified, will sound loud in the speaker.

Some of this might go away if you physically install the pot in the chassis, but I suspect the center shaft of the pot is either connected to the wiper or there is enough capacitive coupling between the shaft and the wiper to give this effect. Trying mounting the pot where it is supposed to go.
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Old 23rd Jan 2020, 3:14 pm   #9
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Default Re: Potentiometer in Andante 1462. Original?!?

Thank you Jim for your very detailed explanation on hum.
I replaced the pot where it belongs and the majority of the hum disappeared.
However still no sound from the bands section (none of them). I only can eat a very discrete sound on two stations of fm (almost no audible).
With a signal in PU socket, the set plays, so I believe output tube and output transformer are ok.
I don’t know where to look...

Thanks once more for your help and effort.

Diniz from Portugal
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Old 23rd Jan 2020, 9:40 pm   #10
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Default Re: Potentiometer in Andante 1462. Original?!?

First things first. Have you removed and replaced the coupling capacitor, C28 0.01uF, shown in schematic? If this capacitor is leaky, it can damage both the output valve, V4 - EL95, and the output transformer.

Second. Clean the band switch contacts. These often become dirty and perhaps corroded and if so, will lead to poor or no performance. This radio has lots of contacts and cleaning them is not easy. Be careful so as not to damage the contacts or the switch mechanism. A good cleaner and then work the switches a lot.

Third. Ensure you have good, known working valves.

Fourth. Check the voltages shown on the schematic. It is normal to have +/- 10% variation from the voltages shown on the schematic. You are looking for big differences from what is expected.
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Old 23rd Jan 2020, 10:29 pm   #11
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Default Re: Potentiometer in Andante 1462. Original?!?

Thanks Jim,

- I had already replaced C28, which I believe is what you commonly call "that capacitor". It was indeed leaking and making output transformer warm a lot.

- I will (again) clean the contacts more carefully.

- I already changed the tubes for working ones, but I will recheck and try different ones.

- Despite the schematic is very poor regarding voltage information, I will try to do that.

Thanks once more Jim, your being very helpfull. Iīll update you on the process

Diniz from Portugal
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Old 24th Jan 2020, 12:31 pm   #12
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Default Re: Potentiometer in Andante 1462. Original?!?

Quote:
Originally Posted by JimMac53 View Post
First things first. Have you removed and replaced the coupling capacitor, C28 0.01uF, shown in schematic? If this capacitor is leaky, it can damage both the output valve, V4 - EL95, and the output transformer.

Second. Clean the band switch contacts. These often become dirty and perhaps corroded and if so, will lead to poor or no performance. This radio has lots of contacts and cleaning them is not easy. Be careful so as not to damage the contacts or the switch mechanism. A good cleaner and then work the switches a lot.

Third. Ensure you have good, known working valves.

Fourth. Check the voltages shown on the schematic. It is normal to have +/- 10% variation from the voltages shown on the schematic. You are looking for big differences from what is expected.
Hi Jim, I just realized that the attachments you sent me have voltage values on it. In my schematic they are not present... Is it possible that you share your schematic with me?
Thanks in advance!
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Old 24th Jan 2020, 2:23 pm   #13
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Default Re: Potentiometer in Andante 1462. Original?!?

I got the schematic from Radio Museum - http://www.radiomuseum.org
I suggest you register with them and then you can download the schematic.
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Old 24th Jan 2020, 2:31 pm   #14
Diniz Diniz
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Default Re: Potentiometer in Andante 1462. Original?!?

Quote:
Originally Posted by JimMac53 View Post
I got the schematic from Radio Museum - http://www.radiomuseum.org
I suggest you register with them and then you can download the schematic.
Thank you Jim, I'll try that.
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Old 25th Jan 2020, 11:59 pm   #15
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Default Re: Potentiometer in Andante 1462. Original?!?

Ok, I check the majority of the voltages, and everything seems to be ok, except for the Pin 8 of ech81, where there is no voltage?
Any tips for what can be causing that?
Thanks in advance.
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Old 26th Jan 2020, 12:14 pm   #16
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Default Re: Potentiometer in Andante 1462. Original?!?

Check that FM isn't selected when checking the AM oscillator anode voltage (pin 8 ECH81) If still no voltage then check the voltage on both sides of R7 with medium wave selected.

Lawrence.

Last edited by ms660; 26th Jan 2020 at 12:28 pm. Reason: additional info
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Old 27th Jan 2020, 10:37 am   #17
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Default Re: Potentiometer in Andante 1462. Original?!?

OK, thank you Lawrence. You're right, with the AM selected I have voltage in the pin8 of ECH81.

I rechecked the voltages in all the tubes and I detected no voltage in pin8 of EF89. I checked the resistors nearby and I found R12 (100kohm) in open circuit... I replaced it and the set is now playing.

I will reassemble it and let you know the final result.

Thank you so much Jim and Lawrence for your help.

Kind regards,
Diniz from Portugal
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Old 27th Jan 2020, 9:08 pm   #18
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Default Re: Potentiometer in Andante 1462. Original?!?

Congratulations!
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