korting: Troubleshooting Korting 1091

I am new to working on radios and have started with this Korting, which I realize is not a great place to start for a beginner, but it's what I've gotten into and would like to see it through to completion. Unfortunately, I am new to reading schematics and the only testing tool I have is a multimeter.

 I have an intermittent noise problem that I'm trying to get to the bottom of.  Here are the steps I've taken since acquiring the radio.  

1.  Corrected a mechanical issue with the off switch, which was broken in the off postion.  The radio powered up and worked after that, but I did not test it for any length of time.

2. With that good news and the help of a contributor's videos on the same model, I cleaned it up, used deoxit on the tube sockets, switches and pots.   I then went ahead and replaced some of the very ugly paper capacitors.  Put the chassis back in the cabinet and it played beautifully for a week until a shrieking sound appeared that was not controllable by the volume control.  It is not a static type of sound, it's more of an annoying shriek.

3.  To hopefully solve the problem, I decided to go ahead and replace the remaining paper and electrolytic capacitors, except for 2 ceramic caps at the transformer (C86 and 87). 2 mustard caps (C72 and C83) in the tone control output section I believe, C25 at the antenna, C62 and C67 at the output I believe, and 2 .015 uf 125V caps at the volume pot (I can't identify them on the parts list by those values).  I did not replace those mentioned because from what I've read they tend to be reliable and/or they are in more difficult places to get to.

4.  Radio powered up and played great.  Again about a week later the same shrieking noise started up.  The noise can begin right upon start up or happen after it's been playing fine for minutes or hours.

5.  Upon the recommendation of a restorer, I starting tapping around with a chopstick while the radio was on.  I thought I had found a section of chassis board that was causing trouble so I resoldered a couple of joints in that area and I thought it was fixed.

6.  Played it daily for about a week again and the same thing has happened.

 

My current observations are:

1.  The problem can happen on AM, FM or with an MP3 player plugged into the phono jack.

2  The noise will not change with the volume control, but, at times I have been able to hear a radio station at the same time with the noise if I turn the volume way up.

3.  The problem can't be consistently replicated.  At times I can tap on tubes, components, the chassis board or anywhere and nothing happens.  At other times I can tap just about anywhere, even just a light touch to a wire, and I get noise or a significant change in volume.  I have not been able to spot any pattern.

4.  I have searched for shorts and bad solder joints but have not stumbled on any issues. 

5.  I do not know if a tube is bad or weak.

6. I have not changed any resistors.

7.  I have not changed the selenium rectifier.

 

It's a puzzle to me.  I've considered going ahead and replacing the remaining capacitors and some resistors but that would be continuing the shotgun approach.  I would appreciate any suggestions the expert community would have to try to figure this out in some type of logical way.  

Any help is truly appreciated

 

Regards,

Mark

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Since your problem arises in all operating modes, it is actually only to be assumed in the LF part. I have not worked intensively on the circuitry of this radio, but I would like to point out that I once had a similar problem with the ECL86. Did you replace the tubes on a trial basis? Even if this hint wasn't helpful, it will bring your problem back up on the forum. Maybe someone else has a good idea to this issue.

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Thanks, Eilert. With the help of another member I was able to get it sorted out by resoldering a number of joints on the circuit board and also securing the board a bit better.  The conclusion was that the PCB was the root of the issue.

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