Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Tech Talk

Forum home - Go back to Tech talk

 Lekmek starting to sound sketchy at low to mid volume after a few minutes?
« Back · 1 · 2 · Next »
 Return to top of page · Post #: 16 · Written at 9:59:59 AM on 16 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

Yes I thought it could be poling in the speaker, from your description. The fact that it has a 75 means it has a diode detector and an "audio" volume control, so that pretty much rules out any scenario that would result in distortion on low volume in the audio amplifier stages. Injecting a known-good signal as I described will prove it one way or the other.

But I should have a little talk to you about how to fault-find, based on a lifetime spent doing it......

1. Don't pull anything apart or replace anything unless you have proof it's the cause of the problem.
2. Disturb as little as possible - collect evidence instead BEFORE you dive in.
3. If you have a hypothesis, test it first. Doing so saves a lot of time and frustration.
4. Think before you act! Use your head, not your hands.


 
 Return to top of page · Post #: 17 · Written at 12:50:09 PM on 16 August 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

I agree with that Ian! A good scientific approach makes good sense, I have a Bakelite Astor Mickey that I made the mistake of "diving in" and don't know where I am with it, but I've been lucky with the Lekmek, it had issues, the candohm originally was open between all sections, well and truly cooked and the cathode resistor 425 ohm, which serves both the 6D6 and 6A7 was open, real evidence that too much juice had been going through carving a path of destruction. What I should have realised sooner is that the power cord was put on to the wrong ie. 22 volt tap (by myself) but the original cord was cut and I soldered a new one to where the old one went... Derrrr! I should have just checked all taps with my ohmmeter and not assumed it was previously correct, especially with the faulty candohm!

Lekmek 540 Console Radio


 
 Return to top of page · Post #: 18 · Written at 8:28:42 PM on 17 August 2017.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5791

Photo uploaded to Post 17.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 19 · Written at 10:42:33 PM on 17 August 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3967

What I like to call it is methodology. Develop a plan of attack which never starts with powering it. As pointed out with the current pair here. one had several errors and it would have never run and the other had a patently obvious, by the contents leaking, dodgy cap. All achieved by looking before you leap. Neither fit to be powered.

I align with an Oscilloscope as the measuring device. It often picks up distortion and is used to track its source down. It is amazing at times how easy & formidable an Oscilloscope & Signal generator can make finding lost signal & distortion at its source.

Anything that eliminates speculation & guesswork is good & often a time saver.

Marc


 
 Return to top of page · Post #: 20 · Written at 10:48:39 PM on 20 August 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

I don't have a signal generator, but I do have an oscilloscope, I just don't know what to do with it?


 
 Return to top of page · Post #: 21 · Written at 11:02:33 PM on 20 August 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

You don't really need either to fix old radios but they can make tricky jobs easier.
A smartphone makes an excellent audio signal generator. The example I set out earlier
in this post is recommended.


 
 Return to top of page · Post #: 22 · Written at 10:31:12 PM on 22 August 2017.
JamieLee's Gravatar
 Location: Clare, SA
 Member since 27 March 2016
 Member #: 1894
 Postcount: 508

Ok I had never considered that, but a brilliant idea, I shall have to do that and get my head around it as I also have a couple of good books.


 
« Back · 1 · 2 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.