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 Radiola B15 Shenanigans
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 Return to top of page · Post #: 1 · Written at 9:27:10 PM on 24 September 2015.
TheDanger's avatar
 Location: Devonport, TAS
 Member since 26 March 2015
 Member #: 1718
 Postcount: 22

Had this on the bench for awhile now with no resolution in site. Threw up a quick video on the youtoobs so you can see first hand the issue.

https://youtu.be/7b6u9j7wH8k

Basically.... station is tuned in, volume turned all the way down and there is crackle and his. Unit has been recapped besides a couple of low value Micas. Any resistors out of spec have been replaced and the speaker replaced due to the cone paper being eaten away.

There is a rogue low value cap which I point out in the video. Would like to know what effect this would have been put in place for? But even more where I should start looking to resolve this last problem? This bad boy has thrown a few different problems at me that I have not encountered before. Been a good learning experience so far Smile

Cheers for any help!


 
 Return to top of page · Post #: 2 · Written at 10:50:40 PM on 24 September 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5461

Crackling & hissing is most commonly the realm of a Silver Mica cap. External interference on a set with an external antenna is easily determined by shorting the antenna terminal to ground with a 0.01mfd cap.

It sounds to me like it could be around the Detector / First audio. Failures of Mica caps in Australian sets is not that common, but it does happen & that why I carry spares.

The most common to fail are the ones with HV on them, or damage. Testing them for leakage with an ohm meter is normally an exercise in futility. If I have one of these with an end out of circuit I will check it with an insulation tester at 250VDC. The best time to test resistors & caps is as you get rid of he wax paper caps & old electrolytics.

Do not over look a bad resistor arcing. In that area I find plate resistors on the first audio and the grid resistors on the OP tube to have a high attrition rate.

Having no circuit is of no help, so if it is say 6B8 or similar look at the Mica / ceramic caps on the diode plates.

When you have sorted that do re align as changing parts in the RF can throw it off frequency all the way along the line & the set will be "deaf".


 
 Return to top of page · Post #: 3 · Written at 11:31:52 PM on 24 September 2015.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 2109

From my experience (although it could easily turn out to be something else), look for mica caps of around 100pf in the area around the 6AV6 valve, and replace them.


 
 Return to top of page · Post #: 4 · Written at 11:58:42 PM on 24 September 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6797

In case someone is looking for B15 schematic and service notes:

http://www.kevinchant.com/uploads/7/1/0/8/7108231/b15.pdf


 
 Return to top of page · Post #: 5 · Written at 1:20:33 AM on 25 September 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5461

Much more enlightening. C19; C24; C25; would be high on my list. The set will likely run with them disconnected. So try that one at a time & see if you get rid of the snap crackle & pop? If it goes its that one.

The grid resistor on 6AV6 is rarely good & bad connections (dirty , sprung) on the valves sockets can also cause issues.


 
 Return to top of page · Post #: 6 · Written at 1:53:01 AM on 25 September 2015.
TheDanger's avatar
 Location: Devonport, TAS
 Member since 26 March 2015
 Member #: 1718
 Postcount: 22

I had to get up and check once I saw the replies Smile Sure enough C24 disconnected and beautiful sound again. 100pF Simplex cap. I'll pay extra attention to these from now on.

Again the knowledge here astounds me! Thank you very much!


 
 Return to top of page · Post #: 7 · Written at 9:23:37 AM on 25 September 2015.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 2109

Good to hear you've fixed it. Those radios sound good and are quite sensitive when working properly. The downside is that the carrying "handle" can break if handled roughly.

My last run-in with a mica 100pf capacitor was when it decided to turn itself into a 600 ohm resistor. Being connected to the triode's anode, there's no surprise that the radio was totally silent.


 
 Return to top of page · Post #: 8 · Written at 1:57:47 PM on 25 September 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5461

As said rare failure, but you really only learn from sets that fail to play ball.

Marc


 
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