View Full Version : Odd Occurrence With My Predicta Barberpole


reichsrundfu
04-20-2017, 10:29 AM
Hi Guys,

A strange thing occurred the other night with my Predicta Barberpole: I switched it on and at first I noticed that the audio, which typically comes up about 2-3 seconds before the picture, came up simultaneously with the picture. After a minute or so the picture and audio began "pulsating" at about 120-130 beats per minute speed. Pulsating meaning the audio got aa bit louder during the pulse, and the picture got a bit brighter with the pulse. When I turned the volume control up further the pulsating ceased entirely. After some minutes I turned the volume back down and there is no longer any pulsating, however, the volume is softer than normal, in other words, you have to turn the volume up about 1/3rd further to get the same level as before. Additionally, now the sound quality is actually BETTER than before - meaning a nicer more natural base and cleaner and higher high frequency sounds.

I'm wondering if a tube in the audio section is on its way out the door or if something else is going on? Since restoring this set it has been a rock solid player, and I typically play it once to twice a week. Due to traveling its sat for about three weeks which is when this occurred.

Any thoughts?????

-George-

old_coot88
04-20-2017, 10:50 AM
Wonder if it possibly uses the "stacked" B+ scheme in which the audio output tube does double duty as voltage dropper for other circuits.

bandersen
04-20-2017, 11:22 AM
Yes, the 12CA5 audio output tube cathode is connected to the +140 volt source.

reichsrundfu
04-20-2017, 04:12 PM
Awesome. So happens I have an extra one in stock. If I get a chance tonight I'll swap it out and report back.

-George-

reichsrundfu
04-22-2017, 02:50 PM
Hi Again All,

Success. You were right on the money old coot 88! Swapped out the 12CA5 audio output tube and everything returned to normal right away. Loads of volume and everything just right. Looks like that put the B+ right back to 140 volts.

Many thanks, guys

-George-