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crackly 47-1230 - Printable Version

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crackly 47-1230 - NostalgiaRadioTime - 12-13-2017

I've had this set for a number of years and even after restoration, it has had a crackle on the AM band that I couldn't get rid of. So, having some time off of work (using up PTO before the end of the year), I decided to pull the chassis and give it a more thorough check. All the wax and paper caps were changed, as well as the electrolytics. Tube swapping didn't make a difference, so I first though the problem was with the 7X7 tube socket. It was very sensitive to touch...it would bark back and honk and sputter at just the slightest touch. So, I replaced the tube socket. That didn't solve the problem. Thought it could be the R101 candohm, so I replaced that with a 150 ohm 10 watt. The problem is still there.  Thoroughly cleaned and flushed out the tuning capacitor. No change. So I'm left with a radio that crackles on the AM band, but mostly on the lower end. When I push the button to change it from radio to phonograph, the crackling stops, indicating the problem is before that switch. It also fluctuates in volume a bit. When the crackling stops, the volume suddenly gets loud. When the crackling starts, it quiets back down. Thought it could be the volume control, so I tried a new pot. No difference.

The last test I did was removing tubes to see if I could isolate the problem. I take out the 6BA6 Tube and the crackling stops. I put that tube in and take out the 7F8 Tube and the crackling is there again.

Voltages are steady but on the high side:

Section 1       VDC normal           VDC tested

(test points
as per SCH)

E-C               240                       278
D-C              190                        213
A-C              310                        352
B-C              -16                          -16

Plate to ground VDC

                         Should Be                           Measured
6V6's                    235                                     274
7AF7                    165                                     184
7R7                      235                                     274
6BA6                    235                                     274
7F8                        95 on AM                           104

Here's the schematic:  http://www.nostalgiaair.org/pagesbymodel/606/M0013606.pdf

Any help would be greatly appreciated. Has anyone else experienced this on this model?

RE: crackly 47-1230 - vinzer - 12-13-2017

You might check c 318 319 320 I had a ge radio that had that problem and they were mica so I didn't change them one was causing the noise.

RE: crackly 47-1230 - NostalgiaRadioTime - 12-13-2017

Thanks for the tip...will check them out.

RE: crackly 47-1230 - Radioroslyn - 12-14-2017

Hi Greg, 
As per your pm I took a look at the schematic, boy it's got a lot of squiggly lines!!!!
Would start with cleaning the sockets and switches w/contact cleaner. Loctal sockets can be trouble in terms of poor connections. 7X7 is sort of a red herring (I think) it's the fm det and 1st audio and that seem to be ok. In terms of troubleshooting one way is to go thru and ground the control grid of each stage (this stops the signal from the previous stage from enter the next). Caps it the plate circuit are suspect like micas. Breaking out the signal tracer and listening for noise at the grid and plate. Sometimes this can backfire if the noise gets into the avc circuit then it can be picked in thru all of the tube that are connected to it. Years ago use to use freeze spray but don't know if you can still get it (Freon). Would start off focusing in on the 7F8 area.

GL

RE: crackly 47-1230 - NostalgiaRadioTime - 12-15-2017

Thanks once again for your thoughts on this, Terry. Before I did anything, since the problem seemed to lie between the 6BA6 and the 7F8, I decided to try another new tube in each. Even though the one's I originally had in tested good. Well the 6BA6 made no change, but the 7F8 made the crackling disappear substantially. It still would turn up occasionally as I'd tune through the dial but never to the degree it was. Plus I can tap on all the tubes now without getting crackling, barking or feedback. I found the strong local station on the low end of the dial...where the crackling was at its worst....and kept it on there playing for almost an hour. Not a one crackle or volume drop. So I think I'm gonna cautiously call it good. Is it fixed...can't say for sure, but it's been behaving for now.