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Sonora RCU-208

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Post by Ben Delk on Tue Apr 23, 2013 8:33 pm

I have a 2nd Sonora RCU 208 I'm working on now. I have recapped and replaced almost every resistor in it but I still getting the motor boat sound. Tube test good
symptom:
When I turn it on low volume the radio is playing with small amount of static like I need to adjust the tuner knob position but if I turn it starts to motor boat.
symptom:
Turn it on low volume (no motor boating) try to check the AC on the 35z5 pin2, as soon as I touch the meter lead to the pin it starts motor boating.

I have tested all the tube pin voltages and they are all running close to spec...........What am I missing here?


http://www.nostalgiaair.org/Resources/524/M0018524.htm
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Post by jerryhawthorne on Tue Apr 23, 2013 10:53 pm

Ben, you have some interesting feedback lines from the secondary of the output transformer. I would check the wiring of caps and resistors in this area. Sounds like something is amiss in this area.
Of course, I know nothing. Very Happy
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Post by Ben Delk on Wed Apr 24, 2013 8:03 am

jerryhawthorne wrote:

Ben, you have some interesting feedback lines from the secondary of the output transformer. I would check the wiring of caps and resistors in this area. Sounds like something is amiss in this area.
Of course, I know nothing.


Yup.....I keep going back to that same area. I have even disconnected the output trf to check the continuity and resistor values. Double checked the solder joints as well. All the symptoms point to the audio section so I guess tonight I'll magnifying glass all the pin sockets, caps and resistors again. I'm sure I will kick myself when I find it but at the moment it eludes me.

Thanks,
Ben
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Post by Dr. Radio on Wed Apr 24, 2013 12:24 pm

Make sure you double-check the grid connections on the audio tubes, often times motorboating can be traced to grid issues--and don't rule out the possibility that the new capacitors could have had a bad one in the bunch.
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Post by Ben Delk on Wed Apr 24, 2013 12:41 pm

Yup....Just recently found a mislabeled axial cap. I guess I'll start a new habit (should have already) of verify the caps before installing.
I'll check your suggestion.

Thanks
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Post by Ben Delk on Thu Apr 25, 2013 10:04 am

Last night I replaced all the caps again but one .0005, it is on order? It is connected to the 1st audio 12sq7. Funny thing, when it starts the motorboating noise it does it even with the volume all the way down. Something is definitely feeding back. Tonight I'll start going over the resistors again.
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Post by Ben Delk on Fri Apr 26, 2013 8:11 am

Last the the rest of the resistors last, not that any of them were bad but if I'm rechecking and resoldering connection it might as well be a new resistor being soldered. Still motorboating when I increase the volume. Still waiting on the last cap to arrive and the volume control. The 1st and 2nd IF trx are reading to specs..
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Post by Ben Delk on Fri Apr 26, 2013 8:49 pm

Sometimes persistence pays off, sometimes not. Today it did. Here’s what I found….actually 2 things:

Symptom …..Tuning capacitor acting strange when changing frq.
Cause ….. found some solder slag stuck between two of the tuning cap fins (most likely, self inflicted)
Repair….. remove the solder with a credit card

Symptom …… speaker motor boating
Cause ….. this model has solder joints grounding each tube socket to the chassis. I found 2 sockets with cracked or broken solder joints at the chassis. Really hard to see.
Repair…..I ran a wire from socket to socket and made a good chassis ground to feed them all. I came to this discovery quite by accident when I found the tube readings would change every time I moved the chassis. One time it would read to specs, the next it was all over the place. This is a good case for why it is important to write down the values as you are testing each pin. I will say I learned that the hard way but my friends here on TRF pushed me in that direction and it finally stuck. The tube\pin readings sheet I posted has worked out very nicely to this process. Thanks again to all those who are teaching us rookies how it should be done.
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Post by fixinmyphono on Fri Apr 26, 2013 9:24 pm

Good to see you got it Ben!
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Post by Dr. Radio on Sat Apr 27, 2013 9:30 pm

I knew you'd get it Ben Very Happy

As an important side note, radios that use rivets to provide a ground connection to the chassis can also be just as troublesome. Sometimes the rivet looses it effectiveness (electrically, even though to the naked eye it looks solid) and then you have a potential difference (voltage) between the tube socket, component, etc., rather then a good reference/ground connection and all kinds of gremlins result, much like your motorboating. Just an FYI--its the little stuff that can drive you nuts. Shocked
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Post by Ben Delk on Sun Apr 28, 2013 9:33 am

Such was the case with this Sonora, rivets were used to hold the sockets to the chassis and then a bit of solder on that to ground to the chassis. The key to finding the issue was all the voltages kept changing. Thanks again .
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