80 Jr Troubleshooting- B+ measurements
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Need some help please. Recapped an 80Jr that was playing originally (at least it pulled in one station when I went to pick it up but I asked the seller to turn it off as soon as I got there ). Also replaced out of spec dogbone resistors. When I power it up now, it is motorboating all across the tuning span. Some voltages:
[Image: https://www.dropbox.com/s/jnru9hgema6p6j....jpg?raw=1] "CORRECTED TABLE"
The strange thing is when I put a test lead on plate of the Det-Osc tube, all the motorboating stops (although I can not tune in any stations). If I put the test lead on the cathode of the Det-Osc tube, it still motorboats, except when I tune it to one strong station we have, which then comes in clearly. Don't have a tube tester but since it was playing originally I'm assuming they are still OK. Any thoughts on what to try next. Thanks!
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Something is wrong with the output tube voltages. The screen voltage should never be less than the plate voltage because they are both fed from the same voltage supply. The plate voltage should be slightly lower because it passes through the resistance of the output transformer while the screen is fed directly.
Check to see if something is miswired, as the supply voltage also seems low.
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With my 80 motorboating was the result of the regenerator's gain regulator setting. Try too find one that removes it. Or at least try to see if that is the reason for the motorboating in the first place.
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Thanks Mondial...I checked my measurement and realized I made a typo. I've corrected the table in the original post. The value was 207 not 107v.
Thanks Morzh...the regen adjustment does not correct the motorboating. I can adjust clockwise until I get the expected squeal but lowering the regen well below where it should be does not remove the motorboating.
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I'd check the osc feedback winding the set should pick up station across the whole dial.
BTW where in pa ru? I'm near Phila. Most of all of these 4 toob sets 80,84,37-84 ,600,and 57 need the all of the caps and resistors replaced. Also the ant pri and ocs feedback winding. After that they will play like Gangbusters!!
Terry
(This post was last modified: 07-07-2013, 07:54 PM by Radioroslyn.)
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Lehigh Valley, just north of you. I'll check it. Thanks.
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OK I pulled the oscillator coil out to look at it and check continuity. It looks good in that there isn't any green crud/corrosion that I've seen in some pictures. The measured values are in yellow on schematic photo. One value is off - 18 ohms (spec is 2.8 ) but its certainly not open, not sure what's going on with it...thoughts? The resistor incorporated into the coil assembly is about 11.4K (14% high) and the 710 pF cap is original...perhaps I should try replacing it/them?
[Image: https://www.dropbox.com/s/uyu76ycmt9ef7a....jpg?raw=1]
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I vote rewind it. It's not hard to do since you got in out anyway. Also Unplug the set from the wall turn the volume control all the way up and measure the resistance from the Ant post to chassis ground. Should measure just a few ohms if not the pri of the Ant coil is open.
GL
Terry
ps The winding I'm talking about is the one tied to the cathode it's 6.xx ohms. That's the one that provides feedback for the osc.
(This post was last modified: 07-08-2013, 05:37 PM by Radioroslyn.)
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Terry, thanks for the info. I think the osc feedback coil is OK. It measures about 5 ohms and seems stable during movement and handling of the coil. Ant coil is also OK, measured about 3 ohms. I replaced the 10k resistor (#6) in the schematic above on top of the coil since it was out of the chassis and put it all back together. Still motorboats but I can get a couple stations and if I carefully tune the station there is a point where it will come in clearly without motorboating. Maybe I need to align, have to make a plastic hex nut tool to do that, back to the bench.
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What you can do is listen for the osc in another radio at the frequency of the model 80 dial + the IF frequency.
Terry
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Terry, morzh and Mondial, many thanks for your advice! I finally solved the problem by tacking a 0.1uF cap across the cap circled in red in the photo. When I pulled the bakelite block associated with that cap I realized I had wired the block backwards, instead of the caps being across lugs 1&2 and 1&3 I had them as 1&3 and 2&3, yikes  Playing pretty well now, can probably use some alignment etc., but at least I'm in the ballpark again.
[Image: https://www.dropbox.com/s/cdmvn6uub5ni1g....jpg?raw=1]
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Glad you found the problem, as something like that can really drive you crazy.
It still seems that at 207 V your B+ voltage is a bit low. Are you applying full AC line voltage? What is the bias voltage generated across the 325 Ohm wirewound resistor? If everything is wired correctly, you might try another 80 rectifier tube.
Getting the B+ supply up to 250 V or so will make a noticeable improvement in performance.
(This post was last modified: 07-11-2013, 08:31 AM by Mondial.)
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There's nothing like first creating a problem and then solving it.
Always elevated my mood for at least two consecutive days 
Congrats!
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LOL morzh! Nothing like a self-inflicted problem, but it was good for me to sort through tracking it down. This is only the second set I've worked on but after viewing one of Bob Andersen's videos I decided to go ahead and take it "almost" completely apart to clean it up. It was pretty rusty so I thought this would be a good set to do some learning on. I know there has been debate on painting chassis but decided to at least do the top since that's where the rust was. Left the underside alone. Some pics in various stages; the last with my attempts at molding dogbone resistors (the 3 in the center)...I think that will take more practice 
[Image: https://www.dropbox.com/s/js614bmbqcizad....jpg?raw=1]
[Image: https://www.dropbox.com/s/mtk2lmnpfclmlx....jpg?raw=1]
[Image: https://www.dropbox.com/s/o9a9vdef8jxnax....jpg?raw=1]
[Image: https://www.dropbox.com/s/wf63s95xdn4urz....jpg?raw=1]
[Image: https://www.dropbox.com/s/5k2ts5p3w90exf....jpg?raw=1]
[Image: https://www.dropbox.com/s/f5i3ov45ng443o....jpg?raw=1]
I also learned in this process that the bakelite cap blocks are not always wired as you would expect them to be from the Bakelite Data Sheet, so you need to double check them.
Now back to tracking down why my B+ is low
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Good you found it!! Those are are the worst to find!! Chassis looks great.
Terry
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