Hi everyone! I hope everyone is well and staying safe.
I have a problem that I have never run into before. Although it does sound like I've said that before. Anyway, I replaced all the caps, checked and replaced any resistors over 20% out. All the tubes check good and it has been aligned. Here's my problem: with the original loop antenna (it has continuity) the only thing I can pick up is my SStran, about 8 feet away. If I connect my long wire to the antenna connector it sounds great and picks up all local stations.
Any ideas where to look?
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Thanks,
Did you adhere to the comment about a connection to chassis when loop antenna is used? See top left corner of schematic. Just a guess on my part.
Assume you connected the wire with the lug on it to the chassis per schematic when the external antenna is not used.
That’s correct. Never disconnected it.
Is your loop antenna intact? Can you disconnect and make sure it at least shows continuity?
The reason I am asking, you might be receiving the SSTRAN even withour it, the signal is strong enough.
It has continuity. I even tried some loops that I had sitting around. One of them Worked a little better but still no good.
Is the secondary of T1 good? If so, there should be AVC voltage at the control grid of the 7A8. Also, check to make sure that C12 is connected between the AVC and B-. C12 is the signal ground for the first 2 amplifier stages.
Rod
>If so, there should be AVC voltage at the control grid of the 7A8.
To be picky generally the control grid would be the one closest to the cathode. In pentigrid converter circuits it's use in the oscillator portion. The rf input is actually on grid 4 (pin 6). This where the avc voltage should be found. Grid 1 would have some - voltage on it if the oscillator is preforming properly.
In reality the pentigrid has two inputs. It's like having a triode on the bottom half. Cathode, control grid, and plate (grid 2). And a tetrode or pentode on top. Grid 3 acts as a screen grid speeding up the eletrons. Grid 4 is the 2nd input (control grid for the pentode on top) Grid 5 is the screen for the pentode section. And finally grid 6 is the suppressor grid which is internal connected to the cathode.
Happy Hunting!
Ok, Terry, thanks for the info, but it was way over my head. I did check all the voltages On the 7A8 according to the chart that came with the schematics and they check ok.
Hey Rod,
Here's a good one. C12 was the problem. It wasn't there! I installed one and it works!
Thanks, everyone for your help.
Not a problem. The reader's digest version of Rod's post abt the avc is that on pin 4 of the 7A8 you should w/a strong station -10vdc or so. With a weak station the voltage is going to be less - so maybe -5vdc. The less - it is the more the mixer tube (7A8) and the 7B7 IF amp tube will conduct. When they conduct more weak signals become louder. And vise versa.
I'm thinking that it's a open ant coil or IF transformer. Both of these could pass some signal but not all giving poor performance. A weak tube or a stage miss aligned could be the issue too.
Just for grins if you happen to have spare tuning cap try connecting one section in parallel with the ant section in the set. Tune the added cap to see if there is a peak in signals.
GL
Excellent, on to the next issue. It's after noon, time for a brew to celebrate.