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38-10 Low volume on stations - Printable Version

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RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

Injecting 470 into the 6K7 full strength on the gen (through a .1uF cap), I would say the volume is at a normal level with the volume all the way up. It's noticeably louder if injected to the 6A8 grid instead. To give you an idea (since I don't know how else to quantify it), the AVC is about -10.0 through the 6K7, and -15.5 through the 6A8.

It's hard to tell on the schematic, but it looks like the pot is rated at .3M ohms, not .5M. Which makes sense with what I'm seeing.

RE: 38-10 Low volume on stations - morzh - 05-18-2014

Yep, .3M. That Riders print quality is pretty awful....

RE: 38-10 Low volume on stations - Nick3092 - 05-20-2014

Not sure if anyone is still paying attention, as I seemed to have burned through all the easy/quick fixes/checks. But I went back tonight, and was going through the alignment again (even though I knew it was futile going into it). Not sure if this will spur any new ideas, but I noticed something new tonight.

I was trying to do the RF alignment again, starting with the SW oscillator. I had the dial at 18MHz, had the sig gen at 18MHz, and hooked up to the red antenna terminal. I had 4B tightened all the way down to start the alignment. As I started backing it out, the AVC spiked, and the speaker started motor boating. At the peak of the AVC spike, the motor boating went way. I checked with my DMM, I had -38v on the AVC line. I disconnected the sig gen, no change. I turned off the sig gen completely, no change. Turning the dial off 18MHz would reduce the AVC.

So, that's my new observation. The radio is going into oscillation and motor boating while adjusting the SW oscillator, even with no SW signal even near the radio. If I touch the grid cap of the 6A8, the AVC immediately drops. As soon as I take my finger off, it spikes back up again. Changing the dial removes the oscillation.

RE: 38-10 Low volume on stations - gregb - 05-20-2014

That sounds like an internal oscillation of some kind. Have you tried swapping out the 6A8 for a know good one? Converter tubes can test just fine and give some grief when in the radio.

Gregb

RE: 38-10 Low volume on stations - Nick3092 - 05-20-2014

I tried another NOS 6A8, same result. In the past, based on someone's suggestion, I put the original 6A8 into a known working radio in place of the second radio's 6J8. The second radio worked fine with the original 6A8 in it.

RE: 38-10 Low volume on stations - TA Forbes - 05-20-2014

Nick: assume you are injecting minimal signal amplitude when doing an alignment? Too high an amplitude will cause the AVC to flame on and screw up your alignment.

Have you disabled the AVC and seen what the result is?

RE: 38-10 Low volume on stations - Nick3092 - 05-20-2014

I'm getting the oscillation and AVC spike with no in injected signal at all. My sig gen isn't even on. I'm not sure what you mean by disabling the AVC?

RE: 38-10 Low volume on stations - morzh - 05-20-2014

Unsolder the resistor that feeds the AVC to the input tubes I guess. Possibly tying it to GND.