[morzh]

I would refrain from changing L2 (antenna coil) for now. It is intact.
I am not sure why, while working, there is a difference of 0.3V between C2 and the grid cap, it supposed to be the same.
When it fails, it is the same, and this is how it should be.

Now, I am not sure what it will tell us, but you could possibly try to see, while the radio is working, where that 0.3V go between the C2 and the grid cap.
Try to measure between them on all 5 pins, that is the C2, 3 taps between the C2 and the Grid cap (all 3 also go to the band switch) and the cap itself. See at what tap it drops from 4.8V to 4.5V. Or maybe it is gradual.
Write it down.

When it craps out, measure the same points again to see, if the drop disapears (all are 2.4V) at all 5 points.


Also. Just for the heck of it.
I think it is the wavetrap (the inductance and trimmer across A to G) shown right at the Antenna strip in the sch.
Disconnect it altogether. It is not needed anyway, and one less variable.

[dconant]

So do my readings confirm antenna coil is good?

[morzh]

I think yes.

We did two things: 1. Confirmed that the voltage at C2 and Grid cap are the same. This is only possible when the end-to-end secomdary coil is good. 2. Your A to G is 24 ohm, again, the primary is good.

Now, could you also measure end to end secondary resistance (powered down, in Broadcast position). I wonder what kind of current it takes to drop 0.3V across the secondary when the radio is working.

[tbone]

In theory you could measure the resistance between the 6A8 and 6K7 grid caps. It will include the resistance of the IF secondary, but would take into consideration the whole RF coil secondary. An open anywhere in the multi tapped coil would show an open between the two points, which I chose because of easy access and less disturbance to the radio.

I would think the voltage should be identical on both grid caps, as there really is any grid current to speak of. Taking readings might affect the tuning of each circuit causing a change, but you get the idea.

[dconant]

Mike, From grid cap of 6A8 to C2 is 7 ohms with radio playing normally. I had already disconnected the wave trap but problem persisted.

Using tbone's suggestion I got 16.1 ohms. With radio playing normally grid cap of 6A8 is swing between -4.1 and -4.2 at 6K7 grid cap swinging between -3.7 an -3.8.

[morzh]

Have you measured the taps' voltages on the Antenna secondary, as I asked?

Now, you having 7 Ohms there, think about it:

The AVC voltage is created via R5, a 1 MOhm resistor and C2, a 0.047uF cap. Forget the cap, it is the filter.
Now, you have, in essence, a 1 MOhm in series with 7 Ohm (your coil), cap to cap (no pun intended).
And you drop 0.3V, if your measurements were correct.

To drop 0.3V across 7 Ohms you need ... you need...you need a whopping 42mA current. There is no one there who can pull that.

So. Re-measure, with all the taps. While working.

[morzh]

Oh...you have somehow put two of the same threads in Want to Buy.
I deleted the redundant one.

[dconant]

I thought I had deleted dup. Want to Buy. Oh well.

I have found all 5 points of the secondary. My resistance readings between coils/taps are .5, 1.6, 3.1, 3.3 for a total of 8.5 but from grid cap to C2 is 7.2. Where did I gain 1.3 ohms? I did this so I would know what points to measure voltages.

Voltages on secondary in order from top down. -4.1, -4, -4.1, -4.3, -4.4. Measured 3 times using a VTVM. Stay tuned for crap out voltages. Probably tomorrow.

[morzh]

Are the resistances measured powered down or while playing?

[morzh]

OK. Have you cleaned your tuning cap?
Also the trimmer H.

I suspect 6A8, but you said you changed it.

I wonder if the dirt on tuning cap does something ne-fart-ious. (it was a typo, but I liked it)
And as long as you are at it, if you decided to clean the cap, the tuning switch could probably use some cleaning? Especially the wafer that switches the taps of that Antenna secondary / oscillator.

[RodB]

That wasn't a typo, it was AI (the spelling assistant). It knows what you were thinking.

[morzh]

I do not use the spelling assistant. So, if it knows what I think, it cannot express it. Worst kind of torture. You know everything but cannot say it.
Like a dog.

[MrFixr55]

Sorry, Gang. I have been jumping in and out. Please confirm the following:

You DO have an Aux / Bluetooth input and when the issue occurs, there is no issue with volume level when aux / Bluetooth is in use?

The signal attenuates but never dies, true? If so, the oscillator is eliminated.

You have change the 6A8 (multiple times) and 6K7? Have you changed the 6H6 Detector / AVC tube?

When using your transmitter, do you get a nice negative AVC voltage? When the issue occurs, does the AVC voltage change?

It seems that if the lowest frequency band is the broadcast band, The wafer switches may not enter the picture as (I believe, but I could be wrong) that the wafer switch shorts out sections of the coil to change bands.

It could be that the OSC Mix circuit is good and your issue could involve an IF secondary or the detector diode. The diodes are in parallel so if one goes wacky, it may attenuate the other. You could replace with 1N34 or 1N60.

[dconant]

In my last post the resistance readings were in powered down state.

The band switch has been removed and cleaned within a inch of its life. All tubes have been swapped. The 6A8 4 times, 6K7 3 times, 6H6 3 time and 6F5 twice.

The signal never dies. Bluetooth plays fine when issue occurs.

I don't have the diodes mentioned and wold not know how to wire them in.

I will fire up the radio this morning and report back on the results of when acting up. it usually is first time for the day.

[dconant]

Just turned it on and it played normally. Now I have to wait for it to crap out before I can get voltage readings while crapped out.