Why poor AVC - repeat

3/10/2009 5:37:48 PMPeter G. Balazsy

I've been recapping this Fada 260 for a customer:
http://www.nostalgiaair.org/PagesByModel/411/M0005411.pdf

It seems to use an early form of the more common AA5 type AVC ckt.

R1= 250k
C24= .05uf

The problem is that it really hardly works at all. Almost useless.
Is this right?.. or should I expect it to operate better.

If I try to measure AVC voltage on C-24 I get almost nothing unless I tune to the strongest station and really turn up the vol control pot in the antenna circuit. Then I do get a few hundred mili-volts negative.. but the volume is blasting.

What good is it?
Or is something wrong.
EVERY component has been replaced with NEW.

3/10/2009 5:40:17 PMEDD please repost

EDD the system hung up when I was replying to your post. Would you please try to re explain what you said.. in simple terms.. thnkx

3/11/2009 10:48:03 PMEdd

WHUT !. . . .you’re not getting SWAMPED on that receiver with those. . . . NOO-YAWK-CITTEE
. . . AM powerhouse stations?

Looking at the sets detector, with its using grid detection, still leaves there being an adequate
level for the AVC buss, but they just happen to only be feeding back to the frontal mixer stage.
One would expect the sets system gain in the mixer stage to be in the order of ~ 15x. . . WHILE. . .
the 1st I.F. would be contributing in the order of ~100x.

Now if you figure in that cascaded gain of both stages, and ALSO include the I.F. stage in the AVC
control loop, THEN you would be expecting to see some serious throttling back of gain, if so
mandated by the controlling AVC feedback.

Looking at the I.F. stage, note that its 1st grid circuitry is dropping down thru the IF secondary
and receiving a fixed bias level via the 250/350 ohm voltage divider resistive pair off the cathode
of the IF amp.

Now, should they additionally be utilizing the control of that I.F. stage, one would normally expect
that low end of the transformer bypassed with an ~.05--.1 decoupling cap and having inserted an
~250k---2.2 Meg resistor going down to the AVC buss.

73's de Edd

Sir Peter, looks like your last post had some bad HTML in it and it dropped the thread , so I
went to archives and extracted it, in order to be able to reply:

Thanks Edd:... I'm not sure what your conclusings were.

Can you spell it out more clearly for me?.. i didn't quite get you thrust.

Do you feel something is wrong or can easily be improved?

I DO have plenty of RF signal strength available at the antenna and get very VERY loud audio
from very little volume control advance.

But when I really increase the volume then the negative voltage on c-24 does show a few
hundred mili-volts... but the volume is much too loud.
So it seems to me that if the volume control were located in the audio circuit then there would
be enough RF / IF signal to generate AVC.

But I don't want to modify the customer's radio very much if at all.. but would like a simple fix
if that's possible.

That sets volume control circuitry is like so many of the very early day superhets, wherein
it is playing with the frontal RF gain to indirectly control the volume.

I usually find that the main volume control effect on that type of circuitry is within the last 1/4 of the control.

I am surprised, that IF you actually have changed ALL the discrete R & C parts, that the
derived AVC level is a mere 100’s of MV’s. Reconfirm ALL of the RC components tied into
the reference schematics [ GREEN ] buss.

The quick confirmation of possibly additionally controlling the I.F. stage would be a temporary
snip loose at [ X ] and the connection of the res and cap as is shown.

73's de Edd

FADA_Model 260. . . . . . SCHEMATIC :

3/11/2009 11:22:15 PMThomas Dermody

With the volume control being in the antenna circuit, the "AVC" circuit isn't going to generate much juice, if any, unless the volume control is turned up all the way. With more modern radios, where the volume control is in the audio section, the RF is left wide open, with only the AVC circuit to regulate it. It generates enough of a signal that if your interstage audio volume control is turned up all the way, the radio will blast.

That said, unless the antenna volume cotrol of the radio we are currently talking about is turned all the way up so that the radio blasts, there won't be enough of a signal getting through to create an AVC voltage. Also, the grid leak method of generating the negative voltage is a weak one. I tried using the grid leak detector as a source in my Majestic 180, and it wasn't capable of fully controlling the RF circuitry for all strong stations. I'm not quite sure why, but it doesn't work well. As is explained in the literature that goes to your radio, the circuit is merely an 'overload' control, that might help keep the radio from blasting when the radio is tuned from a weak station to a much stronger one, since it is otherwise necessary to constantly turn down the volume control when hitting the strong stations with this kind of volume control circuit.

Considering this topic I will do more research with my Majestic 180 to see if in fact it might be possible to use the grid leak as an adequate source of AVC, which would omit the extra modern components (diode) I added to the set. Since I really don't like adding modern components to a set, at least in an undisguised condition, I'd rather find a method of adding AVC that uses original components and circuitry only. With proper circuit design and layout you, too, could add a proper AVC circuit to your radio if you really wanted to. What it would do is keep the RF gain high, thus feeding a strong signal to the grid leak detector, and thus preventing square law detector distortion at low volume levels (the volume control must, of course, be positioned as you would find it positioned in a more modern radio).

3/12/2009 1:27:39 AMPeter G Balazsy

Hi EDD and Thomas:
Edd .. The NA system hung up when I was replying... there was a "disk full" error message.. so I don't think it was bad html code.

Anyway as far as my Customer's Fada 260 Radio goes... I did check the values in the AVC area and all is well.

Thomas seems to have described the actual behavior.

When I have the radio at normal volume to my ears on an average station the vol knob is at about the 1/2 way point.

At that time there is +7vdc on the cathodes of the 6d6 and 76 (top of r-33)
That puts +4vdc on the grid of the 6d6 via the cathode resistor divider junction of r-33/r-34.

The "avc" line at r-1 has less than 1 mv at that time.

When I increase the vol control to the 3/4 point the speaker is much too loud but I do start to observe a few negative volts appearing on the AVC line top of C-24.
With the vol control at max.. I see -3 volts on the top of c-24 when tuned to an average strength station.

When tuned to a very strong station that point goes to -7volts.

So it appears that some good strong avc type negative voltage is possible but only at 3/4 to max vol.

I wouldn't mind a minor modification to get this to behave nicer but it is a customer's radio and I shouldn't try to fully redesign it.