Does anyone know this mod on a Halicrafters SX-99? What's missing?

[Scott SWL]

I just dug out my SX-99 from storage, my workbench is clear, and I'm going to nights again for a while, my Hammarlunds are still waiting for parts,
so now is the time to work on this one.
There is an extra switch inside for AM and FM, and some extra wires hanging out the back.
Is there something missing? I haven't even plugged it in yet, If I have to I can remove the mods and return it to stock for the most part.

[Scott SWL]

more pics

[Scott SWL]

the wire marked IF comes from pins 5&6 of the second IF tube V8 6SK7
The AM and FM switch is connected to V5 6SC7 between pins 4&6 with the AF wire connected to pin 4 also the detector seems to be modified as well.

[Scott SWL]

This is what is inside the detector mod box

[DMOD]

Without a schematic or drawing of the circuit mods it is difficult to determine.

Download a schematic and compare it to your circuit,

However, one of the mods appears to be an extra audio cable to an external phonograph or FM tuner.

Phil - AC0OB

[Scott SWL]

From what I can see, the cable marked IF is always on, connected from the output of the last IF tube, and the AF wire is switched in when FM is selected on the switch, I am guessing the missing piece takes the signal from the IF stage and detects FM signals and returns audio to the amplifier. I don't know if there are any FM signals below 40mhz so this is just supposition.
I will trace out the detector box mod, there is a 1N34 diode in it.
Schematic original, and with the mod box installed Fil in the mod schematic is the same as original. up to the diode.
It looks like it just applies an adjustable negative DC voltage to pin 3 of the 6H6, what that does I don't know.

[WBear2GCR]

It was common to use the "FM" position to access the plug-in socket and use that for something else.

The circuit appears to be simple, so perhaps it is merely an improved AM detector?
Not enough there for SSB.

I'd plug in the radio, see what it does in normal mode and then try the plug-in unit.
IF there is an "IF OUT" on the back panel, I'd guess that is what it wants to see.

Trace out the parts and draw up a simple schematic.
Post it here if you don't know what it is?

                          _-_-bear

OH EDIT: Looks like adjustable ANL level. Quick look/guess.

[DMOD]

                          _-_-bear

OH EDIT: Looks like adjustable ANL level. Quick look/guess.

Either that or a squelch circuit.

Phil - AC0OB

[Scott SWL]

when I turn the pot, it seems to change the sensitivity on weak signals, maybe noise level, on strong signals it doesn't seem to do anything. it also keeps the meter around s3 and higher (AVC on) instead of going to 0, when it is installed

[Scott SWL]

Something new,
What is this?
it seems as though the shielded cable goes to another tube, but is disconnected.

[n4joy]

Something new,
What is this?
it seems as though the shielded cable goes to another tube, but is disconnected.

That is how the gimmick is setup in my SX-85.  It is simply a piece of shielded hookup wire. Definitely leave it be!

[Scott SWL]

 :Certainly, I don't mess with things I don't know.

[Scott SWL]

So what is it supposed to do? carry a signal from the connected tube to the place it goes, and radiate out, or pick up a signal at the open end, and carry it to the connected tube?, or something else?

[KA8WTK]

It is a Gimmick capacitor. On some radios it was used to inject a BFO signal into a circuit. Look at a S-38 schematic and you will see one from the BFO tube to the detector stage. They were usually twisted wires, one wire coming from each stage. The more wire twisted together, the more BFO injection.

Bill KA8WTK

[Scott SWL]

Cool! thanks! loose coupling.. I get it!

[DMOD]

So what is it supposed to do? carry a signal from the connected tube to the place it goes, and radiate out, or pick up a signal at the open end, and carry it to the connected tube?, or something else?

On my S-108 and others of this vintage it is simply a length of single conductor shielded wire from the BFO Osc. to the IF stage where, on the other end, about 1" length of unshielded wire is placed near the grid pin of the IF stage. About 1 to 3 pF of capacitance exists between this exposed end and the grid pin with just enough signal for injection.


Phil - AC0OB

[Scott SWL]

Thanks!