What to do with pesky delta tune pot.

[9Lives]

While working on my same cobra rfx75 radio i had an idea that works pretty well. I designed a small power Mic circuit on a piece of Vero board to inject in the Mic audio wire inside theradio. It gets freakin loud! If i wanted to place the volume knob in place of the delta tune pot what should i do? I know rf gain its easy to replace but delta tune seems useless. Do i need to connect any of the wires once remove?

Also the power gain stage i added gets to loud. Is there a way to tone it back Some? 2 2n3904. It's it gets quite abit of feedback when it's dimed.. maybe a spot on the board to feed it less voltage? Or redesign?

 

[Atlasta]

Remove the delta tune and just solder the wires so it stays on center. Delta tunes are basically useless on AM-only radios. All they do is shift the receiver a Kc or two up or down if you happen to be speaking with someone who is a bit off center frequency. Back in the old crystal-control days it had some value but today it really has no function

I've seen many who remove the RF gain control and put echo controls or variable power in that slot. This has always been a pet peeve of mine. The RF gain IMO is the singularly most useful control anyone can have on a radio. I'd give up the squelch before giving up an RF gain adjustment.

Remove the delta tune and put your variable power there. Don't take out the RF gain.

 

[9Lives]

So i solder all 3 wires together? Wouldn't it require a resistor?

 

[Turbo T]

I think the only reason why they still put Delta Tunes on newer Cobras, is to give you an extra area to put in a knob for an echo board, or an adjustable talk back, or as mentioned, a way to control RF power out, for amp use.

 

[Robb]

I really hate working on radios that someone has hacked up.
It is a genuine PITA and refuse to do it.

If you want to put in a variable power knob; put it in the inside of the radio or on the back. Get a mini-pot so you don't have to hog a huge hole in the back to put it in. If you are lowering the power for an amp, you really only need to set it to one point anyway - especially if you have an RFX in it. If you are putting in a swing mod; then who needs a variable power? The swing mod can lower it to 1w if you just pick the correct resistor value for it.

Some guys on AM never had their radios aligned so they can and will be off-frequency.
Better to keep the Delta Tune in there for that reason alone.

RF Limited makes a fine power/echo mic; the Xtreme2018. Has the best/cleanest echo IC chip available in it from anyone. The mic element and sound are top grade too. Its no secret and you can't do much better for a mobile or base mic for an AM radio - IMO.

Another thing about hacking radios. If you are going to do it; then do a neat job for one. Another is to do the most widely acceptable method to do it. This way, when someone who might have bought/acquired that radio wants to reverse the mod there isn't any guesswork about what has been done and no repairs are necessary. It is the difference between a piece of junk and something that still has some value left in it.

Bought a Cobra 29 GTL/ST from a flea market for $5. Sure am glad I only paid that much for it. It was really a huge mess inside. Parted it out to work on other radios. It was useless to fix - what a junk pile! Solder joints that looked like a Payday candy bar and wires that had been soldered/spliced together in the middle. Didn't even put tape on all the joints. Who knows how long I would have spend just to sort it all out. No way. What a sad thing. I'm no pro; but I sure can recognize amateurish radio work when I see it.

 

[2RT307]

I actually hear quite a few operators that are off frequency on AM. Mostly on channel 19. I prefer to keep my delta tune, thank you!

73,
RT307

 

[9Lives]

Rob, what i did was take a 2 npn transistor improved power Mic circuit and put it inside the radio after the Mic plug on the audio wire. I wanted to add an attenuation to the end and put it where the delta tune knob is.

What i ended up doing is, i left the delta tune pot in tact and left it inside The radio with apot condom on it and replaced the knob with the new Mic gain. It actually works really well except it starts clipping or squelling fairly early on the pot rotation but it adds a lot of gain to the Mic. I wish i had a scope to see what it's doing exactly but it definitely adds on the talk back. (This is only aclue people, don't jump me)

 

[ExitThirteen]

People just laugh when I tell them what I generally do with the Delta Tune... I unlock it and use it as a clarifier! Kinda fun to play around with it.



~Cheers~

 

[great white]

People just laugh when I tell them what I generally do with the Delta Tune... I unlock it and use it as a clarifier! Kinda fun to play around with it.



~Cheers~

I know this thread is a little bit old, but can elaborate on that?

IE: how and why?

I've pretty new and have never heard of that before...

 

[Onthecoast]

I was actually wondering about that the other day? I'm not sure how much adjustment you can get out of the Delta Tune compared to the width of the AM signal, but it would be cool if you could use DT to get closer to in-between channels for talking.

 

[dirtyjob]

i realize this thread is super old but i have a radio where they removed the delta tune and soldered all 3 wires together, it sounds like my rx is off because of this, i do not have a delta tune pot at all, how can i fix this issue ? should all 3 wires be joined to give "center slot" it sure dont sound like it, please help. Thank you.

 

[sunbulls]

i realize this thread is super old but i have a radio where they removed the delta tune and soldered all 3 wires together, it sounds like my rx is off because of this, i do not have a delta tune pot at all, how can i fix this issue ? should all 3 wires be joined to give "center slot" it sure dont sound like it, please help. Thank you.

I personally don’t like the idea of connecting all 3 wires together. If you’re not planning on ever using another 20k adjustable Delta Tune on the front panel as originally intended. I’d replace it with a miniature 20k pot that would be left inside the radio. At least that’s something that could be adjusted. I occasionally run across radios that are far off center on the Delta Tunes adjustment in order to achieve best clarity. That’s usually because it’s compensating for some mistuned circuit, if not an aged crystal.

 

[Handy Andy]

I second Sunbulls approach...there are "issues" of completely removing the pot - best to tape it off...

...or...

By simply doing up a divider using 2 10K resistors of make the 20K pot "fixed" at center detent

- or -

Not recommended - like suggested, short out all the wires...​

There's a disaster in the making with that last option...shorting out the RX is not a good thing - the wire takes RX power from radio - applying it to the 20K pot, IT IS NOT BUFFERED...so you have been warned...

I'd do the 10K to wire both sides from "hot" and "ground" 10 K from each side soldered together in the middle with the wiper arm soldered to it...that is the simplest...at least both Hot and Ground are referenced to the same tap (wiper arm - or - center)

​

Or, just remove Pot, Unsolder W57 (In 29's it's by R99 R112 by L18 / IC stuff - colored coded possibly Green Wire)

​

Unsolder all wires - from Delta Tune Pot - it's a 20K...3 terminal - 2nd one in from the Channel Selector

- locate wire jumper going to W47 on the main board by the PLL

- it will have a resistor of R110 (2.2K) and L22 - save this jumper
- keep it soldered in where it's at by the PLL​

- and install 10K resistor in series on the other end and use this wire

- route over to back side of board where W57 went

- Put 10K in SERIES with wire from W47

- solder to hole left behind from removing W57​

This 10K just puts the CENTER wiper arm - your adjustable - to "Center" and leaves R112 the 5.6K to "pull up" on the Center, as if you had a 10K in there from the "hot side" - you may have to "tweak" L24 to bring radio back to your "Center" but not by much...

 

[Shadetree Mechanic]

I have heard of people disconnecting the wires from the delta tune all together. The pot applies a load one way or the other when turned. In the middle the load is equal. Disconnect it and the load will be equal. I have not tried this but it stands to reason.

 

[Handy Andy]

Well, you can, because of that pull up Resistor R112 (5.6K) and the load resistor R111 (6.8K) Both back by where the W57 wire goes in - you can just clip / Remove / Unsolder the wires - but if you're saving the pot for reinstall - then you might want to do up the divider and tape the wires off so you can use the pot for something else...it's just easier to reattach the pot to the wires versus figuring out how to weave them all back in the loom to make it work.