TRS Challenger 1400 (AMC) - delay in audio with key up

I'm pretty sure I have the picture.

First clue was to press the mike's PTT button slowly so that the transmit pin gets grounded, but before the other side of the switch can connect the cartridge inside the mike to the audio wire.

In other words, keying the transmitter with no mike connected to the audio pin of the mike socket.

Makes it squeal, for as long as you can hold the button in precisely this in-between position. Press it just a little harder and the other side of the switch closes, and connects the cartridge in the mike to the audio wire. The squeal immediately stops. You just have to connect the mike cartridge to the mike cord's audio wire to stop the squeal.

What makes the squeal go away is commonly called "swamping". So long as there is no mike-audio source connected, the audio wire of the mike cord becomes a feedback antenna. When the mike cartridge connects, it puts a resistance of a few hundred ohms to ground from the audio wire. This will carry any tiny stray feedback energy to ground. Keeps it from becoming a squeal.

The "chirp" that's heard when you key the mike is caused by that brief delay between when the transmit wire gets activated and when the audio wire does. Smashing the PTT button as hard as possible pretty much eliminates even that brief chirp. Probably not good for the mike.

Re capping the radio is the one strategy that might work to stop this.

Might.

Only way to find out is to spend the silly money required to do that. Then again, the feedback problem may originate in the basic design of the radio. This one is so rare that nobody out there can tell you how many of the last hundred of them have this quirk. Until you have seen inside that first hundred of them, how would you know? A lot of 23-channel radios were built with feedback quirks like this. And this one is a first-generation 40-channel. Who knows how many leftover 23-channel design quirks it has designed into it?

Besides, it was not made or sold in big numbers. This makes it easier for design quirks to go out the factory door.

Whoever did the warranty service for this radio in 1979 is who you would want to talk to for that kind of inside info.

No idea what that was, though.

73

Are you using a base or mobile mic? Make or break contact distance and tension can be tweaked on most base mikes with a needle nose. To confirm it’s a contact problem I would first try a few different mics. For more mic possibilities get a Midland to Cobra adapter if you haven’t already. Personally, I rewire the jacks on most of my radios over to the more common Cobra standard, so I have dozens of experimental choices.

There are two Pots, One trims AM side, the other the SSB side - both get fed by that Mic Pre-amp.

If one of the pots opened - or gets too dirty, the path to ground is different between the two (SSB/AM)

So the Pre-amp "loads up" the line to this branching tee, but if it is having trouble letting power flow thru both, the circuits they feed, "don't get the right amount".

So if the pot is dirty on the side you're not using - it pushes more audio from the Mic amp into the weaker side - lessening the power present to divide amongst both.

The "mic amp" is a double balanced modulator amp - so it "limits" on it's own.

This thing is ancient...

• Made between Greece's Colossus and the Roman Drive-In/Eat-In Amphitheater.

The "Pot" parts I'm talking about are VR1 and VR2 - the AM and SSB "set and forget" balancing.

Uniden switched to a FIXED divider and a simple transistor shunt to turn one side off and make it appear as a "constant load" so then when you "switched to that mode" the transistor diverting the load when in the other mode - opened up (switched off) opening the floodgates.

The other side gets pinched off from another transistor forcing the AM Regulator "High" (Swamping it's input) all the time during that mode so you don't have that delay. The resistors used between both "split" power so the losses incurred were always equal between the branches FIRST - only one side looked like a "dead short" only to the parts downstream in the strip from that shunt switch.

I located a 1400 Schematic here...
TRS Challenger 1400 (cbtricks.com)
http://www.cbtricks.com/radios//trs_challenger/1400/index.htm

So in radios like this, it is how I learned to make up a Permanent Fix by removing the pots and finding values that "equalized" the power flow - using three resistors with one of them tied as common reference to ground so the trim pots being what they are, when they fail or get dirty - WOOF. Problems....

Now, you asked about AMC and the "inherited delay" - everything in it is set up by level - so the "re-cap" due to this things age, comes to mind. There is more intricacies in the RX side and the SSB mode, then the Am side - which is a feat in itself. The delay may be from the Relay switching - a dirty or slow contact switchover, can cause the voltage drop and that delay as the power hogs that are on the switched line - feeding from the trough hogging up and taking away all the power to feed the rest of the barn...

The AM delay the thing uses a AN7150 Audio amp - and output is just like a typical AM radio - should sound pretty nice. But the "delay" may be from "bootstrapping" issues - "fresh" caps become "slow" caps as they age.

The "Delay" can be from many things...but the bigger clues are found in "if that is the case - why?"

Take a look for and locate this circuit -

• To explain the comment above further...

So a leaky cap - like shown above in C58, can affect the ability of the chip to accept input left floating from ground as a differential signal (AC) for the OP-amp amplifier.

C51 and R57 are the "Emitter" leg load/bias for stability (Pin 1 EQ cap for emphasis) - that 470K is important - it sets DC bias gain for the MIC input so the rest of the amp to amplify what's there.

Pins 8 9 along with 2 set the Output Amp gain factor as well as level. Think of it as an TDA7222AP or TDA2003 needs a reference to set mid - point and taps it to set gain from that mid-point.

Retro CB Guy said:

I'll let Chris know about this info 73's and the best of 4th of July to you kind sir.

Click to expand...

I'm sure he's already readin' the mail.

Best to you and yours - as well.

Stay Safe.

Retro CB Guy said:

might not need a recap per say

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Only if you traveled back to 1979 in your time machine and brought it back with you.

Forty years is forty years. Or, in this case forty-two years.

It would be a statistical fluke for a radio that old to have no issues at all from electrolytic caps well past their 'use-by' dates.

73