Realistic trc465 choppy am modulation

[Uncle Ronnie 336]

I have already done a few repairs on this radio including unspreading the coils. I also decided to do some of the Handy Andy experiments with it around the microphone amp and everything was working good and all modes until I went through it and replaced the rest of the electrolytic caps with higher voltage ones. Receive audio works mint and all modes, I get a good clean loud 18 Watts on SSB, but my am modulation is wicked choppy. Just looking for ideas of where to look first because I have been inspecting for solder Bridges and have come up with nothing. Thanks

 

[nomadradio]

I would look for one that has the polarity backwards.

Frequently the screen-print markings on a pc board have a "+" next to one of the two holes for that part. Trouble is, the capacitors are nearly always marked with a "-" on one side.

Not a tricky analysis, the "-" lead goes to the hole that doesn't have a "+" next to it.

Just leaves a lot of room for error. Simplest explanation is always the thing to check first.

73

 

[Uncle Ronnie 336]

I did give the radio a good look over and I found that c57 was in backwards so I pulled it out and put a new one in its place correctly as to not risk putting anything broken in there and I am still getting choppy am audio. So now I'm wondering what else I should look at around c57. Hopefully I didn't break anything

 

[Handy Andy]

Choppy audio...

Could be many things, including too much bias on a cap...placing DC power on one to pass a signal acts like a limiter. IT only works when you push enough audio into the line to overcome the bias - this can also be related to the PA / CB switch.

Check and make sure the switch - using your schematic, is not shorting out one contact to the bias it "sees" to turn on.

Check your MIC line, your audio wire from the Mic, if it has DC power on it, that's not good, you'll need to look over the work and may need to remove some mods.

AM Regulator can be affected. Mods done here have to be carefully considered and installed...small area to work with can make for bigger problems if a solder bridge is in the way.

This can also be a Limiter issue, I'll have to ask hat or which mods were done to that chassis and where to I can help refine your project.

 

[Uncle Ronnie 336]

Yeah I will list the mods that I have done thus far mostly in the mic preamp area.

560p across q26
.01 cap across r142
82k @R143
47uf 50v c107

 

[Handy Andy]

Ok, I think I know, Did you adjust or change R141 and R142 yet???

These two Resistors R141 and R142 are upgraded - if you do these mods - they are to be changed to higher values - if they don't get changed, it will make the amp section operate like a switch - too much gain.

The mods were for fidelity - not for extra drive...the original resistors used (about 220 ohm or 150/120 ohm) - are too low for proper drive levels and fidelity

To help this along further, the section you looked at under those "Handy Andy" pages, list C109 - since Bennie had done some changes to the security level of the site, I was no longer able to update the website to reflect the more time I had to spend with graphics for the pages.

Note this also reflects the changed Resistor parts to new values.​

 

[Handy Andy]

Another area to look for problems is at the AM regulator.

Might want to keep this as stock as possible...

There is an NPC mod, but it requires you to use parts that have different values.

The NPC mod requires a RESISTOR to be used
Prefer to use 270~330 ohm (1/2 value of the 560)
ALONG WITH the 560 ohm originally there

Along with any changes during the upgrade,
you should do in steps to avoid mistakes in parts locations,
Test your work after each step.
Note C172, Shown as a 47uF 10V - many PC-122's
and their later XL versions used 100uF 16V.
​

 

[Smokinone]

This information is good to know. I have a completely stock 465 I bought some time back. Never did anything with it other than test it...I was collecting a lot of radios at one time.

Anyway, I guess your advice to unka ronnie must have panned out as no replies, But I am glad to get the info for reference at least.