TRC-57 restoration

[brandon7861]

They are not .47pF, they are 47pF. The factory schematic shows C80 and C81 as 39-68pF, which makes sense as under a half a pF of tank would be impractical.

There are large SMD parts like 2512's, but I think a compromise between being able to work with them and being able to easily fit them would be 1206.

A fine soldering iron tip, good magnifying glasses and .015" solder is the way to go. This little helper is how I got started soldering SMD parts.

edit: and pick up a roll of MG chemicals #453 fine solder wick. If you goof and have to try again, that little helper tool is useless with a blob of solder under the part. They will go on all crooked and not flush with the board. Always start with bare tinned pads.

edit 2: And if you are like me and keep your fingernails trimmed short, it helps to leave the nail on your index finger a little longer. That magnetic helper isn't always possible to use so you will find yourself trying to hold them down with your fingernail while tacking one side. A little fingernail there keeps you from getting burned.

 

[brandon7861]

At work now, but I think I was driving the active filter wrong. I don't think it is seeing PWM.

IC6 datasheet says it's a phase-frequency detector (type-2, goes up and down), and looking at the factory schematic, the output is already filtered some.

I am now under the assumption that the signal coming from C322 is a small DC value with some ripple that the loop filter smooths and amplifies. I will try to drive it differently tonight with a smaller DC value going in.

The 10kHz PWM signal that I had tried led to some serious nonlinearity in the VCO tuning voltage that clearly took the active filter from cutoff, through the linear region and into saturation. Maybe if I feed it only enough DC to keep it in the linear region...

It's tricky not knowing exactly what the radio is telling it.

 

[Low_Boy]

I sure miss Mikes videos.