142 gtl help

[Robb]

Schematics.
This is important.
It is KEY to troubleshooting.

Find the SIMPLEST schematic you can - first. A linear power supply - like the one in your 142. Ask yourself: Gee what are all of these parts doing there? Knowing ll of your components in your power supply. If it helps give them a name. Whatever rubs your budda - right? No reason this can't be fun.

Look at where the power enters the xformer. What device does it next feed? What is that next part, and what it is doing? If nothing else at this point OWN that power supply. Memorize it, focus on it and nothing else. Ask yourself: what is this power supply schematic telling me? Am I listening to the devices? A transistor does what? It switches power on/off at the speed of the base/junction signals. What is that zener diode doing there? Go for it . . .

When you get to the point of recognizing all of the parts of the power supply knowing how to test each individual part, and understand the harmony an purpose of the power supply, THEN you can move on to understanding other part failures in other kinds of circuits.

Success is not measured IF you fixed the radio; it is how you fixed it. What you investigated. What you found right. What you found wrong. Because knowing that, you are gaining experience to fix whatever may break in your house in the future. Once you understand one simple schematic, it makes it far easier to understand the next parts of a radio.

ATE = Always test everything.

 

[kaos513]

yeah im doing a visual inspection tonight on it

 

[kaos513]

so far no burnt traces or bad solder joints.

 

[Robb]

Last 142 I'd worked on was actually fairly recently.
Sold it a few weeks ago, just after fixing it and recapping it.
It also had a problem with no AM modulation at first.
Turned out to be a bridged solder joint on the AM regulator - that time.
So I fixed the horrible solder job and replaced the C1419 with an upgrade TIP part.

But there are soooo many different things that can cause that AM no modulation fault. Anything from the mic element all of the way down to where the AM signal reaches the mixer chip - including the mixer chip. SO to ask what the problem may in this instance is to ask which part out of some 20-30 different parts is fault. In fact, it could even be more than one part - too.

When something breaks, it doesn't always break the way you'd hope it would. It is always the weakest link(s) in the chain, either thru abuse or end of service life for a component ('caps'). Some people just love to shove all the modulation they can down that pipe and seldom care about what happens - until it breaks - of course.

Having said that, this is why I pointed out that the schematic is really your best friend. If you can learn to trace thru the path of AM audio on the schemo, all you have to do is start at the very beginning of that circuit ant test everything thru to that end. Often enough, most likely that you seldom have to go the whole distance. But you will always be assured of finding the fault(s) if you do the grunt work.
And it is grunt work.

But once you get that discipline to lean the schematic and test components; then there is no stopping you. You will be fixing everything that comes your way. Broken radios find their way to you and you fix them and turn them around.