Looking for the cause of smoke

[nadona]

R73 of my "buster 2303" makes smoke when the range switch is set to 1A or more range (500mohm left side, 50mohm right side). Other ranges work fine for hours without any problem.

When I first found out this, it was 98ohm, now after a few cook 43ohm.

Looked simple but turned out many nights of coffee

Where should I look?

[Rerouter]

Its acting as an inrush current limiter until the control part of the circuit is powered, complelty remove the relay from circuit and make sure it

1. Energizes
2. The contact resistance is low
3. It can trigger off 7V with a 22 ohm resistor in series
4. that that 22 ohm resistor measures correctly

[nadona]

I pulled out of the PCB and put on the breadboard like in the picture.
Answer:
1) Yes.
2) 80mohm
3) Yes
4) 22ohm resistor on the board measured as 22.23ohm

Now right 1A range doesn't work displaying random number but left side 1A range still accurate number and smoke.

Thank you.

[Rerouter]

Ok next up is make sure the input to the 7805 is receiving a nice solid 7V or greater,

(while you have that relay out may want to confirm what the lowest voltage it will trigger at is,

[nadona]

1.65Volt can hold it. Could be actuated at 4.7V.
I was going to replace the resistor but when I pulled out, it measured 102.45ohm. May I was measuring with transformer coil parallel.
I will put it back all and check 7805.
Thank you.

[nadona]

Yes, bridge rectifier before C12 has damaged. One diode is open both ways.
Thank you very much.

I always check the PS first no matter what but this time the machine seems work fine and as soon as I saw the smoke, my attention was fixed at the smell

Lesson learned.

[nadona]

Too quick. Real big problem.
TR2 7V secondary coil is open:palm:

[Rerouter]

I dont suppose you have any rough estimate what current that 5V rail is meant to pull, as if only that secondary is blown and it is a low current winding you could likely bodge in some $5 230 to 7V Transformer,

[nadona]

230 to 7 OR 120 to 7?
I was thinking this too.
500mA?

Would you select one from this page? http://www.digikey.com/product-search/en/transformers/power-transformers/786735

[Rerouter]

Ah American, so yes 120 to 7, if its 500mA a piddly little 5VA transformer would easily make up for it,