Re: Tech Help Sys80 Circus Power supply

On May 8, 5:35 am, sgerde...@xxxxxxxxxxxxxxxxxxxxx wrote:
Sorry it has been a while since I was able to try something different
with this board!

Here's what has been changed.  CR6, R3, CR7, R10, CR8, SCR1, R13, R8,
R6, R14, Q3, C4.

We are back to blowing the fuse in the cabinet again after changing C4
this last time.  Prior to that, all TP voltages were close except for
TP4 was low, 1.9v.

We retested everything this weekend and noticed a few things.  R8, R14
test ok when out of the board, then test really low when soldered in.
If we cut them out and retest, they test fine.  Why does this happen?

Best way to test voltages out of bridge rectifier?

This has been a nightmare.  Any other suggestions?



Hi Scott,

some random thoughts and questions...

What happens if you try to measure continuity across C3? This is a
stretch, however.

What is the status of the 5V LED? Has it been replaced? If not,
probably should be...

Although tricky to do, do you think you can test the potentiometer?

My best guess for the low resistance measurements on R8 and R14 is
this: The meter measures resistance by sending a small current/
voltage through one test probe, to be picked up by the other probe.
This current/voltage is SUPPOSED to go through the resistor you are
testing. It may be the case that the resistance of the part tested is
higher than the overall resistance if the current/voltage were to go
around the "back way" to the other test probe, hence the current/
voltage following the path of least resistance, as it always does.

I think a good test of the bridge's output would be to probe pins 1
and 2 at the J1 connector, to measure the voltage.

When you are blowing the fuse, is the circuit board on its heat sink
plate? There was another fellow who was blowing fuses in a similar
manner; it turns out the heat sink wasn't on, he would have the board
powered up for a few minutes, the components would get hot (probably
excessively), and then the fuse would blow.

Regards,
Dan
.

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