Blowing Fuses

[Siegfried Meier]

Hi Tim,

All I seem to get is a blown fuse when I plug my 02 in. In your notes for the 01, you say to measure the resistance between pin 1 and pin 2 of the power connector-which connector do you mean? J2? I've tried seeing if there are any blatant shorts but no luck yet. Thanks.

Best regards,
Siegfried Meier

[Siegfried Meier]

Oh. I just reread it and you must obviously mean the IEC power connector. Checked all pins and I see an open circuit. Now, I'm starting to think it may be my power switch that is incorrectly connected. I'm using one with 6 poles, but only using 4 of them. If you picture the back of the switch to look like a 6 sided die, they are numbered:

1 4
2 5
3 6

Most likely a very common switch. Right now I have 3 and 6 disconnected. When connected to J4, 2 goes to pin 1, 5 goes to pin 2, 4 goes to pin 3, and 1 goes to pin 4. If this is incorrect, can you please specify which pole goes to which pin on J4? Thanks Tim.

Best regards,
Siegfried Meier

[tpryan]

That'll blow the fuse for sure. The way you have it wired now, when the switch is closed J4 pin 1 is connected to J4 pin 4. If you have a look at the schematic, you'll see that's equivalent to a short circuit across the AC line. The schematic clearly shows how to wire a DPDT switch like yours (SW2) to J4. In tabular form:

J4 SW or SW
1 2 2
2 3 1
3 6 4
4 5 5

When you're done, it's essential that you insulate the connections on the back of the switch with heat shrink tubing or electrical tape. I have a whole series of nice, new photos that illustrate this procedure, but there hasn't been time to get them up. Also, be very careful if you're testing the supply outside of the chassis, since there are uninsulated AC connections on the bottom of the PC board. Make sure it's on a non-conductive surface and there are no metal tools or wire snippets that could find their way underneath the PC board while AC power is connected.

[Siegfried Meier]

OK. Looked at the schematic one more time and yes, my switch is definitely connected wrong. I rearranged the crimps, plugged it in and no more blown fuse. I'm not exactly sure I understand the voltages, but here's what I've found:

Measured between 1 and 2 of J1-found 27V

Measured between 1 and 6 of J2-found 27V
Measured between 2 and 6 of J2-found 0V
Measured between 3 and 6 of J2-found -6.5V
Measured between 4 and 6 of J2-found 0V
Measured between 5 and 6 of J2-found 48.5V

Something doesn't seem right here. Any ideas? Thanks.

Best regards,
Siegfried Meier

[tpryan]

The voltages look OK with the exception of the -6.5V. First, make sure you don't have a short somewhere around the LM337 (U1). Check the resistor values for R3 and R4. When you sure they're OK, carefully measure the voltage at U1 pin 2. You should see something like -36V. Let us know what you find.

[Siegfried Meier]

Thanks Tim. Can I just put the negative side of my meter on pins 2, 4 or 6? Or is there a better way to measure U1? If I check the 2 resistors, is it possible to check them in the circuit?? Thanks.

Best regards,
Siegfried Meier

[Siegfried Meier]

OK. Checked U1 and here's what I've found. Pin 1 (left) is getting -3.4v, pin 2 is getting -36.5v, and pin 3 is getting -6.5v. How can I properly check the resistors in the circuit? Thanks.

Best regards,
Siegfried Meier

[tpryan]

You won't be able to get a precise measurement in-circuit, but in this case it'll be close enough. Be sure to turn off the power and allow the capacitors to discharge before making a resistance measurement. You can see the capacitor voltage at U1, pin 2. What we're really interested in is whether R3 and R4 got swapped by mistake, or are wildly wrong. Also, check the condition of C1. Does it get hot when the power is on? Does it appear discolored or damaged? Is the polarity correct? If R3, R5, and C1 seem OK, the regulator has probably been damaged.

[Siegfried Meier]

Okay. Now I'm a little confused. I thought R3 and R4 were supposed to be the same value (2K49). Is this not the case? Remember I mentioned that the old parts list said R4 was a 121, but the board said to put in a 2K49, as is R3. I've got the same resistor (2K49) in both R3 and R4. Thanks.

Best regards,
Siegfried Meier

[Siegfried Meier]

Boy, did I make a serious mistake there. I completely misunderstood what you said in the earlier email about that resistor, R3. I changed it to a 121 and now I get -27.2v on the negative rail. Thanks for all the help.

Best regards,
Siegfried Meier