Stereomour II blowing fuse [resolved]

[TexFlood]

First build blowing fuse at voltage check.  Fuse blows immediately when plugged in with no tube glow, no pops, no smoke, nothing.  Fuse does not blow with power switch off and plugged in.  Resistance checks all on point other than a few * values which could be questionable.  Terminals 47 and 48 read 0.L or open according to my meter (had initially thought this meant out of range and thus proceeded to voltage check). Terminals 7 and 14 read 4.69 and 4.7 M ohm, and do not fluctuate.  A few other terminals with * values fluctuate in the M ohms.  I have checked and rechecked all wiring, solder joints (reflowed any remotely questionable), part orientation and values, and checked for shorts with flashlight and magnifying glass.  Any assistance is greatly appreciated.

[TexFlood]

Perhaps some pics will aid in the investigation.

[TexFlood]

A few more.

[TexFlood]

Hey Dan,

Thanks so much for all the help.  Disconnected the red wires and fuse still blows.  Does this mean we've ruled out issues with this section and issue lies in the filter cap section?  I'll wait to hear from you as to how I should proceed.

[Doc B.]

There's one more step, to disconnect the black wires going to the plate chokes next. If the fuse still blows after that, the 2A3 and 12AT7 circuits are pretty much ruled out as the cause and the problem will most likely be somewhere in the power supply. I apologize that I haven't had the chance to look at the photos you posted yet, but I will try to give them a look this evening.

[TexFlood]

Black wires from PCs disconnected and fuse blows.

[Doc B.]

OK. Just to verify, the transformer passed the voltage checks without any weird readings when you completed that step, yes?

[TexFlood]

Yes

[Doc B.]

I don't think this is necessarily the problem since the fuse only blows with the switch on, but I notice that the foil on the shielded twisted pair cable coming from the power switch back to the power entry connector looks very close to the metal of the live connection on the connector, at least from the angle in your photo. Might want to make sure it isn't touching. Also just to cover all the bases, what size fuse have you been testing with?

Next I will suggest inspecting the rectifiers. Amp should be unplugged. Does your meter have a diode test setting? The setting will have a little triangle and straight line symbol if it does. If not, just set the meter to the K ohms range.

Touch the black test lead to one end of one of the UF4007 rectifier diodes at power trans terminal 19 and the red lead to the other end of that same diode. You should see either an OL reading or something roughly around 1.5K ohms. Swap the leads the other way around and check again. If the numbers aren't exactly those it's OK. If you see a very low reading like under 100 ohms, that diode is bad. Check both diodes the same way.

I will also suggest carefully rechecking the connections all of the resistors that are connected under the filter capacitors. A miswire anywhere in there could definitely cause a blown fuse.

[TexFlood]

Hey Doc,

Checked foil and it looks ok - Its not touching.

Fuses are 1.5A-250V fast blow.

Rectifiers checked on diode setting:
0.L - .519 V
0.L - .522 V

Checked connections of resistors under filter caps and they look good.  I did touch up a solder joint at 29L while in there.

Thanks again for all the help.

[kgoss]

Try a slow blow fuse at the same rating as the stock fuse. If there is a short somewhere it won't help. But its worth a try.

[TexFlood]

Fuse shows catastrophic failure.  Is use of slow blow still advisable?

[Doc B.]

Could you measure the resistance of each of the 130 ohm resistors that are connected to the 100uF 450V filter caps? That would be measuring resistance between T44 and T31, T44 and T30, T31 and T34, and T30 and T27. Check that there are no excess leads hanging off at the same time.

Also make sure that the 270K ohm resistors at between T41 and T42, and T44 and T45 are connected to the proper terminals and no excess leads are touching the chassis.

[TexFlood]

Doc,

Between: 
44&31 = 128.9
44&30 = 128.9
31&34 = 129.6
30&27 = 129.1

Checked resistors between terminals 41 and 42, 44 and 45 - they are connected to proper terminals and no excess leads are touching the chassis.

[Doc B.]

OK, we're running out of things to try without detaching a connection or two. Let's take the power supply filter completely out of the equation and just run the high voltage power transformer secondary unloaded. Sorry to do this as it's kind of PITA, but detach the pair of UF4007 rectifier diodes from power trans terminal 19. Also detach the white wire from T20. This will completely disconnect the secondary from any potential shorts in the PS. With the tubes out of the amp try powering up and see if the fuse holds.