Stereomour Diagnosis Testing or Repair?

[audiotecture]

I suspect my hearing is not good enough to hear hum at 60Hz

[Paul Birkeland]

You can put your finger on a speaker cone and feel the 60Hz.

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I can hear/feel the hum from one pair of binding posts (closest to inputs) but nothing from the other pair.

[Paul Birkeland]

OK, so there's an issue either with how the output transformer is hooked up on the tube side or the speaker side.  This could be missing or poorly soldered jumpers on the output transformer, poorly connected or missing wires from terminals 5 and 10 on that output transformer, a loose 3.3uF/630V capacitor, etc.

[audiotecture]

I went back and checked for missing wires (none that I could identify) and resoldered a number of connections in the areas that you described. Blew the fuse when I restarted the amp. The supplied fuse reads F1.5AL250V the manual reads 1.5A GMA slo-blo fuse. I'm having trouble tracking down replacement fuses online. It seems the supplied fuse and the fuse described in the manual are different fuse types? Can you clarify the proper fuse type and perhaps point me in the direction of an online source. Troubleshooting will need to wait until I get replacement fuses.

[Paul Birkeland]

A 1.5-3 amp fuse of the appropriate size is fine (5x20). 

[audiotecture]

Paul...finally received my replacement fuses. I retested all of my voltages and there are a few that aren't quite right. Terminal strip 11 = 160V, 15 = 268V, 20 = 160V, C1 = 160V, C2 = 268V, C4 = 160V. Similar to before there is sound coming out of the A side binding posts, but none from the C side binding posts. The PC3 on the C side warms up when the amplifier is turned on, but the PC3 on the A side remains cool.

[Paul Birkeland]

Heat in the plate choke is determine by the voltage at the HV+ pads on each side of the amp vs. the voltage at pin 2 on each 4 pin socket across the resistance of the plate choke.  So for the heat dissipated in the plate chokes to be different, you need voltages on the 4 pin sockets that are completely different from each other or the DC resistances of the chokes need to be way, way different (which would indicate a damaged choke).

I have provided you the instructions before to check for each of these conditions, and you didn't find either issue.  I would strongly recommend redoing these checks.  For the one plate choke to get hot but not the other, there has to be an accompanying result that indicates why.

So when I went into the manual to see what might be going on with your voltages, I can now tell that your 4 pin socket voltages aren't the same between sides.  Seeing 160V at terminal 11 is not a good sign.  The rest of these are all kind of a result of that.  I suspect that the 5W/10W resistors mounted to the terminal strips on that side of the amp are loose or damaged. 

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Paul...I took the resistor out and when I tested it, it showed 231.8 ohms of resistance. Looks like my local Microcenter has 220ohm 1/4watt resistors in stock. I should be able to pick them up, replace the existing resistor, and test again tomorrow.

[Paul Birkeland]

No, put that resistor back!

There is a 5W and 10W resistor connecting between the two 5 lug strips around the 9 pin socket (a 1.6K and a 4.7K). Either one of those is damaged or one of those is not connected (maybe both).

[audiotecture]

Paul...I think I found the source of the smoke I saw/smelled when I initially powered up the amp with the tubes in the wrong orientation. You can see (in the attached photograph) that the heat from the 10W 1.6K resistor melted the insulation on the red wire between B6 & 14L & singed the socket screw/washer. The 10W 1.6K resistor measures 1.7K ohms of resistance. The 5W 4.7K measures 4.58K. The 220-ohm resistors both measure around 220 ohms of resistance.

[Paul Birkeland]

You can put that all back together, disconnect the "K" wire on the power supply board on that side, then measure DC resistance between ground and pin 1 on the 4 pin socket on that side, then ground to pin 4 on the 4 pin socket on that side (powered off).

-PB

[audiotecture]

Paul...Finally had time to put everything back together. I disconnected the "K" wire on the power supply board. Ground to pin 1 and pin 4 on the 4 pin socket are 1.255K ohms (on the side that's not working) For reference measurements on the Ground to pin 1 and pin 4 on the 4 pin socket on the side that is working are 1.168K ohms 

[Paul Birkeland]

You can leave the wire to K disconnected, recheck DC volatges on all four pins on the 4 pin socket on the bad side and report back. 

That resistance is totally fine, those resistors are good.   There could be something amiss with the wiring to pin 3 of the 4 pin socket that's not allowing for it to get a good ground reference.  That is the 249K resistor and the 220 ohm carbon comp grid stopper.  Be sure those are well connected.

[Paul Birkeland]

You can also swap the 2A3s to see if the voltage issue follows a tube.