Help, my left channel has developed hum [resolved]

[thunderfrenchie]

Could I have a bad 5670?

[thunderfrenchie]

Also in case it matters, when I first discovered the problems, I noticed that the affected side's OT was much hotter than the other.

[Paul Birkeland]

B1/B4 now stabilizing around .007 v AC

What AC voltage do you between terminals 5 and 7?

Could I have a bad 5670?

This is incredibly unlikely.  You don't have heater voltage at your 300B.

I noticed that the affected side's OT was much hotter than the other.

The output transformer is on the bottom of the chassis and the plate choke is up top.  The plate chokes will get warm during operation, but the output transformers tend not to. Can you confirm which was presenting a temperature difference?

[Paul Birkeland]

42U does not look soldered.

[thunderfrenchie]

The OT on the side that was humming. Even after the other side had cooled down the OT was quite warm. At full power it was definitely hotter than the other.

[thunderfrenchie]

OK, resoldered 42U. I found that one half of the 7 resistor was not soldered. Did that and the reading between 5 and 7 is .047v ac. Retested 1 & 4 and that is now 2.73.

[thunderfrenchie]

Also, in the short time I was checking the voltages, the OT on the affected side is warm where as the other side is cold.

[thunderfrenchie]

Sorry, just reread your post. It is the choke that is getting hot.

[Paul Birkeland]

OK, resoldered 42U. I found that one half of the 7 resistor was not soldered. Did that and the reading between 5 and 7 is .047v ac. Retested 1 & 4 and that is now 2.73.

Is that B1 and B4 shows 2.73V AC between them but 5 and 7 show 0.047V AC between them? (This is almost impossible BTW)

Sorry, just reread your post. It is the choke that is getting hot.

If the voltage at 14 isn't considerably lower than it should be, then this isn't a concern.

I would recommend performing the power transformer test found on page 34 of the manual.  Specifically, you have no AC voltage between 5 and 7.  There should be 6V AC there.  You'll want to measure the AC voltage between power transformer terminals 12 and 13.  If there is 6.3V there, then there is a break in the green twisted pair of wires between those terminals and terminals 5 and 7.  If there is no AC voltage there, then power off the amp and disconnect each green wire on those power transformer terminals and retest. 

[thunderfrenchie]

Thanks Paul. I just retested the PT as on pg 34 and everything checked out perfectly. I'll do your green wire test in the morning.

Sorry about the confusion on the last set of measurements. Just redid and I have:

B1/B4: 350v DC, 0v AC
B5/B7: .3mV DC, .041v AC
C1/C4: 4.97v AC

Will let you know how it f=goes and THANK YOU for all the help thus far.

[thunderfrenchie]

Paul, I just realized when you've been referencing 5 & 7 voltages, you mean that of the terminal strips, NOT positions 5 & 7 on the 5670 socket.

On the affected side, which is actually 20 and 22, I am getting the full 6.3v AC, all good there. C1 & C4 hold steady at 4.95v AC. The unit is wired for 4ohm spur taps.

Anything else I should try/inspect?

Thanks!

[thunderfrenchie]

ALSO, just realized I had been measuring B1 and B5 values. The correct B1/B4 reading is 80.5v AC.

[Paul Birkeland]

OK, let's go backwards a little bit.  Can you also give me these AC voltages:

Between 4 and 7?
Between B1 and B4?

If you have OV AC between B1 and B4, that is your affected side, you will get no music out of that channel.  If this is counter to what you have observed so far, perhaps a 300B swap caused the issue to change sides?

[Paul Birkeland]

ALSO, just realized I had been measuring B1 and B5 values. The correct B1/B4 reading is 80.5v AC.

There is no B5.

[thunderfrenchie]

OK, I'm officially an idiot. Where I have been writing "B" I meant "D", the 5670 socket, which is not at all what you have been asking for. OK, once more with feeling... measurements coming...