Airline GIM-9131A knowledge/repair question

[The Dude]

Yes, if the voltage is low, the caps have discharged enough and you should be good to go. You can also do it as g1 said and reverse the secondary at the speaker jack. Either way will tell us if that is the problem.

[loudthud]

I don't think there is a problem with the phase of the output transformer. I think there is a bad filter capacitor that allows the output stage to oscillate through the power supply to the phase inverter and also causes the hum.

[btah]

Well, so far so good. I am going to put the rest of the tubes back in and see what happens. See if I get any output at all.

[The Dude]

Glad you're still with us!

If the problem is elsewhere, the squeal may be worse than it was, so be ready to shut 'er down.

[btah]

Looking much better. All tubes back in and no squeal. Been a good 5 minutes. Previously would squeal immediately.
However, it makes absolutely no sense to me. The solder looked old. I suppose it could have been messed with and then put in a closet for 20 years...
I do have some hum and I will ask those questions tomorrow. I am calling this a major success right now and I am going to end the evening with that.

However, looking at the schematic, something doesn't make sense to me. One the bottom right of the schematic it shows the line of tubes and the wires that go between then. The 12ax7(s) show all the pin 9s connected together going to the pin 2s of the 6L6(s). In reality, the pin 9s of the 12ax7(s) connect together but then it goes to the pin 2s of the 6L6s. The pin 4/5(s) of the 12ax7(s) connect together but in reality go to the pin 2(s) of the 6L6. Essentially, the schematic is reversed as far as the pins for the 6L6(s).

That is all for me tonight.

You all rock...

[btah]

And food for thought for the next question about the hum. Not loud, but there is hum. Actually, I don't know if it is supposed to be dead silent or not or if some hum is expected. But, the hum has a pulsing to it. maybe two or three pulses a second.

[The Dude]

As far as those pins you are talking about. Those are just the filament connections for the tube heaters, so it really doesn't matter. Think of a light bulb. It's just a piece of wire that glows in there. You can put voltage to that wire any polarity or phase and it will glow. Similar/Same thing in a vacuum tube.

[btah]

Instead of trying to describe the hum, I put a video out on youtube. Here is the link (hopefully I can add a link to this, if not, just copy and paste it):
https://youtu.be/eVpeDiAqvRk

[btah]

I should have also said I had the reverb pot (is that what you would call it?) unplugged.

[The Dude]

It sounds to me like you've got a reverb tank issue or connections to the tank. Are there RCA connectors from the tank going into the amp, or is it hardwired? If there are RCA connectors there, unplug them, check the resistance from hot to ground on each and post the values you get. If it's hardwired, check at the tank itself.

Edit: I was typing during your last post. That buzz is likely with the tank unplugged. Is it there with the tank hooked up?

Edit 2: Also see my post #5. This amp is dangerous as is and is also not earthed, which may cause some noise.

[btah]

OK, so a little more information. With the reverb tank plugged in, it acts the same.
When checking the resistance, I checked between the male part of the plug to the shoulder of the plug (those are both technical terms). Infinite resistance on both plugs.

However, on the output plug I get very little (just above zero) resistance between the shoulder and the chassis. But on the input plug I get infinite resistance between the plug shoulder and the chassis, seems like that should also be zero resistance.

[The Dude]

Yes, the one that measures infinite indicates an open transducer or cable or a connection issue. Was that test right at the tank, or at the end of the cable? If cable, test it at the tank itself. You might just have a bad cable. If at the tank, look inside the tank. There are little wires that go from the connector to the transducer. Maybe one of them has broken from the jack. The resistance will be different input and output, but neither should be infinite.

Edit: To be clear, we are trying to verify that the transducer coils are good inside the reverb tank. If you leave the wires plugged in at the tank and check them at the other end, you are verifying both the cable and the tank. If either measures infinite, you either have a bad cable or a problem in the tank. At that point you need to unplug the cable from the tank and check directly at the tank to see if the problem is cable or tank. I hope that makes sense to you.

[btah]

Although with the output plug having a good ground, that would ground the reverb tank, which would in turn take care of the ground for the input plug, right? I did check to make sure and there is zero resistance between the tank and the chassis with the plugged put back in.

[btah]

When I checked the cable ends it was at the end of the cable that plugs into the tank but I had the cables unplugged from the tank.

[The Dude]

I edited my last post. See the edit and let me know if that makes sense to you.