Airline GIM-9131A knowledge/repair question

[g1]

Here is the area of concern marked in yellow:

[btah]

Dense alert here... What do you mean by "inject a signal". If you mean just have the amp powered up does that mean I should check the DC voltage at the B side grid? Black probe to the chassis and red probe to the grid? If that is what is meant, I do this with the volume up and a guitar plugged in?

[g1]

Signal is whatever sound that you put through the amp. Normally it is guitar, but for testing it is easier to follow the signal (measuring AC volts) by using a steady tone from some kind of tone generator or app.
You DC voltages are probably all fine as the reverb runs through the same tubes as the "dry" signal.
Can you come up with some kind of test tone to run into the amp?
Even a keyboard with a key taped down will work.

[btah]

The only thing I have that comes to mind is a guitar. I don't have any other instruments that I could plug in. Other than If I took a stereo and hook up one of the speakers outputs to a quarter in jack that I put in the input???

[btah]

and what type of AC volts should I be looking for. I have the guitar hooked up and strum the strings and I see like .3 volts with the volume pot turned all the way up. That is for both channels 1 and 2 at the connection coming out of the volume pot.

[btah]

ooh, do I need the speaker hooked up or doesn't it matter?

[g1]

Yes, you need to always have a load connected to a tube amp, so keep the speaker connected.
Continue along with your AC volts measurement, strumming the guitar.
You have .3VAC coming out of the volume pot, what do you have at each of the wires on the tone pot (tone turned up full) ?

[btah]

OK, so I am not sure what happened but now I have volume from both channel 1 and channel 2 as expected. Other than poking around trying to trace the circuit I haven't don't anything else. So maybe I moved something or maybe two things were touching?

Anyway, I think there are still two issues. buzz and reverb. Now I am not sure the reverb is working or not. But, I have some buzz going on. Channel 1 has more buzz than before. It is also a deeper buzz. channel 2 has buzz but it is not as loud/deep as channel 1. A good portion of the buzz on channel 1 goes away when I jack the guitar in. A little more goes away when I touch the chassis.

So, can we work on the buzz? I am willing to stick this out if you all want to continue to work with me. What causes buzz. Given that I can touch the chassis an change the buzz and also jack in the guitar and change the buzz, that seems like a grounding issue to me. However, that is a guess.

Should this thing really have a three prong power cord?

[btah]

I think The Dude was trying to tell me to put in a three prong cord in post #5. The ground would get bolted to the chassis? It appears to me the hot wire goes to the reset switch and the neutral goes to a different bank of connections. I can't really find it on the schematic.

[The Dude]

Yes, the ground of the AC cord will go to chassis ground. The other two will go the same place as the original AC cord. Also clip out what looks to be C38 (hard to read on the schematic). It's the cap that goes from one side of the AC input to ground.

On the reverb, I would start with just removing the tank all together and inspecting it. Make sure there are no broken springs and measure the transducer coils to ensure they are not open.

[g1]

The reverb was working before when the ch.2 dry signal was not getting through, but you still got sound when you turned the reverb up (post #54).
So now the Ch.2 dry signal came back, but the reverb is gone?
Disconnecting C4 will cut off the Ch.2 dry signal. Can you then hear the reverb?
With C4 disconnected and reverb turned full, how does the volume compare to when C4 is connected and reverb turned down?

[52 Bill]

I have been following along at home, but didn't want to cause any confusion by adding any posts that would just confirm the advice from G1 and The Dude.

A few things, please do change the ac cord for a three wire one. Until then, try and reverse the orientation of the two wire plug in the outlet and see if the hum/buzz is lessened.

You stated that the two reverb wires were different and that one was a single wire and the other was a shielded one. The single one is always the drive wire that goes to the tank input, the shielded one is always the return that connects to the tank output.

If you are going to use the speaker output from a stereo to drive the input of this or any amp, you need to keep the level of the stereo very low. It would be better if you could take an output from a tape monitor if your stereo has one. If the stereo is solid state, then you do not need to have a speaker connected to the stereo when you are using it as a signal source. If it is a tube stereo, then there needs to be a speaker load connected to the stereo at all times.

[btah]

The first thing that I am going to do is replace the AC cord. I had a three pronger laying around that I am going to use. It appears to me the wire gauge is a little smaller than what is currently on the amp but it will do for now. I am assuming I can hook the ground to ANY lug that has a direct connection to the chassis. Let me know if that is wrong.

I will replace the cord after while and post back.

[The Dude]

It is recommended by many that you ground AC in ground to it's own independent lug. You can certainly use another existing ground. Just make sure it's tight and stays tight. In either case, it's a good idea to use a star washer or something that won't loosen up. While we're on that note, it's a good idea on an amp of this age to go around and make sure all grounds and connections are tight. Check jack nuts, pot nuts, chassis grounds, transformer bolts, etc. It's not uncommon for things to come loose over time and that could easily be part of your hum problem.

[Jazz P Bass]

As the ground wire Can be attached to any lug, the rules specify that it should be on it's own lug & firmly 'bonded' to the chassis.

The third wire is all about safety.
IF the chassis should become electrically 'hot', the current will trip the breaker of the socket that the amp is plugged in to.

I have seen it advised to make the ground wire a bit longer than the two live wires.
If the cord gets yanked out of the amp, the ground wire will be the last to get pulled out.