MOD 102 Kit not powering on at all. Any suggestions?

[vipor3D]

Remove the input jack and just let it hang by the wires. Scrape the chassis until you see lots of shiny bare metal around the mounting hole for the input jack. Reinstall the input jack using a 3/8" internal tooth lock washer on the inside of the chassis and tighten securely.

https://www.tubesandmore.com/products/w ... ring-steel

I filed more around the input jack hole and added a lock washer but it didn't help unfortunately

[LOUDthud]

Let's review the symptoms.

There should be no hum between the time when you turn the power on and when the tubes warm up, please confirm.

Does the Volume control affect the Hum ?

Is the hum very low or zero when the Volume is all the way down ?

Does the hum gradually get louder as you turn the Volume up ?

Is there a place on the Volume control where the hum very low but it gets louder as you move the volume higher or lower ?

Do the tone controls change the hum ?

Any change if you remove the 12AX7 from it's socket ?

[martin manning]

I ordered the parts for the MOD 102+ upgrade along with the original kit and was planning to make that upgrade just to get some experience working on the amp, but I was really wanting to get the original build right before making the upgrades.

Good plan. It's likely that the underlying problem will still be there after making the 102+ mods.

[vipor3D]

Let's review the symptoms.

There should be no hum between the time when you turn the power on and when the tubes warm up, please confirm.

Does the Volume control affect the Hum ?

Is the hum very low or zero when the Volume is all the way down ?

Does the hum gradually get louder as you turn the Volume up ?

Is there a place on the Volume control where the hum very low but it gets louder as you move the volume higher or lower ?

Do the tone controls change the hum ?

Any change if you remove the 12AX7 from it's socket ?

1. When I first turn on the amp, there is a very faint hum that I can hear but it's not the "buzz issue" noise. After about 10 or so seconds, the buzz noise starts coming through the speaker.

2. The buzz is virtually gone with the volume at zero

3,4,&5. The buzz gets louder as I turn the volume up, but once the volume reaches about 80% the buzz noise cuts out and turns into more of a low-level hum. With volume at 12 o'clock, the tone controls don't affect the buzz. However, when the volume is maxed, the treble control increases the buzz a lot as its turned up. I also notice a good bit of scratching as the volume pot goes from about 2 o'clock to 4 o'clock.

6. When I removed the 12AX7, the buzz was completely gone.

I took another video demonstrating the results that I listed above, except for the removal of the 12AX7:

https://www.youtube.com/watch?v=OrGNeZhuN5k

[martin manning]

So the buzz is being generated in the input stage is my conclusion.

[vipor3D]

So the buzz is being generated in the input stage is my conclusion.

That's what I was thinking after removing the 12AX7 but wasn't totally sure. I've swapped the 12AX7 for another I have and still had the issue, so I don't think it's the tube.

So that at least narrows down the problem area. Would that mean the issue is most likely with the input jack or pots? Or could it be a connection issue somewhere with V1?

[Stephen1966]

So the buzz is being generated in the input stage is my conclusion.

That's what I was thinking after removing the 12AX7 but wasn't totally sure. I've swapped the 12AX7 for another I have and still had the issue, so I don't think it's the tube.

So that at least narrows down the problem area. Would that mean the issue is most likely with the input jack or pots? Or could it be a connection issue somewhere with V1?

Possibly... could you share a pic of the back of the front panel. While you're at it, check for continuity between the backs of the pots and chassis ground. Also, still looking at the filament string, do you get the voltages they published? 0.03VAC from one side to ground and 6.9VAC from the other side to ground.

Perhaps also, check for any stray voltage (AC) on the backs of the pots to ground as well. Almost microscopic offcuts of wire floating around in a chassis have been known to cause all kinds of havoc

[vipor3D]

So the buzz is being generated in the input stage is my conclusion.

That's what I was thinking after removing the 12AX7 but wasn't totally sure. I've swapped the 12AX7 for another I have and still had the issue, so I don't think it's the tube.

So that at least narrows down the problem area. Would that mean the issue is most likely with the input jack or pots? Or could it be a connection issue somewhere with V1?

Possibly... could you share a pic of the back of the front panel. While you're at it, check for continuity between the backs of the pots and chassis ground. Also, still looking at the filament string, do you get the voltages they published? 0.03VAC from one side to ground and 6.9VAC from the other side to ground.

Perhaps also, check for any stray voltage (AC) on the backs of the pots to ground as well. Almost microscopic offcuts of wire floating around in a chassis have been known to cause all kinds of havoc

I had a few minutes during lunch today and popped it open to try the continuity test again. I got "beeps" for continuity between all of the pots and chassis ground. I also got a picture of the inside front panel.

When I have more time this afternoon I'll go through and check voltages and also check for AC in the pots

[martin manning]

Try rerouting the heater lead going to pin 9 as shown below. Right now you have the input stage grid lead inside a loop of heater current. If you look at the layout drawing you will see that the heater leads are shown above the AGK leads, which are best run down against the chassis surface.

[johnnyreece]

Looks like one of the 102+ mods is a grid stopper on the input. Maybe that would be worth a try, as well?

[martin manning]

Looks like one of the 102+ mods is a grid stopper on the input. Maybe that would be worth a try, as well?

That will be more about eliminating RF.

[Stephen1966]

I had a few minutes during lunch today and popped it open to try the continuity test again. I got "beeps" for continuity between all of the pots and chassis ground. I also got a picture of the inside front panel.

When I have more time this afternoon I'll go through and check voltages and also check for AC in the pots

Continuity is good. That's shielding your signal running through the pots. If you get any voltage above the ground reference voltage though you have a leak somewhere so...

Check these leads:

Untitled-2.jpg

They look as if they are making contact with the pot cases, the 6k8 will make a ground loop if it is touching the case... they shouldn't. If you desolder these and cut a piece of insulation off a strip of wire cut to length, you can use the insulation to cover the leads and isolate them from the pot backs.

The scratchy pot in your latest vid... sounds like DC on the pots. All DC should be blocked by the caps in the tone stack before the volume pot.

[vipor3D]

Try rerouting the heater lead going to pin 9 as shown below. Right now you have the input stage grid lead inside a loop of heater current. If you look at the layout drawing you will see that the heater leads are shown above the AGK leads, which are best run down against the chassis surface.

Unless I'm missing something, this is how I had it wired initially (please see my last pic on page 4 of the discussion). Based on suggestions here, I rewired the filament wires to V1 so that the wire to pin 9 didn't go across the wires to pins 1-3

[johnnyreece]

Looks like one of the 102+ mods is a grid stopper on the input. Maybe that would be worth a try, as well?

That will be more about eliminating RF.

You're right, of course. I'll try for a more productive prompt: Do the bass and treble controls have any effect with a guitar plugged in? Is it the expected effect?

[vipor3D]

I had a few minutes during lunch today and popped it open to try the continuity test again. I got "beeps" for continuity between all of the pots and chassis ground. I also got a picture of the inside front panel.

When I have more time this afternoon I'll go through and check voltages and also check for AC in the pots

Continuity is good. That's shielding your signal running through the pots. If you get any voltage above the ground reference voltage though you have a leak somewhere so...

Check these leads:

Untitled-2.jpg

They look as if they are making contact with the pot cases, the 6k8 will make a ground loop if it is touching the case... they shouldn't. If you desolder these and cut a piece of insulation off a strip of wire cut to length, you can use the insulation to cover the leads and isolate them from the pot backs.

The scratchy pot in your latest vid... sounds like DC on the pots. All DC should be blocked by the caps in the tone stack before the volume pot.

It's hard to get a good picture, but none of the component leads are touching the backs of the pots. I even tried lifting them away a bit, but no change. The black cap is farther away from the pot that it appears in the new photo.

While I was testing the amp to see if that worked, I chopstick-ed around a bit to see if I noticed any changes. And while I didn't notice any changes with the movement around the pots, I noticed anytime my hand/finger got near either the input jack or volume pot, the buzzing significantly increased. I tried to take a new video of this as well. Skip forward towards the end if you just want to hear "the finger buzz"

https://youtu.be/AFDs__7LxN8