VOX Night Train 15 NT15 Volume and Noise Issues

[misterc57]

Picked up a NT15 head. Seller said it was making strange noises. I did a complete tune-up on it (cleaned pots, jacks, tube sockets, etc), swapped all the tubes.

I do get occasional background noise (crackling) independent of the input signal. When playing a guitar I do not hear it, just hear it at idle.

Notice the noise in Bright mode not in thick mode. (thick mode is a gain mode that bypasses all the three EQs pots, treble, middle, bass).

Here is another issue in Bright mode; middle and bass pots are acting like a volume control, when turned down to zero there is no signal going through. As they are turned up, volume increases.

There is some discoloration (maybe scorching) on the tube sockets but I do not think it is affecting anything, I have good continuity between the tube sockets and the pins on the underside of the board.

I have attached a schematic. Sure could use suggestions.

Thank you, MC

[Jazz P Bass]

Well, from here it appears you either have a leaky coupling cap or a duff control.

Easy enough to rule out a leaky cap. Look for Vdc on the pot wipers.

To prove out whether or not a control is broken, I would measure the resistance from the treble pot/ leg 3, to ground.
The reading should add up to all of the controls added together (1.27M)

[dmeek]

I just fixed an NT15H yesterday with the same symptoms.
It was caused by the white goop they use to glue large components to the circuit board.
Remove it with solvent or heat to make sure it doesn't bridge any traces.
I'm guessing it either absorbs or traps moisture underneath causing short circuits.

[misterc57]

No VDC on wipers.

Not sure which treble pot leg is #3. I get a range at each leg. Reference picture, right side leg ranges from 2.62 ohms to 487K ohms. Left side leg ranges from 246K to 734K. The range is mostly affected by having the bass pot at zero (low number) to 10 (high number). Note that my bass pot is 500K, not 1M like in the schematic.

[misterc57]

I have a lot of that stuff (see picture) on the board.

Did you remove all of it or is there one or two spots to focus on? What kind of solvent or heat can I use to remove it?

[Chuck H]

Since the problem doesn't manifest with the tone controls bypassed I'd say that's as good an isolation of the problem as you might hope for. No DC on the pots, so no leaky caps. But you might have a wonky pot anyway. I've had cold joints and dirty tube pins cause the sort of symptoms you describe. It's probable one of your tone pots or it's connection is compromised in some way.

[dmeek]

Try to remove all of it, especially where it touches any resistor or cap leads. I used a heat gun normally used for SMD soldering.
It has a fairly small nozzle. I also tried "Heavy Duty Flux Cleaner" spray, works well. Acetone should work as well.
A while back I saw Vox NT50 with bad pots - the terminal rivets needed crimping, so that's another possible problem.

[misterc57]

Something I noticed that seems strange to me with the bass and middle pots; when measuring resistance end leg to end leg, I only see the full value when the pot is set to 10, otherwise I see a very low resistance when the wiper is set to the zero position. Shouldn't that measurement always read the value of the pot regardless of wiper setting? These readings are with the pots in place on the board.

[misterc57]

That stuff is on there good, very stiff feeling. Did you have all the same symptoms... Crackling noise, and the bass and middle pots acting like a volume control?

[Dave H]

Something I noticed that seems strange to me with the bass and middle pots; when measuring resistance end leg to end leg, I only see the full value when the pot is set to 10, otherwise I see a very low resistance when the wiper is set to the zero position. Shouldn't that measurement always read the value of the pot regardless of wiper setting? These readings are with the pots in place on the board.

No, check the schematic, The bass and middle pots are wired with their wiper shorted to their cw end so they will only read full value when set to '10' and they will read zero set to '0' If the treble pot is also set to '0' there will be no signal and the resistance from treble leg 3 (cw) to ground will measure 250k (the resistance of the pot)

[misterc57]

My 734K does not quite sum the three pots (250kb, 500kb, 20kb). I will be pulling the board out tonight for further testing and inspection. I will also see if I can get the white gunk off the board.

Thank you for all the input so far, it all helps! Please keep it coming! MC

[misterc57]

Removed the yellow glue where it was making contact with resistors and small caps. Noise is gone. Amp is very quiet now. Yay!!!

However my bass and middle pot still act like a volume control. Do you think this is by design?

Thank you all for the help! Very happy this amp is no longer making noises!

MC

[Chuck H]

Removed the yellow glue where it was making contact with resistors and small caps. Noise is gone. Amp is very quiet now. Yay!!!

However my bass and middle pot still act like a volume control. Do you think this is by design?

Thank you all for the help! Very happy this amp is no longer making noises!

MC

Looking at the schem that shouldn't happen. However, the preamp stage that clips is AFTER the tone stack. This absolutely does mean that the tone controls affect the distortion level when overdriving following stages. This would be especially apparent in "bright" mode as you note in your first post. But this shouldn't stop signal all together. There is definitely something wrong with the pots or wiring in the tone stack.

[misterc57]

I should add that the tone controls are acting fine as tone controls, but also acting as volume controls.

I Will dig deeper into the schematic and circuit.

Thanks

[SunDogAZ]

My 734K does not quite sum the three pots (250kb, 500kb, 20kb). I will be pulling the board out tonight for further testing and inspection. I will also see if I can get the white gunk off the board.

Thank you for all the input so far, it all helps! Please keep it coming! MC

I know this is an old thread, but wanted to comment on pot values. If you measure a pot that is not connected to anything, just the loose pot, you will find that they rarely measure exactly their stated value. Same goes for capacitors. They can vary +- 10% to 20%, and usually measure less than stated value.

I have bought bags of pots for guitar electronics. And I will usually measure each one, then mark the pot with the measured value. Usually, pots with tighter tolerances (+-5%) will cost a little more, and are used in higher quality guitars, amps, etc.

In your case, 734 / 770 = 95.3%, so that is actually pretty good, less than -5%. It would be interesting to know what each of those pots measure out to be.

Just my 2 cents.