VOX Night Train Guitar Amplifier Repair

Here’s some photo’s from a recent repair of a VOX Night Train amplifier.

The amplifier was producing no sound.

Curiously the HT voltage on the first pre-amp valve was only 95 VDC, way below spec.

I removed the valves and the HT on the first valve stage was still only 95 VDC. On further investigation I found that there was only 10k of resistance from the HT to ground on the first valve, ie something was shorting to ground.

Finding the problem was an interesting exercise.

The filter cap was removed, tested and given a clean bill of health.

I then traced the HT circuit around the PCB, NOT helped by the brown gunge liberally spread over the board.

After mucho careful scraping I found resistors R23 and R24, which lie over a PCB track with HT on it.

 

VOXNT1

 

On lifting these resistors from the board, the resistance to ground was unmeasurable, and of further inspection, you could see that there was a short between R24 and the PCB track underneath.

 


VOXNT3

 

I replaced R23 and R24 with 1W MF resistors (all I had in the relevant values), and stood them off from the PCB trace to avoid any arcing/ shorting in future.

VOXNT2

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7 Comments

  1. Ned
    Posted September 2, 2013 at 22:32 | Permalink

    Hey,
    As far as I know, that brown gunk is to tell if a part is burned out, it turns black and crumbly if a part fries…
    Anyhow, I got one of those night trains a month ago, DOA, the guy took it to a guitar center tech that just mangled it, totally jury rigged, after fixing that, and replacing all of the burnt components that I could see, and making sure tubes were good, I’m getting no sound, some of that yellow gunk is brown/black right at the edge of where it covered r23 and r24, I’m thinking that it’s the same issue… what did you use to scrape that gunk off?
    curiously enough no fuses are blowing, so I know that it’s not a major short or anything, I was at wits end, thanks for posting this, I’m going to try pull those resistors and see if there’s a short there.

  2. Posted September 2, 2013 at 22:48 | Permalink

    A short in this part of the circuit won’t blow fuses as it can’t draw enough current.

    R24 and R25 are the likely culprits as the HT runs under them. Any current leakage from the HT to these resistors will bias the first gain stage into cut off and hence no signal.

    If you would like a schematic, you can send me an email:

    [email protected]

  3. Ned
    Posted September 3, 2013 at 06:04 | Permalink

    Hey,
    Thanks for replying so fast!
    Yeah, I was hearing a weird pop/crackle through the amp too, very subtle, but definitely sounded like mild arcing, which was strange as I couldn’t see any arcing, But sure enough, under R24 there was a small hole to the HT trace, looks almost exactly like your picture, I’m going to get some new resistors and pop them in tomorrow to see if that fixes it, there was also a trace that needed a jumper wire from R1, I jumped it and checked those resistors, that should fix it all up.

    I’m good on the schematic, I have a nice pdf of it 🙂

    thanks again, and hopefully this helps anyone else looking on google with a similar amp problem :a vox night train that makes no sound despite having good tubes and no blown fuses (just throwing that in there to make it show up quicker on google)

  4. Ned
    Posted September 4, 2013 at 00:52 | Permalink

    Wow, well I’m stumped,
    Even though that resistor was arcing with the HT line on my night train, I still get no sound, I replaced r24 with the correct 6.8k 1/4 watt resistor, and r23 with a 1/2 watt 1.5k resistor and reaised them off of the board. It stopped that weird crackle that sounded like arcing, but now I still get no sound, no blown fuses either, so I guess I’ll just have to keep looking for another short that could be drawing current from v1.

  5. Ned
    Posted September 4, 2013 at 02:23 | Permalink

    New update,
    I replaced R25 and at least started to get sound through it, it’s pretty quiet, I can’t get any distortion, if I have gain on half, I have to put the volume on half to get to television volume, and I’m getting a motor boat puttering/crackling through the amp, regardless of whether I’m sending a guitar signal through it or not, gain on 10 gives me fender cleans, at least now that a signal is going through I can better diagnose things, my HT reads 25 megs to ground, I’m guessing that I missed a capacitor somewhere and dc is leaking where it shouldn’t be or something, either way, thanks, your page got me a step closer to fixing this thing.

  6. Ned
    Posted September 4, 2013 at 02:33 | Permalink

    Yeah, I’m guessing that I may have missed a bad capacitor or something, the choke resistor was burned enough to make the board black on one spot and totally burn away the trace, not sure how the previous owner managed that, anyways, I think that I may have either missed a bad capacitor or a bad solder joint, I’ll check those and then post back,
    thanks,

  7. Ned
    Posted September 4, 2013 at 04:30 | Permalink

    Okay, sorry to waste your time with so many comments,
    But I got the amp up and running, I’ve tested the tubes by plugging them into an epiphone valve jr, so I know that all tubes are good. I replaced r25 as well, and double checked my solder joints, replaced another one of the bad 22uf caps, and now I can get a signal through, but the amp is quiet, I get no overdrive, even with gain on 10, it’s just clean, and stays around television volume, gets kind of loud if I put gain on 10 and crank up the volume, but not like a 15w push pull amp should be, its got me stumped, any idea what could be causing this, these are supposed to be loud gain monsters, but mine sounds opposite right now… considering that could it be a preamp problem? I’m going to check the cathode resistors and capacitors, but I really don’t know what to do at this point, any input from you would be greatly appreciated,
    thanks,
    -Ned

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