Carvin Nomad with no sound

[Puddles]

Hi all,

Recently I was given a Carvin Nomad that apparently is relatively new however wasnt working - I accepted it assuming I'd be able to sit down and work out what was wrong with it. Without having ever owned a tube amp, I thought it might be a nice addition if I could get it working. Inherently this also means I don't have much experience with these things, so please bear with me...

Turns out there was a cracked preamp tube (12ax7) so I thought alright, I guess that's all that's needed. So I went out and picked up a new tube and put it in.
Sure enough - sound! Sweet. Plugged in and had a play, sounded good, nice and warm compared to my solid state Fender M-80.

Two days later, I switch it on, lights come on, warm hum rolls in and I go to play again - nothing.

Cutting the story short - It looks like only the tube in the socket that I replaced is glowing while now none of the other ones are. I don't think the tubes are bad as I switched each one into the working socket and they all took turns lighting up while the others didn't.
The tube position in question is the furthest one away from the input.
There's a faint hum and what sounds like the ocean rolling in the distance.

The only other things I can provide is that the amp hadn't moved since I got it working. I did accidentally leave the power on (but on standby) for a few hours after playing before I realised. I have tried another speaker resulting in the same symptoms. I have also tried plugging directly into the FX return to no avail.

I'm not too bad with a multi meter, but I work on cars normally, so PCB's are a bit unfamiliar territory - is there anything else I can try or test?

 

[wanderin kind]

are the power tubes glowing? they run off 6.3 AC, while the preamp tubes run off DC, if i have the right schematic.

chances are when you left the amp on, the filter caps got hot and started to suck voltage, or the rectifiers blew (see schematic, blue circle) ,

most likely it is the caps, (red arrows)

try starting the amp with the tubes installed after it has cooled off overnight and see if the tubes light then slowly go dim,

it could just be the solder connections on the caps, if the pc board traces get tweaked, then the caps do not raise the pulsating dc to near peak voltage, so you might have 4 volts on the tubes, but not 6.3 like you need, feel the tubes which are faint, do they feel warm at all? if so, you need to fix the pc board traces to get that voltage back up high enough to light the tubes,

the reason they use a DC supply is to heave less hum. if you have a soldering iron and can figure out how to lift the pc board hi enuff to touch up the joints, then you are in luck, if not, somebody else will have to do it and they might want money.

 

[Puddles]

Yep, the power tubes look to be glowing as I would expect is a normal operating brightness. The preamp tubes that aren't lit feel cold to the touch and no indication of being lit at all.

Schematic looks correct, however mine has the addition of V3 as well

I've just tested the rectifier and am getting a reading of 052 in both directions, I assume this indicates being blown?

I should be able to lift the board myself, looks to be a couple of screws and just the pots on the front for the most part. I'll double check the solder of both the rectifier and the caps in question (along with everything else while I'm at it) and give some feedback.

Thanks

 

[Solaris moon]

Here are a couple of "quick fixes". Is there a " Slo-blo" fuse in it? Does it have a hum pot in the back to adjust the amount of sizzle going through the speaker? Try those first. It sounds as if you might need to BIAS those tubes. That's the last thing I'd check. Let us know what goes on.....

 

[clintj]

Quick question: what diodes are they using for that circuit?
I've seen poor choices made by reputable companies as far as current capacity before, and it becomes a repeat repair issue.

 

[Platefire]

Leaving the amp on standby for a few hour hurts nothing--only heater voltage is running and high voltage DC is cut off. The preamp tube furthest away from your input would be the phase inverter tube just next to the power tubes. If it's messed up, it will not drive your power tubes. If it was working before, sounds like a short. I would first try some contact cleaner (Deoxit 5) on the tube sockets and tube pins. One corroded connection could cause it to do that. Also a preamp in/out jack with a switch on it could do that. Put contact cleaner on a 1/4" plug and push in and out of each input/output jack.

Also think of exterior issues---like a bad cord or a guitar cutting in/out. Eliminate all the possible exterior things before opening up the amp. Platefire

 

[wanderin kind]

diodes are blown, change 1n4007 to DGP30

 

[Puddles]

Thanks for the reply's everyone. Looks like I've got a good handful of things to check today.

Here are a couple of "quick fixes". Is there a " Slo-blo" fuse in it? Does it have a hum pot in the back to adjust the amount of sizzle going through the speaker? Try those first. It sounds as if you might need to BIAS those tubes. That's the last thing I'd check. Let us know what goes on.....

Looks like the fuse is good (only one in the circuit from what I can see) No pots on the back for adjustment - I'll double check biasing as a last resort I think as the amp is basically stock and relatively new (couple of years) But I wont rule it out.

Quick question: what diodes are they using for that circuit?
I've seen poor choices made by reputable companies as far as current capacity before, and it becomes a repeat repair issue.

diodes are blown, change 1n4007 to DGP30

Schematic states that "All diodes are type 1n4003 except as noted" - seeing as there's no note on the circuit in question I'm assuming that's what they are. I'll be pulling them from the circuit to test properly and will confirm/replace with a substitute if required.

Leaving the amp on standby for a few hour hurts nothing--only heater voltage is running and high voltage DC is cut off. The preamp tube furthest away from your input would be the phase inverter tube just next to the power tubes. If it's messed up, it will not drive your power tubes. If it was working before, sounds like a short. I would first try some contact cleaner (Deoxit 5) on the tube sockets and tube pins. One corroded connection could cause it to do that. Also a preamp in/out jack with a switch on it could do that. Put contact cleaner on a 1/4" plug and push in and out of each input/output jack.

Also think of exterior issues---like a bad cord or a guitar cutting in/out. Eliminate all the possible exterior things before opening up the amp. Platefire

That's the one! Couldn't think of the name. This was the one that was blown, but the others were fine. As soon as I put a new tube in, the others dont work.
Leads and instrument are all good as I checked them with my other amp.

 

[Solaris moon]

As soon as I put a new tube in, the others dont work.
Leads and instrument are all good as I checked them with my other amp.

Um THAT sounds like someone has wired the value (tube) socket backwards!! I'd spray it with some type of electronics cleaner. That might help, but if it doesn't you might have to open up the chassis to see what might be wrong. You also might have a shorted diode, or resistor. See with TUBE AMPS there are a lot of things that can go wrong which is why I decided to go solid state after 29 years of tubes I couldn't bring myself to waste any more money on the damned things or put up with the shenanigans of obsolete tube technology!

 

[clintj]

+1. The 1N series is a bit too light duty for powering three tubes IMO. The heater current inrush when you switch on has a good chance of killing a diode.

diodes are blown, change 1n4007 to DGP30

 

[Puddles]

Um THAT sounds like someone has wired the value (tube) socket backwards!! I'd spray it with some type of electronics cleaner. That might help, but if it doesn't you might have to open up the chassis to see what might be wrong. You also might have a shorted diode, or resistor. See with TUBE AMPS there are a lot of things that can go wrong which is why I decided to go solid state after 29 years of tubes I couldn't bring myself to waste any more money on the damned things or put up with the shenanigans of obsolete tube technology!

The strange thing is, is that it did work and sounded great. It was only when I turned in on a couple of days later it all stopped working. I'll give them a clean anyway and see how we go. Additionally, I did also tighten the sockets in the hope that it was a loose connection between the pins.
I must admit, I do love my M-80 Chorus I got of my father. Especially with the Ibanez. I was given this amp as the guy couldn't be bothered fixing it... it would be nice to have working though.

+1. The 1N series is a bit too light duty for powering three tubes IMO. The heater current inrush when you switch on has a good chance of killing a diode.

Well it definitely seems like these are the problems as I checked the other diodes on the board and they all read "healthy" (Current one way and a "1" the other). I'll desolder them today and replace them with something else

 

[Puddles]

Update: I've desoldered the rectifier diodes and checked them in isolation, they're reading healthy... I'll have to leave it for today now. Would be nice to have it working again, but no real loss if I can't

Update 2: I soldered the diodes back on just to put things back to where they were... plugged it in for my own amusement. We have sound! Clean is back and sounding fine, however still no sound from drive. Speaker noise increases with volume so it's trying to do something.
The fireplace also spontaneously caught alight after trying to get the fire going all morning. Must be the full moon.

Update 3: Drive's back after giving V3 a slight push. So either a dodgy solder in the rectifier, or one of the tubes wasn't sitting right (I had them all out when taking the pcb out)

Thanks for the help everyone, while the cause isn't known specifically I learnt something new.

 

[Platefire]

I've always kind of admired those Carvin Nomads. Think you probably got yourself a pretty good tube amp. I'm a DIY tube amp builder and love good tube tone. Platefire

 

[Puddles]

Nice, well you're probably the first person I've heard to say something good about them. The clean sounds really nice - I just hope it hangs on for me now!

 

[wanderin kind]

Solder all the tube sockets.
Wave solder tanks are set so that the boards do not get too hot and warp when they pass through, companies go cheap and stuff the sockets instead of applying solder mask and hand soldering large parts that take more heat to properly solder, thus, lots of cold solder joints appear after a while on large parts like pots and tube sockets, so the first thing i do on any pc board amp is hit all sockets, pots, and in/out jacks and big filter caps and power resistors.