How To Fix VOX AC15C1 No Output

Totally dead on the output. I went down a slight blind alley on this one but eventually got it working!

[@merwinjm]

I just watched a psionic video of repair and upgrade of the AC15C1 I guess he's seen a few of these and it's a common failure mode. He recommended preemptively replacing the 470 1W screen resistors at R76 and R83 with 1k 2-3 W and mounting them off the board a touch to get some additional cooling as well as hopefully protecting the traces below if the resistor does fail.

[@stuartukguitarampguy5830]

Ok thanks John, useful info.

[@alexdeleon7135]

This episode was a great admission to the trials and tribulations experienced by amplifier techs. Hopefully this justifies exactly why the cost of repairs are as such. I believe your inclusion of the red herrings [sic] were necessary to demonstrate that even with a schematic handy, the repair doesn't always fall easy. Stay safe, and I'm happy to hear you have plenty of amps to sort out.

[@stuartukguitarampguy5830]

Thanks Alex!

[@MrSpacejase]

I am going through this now. My brother found the same type of amp in the trash. It is in near mint shape like the one in the video. Very strange to throw a vox AC 15 out. All of the tubes light up but no sound. I took the back panel off and found that the 250 volt fuse is scorched inside the glass. I assume that it is bad. watching this video, I took another look inside at the circuits and none are burnt. I do realize that it is much more complicated than that. The panel looks very clean. I hope it is just the fuse. Man I was hanging on that last repair you did. Made the sign of the cross..held my breath. lol Victory! you did it. I subscribed. Cheers from the USA.

[@el_micha]

Usually there is a reason the fuse is blown. So possibly its going to blow again since the fault is still unsolved. Good luck!

[@jesselackey5333]

Guess who just bought one of these amps real cheap? Guess who just figured out how to repair it? This guy! Thank you, the first resistor in question is burnt on mine as well.

[@michaelevans3852]

This repair is one of my favorites. I so enjoy watching you chase down the issue when it seems to want to hide. The explanation with schematics is wonderful as it shows how you find the issue and helps us see the value of the schematic. Bravo Stuart!

[@stuartukguitarampguy5830]

Thanks again Michael.

[@bobkraft9029]

I enjoy your videos and do appreciate you showing the full troubleshooting process. Good stuff, Stuart. Thanks!

[@fiddlix]

I don’t know How I missed this one, a brilliant fix in my opinion. Your fix will definitely last the lifetime of that amplifier.

[@stuartukguitarampguy5830]

Thanks David.

[@johnkohlhauff9061]

Great video! Really enjoy the way you diagnose and repair. No smoke and mirrors. You see it as it happens. Thank you!!

[@stuartukguitarampguy5830]

Cheers John

[@rodneykroetsch2924]

Stuart, I enjoyed this one. Shows we are all prone to get snookered now and then. I failed to fix one of these a few weeks ago. After watching this, I may have another go at it....thx!

[@stuartukguitarampguy5830]

Keep at it, I say! Thanks Rodney I hope you get it fixed.

[@4034miguel]

Always relaxing to follow an amp repair. Thanks.

[@alexstrong6310]

Honest old school fault finding! Brilliant

[@stuartukguitarampguy5830]

Thanks Alex.

[@hookydoo]

Hello Stuart! When I am in a situation where I have clearance issues with screws I pull out an old set of right angle ratchet screwdrivers that were given to me by my father. They are still very easy to come by and will get you out of silly design issues like this. Cheers!

[@stuartukguitarampguy5830]

Ooo nice. I must get some!

[@edwardhannigan6324]

Excellent video, red herrings & all..! Good to see the faults unfold and your technique in solving them. Thanks for sharing your knowledge..Ed..uk..😀

[@stuartukguitarampguy5830]

Thanks Ed I always appreciate your feedback.

[@ferraridinoman]

Nice one Young Stuart!! No wonder ALL you amp repair guys HATE Circus boards!!!

[@stuartukguitarampguy5830]

YEs we defo p[rfer point-to-point but of course a pcb makes for far cheaper construction and hence lower price in the shop. I'm not dead set against pcbs. Surface mount though, that's another story!

[@rgbplumbinghilton]

Another great video. Knowledge is everything in most situations 🙂

[@stuartukguitarampguy5830]

Thanks Richard!

[@tubejay1]

Thanks for this. I'm a bit of a novice and have an AC30CC2 that I'm working on. I'm watching all of the VOX AC repairs I can find, and it definitely helps to see the though process and red herrings.

[@stuartukguitarampguy5830]

Great. We all started as novices. The best way to learn is hands on. Good luck.

[@chrisdee5032]

What are you doing to the AC30CC2?

[@3MonkeysGarage]

I have run into that problem with resistors myself. I love it when you sit on a project a week waiting on a new resistor to come in and the original turns out to be fine. Something to do with the solder I guess.

[@Avi8torDavy]

Thanks again for another honest troubleshooting video! I know I made a similar mistake with some of the modern blue resistors that I measured before putting in an amp. They had the resistance reading I was expecting to see on the meter, except it was supposed to be in K ohms and I didn't catch it. It cost me quite a bit of time in troubleshooting the amp later, but lesson learned.

[@not28]

that was pretty interesting. thanks for sharing with us.

[@stuartukguitarampguy5830]

I'm glad you enjoyed it.

[@stefanomezza9638]

Great Stuart. You are an honest man.

[@stuartukguitarampguy5830]

You obviously haven't seen my secret stash of stolen biros and legal pads....

[@gmonay2k]

Great video. Will be troubleshooting a VOX Valvetronix AD120VT. I get sound out of it (minimal) but the master and gain have to be all the way up or there’s nothing. There’s also a power knob on the back that needs to be up half way in order to get sound.

[@stuartukguitarampguy5830]

I haven;t had a lot of luck with Valvetronix as they are very modern with surface mount components etc.

[@Patbwoy]

There must be a reason for the resistor to blow in the first place. So I wasn't surprised it didn't work right way after changing that resistor.

[@s9plus20]

Nice job!! Excellent video!

[@stuartukguitarampguy5830]

Thanks Frankie

[@mitchmason8386]

Where did you find that schematic? Great video thank you.

[@stuartukguitarampguy5830]

Do you need it?

[@tomfoolery2082]

I have an ac10 which i like . I like how this one looks inside better . It appears to be higher quality, more professional. But what would i know .

[@stuartukguitarampguy5830]

There are SO many different versions of VOX amps tbh, it's hard to know where to start!

[@roygangoo1744]

thank you for sharing

[@stuartukguitarampguy5830]

Thanks Roy

[@dennisflock3958]

Good morning from Chicago!

[@stuartukguitarampguy5830]

Ah... been there loads of times. Spent hours in the science museum, Field museum etc. Had a good friend in Hyde Park.

[@Shiloh7377]

Great job!!! I loved tha comment about superman...

[@stuartukguitarampguy5830]

Where's my cape when I really need it!?:

[@Shiloh7377]

@@stuartukguitarampguy5830 it's quite refreshing to watch a video by someone who includes themselves making a mistake as well as doing good. To many people wanna show tha good not tha normal ,ya know.

[@weschilton]

@@stuartukguitarampguy5830 No capes! ;P

[@stringlocker]

Yeah that was a great comment. I've learned if I fail on the first pass instead of troubleshooting what I just did for me it's better to start over from the beginning again. I used to Chase my tail but I've learned.

[@Shiloh7377]

@@stringlocker I've done that a few times. I messed it up and started fresh....

[@rods4548]

Would you know the value of the MOV or the ICL surge protector of the Vox AC15C1 soldered between the A/C receptacle in the back and the on/off switch? Thank you!!

[@zbaby82]

Great video. Yeah some say that the Chinese (probably Shuguang) valves damage circuits when they fail. I have use two sets and they sounded really well. But the second set blew a fuse after only a couple of hours of play. So I will no longer buy them. I wish the British and Americans would start making tubes again.

[@weschilton]

Yep, same here, one of the EL84s fried itself within a few days of owning the amp. Fortunately I heard it happen and turned the amp off so I didn't lose anything else on the board. I took out all of those crap tubes and replaced the 12AX7s with JJs and the EL84s with newer (russian) Mullards and happy days.

[@stuartukguitarampguy5830]

There's a worldwide shortage of tubes at the moment with insane lead times!

[@tjsogmc]

@@stuartukguitarampguy5830 ..smiles in solid state.....

[@goodun2974]

@@tjsogmc , I have many hundreds of NOS tubes and its uncommon for me to have to buy any from Russia or China.....

[@watkinscopicat]

Old tubes can do that too! i had a set of Mullards in my Laney and they blew the transformer because of the different way they’re wired compared to modern EL84s

[@pda49184]

Great video Stuart.. Many thanks ..

[@stuartukguitarampguy5830]

Cheers Pete!

[@Jedum]

amazing work sir! question, I just noticed on my circuit, there is NOTHING connected to the J12 pin...I see yours is connected, but to what? also, would that have any major repercussions? at the moment I have no signal and I'm trying to figure out why and find it strange there's nothing there...

[@stuartukguitarampguy5830]

Hi Sorry but it's been a couple of years since I did this amp so can;t remember. Good luck.

[@peterm3964]

So much for the legend of the AC15 . Modern reissues are NOTHING LIKE THE ORIGINALS . But hey they are cheap …. and nasty .

[@stuartukguitarampguy5830]

Agreed. Mind you VOX has had a long and convoluted history of ownership.

[@kookoogearkook]

Great vid 👍

[@stuartukguitarampguy5830]

Many thanks!

[@goodun2974]

Shouldn't a red herring more properly be called a red kipper on your side of the big pond?

[@stuartukguitarampguy5830]

Only if smoked. And with some of these amps, that might actually be the case!

[@goodun2974]

Stuart, you used a carbon film replacement screen resistor? Should be metal oxide or metal film. I probably would have gone ahead and replaced the other screen resistor while I was at it just because of how badly the board got scorched when the first one burned up. Why risk making the board conductive? Modern amplifiers, especially Vox amps, have a reputation for eating crappy modern tubes and the chances are good that this amp will again burn a screen resistor or two at some point in the future.

[@stuartukguitarampguy5830]

Yes I did think of that but for some reason didn;t do it.

[@wesleyc.4937]

Good job, old boy.

[@stuartukguitarampguy5830]

Cheers. Less of the 'old boy' if you don't mind. I'm only 29....

[@wesleyc.4937]

@@stuartukguitarampguy5830 I've got a bigger problem than you, I only feel 14!

[@stuartukguitarampguy5830]

@@wesleyc.4937 Ah that does creep up though. When I was 40 I felt about 25. 50 I felt about 30. 60, maybe 35. Just hit 70 and feel about 40-45. Stuart

[@wesleyc.4937]

@@stuartukguitarampguy5830 I'll settle for 50 any day of the week. Cheers🥃

[@clivesilk3501]

I just bought a dead vox amp just like this from A carboot sale for £30 . I hope I don’t have to all of this to it 😢

[@stuartukguitarampguy5830]

Hi CLive Let's hope it's something simple!

[@clivesilk3501]

i was lucky it was just the preamp valve ECC83 ! im very happy @@stuartukguitarampguy5830

[@KarenBasset]

How much did this repair cost? I may need a similar repair done to my amp. Thank you for the video!

[@stuartukguitarampguy5830]

Hi Karen I think it was about £75. Are you thinking of bringing the amp to me?

[@petercornell2002]

Excellent video, thanks Stuart

[@stuartukguitarampguy5830]

Glad you enjoyed it Peter.

[@markjamesgoddard2247]

👌👍😊🎸Well Done Sir🤛

[@stringlocker]

Sometimes it seems they do it backwards. They build the cabinet around the board.😃

[@stuartukguitarampguy5830]

Aaagh! Shoot me!

[@gamjammallota78]

Did he have both hands inside the chassis of a live amp whilst measuring valve currents at the same time in a finger reach of capacitors at about the 20:28 mark? Well, I suppose being as young as he is, his youth will protect him from any sort of charge unlike myself. I'm way too old for that...but back in the day, I'd just touch it to me lips or tongue to see if it were live or not.

[@stuartukguitarampguy5830]

Ha ha, yes I remember the old 9V battery on the tongue trick to see if it had any charge left! You'll find that any tradesperson with decades of experience will do things not recommended for a beginner or someone less experienced. Been at it 50 years now so know to keep myself out of danger. All the best.

[@gamjammallota78]

@@stuartukguitarampguy5830 I've been bitten more than once by my microphone whilst playing in the old days. Keep up the good work. By the way, I still use the 9v tongue trick. Cheers.

[@jn1lfd]

You may be satisfied, but in my case, I reject that approach. I prefer to remove and replace the PCB as normal. Do not use lack of tools as a reason. You should have more carefully first considered why the grid resistance burned in the first place. You can still get a TV-7 (vacuum tube tester) for a reasonable price.

[@tjsogmc]

Well now, I'm happy to say that I've never ever had a VOM on the wrong setting or read a number incorrectly....or misread a part number.... (eye roll)

[@stuartukguitarampguy5830]

Yes that was an absolute first for me too.... cough....

[@goodun2974]

Yup, I've done it too. Or, having the meters set to DC when you're looking for AC or vice versa. Worst case scenario is having the meter in current measuring mode and trying to measure voltage with it and either blowing the internal fuse or blowing up the meter! As for erroneous resistance or voltage measurements, an auto-ranging meter can be helpful but sometimes the display bouncing around and taking a couple of seconds to settle down on a reading turns out to be annoying.

[@papahjoshscene4234]

I have a vox vt30, but now there is a problem. if I play for 30 minutes, the sound suddenly decreases and hoarse. what's the problem?? Please help me. Thank you

[@stuartukguitarampguy5830]

No Idea I'm afraid. I just had a VT in for repair and I couldn;t fix it. These amps are all micro- miniature surface mount components and not easily repairable. Good luck.

[@ryanwatson2007]

Hi, I am having the same problem as what you had at the 4:00 minute mark. I put in a new resistor with the same value and it burnt out as well. Would the tube possible have anything to do with it? I noticed you swapped them out as a precaution.

[@stuartukguitarampguy5830]

Hi Ryan Yes definitely suspect the tube in this case. Those are 470ohm 2W screen resistors.

[@ryanwatson2007]

@@stuartukguitarampguy5830 thank you for the response. I actually swapped the tube out and put in another resistor and the amp sounds good as new. So thanks for a great video!

[@larriveeman]

can you link the schematic?

[@stuartukguitarampguy5830]

Hi there's a splash page at the end of each video and my email is on it. Please contact me via that.

[@vincentl.9469]

What annoys me looking at this is how close the tubes are to the speaker ..there has to be a better way. These things get hot inside too-as do most Vox

[@stuartukguitarampguy5830]

Yes I come across SO much basic bad design.

[@vincentl.9469]

@@stuartukguitarampguy5830 The AC10 C1 is another example . That closed back with 2 small vents. That gets warm. In hot weather, it gets hot. .but I've raised this on some forums and people suggest it's not an issue. It comes down to price point and as long as people keep buying them , Vox wont change anything.

[@motokev2727]

Hmmm, I better change my power tubes frequently on this amp.

[@krauz111]

I assume companies just expect people to throw away amps and buy a new one instead of repairing them. The design is terrible for techs

[@stuartukguitarampguy5830]

Yes you've got it in one. Blackstar are shockers for this. They've stopped selling replacement boards for their HT1 and HT5 which means THOUSANDS of them are heading for landfuill.

[@1Dougloid]

Just had one, same deal, I got lucky-bad new JJ preamp tube section.

[@stuartukguitarampguy5830]

Nice one, well done. Got a nightmare at the moment. Marshall DSL40CR. Some sort of sticking relay problem on the O/D switching. Can;t find a schemtic and there are 6 relays in there!

[@1Dougloid]

@@stuartukguitarampguy5830 Is that different from the DSL40C? I have the DSL40C schem.

[@stuartukguitarampguy5830]

@@1Dougloid Yes quite a bt different, it's a much more modern amp. Amazngly I did manage to get the schematic from a friend!

[@1Dougloid]

@@stuartukguitarampguy5830 Great. I get that it's different, but is it better? Do they use an eprom for the switching commands?

[@stuartukguitarampguy5830]

@@1Dougloid I don;t thin it's an EPROM but it's some fancy digital switching I don;t fully understand! No idea if it's a better amp.

[@hippyamp]

did you check bias after new tubes?

[@stuartukguitarampguy5830]

I can;t remember tbh. I usually do though.

[@user-rf9mx9tu1d]

Why don't you use Russian lamps, are Chinese ones better?

[@stuartukguitarampguy5830]

I dont use chinese. There are no chinese valves any more since the factory burned down 3 years ago. I usually use JJ (Slovakia) because I used to sell them and hence have a lot of stock. Rusian valves are good too.

[@infectionsman]

That amp is a bad design, they should have put 1k 2w instead of those pesky 470r 0.5w They also should have mounted them with long legs so they were up in the air away from the board.

[@stuartukguitarampguy5830]

Yes it's so annoying to see such basic design errors time and again.

[@infectionsman]

@@stuartukguitarampguy5830 I recall these amps eating power tubes by overbias too, that 120r cathode resistor should have came stock at 150 ohms. Especially with such cheap tubes.

[@stuartukguitarampguy5830]

@@infectionsman Yes I usually check the bias current and upgrade the resistor if needed.

[@MrVanalen]

What an awfull job 😳

[@chrisdee5032]

???