Help with VOX AC4C1-12 - damaged during transport (SOLVED!)

[TonyClifton]

Hi guys, hope you can help. I sold a VOX AC4C1-12 a week ago and the buyer received it.... no sound....

The buyer insisted I ship in a box without removing the valves, big mistake. Shipped super well packed. I had the buyer remove the chassis and send it back to me. Tubes OK, jewel light ON, no loose or disconnected cables, but super low and distorted output (you can hear the guitar very very faintly and super distorted when the amp is cranked).

I threw in a fresh new set of valves before trying anything else and same issue - super low output and fizzy/distorted sound. Next step is to start prodding with a chopstick see if that does the trick and I find the culprit. The only time this happened to me was with a vintage Fender amp with a blown screen resistor on one of the power tubes, don't think this is the case.

Has anyone ever experienced this? I'm going to use the signal tracer to see where the signal stops, but I would greatly appreciate any feedback about this issue if you have experienced this.

Thanks guys!

[shooter]

was it working fine before?   claim damage n call it a day
have you worked on it before?
got a schematic?


this is the only AC4 I have for reference

[TonyClifton]

was it working fine before?   claim damage n call it a day
have you worked on it before?
got a schematic?
this is the only AC4 I have for reference

Hi Shooter,

I've shipped loads of amps before and never had a problem... so this one I send uninsured. Claiming insurance in Spain is virtually impossible, I simply pack very well.

Yes, the amp was working fine before I shipped it. Never worked on the amp before, I purchased it second hand but I would say it is mint, came with the original box, almost untouched. Great sounding little amp by the way.

Thank you very much for the schematic, it's the correct one! Mine is an AC4C12 but I guess the model name is different because of the speaker, but I'm checking and it has exactly the same components.

Your help is much appreciated :)

[Latole]

Low output, distortion ;if you replace tube by know goof ones : IMO it is a voltages issue. Check carefully the power supply : all B+( tubes plates )  and grounds.

Wouldn't the amp be an AC4C1-12 It have 2 preamp tubes. AC4 have one.
For someone experienced in amp troubleshooting, this is of little importance.

I don't have a better picture. It is a screen print from here ; https://www.tdpri.com/threads/vox-ac4c1-schematic-and-mod-ideas.461204/

[TonyClifton]

Low output, distortion ;if you replace tube by know goof ones : IMO it is a voltages issue. Check carefully the power supply : all B+( tubes plates )  and grounds.

Wouldn't the amp be an AC4C1-12 It have 2 preamp tubes. AC4 have one.
For someone experienced in amp troubleshooting, this is of little importance.

I don't have a better picture. It is a screen print from here ; https://www.tdpri.com/threads/vox-ac4c1-schematic-and-mod-ideas.461204/

It is an AC4C1-12 indeed, my bad, I edited the post, thanks for pointing that out.

I would say the schematic provided by Shooter is the correct one. Mine has two preamp and one power tube

Probably the only difference with the other model is the cabinet/speaker size?

Thank you for the schematic anyways, I will check the plates and grounds first. Just wanted to know if this ever happened to someone else before and where to look first.

Thanks a lot !

[Latole]

https://www.voxshowroom.com/uk/amp/ac4c1bl_hood.html

[Latole]

Shooter's schematic  ;

[TonyClifton]

Shooter's schematic have just one preamp tube not 2 as the one I show

You may not know , a preamp tube have two section in one tube.

Shooter's schematic has two preamp tubes, V1 and V2 :)

Yes, the two "sections" appear in the schematic - you can see the plate, grid, cathode and filament of each triode.


Yup the image of the voxshowroom looks just like my amp (chassis, not cabinet, mine's a 12")

[Latole]

. Just wanted to know if this ever happened to someone else before and where to look first.

Thanks a lot !

The same problem can have different causes. Other people's experience means nothing.
Follow my previous advice: check the voltages.

And before you get an answer, if there is one, you have plenty of time to solve the problem.

[Latole]

Shooter's schematic have just one preamp tube not 2 as the one I show

You may not know , a preamp tube have two section in one tube.

Shooter's schematic has two preamp tubes, V1 and V2 :)

You are right , sorry . I will edit  my answer

[TonyClifton]

You are right , sorry . I will edit  my answer

No problem Latole, thank you very much for your time in trying to solve this mystery!

[Latole]

First check voltage at  the 22K resistor and report. The last one on the power supply.
R6b on the Shooter schematic.

[TonyClifton]

First check voltage at  the 22K resistor and report. The last one on the power supply.
R6b on the Shooter schematic.

Thanks Latole, here's what I've got:

Heaters OK 6.3VAC
Plate voltages preamp tubes OK (190-200VDC)
Plate voltage power tube OK (300VDC)
B+ OK 315V
R6 OK (200 VDC)

I guess it's signal tracer time....

[Latole]

 "....super low output and fizzy/distorted sound. "

Do you have a scope and audio signal generator ?

Signal trace may not tell where you have the issue

[kagliostro]

Give a try to this

Refresh all the soldering, may be the problem is a frozen solder

One other thing may be a pin in a socket that must be tightened

The speaker if connected to one other amp performs correctly ? 

Franco

[Latole]

First check voltage at  the 22K resistor and report. The last one on the power supply.
R6b on the Shooter schematic.

Thanks Latole, here's what I've got:

Heaters OK 6.3VAC
  ....

Check at each tubes.

[Latole]

Output transformer issue ?

[kagliostro]

Also I've think to an OT problem, but seems odd that a transport damage an OT


Franco

[Latole]

Also I've think to an OT problem, but seems odd that a transport damage an OT


Franco

Right ! 

As for the other parts, transport cannot damage the printed circuit board or any other components either, unless the buyer has received the box very damaged. This has not been mentioned.
The transformer: has the buyer misused the amp by connecting it to the wrong device?

[tubeswell]

buyer misused the amp by connecting it to the wrong device?

Or plugged the wrong tubes into the wrong sockets.


Or any number of other possible situations that resulted in the amp getting broken.


That they are not admitting.

[Latole]

buyer misused the amp by connecting it to the wrong device?

Or plugged the wrong tubes into the wrong sockets.


Or any number of other possible situations that resulted in the amp getting broken.


That they are not admitting.

Right.

I'll try another speaker.

[TonyClifton]

Thank you all for your time!

Tubes were plugged in correctly, I hope it's not an issue with the OT! the amp is almost brand new, I bought it from a guy who had it in the box for years. And I've played it a lot with no issues whatsoever. It stopped sounding after shipping.

Package arrived safely, with no damage or signs of being knocked around. I had the seller remove the chassis and send it back to me.

I have the amp right here with me, tried with a new cable, with my test valve set (a fresh new tested set that I only use for testing amps), plugged to a new speaker...

I will try to spend some more time with the amp because this is a mystery to me. @kagliostro mentioned a cold solder joint, that could be the case, very rare that this happened after working perfectly, but who knows... or maybe during transport something vibrated too much... dunno.

Thanks guys!!!

[kagliostro]

Give an inspection also to the input jack

Franco

[TonyClifton]

One other thing may be a pin in a socket that must be tightened

Refresh all the soldering, may be the problem is a frozen solder

BINGO 

OK; so this is definitely a first.... never ever ever come across something like this!!

I had never had to tighten pin sockets on a new amp.... but I did that first. I also tightened the fuse holder, it was wiggling a bit inside, but it read continuity in the tester anyways.

I started prodding with the chopstick and found two issues, dunno why/how the hell this happened:

- R8 plate on V1 wiggled about too much, when injecting signal it sorta kicked the sound in and out... intermittent error, so I resoldered that

- R19 on the treble pot similar issue, I prodded with the chopstick and suddenly BOOM - sound! Resoldered that.

Working like a charm now 

BIG THANK YOU GUYS! Thank you for helping me out with this one

[Latole]

Congratulation !

[Willabe]

I had never had to tighten pin sockets on a new amp.... but I did that first.

The metal they use on new tube sockets doesn't seem to have the strength/memory to stay in place when their formed. 

[kagliostro]

 

Franco

[TonyClifton]

The metal they use on new tube sockets doesn't seem to have the strength/memory to stay in place when their formed. 

Interesting... I'm not an expert by any means and I repair few amps a year, but this is a first for me. I've had to tighten sockets in old amps, but never in a new one !!! I think it might have been a combination of this and the solder joints on the resistors.

It's sad to see how the quality has dropped... I simply cannot believe that shaking an amp during transport would cause a bad solder joint? I've sold/shipped plenty of amps in the past 20 years, this is the first time this happened. I've even had amps shipped from the US with the tubes on, sent inside a box with no protection whatsoever and they arrived fine! But mainly vintage Fenders that are built like a tank

Franco

Thank you Franco! Great tips

[kagliostro]

May be that the problem was present only partially and shaking the circuit it revealed his presence

Franco