Vox Cambridge 30 Reverb busted

[zedrokz]

So I have a Vox Cambridge 30 Reverb that was bought in japan and works with 120V. The normal line gives 230V here in Sri Lanka. I normally use a step down converter to get the 230V to 110V.

Now a week back i left it at the studio i practice because i was having no way of taking it home that day. The next day when i came to pick it up, some idiot had straight away plugged the Vox into the 230V and had busted it. I brought it home and opened it up and found the fuse blown out. So i replaced it.

After replacing it the on Led was working and the speaker gives out a hum, no matter what knob i turn the volume doesn't change, the hum stays the same. the problem is that although i have some electronics skills, I don't know about Tube amplifiers. So i need some guidance on what and where i should check for faults or burnt out components..

[52 Bill]

After replacing it the on Led was working and the speaker gives out a hum, no matter what knob i turn the volume doesn't change, the hum stays the same. the problem is that although i have some electronics skills, I don't know about Tube amplifiers. So i need some guidance on what and where i should check for faults or burnt out components..

Is this a tube amplifier or is the output stage a power amp chip?

[zedrokz]

Is this a tube amplifier or is the output stage a power amp chip?

This is a tube amplifier.

I have attached the service manual.

[Fragger]

I believe this amp to be a solid state amp with a "12ax7 tube in the preamp section to enhance the tone of channel two."

The VOX Showroom - The Vox Cambridge 30 Owner's Manual Page One

[Enzo]

Indeed, this is a solid state amp with one tube in the preamp. It is not a tube amp.


My impression is that your output IC has probably failed. That is the 5-legged TDA2050 on the het sink. I'd wager it is putting DC on your speaker. And that is not good for the speaker.

But the approach to take is to disconnect the speaker, then start with power supplies. The power amp has two voltages. I don't know what they are exactly, but they will be somewhere close to +30v and -30v. Are those present and free of ripple? Then the preamp and stuff runs on a pair of 15v supplies zener derived from the 30v. Are both of those OK? and finally, there is a further drop to a pair of 9,1v zeners for the tremolo supply. Are those OK?

Look at the output pins for the speaker. Is there DC voltage there?

[zedrokz]

Thank you Enzo for your suggestion. Few hours ago i busted my multimeter(a cheap Chinese one) trying to read the voltage at the transformer HV output. i just wanted to see if that part was working because the tube didnt light up when it was powered(my dad said that it should light up). Next i checked the continuity of the heater pins of the tube to check if it is ok and it is ok.

I will check these voltages tomorrow after borrowing a friends multimeter and will let you know about my findings. Thank you for your guidance.

[Randall]

If the tube isn't lighting up, the first thing you need to do is measure the voltage on the heater winding, pins 4/5 and 9. Supposed to be 6.3vac.

[Mick Bailey]

If the tube isn't lighting up, the first thing you need to do is measure the voltage on the heater winding, pins 4/5 and 9. Supposed to be 6.3vac.

Is that right - thought they had a DC heater supply?

[Enzo]

Yes, the tube heater has a DC supply.

The tube is only in one channel, so if the amp only makes hum on both channels, the tube is not the issue. The tube may well be bad, but won;lt be the only problem. Since the amp took double voltage, the tube heater could be even burnt out. It may also simply be hard to see. Turn out the room lights and see if the glow can be seen.