Vox Cambridge 30 disaster

[guiguy]

Hi everyone,
I have a Cambridge 30 Reverb that I brought to China and plugged it in to 220V by mistake. I thought that the apartment had sockets wired for 115v because they looked like our Canadian sockets. Anyway, now the thing hums when you turn it on. The led light turns on and the fuse is not blown. Any idea where I should start?


Here are the schematics:

Vox Cambridge30R.pdf

[nosaj]

Hi everyone,
I have a Cambridge 30 Reverb that I brought to China and plugged it in to 220V by mistake. I thought that the apartment had sockets wired for 115v because they looked like our Canadian sockets. Anyway, now the thing hums when you turn it on. The led light turns on and the fuse is not blown. Any idea where I should start?


Here are the schematics:

[ATTACH=CONFIG]46812[/ATTACH]

Well for one do you have 115v to use or still just 220v? First thing I would do is disconnect the transformer from the pcb and check what voltages you have there.
Also does anything look destroyed or smell burnt?

Thanks,
nosaj

[guiguy]

Well for one do you have 115v to use or still just 220v? First thing I would do is disconnect the transformer from the pcb and check what voltages you have there.
Also does anything look destroyed or smell burnt?

Thanks,
nosaj

Nothing obviously burned.
I suspect the noise tells me that there is something not blocking the hum from power source?
I tested the voltages with the tranny connected. There are four wires. I think two were at 140v and the other two were around 20 maybe. I am going on memory...a bad memory.
Pat

[nosaj]

Nothing obviously burned.
I suspect the noise tells me that there is something not blocking the hum from power source?
I tested the voltages with the tranny connected. There are four wires. I think two were at 140v and the other two were around 20 maybe. I am going on memory...a bad memory.
Pat

What voltages do you have at c52 and c53, c60 and , c55 and c54, c58 c59? Take notes also.

Thanks,
nosaj

[DrGonz78]

I suspect the noise tells me that there is something not blocking the hum from power source?
I tested the voltages with the tranny connected. There are four wires. I think two were at 140v and the other two were around 20 maybe. I am going on memory...a bad memory.
Pat

Okay hum might be an indication that there is DC voltage present on the speaker. If that be the case then excessive DC voltage will kill your speaker. So disconnect the speaker and measure across the speaker leads with the meter set to read DC volts. The over-voltage might have damaged the transistors or the TDA2050A power amp IC . If you measure voltages at the capacitors that nosaj pointed out then do it with the speaker disconnected. But foremost check to see if there is DC voltage on the speaker leads.

I am hoping that you are now plugging the amp into a 120v socket and pretty sure you must be. Still the comment about "two were at 140v and the other two around 20 maybe" makes me wonder what we are measuring. I would guess you measured 120vac (primary) and 20vac (secondary) on the power transformer.

[guiguy]

What voltages do you have at c52 and c53, c60 and , c55 and c54, c58 c59? Take notes also.

Thanks,
nosaj

Ok, so here is what I have so far:

Transformer output at yellow wires - 145VAC
Transformer output at red wires - 22.9VAC

Voltage at R92 is 29.5DC and 16.6DC
Voltage at R93 is 29.5VDC and 17.2VDC
Volts across R90 is 16.6VDC and 9.2VDC
Voltage across R91 is 17VDC and 9.2VDC
Volts across C54 is 16.6VDC
Volts across C55 is 17.3VDC

DC Volts at speaker - 39VDC

at this point I cannot access the C56 and C57 capacitors to test them.

it looks like the voltage is a little higher(lower?) on negative side of the power supply. Also, there is DC at the speaker.

What do you suggest checking next?

Pat

[DrGonz78]

Transformer output at yellow wires - 145VAC
Transformer output at red wires - 22.9VAC

Oh okay the 145vac is for the plate of the 12ax7 preamp tube, forgot for a minute that was in there. 22.9vac is the voltage that feeds into the bi-polar power rails on the power amp chip and the preamp.

Voltage at R92 is 29.5DC and 16.6DC
Voltage at R93 is 29.5VDC and 17.2VDC

Note that each side of R92 would be 29.5vdc & 16.6vdc but on R93 it is -29.5vdc & -17.2vdc. It is bi-polar power supply so there will be a positive and a negative rail voltage from the power supply. Good news is that the power supply seems healthy still.

DC Volts at speaker - 39VDC

When you say "DC Volts at speaker" I hope that means where the speaker would be... If not get that speaker disconnected right away and do not connect it back up until the repair is complete. You should have only millivolts of DC on the speaker of a working amp, ideally zero, but typically there will be a little DC offset in millivolts.

With the amp off please take a look at C48, which as I recall is right on the little board that has the TDA2050A chip. With such high voltages hitting the amp I would also test that cap to make sure it has not been damaged.

[guiguy]

Oh okay the 145vac is for the plate of the 12ax7 preamp tube, forgot for a minute that was in there. 22.9vac is the voltage that feeds into the bi-polar power rails on the power amp chip and the preamp.



Note that each side of R92 would be 29.5vdc & 16.6vdc but on R93 it is -29.5vdc & -17.2vdc. It is bi-polar power supply so there will be a positive and a negative rail voltage from the power supply. Good news is that the power supply seems healthy still.



When you say "DC Volts at speaker" I hope that means where the speaker would be... If not get that speaker disconnected right away and do not connect it back up until the repair is complete. You should have only millivolts of DC on the speaker of a working amp, ideally zero, but typically there will be a little DC offset in millivolts.

With the amp off please take a look at C48, which as I recall is right on the little board that has the TDA2050A chip. With such high voltages hitting the amp I would also test that cap to make sure it has not been damaged.

Yeah, sorry about leaving out the "-" on the power supply negative side.
Yes indeed. I disconnected the speaker before doing the testing as suggested.

I have 58.5V across C48. What does that tell me?

[DrGonz78]

I have 58.5V across C48. What does that tell me?

To be honest I am not sure. With a fault in the circuit it might not be that C48 is a problem. Does C48 have any brown spot on it at all? Had similar issue with that C48 on a repair and the cap tested okay, but still it had issues when powered up. Reverse leads on the multimeter and test if it turns to -58.5vdc (be careful while measuring voltages not to short your leads to anything). I think measuring across the cap would show 58.5 volts one way and -58.5v the other way. With the amp off you could try testing the cap for resistance but still hard to test these little buggers. There is a fault condition here so things might not appear what they should be. In diode mode test diodes D3 and D4, do all resistance and diode tests with the amp off.

Edit: "I would think that C48 should not have a very large voltage when measured across it as 29.5vdc and -29.5vdc would cancel out as it is decoupled by the capacitor." I removed this statement since it is now contradictory to what I wrote as an edit.

[g1]

I think the AC line voltage needs to be checked. Maybe there is more than 120 or 125V there.
C48 is shown as a 50V cap. TDA2050 is +/- 25V max. With +/- 29V the chip and cap may both be damaged.
I guess it could be a meter inaccuracy also. There is 17V measured across a 15V zener ZD4.

[DrGonz78]

I think the AC line voltage needs to be checked. Maybe there is more than 120 or 125V there.
C48 is shown as a 50V cap. TDA2050 is +/- 25V max. With +/- 29V the chip and cap may both be damaged.
I guess it could be a meter inaccuracy also. There is 17V measured across a 15V zener ZD4.

It's the amp design that is bad I think. The C48 cap needs it rated voltage to be increased to 63v. Can't believe they designed it with voltages that exceed the safe operating limits of the TDA2050 chip.

Look at post #3 on this thread. I researched everywhere about this amp when I was fixing that one. I replace the TDA2050 chip with a LM1875.
http://music-electronics-forum.com/t43905/

[J M Fahey]

Thereīs some very inconsistent voltage values posted here which make me not trust bthem.
Please measure and post results:
1) mains voltage at *real* wall outlet. (where you plug "everything else": fridge/lamps/computer/TV/PC/whatever)
2) how are you getting 120V for the amp?
Do you use an autotransformer? ... Please post a picture and show which labels, if any, does it have.
Does it have a 120V outlet?
What is the real voltage you measure there?
3) what is the real voltage across PT primary?
4) what is the AC voltage at each transformer secondary wire?
Itīs weird that the service schematic does not show a secondary center tap, yet the "official picture" does: 2 red wires PLUS a presumably grounded black one:

Iīm not worried about the yellow HVAC wires yet.
5) what is the raw rectified +V and -V rails voltage?
measurable at C56/C57 ends and/or BR1 + and - ends and/or R92/93 left ends (as shown on schematic, they may point anywhere in the Real World)

To recheck doubtful values, please remeasure voltage across 15V Zeners.
IF yoou have a fresh 9V battery also measure it and post result.
IF you have another meter, check both measure the same at the same points.
Thanks.

[guiguy]

To be honest I am not sure. With a fault in the circuit it might not be that C48 is a problem. Does C48 have any brown spot on it at all? Had similar issue with that C48 on a repair and the cap tested okay, but still it had issues when powered up. Reverse leads on the multimeter and test if it turns to -58.5vdc (be careful while measuring voltages not to short your leads to anything). I think measuring across the cap would show 58.5 volts one way and -58.5v the other way. With the amp off you could try testing the cap for resistance but still hard to test these little buggers. There is a fault condition here so things might not appear what they should be. In diode mode test diodes D3 and D4, do all resistance and diode tests with the amp off.

Edit: "I would think that C48 should not have a very large voltage when measured across it as 29.5vdc and -29.5vdc would cancel out as it is decoupled by the capacitor." I removed this statement since it is now contradictory to what I wrote as an edit.

I checked C48 and it looked good, however, I changed it just in case. I also changed D3 and D4. I still have 30V DC at the speaker wires.

Hmmm.

I now have the board out of it so I can test anywhere because I can see all solder points. Where would be a good place to start?

[nosaj]

please answer juan's questions so that we may move forward. Randomly changing parts is a waste at this point. We should start at the wall socket voltage and move through the circuit from there. Answering Juan's questions will provide a lot of informative info .

nosaj

[Jazz P Bass]

If you have 30Vdc on the output then the TDA2050 ic is bad.

At least.