Vox Cambridge 30 disaster

[guiguy]

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

Hey guys,

I must have missed Juan's post.

I think I need to clarify something. I blew this amp in China a few years ago. My wife was posted there so I sent an amp with her. I am back in Canada now and am testing it with a real live 120V system. So here are the new numbers that I just retested:

Line voltage - 119V
Voltage going in to Tranny - 119V
Secondary at red wires - 46V from red to red or 23V from each red to ground
Yellow wires - 292V between each and 146V between each wire and ground

Ground to ZD4 - 0V on one side, -17.1V on other
Ground to ZD3 - 0V and 16.8V
Ground to ZD1 - 0, 9.3v
Ground to ZD2 - -9.2, 0V

Ground to R92 - 16.8V and 29.6V
Ground to R93 - 17.1V and 29.6V

[J M Fahey]

1) by any chance,did you buy that amp in Japan?
because all of your voltages are about 15% high ...Japan version asks for 100VAC straight, not 110/115/120 .

2) or maybe your meter is low on battery (in that cases meters tend to read higherv than normal) or itīs plain poorly calibrated.

In any case, even if those voltages are real, your amp can *still* be repaired and will work ... only problem is that it will be on the edge and blow again sometime in the future.

3) build a lamp bulb limiter (search the Forum), use a 40/60W lamp there (oldstyle filament or semi new style quartz-bulb-inside-a-larger-bulb , we need a red hot wire inside, so no LED/CFL/etc. and plug bthe amp there.
All controls on 0, no speaker or any other load connected.

4) remeasure voltages which will be somewhat lower because bthe lamp eats something, and check voltage at each of the 5 pins of TDA2050.

If you still have undesirable high DC voltage at speaker out, replace TDA2050.
Buy it from a reputaboe seller, such as Mouser or Digikey or Farnell, etc. , NOT from EBAY.

If you canīt get it, buy an LM1875 , again from trusty suppliers.

Replace it on the PCB, remount with fresh mica and grease, etc, turn amp on and remeasure voltages.
They *should* be fine.
Post results.

[DrGonz78]

1) by any chance,did you buy that amp in Japan?
because all of your voltages are about 15% high ...Japan version asks for 100VAC straight, not 110/115/120 .

This was exactly what I thinking too, makes sense. I had very similar issue with one I was working on and yes it was USA model 120v. The voltage reading on the amp I was working on was -/+28vdc, -/+16vdc and -/+9.1vdc. Could it be possible that they screwed up on many USA 120v models by installing Japanese 100v power trannys? Makes me wonder a bit. Also, the Cambridge 30 twin is like the same amp and there is a schematic where someone added what the "HV" was and it was -/+27.1v. Look at Cambridge 30 Twin schematic attached... So there are accounts of voltage exceeding the rated max value for the 2050 chip.

Definitely do not put a 2050 chip in the amp at this point and go with the LM1875 from Mouser. Change C48 to 1uf 63v while your at it.
https://www.mouser.com/ProductDetail...MPC3hA8Z5YI%3d

Edit: The thing to note about the LM1875 chip is that the max voltage rating is 60v and not 50v like the TDA2050. So, -/+30v power supply max will be okay with what is in the amp. However, the way the 1875 chip mounting orientation is different and you have to carefully bend the legs to adjust it to fit. Order a few just in case the bending process goes bad. The LM1875 I installed just barely fit into place but it did work out. I have messed up a few IC chips by bending the legs badly over the years. At this point I got it down but when I first started... OH MAN!!

[DrGonz78]

I must have missed Juan's post.

Guiguy look at the top of the page thread on the right for "Display". Click it and select "Linear Mode". Many people miss postings when set to Hybrid or Threaded mode. Probably why you did not notice his post.

[guiguy]

This was exactly what I thinking too, makes sense. I had very similar issue with one I was working on and yes it was USA model 120v. The voltage reading on the amp I was working on was -/+28vdc, -/+16vdc and -/+9.1vdc. Could it be possible that they screwed up on many USA 120v models by installing Japanese 100v power trannys? Makes me wonder a bit. Also, the Cambridge 30 twin is like the same amp and there is a schematic where someone added what the "HV" was and it was -/+27.1v. Look at Cambridge 30 Twin schematic attached... So there are accounts of voltage exceeding the rated max value for the 2050 chip.

Definitely do not put a 2050 chip in the amp at this point and go with the LM1875 from Mouser. Change C48 to 1uf 63v while your at it.
https://www.mouser.com/ProductDetail...MPC3hA8Z5YI%3d

Edit: The thing to note about the LM1875 chip is that the max voltage rating is 60v and not 50v like the TDA2050. So, -/+30v power supply max will be okay with what is in the amp. However, the way the 1875 chip mounting orientation is different and you have to carefully bend the legs to adjust it to fit. Order a few just in case the bending process goes bad. The LM1875 I installed just barely fit into place but it did work out. I have messed up a few IC chips by bending the legs badly over the years. At this point I got it down but when I first started... OH MAN!!

I ordered a LM1875 chip. It should take a few days to get it. Is the issue that the pins are spaced differently of do they have to cross over to a different hole?

[DrGonz78]

No the pins spacing are fine and the legs match the other chip just fine too. It's the way the chip mounts. For example, I would say the chip in your Vox amp is standing up where as this chip would be lying down. The legs on these chips come pre-bent to help you mount it up easier. This LM1875 chip will have the legs bent different and it might make it more difficult to mount it. If you take your time with it then you'll be just fine.

[J M Fahey]

According to datasheet they are pin compatible.
But TO220-5 packages come in 2 versions; the most common one id the "V one, meant for "vertical" mounting, so pins are straight and it looks like a regular TIP31 or 7815 which grew 2 extra legs.
The "H" version is meant for "horizontal" mounting so legs are bent accordingly.
In any case you can go from one version to the othjer in less than 5 minutes by using longnose pliers and a little skill.

H:


2 versions of V: staggered and straight. No big deal.




pin numbers and functions remain the same.

[guiguy]

So I replaced the chip and there is no more noise from the speaker and I can get the amp to work like it is supposed to. Yay! Thanks folks!

However, there is a strange kind of distortion that is most apparent on the clean channel. There is no way to get a true clean,warm sound. The clean channel has a bit of a robotic quality to the notes. It is hard to describe. With the overdrive kicked in in channel 2 you can hide it but the clean channel is rather quiet and has a warbly crustiness that was not there before I abused it.

I know I am being vague but does anyone have any insights?

Pat

[DrGonz78]

First off you need to rule out the speaker as being an issue since it was exposed to excessive DC voltage. So quickest way is to hook up another speaker and test it out. That would be the first thing to test. Once you have ruled out the speaker then let us know the outcome. Also, research the tremolo adjustments for that amp. Letting too hot of a signal to pass through the tremolo bias circuit might make it sound rough too. Then again turn one of those bias pots down too much and it kills the signal.

[J M Fahey]

Here an overheated speaker voice coil developed a bad case of bubbles, literally, which scratch the center pole when playing and cause all kinds of funny ugly noises.



Unfortunately, voice coil is not technically "burnt", speaker keeps working, but even a small single bubble ruins it.

To rule this out (or confirm it) try your amp with other speaker, any speaker, we are only testing for presence/absence of funny noises.

You can also test your original speaker with any other amp you have.

Just play a full chord on the clean channel and let it die, it should stand up.

[guiguy]

Here an overheated speaker voice coil developed a bad case of bubbles, literally, which scratch the center pole when playing and cause all kinds of funny ugly noises.



Unfortunately, voice coil is not technically "burnt", speaker keeps working, but even a small single bubble ruins it.

To rule this out (or confirm it) try your amp with other speaker, any speaker, we are only testing for presence/absence of funny noises.

You can also test your original speaker with any other amp you have.

Just play a full chord on the clean channel and let it die, it should stand up.

So, I tried another speaker. It is not the speaker. The sound I am getting is a robotic kind of distortion when playing any note at any volume with the clean channel. It is more obvious when playing at low volumes.
Hmmmm.
Pat

[g1]

How about if you set up the dirt channel as clean as possible (volume full, slowly bring up gain control) ?
Can you hear the distortion or not?

[guiguy]

How about if you set up the dirt channel as clean as possible (volume full, slowly bring up gain control) ?
Can you hear the distortion or not?

Yeah, it is the same in both channels.

[guiguy]

First off you need to rule out the speaker as being an issue since it was exposed to excessive DC voltage. So quickest way is to hook up another speaker and test it out. That would be the first thing to test. Once you have ruled out the speaker then let us know the outcome. Also, research the tremolo adjustments for that amp. Letting too hot of a signal to pass through the tremolo bias circuit might make it sound rough too. Then again turn one of those bias pots down too much and it kills the signal.

Hey there,
Well, I have checked all those things. It is still sounding rather harsh. After adjusting the tremolo trim pots, I got it a little bit better but it is not the warm amp it once was. Would the tremolo opto coupler be a good thing to try replacing? Are they also sensitive to over voltages?

Pat

[g1]

The opto-coupler was notorious for causing various faults in these units. You could just try bypassing it. You will not have tremolo, but if the 'bad' sound disappears, then you know it was caused by the opto.