Vox Cambridge 30 Reverb - blown amp

[bluesky6]

I just got in a new dead Vox Cambridge 30 Reverb. This is the hybrid one. Eb*y lurkers may know it as the recently ended one that needed TLC and which would be all spanking new again with a capacitor.

Yeah right. Someone's going to get negative Eb*y points for misrepresentation. Not that I'm even going to try to ship the thing back.

Anyway, I bought it as a repair project.

I plugged it the amp, with all knobs turned down, stuffed cotton wool into my ears and flipped the power switch. No sound, no light.

I opened up the back of the amp. First thing I noticed was the blown fuse.

I started to scan the circuit board with a flashlight (and glasses/spectacles off, obligation of age) and the next thing I see is a burnt TDA2050. The package is cracked and pin 4 (output) is black/charred.



That was easy.

I'll probably go get a couple of TDA2050s just-in-case and also since almost every SS amp I have uses some variant of it.

Question for the pros on the forum: what else should I look for?

[Jazz P Bass]

It may simple be a blown TDA IC.

With the IC removed, you may want to check the power supply voltages.

[bluesky6]

Thanks. What should be TDA2050 power supply voltage be? The schematic I have just says MV.

[teemuk]

Thanks. What should be TDA2050 power supply voltage be? The schematic I have just says MV.

What's the voltage rating of main filter caps? The voltage rails will be close to that, maybe few volts lower. Overall you will be more interested in seeing that both positive and negative voltage are "symmetric" and that the voltage isn't "alarmingly" lower than that of the filter cap ratings. e.g. If filter cap ratings are, say, 35V expect to measure maybe 30 - 35 volts, be alarmed if you measure something like 18 volts. Similarly, be alarmed if one rail measures, say, +30VDC and the other only, say, -25VDC. You want to see such "symmetry" of these rails that ensures that their "mid point" is about zero volts.

[bluesky6]

The new TDA2050s arrived. So I'm in the process of taking stuff apart. The fuse calls for a 1.25A 250V slow blow but Radio Shack only has either 1A or 2A. Ugh.

The power supply voltages are somewhere in the +/- 28V. That's a little high for the TDA2050, it seems.

I'll swap in one and see what happens. If the amp blows again, I'll be sorely tempted to put in one of those little LM3886 kits that one can get on Eb*y for less than US$20 shipped. Since I play only at living room volumes, there'll be no risk of overloading the PT.

[52 Bill]

The new TDA2050s arrived. So I'm in the process of taking stuff apart. The fuse calls for a 1.25A 250V slow blow but Radio Shack only has either 1A or 2A. Ugh.

The power supply voltages are somewhere in the +/- 28V. That's a little high for the TDA2050, it seems.

I'll swap in one and see what happens. If the amp blows again, I'll be sorely tempted to put in one of those little LM3886 kits that one can get on Eb*y for less than US$20 shipped. Since I play only at living room volumes, there'll be no risk of overloading the PT.

Use the 1 amp for testing. Once the amp is working get the correct fuse.

Do you have a light bulb limiter set up? It will help save your fuses and perhaps your new output chips as well.

[bluesky6]

So everything works. Although it was TV time when I put everything together, so I didn't spend too much time on the amp or cranking up the volume.

I measured the output pin of the TDA2050 daughterboard and saw 8 ohms on the meter which matches the impedance of the speaker (direct coupled).

Soldered in the new chip, cleaned up the flux with an alcohol wipe, plugged in a 1A fuse, zero-ed all the knobs, plugged in the mains, flipped the switch and ducked. Other than the power-on thump, things sounded ok. Plugged in the guitar, upped the volume, strummed a few notes, got yelled at, and things still sounded ok. The reverb sounded like it should. Trem was a little weird, but the opto had been replaced with a 5-legged unit, so I assumed that the adjustment was out.

I'll go hunt for the right fuse today and will play more.

The next question obviously if whether this is a keeper. I already have a Twin which I like a lot.

[Jazz P Bass]

[QUOTE=bluesky6;348621]So everything works. Although it was TV time when I put everything together, so I didn't spend too much time on the amp or cranking up the volume.

I measured the output pin of the TDA2050 daughterboard and saw 8 ohms on the meter which matches the impedance of the speaker (direct coupled).

Soldered in the new chip, cleaned up the flux with an alcohol wipe, plugged in a 1A fuse, zero-ed all the knobs, plugged in the mains, flipped the switch and ducked. Other than the power-on thump, things sounded ok. Plugged in the guitar, upped the volume, strummed a few notes, got yelled at, and things still sounded ok. QUOTE]

Flipped the switch & ducked!
Upped the volume, strummed a few notes, got yelled at.

Hardy, har, har.
Made my day with those statements.

[bluesky6]

Flipped the switch & ducked!
Upped the volume, strummed a few notes, got yelled at.

Hardy, har, har.
Made my day with those statements.

True fact, though.