Vox VT30 - loud humming

[jondoe]

VT30_SManual.pdf

I bought a Vox VT30 that was supposed to have an input jack problem, but it turns out the seller had been inside and disconnected the speaker to make it just seem dead . Once i'd plugged it back in the amp was putting out a very loud hum with no trace of the input signal. The DSP board is working fine with headphones everything functions as it should. I removed all the connectors and valve and the problem persists with just the power circuit board, so at least I've narrowed it down to there.

I've replaced the main filter caps c109, c108 and c126, c127 at the TD2050, nothing changed, I removed the TD2050 and it's not shorted. I used my DIY signal tracer and I can hear the same hum coming through the +18v/-18v DC rails feeding the TD2050. I don't have the hum on the 9 volt rails.

I'm not sure I should be using the signal tracer on the 18v rails like that, should I? I'm still new to this

Any pointers of where to look next gladly taken

[J M Fahey]

1) disconnect speaker NOW, youŽll burn it.

2) with speaker disconnected, measure DC voltage across speaker cable terminals, I bet youŽll find many volts DC there.

3)

I removed the TD2050 and it's not shorted.

itŽs not a simple component like a diode or transistor but has a cpuple dozen parts inside, so a simple short testbtekls you little to nothing in this case.

The test is "functional": measure voltage at each pin and post here.

4)

c109, c108 and c126, c127

mean nothing without a schematic, sorry.

[jondoe]

Thank's for the reply The schematic is attached to my first post, but I've noticed it's not very obvious, it's right at the very top, the first thing.

I'll get the TD2050 soldered back in and test for DC on the speaker in a bit.

[jondoe]

Further to my previous post, I've popped the TDA2050 back in and I am getting 24 volts DC at the speaker output.

Voltage readings on the TDA pins are as follows;

1 0
2 -9.1
3 -24.4
4 -24.4
5 +24.5

[J M Fahey]

Ouch!! didnŽt see it the first time.
That said, I opened it and pages 5 and 6 , the schematic proper, are blank on my side.
But Adobe Reader is nagging me about an update, so maybe thatŽs the problem, IŽm updating now.

IF not possible for any reason, IŽll tell you how to turn those pages into a regular .gif or .jpg which can be displayed here straight inside the post.

In any case, you have a very dead TDA2050, try to get another.

One caveat: itŽs out of production, most were swept from dealerŽs shelves about 2 years ago, most remaining ones are fakes (relabelled TDA2030) , specially Oriental ones (EBay, Ali Baba, etc.)

Try your luck at some respected supplier: Farnell, Mouser, etc. but donŽt hold your breath.

If fully unavailable, get an LM1875 (again from a respected supplier) which are still in production and "almost" the real thing.

>>>>>>> nope, still blank pages 5 and 6

1) open your service manual copy

2) go to page 5 (blank in my copy) ... click the middle of the page, so Adobe knows you are there.
Set Zoom to 100%

3) open some Graphics editor in another window.
I still use old reliable Irfan View, but any one will do, including Photoshop of course.
Not sure about Windows Paint (does it still exist?) but it will probably do, and leave it open, then go back to Acrobat Reader.

4) on top menu click Tools > Select & Zoom > Snapshot Tool.

5) cursor shape will change, click and drag without lifting your finger the top left schematic corner, slowly drag it down right towqards the opposite corner until you covered the full schematic.
Give it some time, since schematic is larger than screen it will take a few seconds.

6) when you release your finger, Adobe will tell you "Snapshot taken" or something similar.

7) click the middle of the screen, then on Menu: "Paste Image"
Image should appear on screen.

8) on menu: "Save Image" or equivalent.
Choose "save as" (say, on your desktop) "VT30SCH01.gif"

9) do same with next schematic page, saving as "VT30SCH02.gif"

10) upload both as images inside a post.
If not as "images", then try uploading them as "attachments": open "Go Advanced" and upload them clicking the small paperclip on the new menu.

You should be able to upload something like this:

[jondoe]

I've ordered up some supposedly new old stock of TDA2050's, two, just in case the first one goes bang

I've printed pages 5 and 6 into a new PDF document via a different PDF creator, this might help >> 5 and 6.pdf << it's quite a large detailed set of pages so I'm not sure screen shots would be detailed enough, but if that doesn't work I'll go down the JPG route. I think it breaks due to embedded Chinese fonts, which aren't embedded in the version I've created.

Thanks!

[g1]

That pdf (post #6) works.

[jondoe]

While waiting for the part to arrive I've cobbled it back together and tested for a signal tone and DSP function all the way up to C10, just before the TDA2050, so at least I know the DSP board and the rest of the preamp's are working.

Fingers firmly crossed.

[jondoe]

OK, so the plot thickens, new chip has arrived.

A little background, when I took this apart I noticed the heat-sink compound seemed very "wet" and new, which given it's age seemed strange, I'd expect it to be fairly dried/thick by now. I also noticed the joints on the TDA2050 looked a little fluxy. Anyhow, I fitted the new chip, powered it up and the sparks flew, it turns out the heat sink was conducting -23v from the chip tab to ground, it's not insulated, but I rebuilt it with everything that came off it originally. I've checked my parts try and I didn't remove any insulating washers, so now I'm thinking someone else fitted a new chip, left off the insulating washer, shorted it to hell (as it's not fused) and the chip went bang.

[g1]

Agree. The svce manual you posted shows a silicon pad #30 on parts list, but you can also use mica & heat sink grease.

[jondoe]

Ahh yes, well spotted, part 29, insulating bushing, thankfully the silicon pad is still on the heat sink, along with a large amount of silicone on grease everywhere, it's even slapped all over the base of the heat sink where it makes contact with the chassis, which seems excessive.

[Enzo]

I'm thinking someone else fitted a new chip, left off the insulating washer,

thankfully the silicon pad is still on the heat sink,

I am confused, which is it?

The chassis is part of the heat sinking, so thermal grease between heat sink and chassis helps heat to flow into the chassis. This aids cooling.

[jondoe]

Sorry, maybe I didn't explain that so well. I'll use a picture.



I have part 30, but not part 29.

I've just pulled apart an old ATX power supply and found a suitable bush, will reassemble and test.

[The Dude]

Check for DC before you hook up a speaker. There's a good chance took out your new part.

[jondoe]

Good call, I did test that, no DC on the speaker output, signal getting all the way through, background hum seems a little louder than I'd expect, this amp has a power level adjust on the back, 0.2w to 30w, at full 30w she's a little hissy and 100hz hummy, but it's working. I'll tinker more tomorrow, a cup of tea and some sleep are required