Vox Tonelab SE "No Sound"

[Lostfollicles]

Hi Guys

Decided to buy a broken "Vox tonelab SE" for 50 quid thinking it was something simple to fix as there was no sound; Doh! Wrong!!!

I turned it on and all seemed OK all the led's lit up digital dispaly showed program number ect...... but there was no sound getting through the unit from my guitar although there was a hiss which audable from the output and could be attenuated by the volume control on the rear of the unit! OK I thought let's have a look inside for "hopefully" something loose! Wrong again! However I did find that one of the two DSP chips on the main PCB KLM2455(A) a Texas Instrument TMS57070FPJ was so hot that you couldn't keep your finger on it!

Now I know that this chip is now discontinued and there are no data sheets available either but it seems readily available from many different sources, however before I decide to buy one and have a go at soldering those tiny 100 little connections, I've never tried a heat gun!

I thought I should ask you guys a couple of questions: -

1: What do you think caused this chip to fry?
2: Could it be a problem with the power supply to the ic/ic's?
3: Could an overload on the input fry the chip?
4: Is this a common problem?
5: Is there a way to tell if other chips are affected?
6: And finally! Do you think it's worth fixing!

Regards and thank you in advance for any help or suggestions!

Lostfollicles

[Enzo]

I don't know that it is fried, it is just hot. Might be, but never assume.


If it is bad, you may never know why it failed. There may not be some one to one reason for it.

Check your power supply voltages if you suspect them.

As a DSP IC, I suspect it is in the middle of the circuit, not exposed to the outside world. Does it have ADC/DAC on the IC or are they separate?

APPly a signal to your unit, and see how far along the signal gets.


And if the part is obsolete, but you find it other places, be careful. Is this DSP chip programmable? If so, it may be available blank but not programmed. Like a ROM chip you can get blank but not with the program burned into it for some old system.

[Mick Bailey]

I'd start by downloading the spec sheet for the chip and checking the DC voltages as well as the logic states of any error lines. Components can run pretty hot and still be within spec, so what appears to be hot to you may not necessarily be hot for the chip. If you have freezer spray try chilling the chip to see if the signal kicks in.

Some chips provide DC outputs for support circuitry - check to see if your chip has them and make sure they're ok.

A hot chip can mean a faulty external component. If the chip has the capability to drive something (such as an LED, or provide a switching output of some kind) then a load on that line due to a low resistance or short can cause excess current draw.

[Jazz P Bass]

Schematic: http://www.valvetronix.net/docs/Tone...ice_manual.pdf

[Lostfollicles]

Thank you Enzo /Mick Bailey/ Jazz P Bass
Although I do have the correct service manual for the Tonelab(SE) Jazz P Bass, I am afraid this stuff is way out of my league!

As for "Hot" Enzo the chip is so hot that you cannot leave your finger tip on it, the other (same spec) DSP chip is warm but not hot!
I agree with Mick Bailey regards an external device possibly causing the chip to over heat, unfortunately the spec sheet is not available but also my electronic skills and lack of test equipment stop me in my tracks, even with the service manual!?

It looks increasingly like this thing is destined for the bin as I really cannot afford to send it for repair (I live in Andalucia Spain) with no Vox service centre anywhere near me that I know of?

It looks like I have had to learn the hard way, this will teach me a lesson in trying to buy a bargain, I suppose with nothing to lose I could replace a few suspect components related to this DSP but I may do even more damage and waste my time and hard earned money!

Thanks anyway guys for your help!

Regards

Lostfollicles