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Vox BM1 - battery/mains Escort - no output

Started by Stevo1987, May 25, 2018, 06:51:13 AM

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Stevo1987

"If I am not mistaken, the heatspreader has tabs that solder to the ground tabs." - Correct. Hopefully I'll be able to have a good look soon as I'm a bit busy at the moment.
Thanks for all your input.
Steve

solderer25

"If I am not mistaken, the heatspreader has tabs that solder to the ground tabs."
Better not try and pull it off then - my mistake sorry.  xP

Stevo1987

No problem. Hopefully I can have a go at this come the weekend.

Stevo1987

I think I have manged to work out which pins are which. There is no voltage on pin 1 or 4. i.e.no supply voltage to the chip. From the bridge rectifier (18Vdc) it goes to a 1000uF 25v capacitor. I get 18v on one leg and 0V on the other leg.  That's how far I have got....                                                                                                                                                                                                                                 

Jazz P Bass

Just to double check that you have the pins correctly identified:
Amp Off. Meter on Ohms scale.
Pin 1 & 4 should be 100 ohms apart.

If Pin 4 is at the same potential as any of the tabs, that would possibly make it pin 10 instead of 4.

Stevo1987

 Checks out as 100ohms between pin 1 and 4.  :dbtu:

Jazz P Bass

So, either the power supply is not supplying or the switch is bad.

Of coarse you could have an open connection from the switch to the ic.

J M Fahey

Quote from: Stevo1987 on May 28, 2018, 05:29:04 PM
Yes, I know that.  :) I just can't see it because of the heat sink (?) or grounding arrangement on top of it.
Ok, do this:
1) check that you have proper voltage at the power supply, we expect something around +18V DC

2) you have 2 *possible* Pin 1 , on opposite diagonals.
*One* of them will have same voltage as from supply ... that is Pin 1.
Mark it somehow, a felt marker dot, a drop of nail enamel, and you´re done.
Then count other pins relative to this one, following the datasheet.

3) measure DC voltage at ouput pin 12, we expect 1/2 the supply voltage, so some +9V DC

4) measure bootstrap volotage on pin 4 , we expect 3/4 of supply voltage or around +12/13V.

5) now to preamp: we expect some 6 or 7 V on Q1 collector.

IF DC voltages are reasonable, then active components are working, lack of audio most probably comes from connection problems, dirty/open pots and jacks, etc.

Do NOT replace parts at random, no matter whether they are "cheap" or even free, you are damaging the PCB, the *only*  component you can not buy over the counter or even at the (long defunct) Factory.
***THE PCB IS THE AMPLIFIER, PARTS ARE JUST ACCESSORIES***

Stevo1987

Showing 18V at power supply.
1  17.8
4  17.8
8  0
12 0.22

collector - 9.9V I think this is correct very hard to see. The two other legs had about 0.2v. The transistor is a PN930.

Pins 1/4 showing 17.8V is a change though. Before I couldn't get anything I did have a look at the batt\mains switch as it was loose.

J M Fahey

Quote from: Stevo1987 on June 04, 2018, 04:51:11 PM
Showing 18V at power supply..... check
1  17.8 ..... check
4  17.8 ..... would have expected between 9V and, say, 12 or 14V
8  0  ...... check
12 0.22 ..... too low.
Quote
collector - 9.9V I think this is correct very hard to see. The two other legs had about 0.2v. The transistor is a PN930.
Check.

QuotePins 1/4 showing 17.8V is a change though. Before I couldn't get anything I did have a look at the batt\mains switch as it was loose.
Good.
Pity chip amp looks dead.
IF you can get another for a reasonable price, order it.

Stevo1987

I have ordered two.  :) I'll let you know how I get on. Thanks for everyone's help. Could be a week or so before I know but I will be back.

Stevo1987


J M Fahey

Just in case we are missing something , let´s recheck voltage on all pins.
It doesn´t hurt and we might get a surprise.
So check and post voltage on *all*  pins, even those which "officially" are not connected.

Main problem on the old one was 0.22V on Pin 12, which is speaker out, and should be around +9V, but something I found somewhat weird was you found full 18V on Pin 4 ... it´s a bootstrapped output, is fed by a 100 ohm resistor and some current passes through it, so thanks to the series 100 ohm resistor it should be at least 1V below 18V ... yet you reported 18V with no losses.

Of course, since chipamp was damaged, that was possible.

But with a new presumed good amp it should show what I expect.
In fact in my original post I suggested "3/4 of 18V".

So remeasure and repost, but feel confident that absolute worst case we can Mod this amp to use an easy to find TDA2030 or LM1875.

But let´s try once more to make the original work.  :tu:

Almost forgot: use a loupe or strong looking glasses (+3 or +4) and look around all chipamp solder pads and tracks leading to them ... we might have a broken/cracked track.

Stevo1987

Will do. I'm not for giving up. Thanks for getting back to me.