Vox Pathfinder 15R: +15V rail pulled down

[glebert]

http://musikding.rocks/wbb/index.php...-gif/?e7a10a6b

I've got a Pathfinder 15R that has no power light and no sound other than a slight thump to the speaker on power up/down. Probing the voltage rails I find that the +/- 20V supplies are good, as is the -15V. The +15V rail is sitting at a fraction of a volt. Anyone ran into this problem? I don't want to just start lifting ICs and jumpers if there is a known issue. R47 is OK. I have run into similar issues with a zener "regulated" 15V rail from a failing electrolytic cap to ground, so I lifted C47 (even though not electrolytic) but that wasn't it. I also tried probing at various ICs to try to find the lowest voltage (assuming closest to short) but it is in the noise with my DMM. The power IC only runs on the 20V rails so it shouldn't be that. Maybe the zener? Looking at the board it looks like IC2 may have been reworked or installed by hand (some flux crud on the bottom).

Thanks in advance,
Greg

[DrGonz78]

Test ZD1 using the multimeter on a diode check setting to see if it has shorted out. At least get that one out of the way first.

[g1]

You will need to lift an end of the zener to check, as anything pulling down that line will show across the zener.

[The Dude]

If not that, then check to see if any op amps or other parts tied to that rail are getting hot.

[glebert]

Man, I am an idiot. Of course the problem was the electrolytic C33, right next to the zener. There's a story of why I didn't think it was bad, but let's just stick with the idiot excuse. Put in a new cap and it fired up fine, nice sounding little amp.

[The Dude]

Well, it could have been anything connected to that rail, so I wouldn't beat yourself up too much. Glad you got it!

[Familyortiz]

Man, I am an idiot. Of course the problem was the electrolytic C33, right next to the zener. There's a story of why I didn't think it was bad, but let's just stick with the idiot excuse. Put in a new cap and it fired up fine, nice sounding little amp.

I was curious... does the bad cap show a short when you measure resistance across it? I've found that even leaky caps can show a relatively low resistance (<100kohm) when measured and this can be a handy method of finding leakers. Good job! When something gets repaired, there's no room for self immolation!

[glebert]

It measures as less than 1 ohm resistance. The whole rail was measuring as essentially that when it was in the circuit. The last time I had this happen it was on a little Ashdown combo and they had used a 16v cap on a 15v rail, so pretty tight to spec, but this one it was a 25v cap but still failed anyway.

[J M Fahey]

I was chiming in to suggest you "why did you pull C47 while the first cap you find is C33?" but I see you found it all by yourself.
Congratulations