Bi-polar Power Supply Questions....

[Deric]

Trying to make something cool out of garbage.... I have a dead Fender Super Champ XD. The digital board died and Fender does not have replacements. Was on it's way to the dumpster. So I have a chassis, cab, speaker and transformers. Plan is to build a Brown Princeton and add solid state verb.

I copied the +/- 12vdc supply from the Super Champ schem. Same as the Super Champ X2 and easier to read schem on page 15 of this PDF...

file:///C:/Users/D/Downloads/Super%20Champ%20X2%20Service%20Manual.pdf

See the bottom right corner of page 15. I'm also using the doubler (top right of page 15) for the bias supply. Built up a PCB and it all works. Test voltages match them schem. But...I have one issue. C62 on the schematic gets hot (C3 on my layout). Really hot. I've double and triple checked, replaced parts etc. and it still gets hot. Just noticed that I don't have C63 but I don't really think it's needed.

Any ideas where to start? Pic and PCB layout attached (my PCB component numbers do NOT match the schematic).

Thanks for looking. All comments/help appreciated.

[LOUDthud]

The link to the pdf isn't right. It might work for you and pull it off your hard drive.

The layout looks correct. Either the board doesn't match the layout, or you have a bad part.

[sluckey]

Caps should not get hot. It's either bad or connected backwards. Either way, I'd replace it because it's definitely been stressed.

[Deric]

Thanks for the look guys.

Caps should not get hot. It's either bad or connected backwards. Either way, I'd replace it because it's definitely been stressed.

Agreed. Already replaced all caps. I've checked orientation several times but will have another look when I get home.

Not sure what happened to the link... I'll scan and post schematic when I get a chance.

[JazzGuitarGimp]

The layout is correct. Things to check:
1. Make sure C3 is not installed backwards.
2. Make sure D2 and D4 are installed the right way.
3. Check the DC voltage on the minus end of C3 w/ respect to GND to make sure it doesn't evceed the cap's voltage rating.
4. If you have an iscilloscope, look at the minus of C3 w/ respect to GND to make sure there isn't a large AC voltage present (which would imply a shorted D2 or D4).

[Deric]

The layout is correct. Things to check:
1. Make sure C3 is not installed backwards.
2. Make sure D2 and D4 are installed the right way.
3. Check the DC voltage on the minus end of C3 w/ respect to GND to make sure it doesn't evceed the cap's voltage rating.
4. If you have an iscilloscope, look at the minus of C3 w/ respect to GND to make sure there isn't a large AC voltage present (which would imply a shorted D2 or D4).

Lou,

Thanks for the thoughts....

-C3 double and triple checked polarity.
- D2 and D4 installed correctly
- minus side of C3 is -12.5vdc. Cap is rated at 50v
- no measurable AC on minus side of C3 - a VERY tiny amount of ripple on the scope. Same as the plus side of C4.


After double checking everything - again - I fired it up and let it run for 30-35 minutes. With a digital temp gun C3 gets up to about 175-180 F and stays there. Only takes 5 minutes ore so to get there then doesn't seem to get much hotter.

Replace D1, D2, D3 and D4. Disconnected the reverb driver part of the board and fired it up again. Same thing - climbs to 175-180 and stays there.

The board layout doesn't allow me to disconnect the bias portion without cutting a trace. Any chance the bias supply could be an issue? I guess I'll try lifting the ground of the bias supply and see what happens....

[JazzGuitarGimp]

The four wires entering the left side of the board come from your PT, correct? Do you have a spec for the PT? Also, a schematic would be helpful. The pdf is not working over here.

I am guessing the lower grey wire is the center tap for the two red wires. What doesn't make a lot of sense is the upper grey wire seems to be associated with one leg of the low voltage winding for the bipolar supply.

[Deric]

Here's the link to the Super Champ XD schematic. A little harder to read than the original link (Super Champ X2) but I think this one works (and it's the actual schematic for the amp I'm using)....

http://support.fender.com/schematics/gu ... ematic.pdf

The blue wires (from the PT) are the 30-0-30 vac secondary. The yellow wire is the center tap of the 30-0-30 vac. The black clip lead is grounding the CT. The green wire at the top is grounding the bias supply.

I can't find any specs for the PT.

[Deric]

Correction:

The blue-yellow-blue secondary measured 17-0-17 unloaded.

[JazzGuitarGimp]

Is the output of the bias supply giving you a believable voltage?

[Deric]

With the bias pot set most-negative I get a very steady -42.5 vdc.

All test voltages match the schematic.

[JazzGuitarGimp]

What happens if you lift one end of D8? You'll want to pop the TL072 out of the socket. The voltage at C3- should now be about -23V. But does C3 still heat up?

I apologize: I overlooked your question. I don't think the bias supply is causing C3 to overheat. After looking at the schematic, I am no longer confused about the bias supply. And from your most recent report, the bias supply is working as expected. However, you would only need to disconnect one end of the green wire to know for sure. At this point, nothing would surprise me - I must admit, I'm scratching my head on this one....

[JazzGuitarGimp]

And the layout you pisted "Super Champ PS Verb Rev2" - is this your layout, or Fender's layout? If it's Fender's layout, I have two more questions:
Can you post your layout?
Are you certain C3+ us connected to GND?

[Deric]

The board layout is mine. Attached some better pics of the board.

Verified C3+ is connected to ground.

Lifted D8. After 20 minutes C3 is running cool but notice C4 is pretty warm (~140 F). I assume it's been getting warm the whole time but with C3 getting hotter I didn't notice. Lifted D7. Now running room temp (74 F). Measure a steady +/- 23vdc. Bias voltage crept up a couple volts (now -44vdc).

[JazzGuitarGimp]

I would expect R1, R2, D7 and D8 to get warm, but not the caps. Have you noticed these four parts heating up? If so, is there a chance thet are radiating heat over to the caps?