Fender Superchamp XD, no sound

[rbhill]

I am trying to repair a fender superchamp XD, it has no sound from the speaker but the line out does work. The tubes have been replaced with no change. I am getting ready to check the test points since I found schematics I can read on this forum. Any help would be appreciated

[Enzo]

So connect the amp to a different speaker. ANy sound there? And disconnect the existing speaker and test it with some other amp.

[rbhill]

Thanks for the advice I tried that and I have sound and control on the line out of the amp but not the speaker out. If I hook the superchamp speaker up to another amp it works fine. While testing the voltages at the test points on the schematic everything is fine up to test point 9 which is a capacitor on the 12AX7 tube circuit, so I am now back to studying the schematic to see which component might be the cause.

[Enzo]

TP9 is the cathode of the 12AX7. Should see about +1.7VDC there. Close is good enough. You don't get that? If you get zero, then the tube is not conducting. If the heaters in the tube are not glowing, that explains it. Otherwise, try a different 12AX7. And if the high voltage supply is absent that would do it too.

Are the tube heaters glowing? Nothing will come through if they are not.

Worry about the square test points first, the DC voltages. So TP9 is not right. How about the rest of the power amp? Got high voltage?

If the 230/270v on the plates of the 12AX7 are more like 350, then again, the tube is not conducting. And at the power tubes. Is there B+ voltage on pin 3 AND pin 4 of each tube? 375v it says. Won't work without that.

You could have a bad component, but when repairing stuff, don't lock yourself into thinking it is always a bad part. Bad connections, cracked solder, cracked pc board traces, and other things are very common problems.

[rbhill]

The heaters in the tubes are glowing and I have changed the 12AX7, both tubes were glowing. When I get back to troubleshooting it I will check the DC voltages. Thanks

[rbhill]

Checked DC voltages to the tubes found TP30 0vdc and TP29 370vdc visually
R79 is discolored and appears to have gotten hot, and shows open when checked.

[Enzo]

Well, that will do it. replace the bad resistor, but it died for a reason. Most likely reason is a bad power tube. Make sure to check the two 470 ohm screen grid resistors in the power tube stage, R4, R25. If either is damaged, the tube in that socket is probably bad. Other possibilities are less likely but still possible.

[Jazz P Bass]

Fender Super Champ XD Service Manual

Here is the service manual.
Unusually, on page 5, they spell out the signal path.
The bias should be set with the tubes installed & warmed up.
Test point 22 indicates 40mv's, cathode voltage.

[rbhill]

Thanks for the link to the service manual. After replacing R79 the amp has worked fine with no more problems but I suspect I will be back into in the future

[jrfrond]

Cheap Chinese tubes installed in these amps caused lots of open R79's, sometimes with an associated filter cap going bad. Very common. If you didn't replace the original tubes, get 'em out of there...fast!

[Captain]

This link appears to be broken, and I've been unsuccessful in a search for it elsewhere. Would like a copy of the Super Champ XD Service Manual if anyone can send a copy to me. Thanks.

[Enzo]

From Google:

http://support.fender.com/schematics..._schematic.pdf

http://bmamps.com/Schematics/fender/...amp_XD_sch.pdf

[Captain]

Thanks Enzo. I had no trouble finding the schematic. What I cannot find is the complete Service Manual. There are posts which refer to "page ##" of the Service Manual, which supposedly includes a full circuit description (or at least signal path description.) I would like to obtain a copy of the complete Service Manual if one is available somewhere.

Also, can someone please confirm the value of R79? The schematic is not quite clear, but appears to say "Fr1k" (1000 ohm?) 2 watt resistor. I've never seen the "Fr" designation. What does that mean? My R79 is burned at the third band (should be red for a 1k I think) but the third band looks brown, not red. I think it's just a discoloration from overheating, but want to verify the value before ordering replacements. Also wondering if I should go for something larger than a 2-watter? These are likely wire-wound resistors?

I should have added that one of the Fender branded Groove Tube 6V6's showed a short on pin 4 (grid 2) after the failure occurred. The second 6V6 tested fine on my ancient Heathkit IT-17.

[Jazz P Bass]

Here is the Service Manual.

Super_Champ_XD.zip

R79 is a 1K, 2 watt metal oxide (flameproof) resistor.
http://www.farnell.com/datasheets/423658.pdf
I would not advise upping the wattage.
In this case, you wanted it to fail rather than something else, (like a copper trace)

[Captain]

Thanks so much Jazz P Bass for the link and the info. Will order a couple today, along with some new tubes.