Mods for Fender Champion 30

[J M Fahey]

TDA1514;
no load? no problem !
Shorted? ..... mmmmmhhh !!
In theory it has a short protection, in practice I don't trust it *that* much.
But you can test it at once, just reconnect it to the speaker and play.
What happens?
With amp off measure continuity from 1514 output pin to speaker hot leg, and from speaker ground leg to actual ground, maybe you have a jack or other connector problem.
And check that the speaker itself is not open.
As a parallel check, now turn amp on, measure DC voltage at 1514 output leg (a few mV) and try to have same at speaker hot leg, and "0" mV at the grounded leg.
Also remember TDA1514 has a mute function, don't have its operation very clear, description is somewhat confuse, but seeing how it's an RC wired to the +V rail, I think the pins 2+3 need to be a couple volts more positive than pin 4 (which also is the -V rail).
Post what you read there. V2/3 to V4 difference, not voltage to ground.
Apparently if that's not so, a perfectly good chip mutes.

As of all those TDA1514 from China, they stink more than Pliobond 25, try to avoid them if possible.

[km6xz]

If you need TDA1514's in the future, most of the large parts houses like Digikey do not stock them but consumer products/repair parts houses stock them. Try MCM Electronics, where they are $7.
As others have stated, a solid state amp does not need a load so any amp testing should be done with no load until it is determined the amp is working correctly, and only then, connect the speaker or dummy load.

[Steve A.]

Running a SS power amp into an open circuit should do it no harm at all. Now if the speaker wires had been touching each other, that would be bad.

After reconnecting the Jensen C10Q speaker one of the terminals broke off without me knowing it (not the first time for me with those new Jensens!) and that may have shorted out the two leads together. [Any suggestions for repairing broken Jensen terminals? I have another speaker like that.]

I was wondering about a good mnemonic trick to remind me to reconnect the speaker leads. I was thinking of an HVAC service tech I knew who would clip his keys to the disconnect box for an AC on the roof- to remind him to make sure that the power hadn't been left off when he left. (You would not believe how often that happened!)

What I came up with after the fact- I am great at closing barn doors after the cow gets out!- was to stick a piece of bright orange electrical tape over the power switch.

Thanks to everybody for the tips!

Steve Ahola

[Steve A.]

If you need TDA1514's in the future, most of the large parts houses like Digikey do not stock them but consumer products/repair parts houses stock them. Try MCM Electronics, where they are $7.

Thanks for the tip! The shipping charge for the $7.01 part is $9.49 so I probably ought to order something else that I need from them.

With amp off measure continuity from 1514 output pin to speaker hot leg, and from speaker ground leg to actual ground, maybe you have a jack or other connector problem.

Bingo! It looks like the shorting speaker jack done got shorted out. I get continuity from the 1514 output pin up to the ext speaker jack but no further. When I tried inserting a plug into the plastic jack it did not go in smoothly (it had worked properly before) so I have a hunch that the switching contacts got very hot.

Steve Ahola

[Steve A.]

It turned out to be a mechanical failure of the ext spkr jack!

When I took the pcb out of the chassis for further testing I once again had continuity from the 1514 output to the white speaker lead. I put the pcb back in and guess what- I lost the continuity. Just tightening down the nut on the jack evidently caused the switch to malfunction.

That was probably my own fault because when I was reassembling the amp one of the plastic ferrules that go around the pot shafts wasn't set on straight so when I tightened down the nut on the ext spkr jack it probably knocked something out of whack. Until I can get a replacement jack I replaced the old one with a jumper so that the internal speaker doesn't start cutting out. I managed to lose one of the metal jack nuts so I was going to have to track one of those down anyway.

Steve Ahola

P.S. The last 2 mods I did were to raise the capacitance of the caps associated with the treble pot to make the amp less bright, which it did. However after doing that the bass got much louder and would rattle the speaker and the cab at higher volumes (something it hadn't been doing before.)

There is a .33uF cap (C25) in the reverb splitter circuit which passes the audio signal through to the next IC stage. I lowered the value to around .12uF. That cut the boomy bass very nicely and I guess it added a little sparkle to the reverb circuit by reducing the isolation between the input and output. I love it- it is definitely not your typical sterile solid state reverb! The Normal channel is now getting a really nice distortion when you crank it all of the way (it could have something to do with the diode clippers ahead of the power amp chip.)

I think that M Cozza at Fender came up with a really nice design for this amp, especially with a little tweaking for my own personal tastes. I like how the normal channel distorts when you crank it up- it sounds very organic to me! As opposed to switching to a different model or switching in a distortion circuit. And it cleans up nicely when you back off the guitar volume control. In other words it pretty much works as you'd expect a guitar amp to work.

[Steve A.]

With the Champion 300 working great I just moved on to its "big brother," the Princeton 112+. The two amps have a similar vibe and I like the idea of having a small amp for small venues and a larger amp when I need more power- and the Princeton 112+ has plenty of that! The only mod I had to do to that was add a master volume so that I can throttle back the power when I don't really need 65 watts. I replaced a 100k resistor to ground (R39) right before the Preamp Out jack with a 100k pot. These amps are great for blues- especially when you don't want that damned Dual Rectifier sound that so many blues rockers use...

Thanks for all of the help!

Steve Ahola

EDIT After wiring up the MV as spelled out above (which did not sound very good at low settings) I took another look at the schematic and found a much better solution: R47(470k) is the NFB resistor for U3A right before the Preamp Out. I replaced it with a 500k linear pot which works perfectly- I think that my first MV was loading down the next stage when you turned it down.
Getting back to the Champion 300 I could add a similar MV to U4A with R47(10k) being the NFB resistor. It is bypassed by C31, a 1000pF cap which I think means that if I replace R47 with a 10k pot the signal will get brighter as I turn the MV down. For me that should be okay since I'm not adding a MV so that I can get super-distorted sounds at lower volumes- I just want to be able to cut the overall volume down a bit. If I need to cut the treble to compensate for the brighter signal output from U4A that isn't a problem. One more "wish" for the Champion- a Preamp Out. I'll try copying the circuitry from the Princeton 112+.

[Steve A.]

I just added the Master Volume!

I still wanted to add a MV to the Champion 30 because it is LOUD. With the Normal channel once you hit 3 the walls are shaking and the windows are rattling. Backing off on the MV a bit gives me a good tone at a lower volume. The other goal of the MV was to reduce the background noise of the amp for recording but as 52Bill here suggested the noise was in the 1514A power chip and not the preamp circuitry. No problem, but I do wonder if there might be a different brand 1514A that is quieter...

Here are the details of the MV. U4A is the last op amp stage before the 1514A, with R47(10k) being the NFB resistor. So I removed R47 and wired up a 10kB pot which I mounted on the rear panel in the vicinity of the power switch. The pot turned out to be only 9.25k so rather than downgrade the power of the amp I decided to add a 3k3 resistor in series to bring it up to 12.5k for a little boost when dimed. I measured the resistance before wiring it up and installed it so fully clockwise was set at 6:00 for 12.5k; 3:00 corresponded to 10k. 12:00 and 9:00 were two notches I didn't measure, but they are good settings. Fully counterclockwise was roughly 8:00 and with only the 3k3 resistance of the resistor it was too weak to be usable. (I can't imagine how it would have sounded without the resistor.)

Here's the wiring trick I used so that I did not have to remove the PCB again. I cut out R47 and then connected the two leads going to the pot to the "back" leg of R46 and the "front" leg of R62 (those two points are in common with the copper pads for R47.) Back and front are in reference to the layout drawing. You could have run the leads into the vacated copper pads for R47 but I do like to be able see the solder joint to make sure it is secure.

I'm really happy with the amp now- it does what I expect it do for a very rootsy blues/country sound. In some ways the Normal channel is more versatile than the one in the Princeton 112+ which is wired like a BF tonestack after the first gain stage, with the volume control installed in the NFB loop of the 2nd gain stage. With the Champion 30 you have 3 gain stages for shaping the sound before you reach the tone controls which are a lot more complicated than the BF tone stack and with the volume control ahead of them in the feedback loop of the 2nd stage. So while the P112+ Normal is closer to a BF amp I think that the Champion 30 Normal channel is more versatile.

For one thing the P112+ Normal channel doesn't do really well with high gain pedals since the BF-style tone stack has a scooped midrange and what you get is like a fuzz box plugged into a BF Twin Reverb, not the most refined sound. I think that a Texas Tea control would work great here! (On the output of the first tube stage you have a 0.01uF coupling cap in parallel to the treble cap which goes to a 3M pot wired as a variable resistor connected to the output of treble pot which is the input to the volume control. I probably have to scale the values a bit for the solid state circuit. When set to max the full signal from the 0.01uF cap is sent to the volume control which is like many tweed designs. The BF tone stack is still there but it is overpowered by the TT control when it is maxed.)

Thanks again for everybody's help on this!

Steve Ahola