1973 Champ Gut or Repair?

[ndross475]

So I am heading down the amp building rabbit hole.... I have considered this for a while, been doing some research and watching forums like this and decided to take the plunge.

After consulting a local amp builder who advised me to go with a SF / BF chassis build as they were a bit bigger / easier for a noob to work on than some of the smaller 5f1 tweeds,  and he really liked Hoffman Turret boards. I began looking in that direction. I was looking at Vibro Champs and of course PR but I have a SF PR RI so didn't want "another" and that is a bit more involved for a first build so I was just reading....

 I stumbled across this "sometimes working" 1973 SF Champ and decided it fit the bill.  I liked the fact that it doesn't have tremolo and reverb circuits to contend with and its in pretty good cosmetic shape.

I had an idea of kit pricing so the sum of the parts was in line.... I will say,  I don't think amp building is really about "saving" money... Just more fun than golf.

I received it and shot some videos and pictures that I have hosted on Smugmug. https://nickdross.smugmug.com/73-champ/n-T4TjKs/i-KWcGqZW

Some of the video is a bit long,  was just documenting everything,,,

Once I opened it and saw what was inside I decided not to even try to plug it in... the burst paper cap still rolling around inside suggested a bit further inspection was required. 

I have spent some time looking at schematics and layouts and figuring out the R values,, Its slow looking them up but I am learning... It looks like someone got creative... There are some instances that match up with schematics and layouts from Triode and Fender and some, I don't have the experience to know...  The PT is original and dates, the OT is a Hammond replacement,  they obviously replaced the can with 3 Atom 20uf caps. I assume this is a new board... I don't know what an original looks like other than pics but notice the 2nd instrument jack has never been wired and the spot on the board looks never soldered... but some of the resistors look pretty old,,, Foresnsic Science here.  Why would someone pay for the Sprague Atoms and Orange drops but reuse the old parts? Tone?   Its a puzzle,,, but this puzzle will get loud someday.


I am enjoying the process and will keep updating.  I have a tech that will be providing some "parental guidance" but until we get together I thought I would post this and pose this question.

Since the " keep it original " ship has clearly sailed.... and this isn't the cleanest build, but there are some quality parts in there....

Would you start with a new board and source a small parts kit (salvaging what can be saved)?

Or trace out the issues and try and clean this one up.... Either way it will be an adventure....

Nick

[sluckey]

I would fix it just as it is.

Here's the schematic/layout...

     https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schem.pdf

[PRR]

Ditto.

The AA-Champ is a jewel, and worked great as built. This one is clean inside. It has been hacked by a small boy. All the hackery looks to be easily reversible. Go eyelet by eyelet. Is the solder joint good? Are the wires a good gauge and appropriate insulation? (Summa that looks like 9V fuzzbox wire.) Are the parts in the eyelet the right value, rating, and appropriate type? I would replace any 1974 Composition in a high-stress point (mainly plate resistors) with a up-rated Carbon Film (Leo wudda, if he had them, and if he was still there). Some of these parts look *older* than 1973-- clearing out the factory bins? Or later "tweaks"? Make it as new only better.

There's smutz on board near 6V6. Cockroach? eCap guts? That needs to be cleaned; I'd go over the whole board with alcohol and toothbrush. You can do this without stripping the board.

The red/black HV CT from the PT should return DIRECT to the - end of the first filter cap, not inject current spikes through 2 inches of chassis and a bolted joint. _I_ would tend to keep the separate-caps conversion because can-caps are gone out of style. (You can get them now, but $$$, and how about for the 2029 re-build?) I do wish the caps were smaller so their - ends could terminate on the board for easy removal. These seem to be >15 years old and there are better smaller fresher caps now.

Keep the Orange Drops. They may not be original but are blame-free. Check values- that one looks awful big and the codes are confusing.

[DummyLoad]

undo the crap work. restore it. that circuit in stock trim has mucho mojo amigo.


--pete

[mresistor]

I too can echo the posters above , restore it - it will be worth more..  I would also keep the power supply filter caps.. 

[jjasilli]

Champ Gut or Repair?
Given the small BOM, that's a distinction w/o a difference!    The Champ is a great way to get into amp work.  The best thing usually is to restore an amp to stock, then see how you like it.  Once it's working properly to spec, there are options to mod it.

[dude]

I have a 67 Vibro Champ, did basically what everyone above posted. Fantastic little amp. Check the speaker, that's it the original in good playing order or a spec'd replacement.
The only thing I did from all original was replace the stock OT, Leo liked to save $ and that 5 watt OT breaks up pretty soon. But you mentioned that it's a Hammond now. Check out that it's the correct replacement, but a slightly bigger OT does magic in my opinion. I found a paper wound small 8 or 10 watt OT on ebay under vintage electronics that gave me a little more headroom but make sure the impedance matches the 6V6.


al     

[ndross475]

Thanks for all the replys.  Its good info to digest. @prr was spot-on.  Much of it is smaller gauge wire and it was the remnants of a blown cap. The hammond OT is a 125cse, which appears to be a 8w replacement... The speaker is a common Warehouse replacement... I asked if seller had the original basket and no go... He did provide a champ OT, it isn't the original to this amp, dates 79, so I see no reason to look that way unless the Hammond doesn't work out...

I agree with all sticking to the original circuit is how I would like to go,,, 

The challenge for me being a novice is the Fender Schematic and Layout don't jive with what's here, i.e. the blown cap was a 22uf and spec was a 25,  some caps are dif spec than original, and there are others... now some of these changes may be intentional "voicing" changes, and some may be bass-ackwards hackery,,, I don't have the experience to know... Some of the values jive with a Triode layout I found for the Champ, but not all..

One thing I have been looking for, maybe in the wrong place is a wiring diagram / info for converting the can to these 3 atoms, thats a change that I don't know if is wired right or not as I don't have a layout... still looking if anyone can point me to a link for further study that would be appreciated.

I did read about "one grounding point to chassis" and it appears there are about 5 or 6 on this one....

It "appears" the only thing different or "wrong" than the way it was wired by "small boy" (@prr I still laugh about that one) is the blown cap,, now why it blew is another question...

If the plan is to keep this board, Which doesn't appear to be original, maybe replacing the blown cap, following a "first start checklist" and see if she will run stable long enough to trouble shoot....

The checklist pinned on this board is pretty detailed and following it will provide plenty of learning opportunities, which is why I got into this amp...and as long as I don't smoke the PT or OT (or me) there doesn't appear to be much here to lose...

Thanks again for all the suggestions...

Nick

[PRR]

> the blown cap was a 22uf and spec was a 25

All the same.

The actual need is "more than 5 or 10uFd". But a twenty+ is not much more cash, and can rot longer before you lose bass.

The old number-system had "25" as a Preferred Value. A later (still current) number-system has "22" as a Preferred Value. Just like we replace "40uFd" caps with 47uFd. Much like we replace old car tires "6.75"(inch) with new ones marked "175"(mm). (I used to know what the metric was for 1961 Willys or 1967 Cougar... I recall rounding-up to 275mm on a T-bird.)

[sluckey]

The 3 Atom filter caps are wired correctly. I compared your actual board to the board in the schematic/layout I posted. They are the same, so no need to replace the board. There are several resistors that are just "close" to the original values. I would say just replace resistors with the values shown on the layout drawing.

The NFB resistor has been removed from the board and the wire connecting that resistor to the speaker jack is missing as well. This is probably related to the OT change. There was probably a squeal with the new OT so rather than reverse the OT wires they just disconnected the NFB wire to stop the squeal. This is just a guess. Of course it's possible they just liked the sound better without the NFB.

It would probably take a knowledgeable tech a couple hours to neatly install the right components on the board and correct the wiring differences to return the amp to stock Fender design. This job can also be done by someone without any electronics background, but good soldering skills would be good. I believe you can handle the job, especially with your tech looking over your shoulders. Even without a local tech, we can help you get it done right. Just ask and post more hi-rez pics if you get stuck.

[ndross475]

@sluckey Thanks,  I did some digging last night and compared a few amps and felt pretty good about the atoms, thanks for confirming. 

I have picked up some test equipment and putzing with that today and gonna put together a current limiter .  I ordered some caps, resistors and wire last night should have it by the weekend.. more time for me to study....

On my list is to download a schematic of the Hammond and compare that to the original and see what that is about... I will look for reverse wiring....

I put a link in my original post to my photography website... I can host larger images there than can upload here... I will keep doing both....

I noticed a fair amount of corrosion / gunk on the 6v6, and on the socket...and it was kinda loose so I cleaned both up and tightened the pin receptacles on the socket a little.  I tested the 6v6 on my Heathkit, it tested good,  assuming the Heathkit is accurate....

Some time on youtube I found 2 aa764 champ repairs and the same cap was blown in both of theirs and the cause was directly related to issues with the 6v6... 


Nick

[mresistor]

I'm curious as to what the white wire is near the power cord grommet where the power cord enters the inside of the chassis? Is it going through the grommet to the outside of the chassis? Where does it go?  It's not stock.

[ndross475]

the white wire is pointless... don't know why it was there... its grounded to the chassis under a monster glob of solder.... Maybe easier to leave than remove?  Maybe a ground wire for a rainy day???

[mresistor]

I would remove it.. just clip it off and remove

[ndross475]

Yes, I agree... That and a good bit of the extras....  The plan is to get her fired up and stable and see what all is there,  then start troubleshooting issues and cleaning up previous messes.

I am struggling with keeping this board... I can be pretty OCD and the fact that things are neat doesn't sit well... i.e. the labels on the Atoms, not being straight and lined up....

[jjasilli]

Here's the inside of a Silvertone 1482. 

[sluckey]

Here's how that amp looked at one time...

[ndross475]

@sluckey... "at one time,,, long,,, long ago"...

That pic will be helpful Thanks.... Now for the Noob questions..

Following the advice of checking the NFB resistor and OT wiring. I pulled the Hammond schematic,  attached, as well as the MOJO OT schematic which I think matches original Fender wiring colors... Hammond does NOT....  So if I did this right... Hammond Brown = Fender Red and Hammond Greeen = Fender Yellow. Blacks and Blues match.... If thats the case then the Brown and Blue wires are reversed on my board vs the Fender AA764 Champ Layout....   I assume this is the reversal of the OT wires @Sluckey mentioned?  Score another for Sluckey!!.  Parts arriving Thur... but I missed the 2.7k NFB resistor... maybe I will be able to scrounge one around town... Until then more Uncle Doug YouTube videos and memorizing resistor codes...  Dave Hunter's book has been very helpful as well.

The patience I am exercising waiting to get this thing working is surprising me....

[PRR]

The Brn/Blu primary colors are NOT standardized. Even when you know which way it should go, sometimes the factory does a run the other way.

Leave NFB disconnected. Smoke test. Play test. It will be a little loud and raw.

Then connect NFB. If it howls, or gets louder/rawer, reverse the primary and try again.

The OT on a Champ is nominally 7K. That is between the choices of the Hammond 125SE series. It will work at 5K or 10K, perhaps very well, also depending if your speaker runs high or low of nominal. Use the 125 for now, but unless you want to deviate from what Leo liked, there may be a Champ Replacement OT in your future. (You can even get them from Hammond now.)

[ndross475]

So impatience got the best of me.... found some parts local... wired them in,, quick power up with the current limiter and no shorts, no smoke, and the jewel light came on...

After doing a thorough trace before power up I am not sure what's going on in the Tone stack but thats for later.... going to run through the "first power up tests" and see what voltages look like...

Nick

[ndross475]

She is working.... I triple checked voltages and swapped tubes and bias is at about 21watts...
Resistor: 465ohms
Plate: 386v
Drop: 26.5

keeping her shut off for now but happy with the progress...

[sluckey]

She is working.... I triple checked voltages and swapped tubes and bias is at about 21watts...

That's a hot champ. What output tube are you using?

[ndross475]

6v6gt.

Now the fun begins...

Old Tube:6v6gt Sovtek
Resistor: 470ohms
VD 24.9
Plate: 402

Replaced the tube:(6v6gt) Zenith
Resistor: 470ohms
VD: 26.5
Plate: 386

Changed resistor same 6v6gt Zenith
Resistor: 840 ohms
Plate 424
VD 32.9

Dropped from 21W to 16, but the plate and VD went up, if they stayed constant then the 840 would have brought me to 12w....   

here is where the education begins.... school me.

thanks for all the help.

nick

[ndross475]

went back to original Sovtek 6v6 and replaced the 470ohm resistor w/ a 1k 5w resistor and came up with the attached bias calc....

Replaced a tone pot and that's it so far...

Anyone see anything I should be wary of?

Thanks for all the help.

Nick

[PRR]

Does it play "Layla" yet?

[ndross475]

@PRR "Layla"  .... Not exactly... I got her fired up and played through her and she lives... and plays and sounds like a champ, or at least what I think one is supposed to sound like....

However she is still sick... Some hmmm any where past "5", especially in the "treble pot" and the output tube is loose and noisy and it gets pretty hot fairly quickly,,, didn't "test" it for long... But I did have everything at "10". That is how a Champ is supposed to be played right?

Now instead of "does this thing work?" I am in trouble-shooting a sick amp mode ... There are some values that don't jive with the schematic and I have been trying to figure out if any of them have anything to do with it...

Figured out wiring up an old scope and signal generator to her and all that is working, and doing its job showing me she needs some help...

I am pretty happy with the experience so far, have learned a lot, haven't smoked anything or me....

Part of me wants the immediate gratification of having someone "show me the answer" but the fun is in the puzzle,  for now...

Going to keep digging through the forums and Youtube and hopefully have a small parts list order in Sun night... I believe there may be a leaky cap somewhere and that both the 5y3 and 6v6 tube sockets could be replaced.. In my "professional" opinion ;-)