Fender Champ AA764 I think '73

[SAG Electronics]

This has been my nightmare of a first repair. Friend (customer) asked me to fix his Dad's old amp from college. Feeling cocky and fresh outta school (ish) I told him that I thought it was the tubes and I would give it a go. Replaced the tubes, no luck, blew a fuse, so I took it apart (removed the head.) Saw right off the bat a burned resistor so I began testing all the resistors and replacing them as needed. Read forums about the problem and was pointed toward the capacitors. Replaced all the capacitors with modern equivalents. Still no luck (fuses 3-5.) Kept reading different forums and testing other parts of the circuit and components. Replaced the power transformer (It was literally warped) and the choke (fuses 5-10.) The darn thing still blows 1A slow blow fuses as soon as I add a tube. I have tested every element along the way, from new resistor values to new capacitor values to voltages from the new transformer, to resistance of the choke's primary winding (impedance?) I am almost at my wit's end. I know that I went about this in the wrong order (now, at least,) but I have quite a bit invested and still cannot make the amp work. Any suggestions from you fellow solder sniffers?

[52 Bill]

Slow down and start thinking logically. Build a light bulb limiter and stop blowing fuses.

I don't think that Champs have filter chokes, please explain.

[The Dude]

No disrespect intended, but this is a perfect example of why it's not a good idea to shotgun replace parts as a repair method. You need to, as Bill says, logically troubleshoot the amp. Let's start with this. Is this the correct schematic for your amp?

https://elektrotanya.com/PREVIEWS/63..._sch.pdf_1.png

[Enzo]

OK, which model and year are you getting from the amp itself? Does it say 1973? AA764 indicates a design from 1964. But you have a choke, and if I recall well, only the 5E1 had a choke.

One mantra I give my students is this: it isn't about the parts, it is about the circuit.


You can test every part on it, and it won't help you find a tube socket with a pin shorted to ground.


You mentioned the "primary" on the choke. Chokes do not have primaries, they only have one winding.

Parts with "wrong" values rarely blow fuses, they just affect signal QUALITY.

A choke winding may have resistance, but what matters is the winding does not show shorted to the frame.



SO putting in a tube blows fuses? OK, does that mean it wil sit there with power on and not blow a fuse if ALL tubes are out of it? Then you say one tube is enough to blow fuse. Does it matter which tube? Seems to me if you leave out the rectifier tube, then the other two tubes wouldn't have any high voltage to short out.

The problem with replacing a zillion parts is you now have a zillion new places for a wrong connection.

[The Dude]

...... and FWIW, the actual chassis/model # should be stamped on the tube chart inside the cabinet.

[SAG Electronics]

Yes that is the same schematic as the one I have been referring to. Tube chart was partially missing had to do a lot of cross referencing to figure out the model and year. I have made painstakingly sure each part that I have replaced has the same values as the original (according to schematic and/or actual.) Thanks Dude, no disrespect felt, I know (now) what I have done wrong and how and why I did it that way. As I said, I have learned a LOT rising to this challenge. My growth on this venture has made me want to start building these things for christsakes. I have made a lot of new contacts for parts and gained a lil bit o' clout by taking this on.

Bulb limiter I will look that up.

As far as choke, that is just what I have learned to call the smaller transformer. I called it a primary cuz it has 4 wires suggesting a primary and a secondary and differing winding resistances between the two.
I would post a picture, but it will not let me.

[Helmholtz]

As far as choke, that is just what I have learned to call the smaller transformer. I called it a primary cuz it has 4 wires suggesting a primary and a secondary and differing winding resistances between the two.

That's your output transformer (OT). Secondary connects to the speaker jack.
Transformer and chokes ( = inductors) may look similar but are different and are used for different purposes. Both are inductive components having a magnetic core.

[SAG Electronics]

Thanks for clearing that up. Terms folks use for things can get confusing. Try common names for plants in a native gardening group! lol! Drives me nuts. Yes, Output transformer. At least I knew it was a transformer.

[SAG Electronics]

I WILL build a Lightbulb Limiter! Thank you Bill!

No model info on the tube chart. Boo. however, the wiring in the head perfectly matches the schematic that matches the schematic in the above post. Glad I had this one in a book for quick reference.

[Helmholtz]

Output transformer. At least I knew it was a transformer.

A choke only has 2 (used) terminals and is not a transformer. Generally transformer need to have at least 3 (but mostly 4 or more) used terminals.
Chokes are often used in tube amp power supplies to smooth out ripple current.
Transformers have a primary and at least one secondary winding and are used to transform (step up/ step down) AC voltages or impedances and/or to isolate two circuits.

[SAG Electronics]

Then the fault was in where I got my information from initially. Thank you for your input. I may now be able to tell the difference when I encounter one in the future.

[Leo_Gnardo]

Bulb limiter I will look that up..

Our friends at The Amp Garage have an excellent sticky on the Light Bulb Limiter. Here's the link:

https://ampgarage.com/forum/viewtopic.php?f=6&t=20341

[SAG Electronics]

Limited information there, as I am not a member. Thanks anyway. Headed to the hardware store for the bits I do not have on hand.

[g1]

I hope you kept the transformer you replaced. Warped is not a description we would recognize so maybe post some pics?
There may not be anything wrong with it.

[Helmholtz]

Limited information there, as I am not a member. Thanks anyway. Headed to the hardware store for the bits I do not have on hand.

Just google for "Lamp Bulb Limiter".