Fender Champion 600 Low Volume

[themusic]

I recently purchased a used champion 600 with a tone mod.
When I turn it on I only get about 25% volume.
Replace tubes , checked speaker , check all solder joints (twice)
Check circuit board traces for continuity ok.
Pulled and tested replaced parts (there was a mod diagram inside )
R2 , R8 , C3 , R19, C1, all tested good.

I chopstick all components and board while running with guitar signal.. same low volume.
I just found the schematic..Any suggestions on were to start...Thanks



.

[Enzo]

Start by checking the DC voltages at the tube pins, are they reasonable? The schematic also has AC signal voltages, set up the conditions, are they similar to yours? Your volume control and jacks are off the board with a 5-pin connector, explore that. Have you tried both input jacks, are both affected?

[JHow]

I recently purchased a used champion 600 with a tone mod.
When I turn it on I only get about 25% volume.
Replace tubes , checked speaker , check all solder joints (twice)
Check circuit board traces for continuity ok.
Pulled and tested replaced parts (there was a mod diagram inside )
R2 , R8 , C3 , R19, C1, all tested good.

I chopstick all components and board while running with guitar signal.. same low volume.
I just found the schematic..Any suggestions on were to start...Thanks



.

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

Is this the schematic for your amp? Did you check the DC voltages as shown here? ( I assume you can do this if you are chopsticking things?) What does/is the mod supposed to do?

[JHow]

Ha! I see Enzo posted faster! Ha ha.

[themusic]

Yes that is the schematic. Here are my readings
12ax7 pins 1 & 6 260vdc 173 vdc
pins 3 & 8 .023vdc 1.35vdc
6v6 pins 3 353vdc
pin 4 352vdc
pin 8 21.9vdc
pin 2&7 3.8 & 3.1 ac
Here are my test point readings tp1-12.9mvac tp2-varies irratic 13-20mvac tp3 45.8mvac tp4 9.2mvac tp5 24.3mvac tp6 .023vdc tp7 1.34vdc tp8 21vdc
tp9 3.68vac Thanks

[eschertron]

Yes that is the schematic. Here are my readings
12ax7 pins 1 & 6 260vdc 173 vdc
pins 3 & 8 .023vdc 1.35vdc
6v6 pins 3 353vdc
pin 4 352vdc
pin 8 21.9vdc
pin 2&7 3.8 & 3.1 ac
Here are my test point readings tp1-12.9mvac tp2-varies irratic 13-20mvac tp3 45.8mvac tp4 9.2mvac tp5 24.3mvac tp6 .023vdc tp7 1.34vdc tp8 21vdc
tp9 3.68vac Thanks

Pins 1 and 3 of the 12AX7 look like a place to start troubleshooting. If pin 3 (cathode) is 0.023vdc then it's way off. should be, like pin 8, somewhere around 1.5vdc for a normal bias point. Pin 1 (plate) is riding high because there is very little current flowing through the gain stage.

I see that you've verified values of R2 and C3. I still suspect something in that area. Check the jumper. Is the jumper not making good connection? Is it off by 1 pin (or connected backwards)?

FWIW, I have enjoyed modding my champion 600, and it has been a great learning tool. I'm sure you will get yours going and learn a bunch in the process!

[g1]

First half of V2 does not seem to be working. There should be over a volt at the cathode (pin3) like there is at pin8. Are both sides heater lit for that tube?
You said you checked R2 and R8 for that tube, did you resolder that socket?

Edit: Simul-post with Eschertron, different things to check for the same symptom (V2A not conducting).

[JHow]

Pins 1 and 3 of the 12AX7 look like a place to start troubleshooting. If pin 3 (cathode) is 0.023vdc then it's way off. should be, like pin 8, somewhere around 1.5vdc for a normal bias point. Pin 1 (plate) is riding high because there is very little current flowing through the gain stage.

I see that you've verified values of R2 and C3. I still suspect something in that area. Check the jumper. Is the jumper not making good connection? Is it off by 1 pin (or connected backwards)?

FWIW, I have enjoyed modding my champion 600, and it has been a great learning tool. I'm sure you will get yours going and learn a bunch in the process!

I was wondering about J19-J20 jumper as well, like maybe reversed or off a pin.

[Enzo]

FWIW my first response to the voltages is the one side of the tube is dark, no heater.

[JHow]

FWIW my first response to the voltages is the one side of the tube is dark, no heater.

Your FWIW is better than most. So, heater is burned between 4 to 9 or 5 to 9? Or since this is In a board, sone heater connection or trace is open? Did the OP try swapping the preamp tube?

[Enzo]

Open heaters are possible, but rare, so I'd be thinking broken connection on socket or bad contact on socket pin. But a break between the cathode pin and the top of the cathode resistor is also possible.

[themusic]

check pins and sockets all ok. new 12ax7.
Question I get continuity from pin 3 to ground all the way through c3 ,r2
I've pulled and I thought they tested within specs ?? Thanks

[Enzo]

If pin 3 measures about 1500 ohms to ground, then teh circuit is intact, tough that doesn;t verify the socket makes good contact on the tube base pins.

SO then with the tube in place and the amp on, do BOTH little orange spots glow in that tube?

[themusic]

Both orange spots glow . Made a test probe from guitar and was able to get
normal volume from probing R20,R22... off of pin 8 Gotta be in that stupid c3 r2 area.
I've wriggled tube, poke ,prodded.
I'm still confused that pin 8 path thru c10 & r3 seem normal I can't get continuity to ground on the + side.
But on pin3 thru c3 & r2 it's continuity all the way to ground . Should n't I have the same resistance
I guess I'll pull C3 and r2 and replace with fresh...thanks

[Enzo]

Let us not use the term continuity. Let us use ohms instead. From pin 3 of the tube, we should get 1500 ohms to ground, more or less. We don't know how many ohms "continuity" is. From pin 8 we should also get 1500 ohms (well 1500 plus 47) to ground.

DOn't just throw parts in there. If you think R2 is open, measure R2, right at the resistor itself. If the resistor measures 1500 ohms, then replacing it with another one that measures the same won;t do anything. If the resistor checks open right at its own leads, then it is bad.

If the resistor itself is OK, and you have no proper resistance from the tube socket, then measure each part of the circuit path. IN other words, ground your meter, now use the red probe and measure resistance to the ground end of the resistor. It OUGHT to measure about zero ohms - a direct connection. If not, find the break. Then move the probe to the top end of the resistor, you now ought to get 1500 ohms. If you get 1500 ohms there but not at the tube socket, then there is a break between the resistor and the socket. If the circuit trace is broken between resistor and socket, replacing resistors is a waste of time and resistors. FIND OUT what the problem is and fix it. DOn;t just guess and toss parts at it.

ANything is possible, but in my experience, an open 1500 ohm cathode resistor is exceedingly rare.